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Title: The Student's Elements of Geology
Author: Sir Charles Lyell
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Produced by Sue Asscher asschers@dingoblue.net.au
THE STUDENT'S ELEMENTS OF GEOLOGY.
BY SIR CHARLES LYELL, BART., F.R.S.,
AUTHOR OF "THE PRINCIPLES OF GEOLOGY," "THE ANTIQUITY OF MAN," ETC.
(FIGURE A. FROM BOTTOM TO TOP:
PRIMARY OR PALEOZOIC. Bronteus flabellifer.
SECONDARY OR MESOZOIC. Ammonites rhotomagensis.
TERTIARY OR CAINOZOIC. Nummulites laevigata.)
(FIGURE B. Thecosmilia annularis.)
WITH MORE THAN 600 ILLUSTRATIONS ON WOOD.
PREFACE.
The LAST or sixth EDITION of my "Elements of Geology" was already out of print
before the end of 1868, in which year I brought out the tenth edition of my
"Principles of Geology."
In writing the last-mentioned work I had been called upon to pass in review
almost all the leading points of speculation and controversy to which the rapid
advance of the science had given rise, and when I proposed to bring out a new
edition of the "Elements" I was strongly urged by my friends not to repeat these
theoretical discussions, but to confine myself in the new treatise to those
parts of the "Elements" which were most indispensable to a beginner. This was to
revert, to a certain extent, to the original plan of the first edition; but I
found, after omitting a great number of subjects, that the necessity of bringing
up to the day those which remained, and adverting, however briefly, to new
discoveries, made it most difficult to confine the proposed abridgment within
moderate limits. Some chapters had to be entirely recast, some additional
illustrations to be introduced, and figures of some organic remains to be
replaced by new ones from specimens more perfect than those which had been at my
command on former occasions. By these changes the work assumed a form so
different from the sixth edition of the "Elements," that I resolved to give it a
new title and call it the "Student's Elements of Geology."
In executing this task I have found it very difficult to meet the requirements
of those who are entirely ignorant of the science. It is only the adept who has
already overcome the first steps as an observer, and is familiar with many of
the technical terms, who can profit by a brief and concise manual. Beginners
wish for a short and cheap book in which they may find a full explanation of the
leading facts and principles of Geology. Their wants, I fear, somewhat resemble
those of the old woman in New England, who asked a bookseller to supply her with
"the cheapest Bible in the largest possible print."
But notwithstanding the difficulty of reconciling brevity with the copiousness
of illustration demanded by those who have not yet mastered the rudiments of the
science, I have endeavoured to abridge the work in the manner above hinted at,
so as to place it within the reach of many to whom it was before inaccessible.
CHARLES LYELL.
73 Harley Street, London,
December, 1870.
CONTENTS.
CHAPTER I.
ON THE DIFFERENT CLASSES OF ROCKS.
Geology defined.
Successive Formation of the Earth's Crust.
Classification of Rocks according to their Origin and Age.
Aqueous Rocks.
Their Stratification and imbedded Fossils.
Volcanic Rocks, with and without Cones and Craters.
Plutonic Rocks, and their Relation to the Volcanic.
Metamorphic Rocks, and their probable Origin.
The term Primitive, why erroneously applied to the Crystalline Formations.
Leading Division of the Work.
CHAPTER II.
AQUEOUS ROCKS-- THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral Composition of Strata.
Siliceous Rocks.
Argillaceous.
Calcareous.
Gypsum.
Forms of Stratification.
Original Horizontality.
Thinning out.
Diagonal Arrangement.
Ripple-mark.
CHAPTER III.
ARRANGEMENT OF FOSSILS IN STRATA-- FRESH-WATER AND MARINE.
Successive Deposition indicated by Fossils.
Limestones formed of Corals and Shells.
Proofs of gradual Increase of Strata derived from Fossils.
Serpula attached to Spatangus.
Wood bored by Teredina.
Tripoli formed of Infusoria.
Chalk derived principally from Organic Bodies.
Distinction of Fresh-water from Marine Formations.
Genera of Fresh-water and Land Shells.
Rules for recognising Marine Testacea.
Gyrogonite and Chara.
Fresh-water Fishes.
Alternation of Marine and Fresh-water Deposits.
Lym-Fiord.
CHAPTER IV.
CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.
Chemical and Mechanical Deposits.
Cementing together of Particles.
Hardening by Exposure to Air.
Concretionary Nodules.
Consolidating Effects of Pressure.
Mineralization of Organic Remains.
Impressions and Casts: how formed.
Fossil Wood.
Goppert's Experiments.
Precipitation of Stony Matter most rapid where Putrefaction is going on.
Sources of Lime and Silex in Solution.
CHAPTER V.
ELEVATION OF STRATA ABOVE THE SEA.-- HORIZONTAL AND INCLINED STRATIFICATION.
Why the Position of Marine Strata, above the Level of the Sea, should be
referred to the rising up of the Land, not to the going down of the Sea.
Strata of Deep-sea and Shallow-water Origin alternate.
Also Marine and Fresh-water Beds and old Land Surfaces.
Vertical, inclined, and folded Strata.
Anticlinal and Synclinal Curves.
Theories to explain Lateral Movements.
Creeps in Coal-mines.
Dip and Strike.
Structure of the Jura.
Various Forms of Outcrop.
Synclinal Strata forming Ridges.
Connection of Fracture and Flexure of Rocks.
Inverted Strata.
Faults described.
Superficial Signs of the same obliterated by Denudation.
Great Faults the Result of repeated Movements.
Arrangement and Direction of parallel Folds of Strata.
Unconformability.
Overlapping Strata.
CHAPTER VI.
DENUDATION.
Denudation defined.
Its Amount more than equal to the entire Mass of Stratified Deposits in the
Earth's Crust.
Subaerial Denudation.
Action of the Wind.
Action of Running Water.
Alluvium defined.
Different Ages of Alluvium.
Denuding Power of Rivers affected by Rise or Fall of Land.
Littoral Denudation.
Inland Sea-Cliffs.
Escarpments.
Submarine Denudation.
Dogger-bank.
Newfoundland Bank.
Denuding Power of the Ocean during Emergence of Land.
CHAPTER VII.
JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH'S
CRUST.
How we obtain an Insight at the Surface, of the Arrangement of Rocks at great
Depths.
Why the Height of the successive Strata in a given Region is so disproportionate
to their Thickness.
Computation of the average annual Amount of subaerial Denudation.
Antagonism of Volcanic Force to the Levelling Power of running Water.
How far the Transfer of Sediment from the Land to a neighbouring Sea-bottom may
affect Subterranean Movements.
Permanence of Continental and Oceanic Areas.
CHAPTER VIII.
CHRONOLOGICAL CLASSIFICATION OF ROCKS.
Aqueous, Plutonic, volcanic, and metamorphic Rocks considered chronologically.
Terms Primary, Secondary, and Tertiary; Palaeozoic, Mesozoic, and Cainozoic
explained.
On the different Ages of the aqueous Rocks.
Three principal Tests of relative Age: Superposition, Mineral Character, and
Fossils.
Change of Mineral Character and Fossils in the same continuous Formation.
Proofs that distinct Species of Animals and Plants have lived at successive
Periods.
Distinct Provinces of indigenous Species.
Great Extent of single Provinces.
Similar Laws prevailed at successive Geological Periods.
Relative Importance of mineral and palaeontological Characters.
Test of Age by included Fragments.
Frequent Absence of Strata of intervening Periods.
Tabular Views of fossiliferous Strata.
CHAPTER IX.
CLASSIFICATION OF TERTIARY FORMATIONS.
Order of Succession of Sedimentary Formations.
Frequent Unconformability of Strata.
Imperfection of the Record.
Defectiveness of the Monuments greater in Proportion to their Antiquity.
Reasons for studying the newer Groups first.
Nomenclature of Formations.
Detached Tertiary Formations scattered over Europe.
Value of the Shell-bearing Mollusca in Classification.
Classification of Tertiary Strata.
Eocene, Miocene, and Pliocene Terms explained.
CHAPTER X.
RECENT AND POST-PLIOCENE PERIODS.
Recent and Post-pliocene Periods.
Terms defined.
Formations of the Recent Period.
Modern littoral Deposits containing Works of Art near Naples.
Danish Peat and Shell-mounds.
Swiss Lake-dwellings.
Periods of Stone, Bronze, and Iron.
Post-pliocene Formations.
Coexistence of Man with extinct Mammalia.
Reindeer Period of South of France.
Alluvial Deposits of Paleolithic Age.
Higher and Lower-level Valley-gravels.
Loess or Inundation-mud of the Nile, Rhine, etc.
Origin of Caverns.
Remains of Man and extinct Quadrupeds in Cavern Deposits.
Cave of Kirkdale.
Australian Cave-breccias.
Geographical Relationship of the Provinces of living Vertebrata and those of
extinct Post-pliocene Species.
Extinct struthious Birds of New Zealand.
Climate of the Post-pliocene Period.
Comparative Longevity of Species in the Mammalia and Testacea.
Teeth of Recent and Post-pliocene Mammalia.
CHAPTER XI.
POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS.
Geographical Distribution, Form, and Characters of Glacial Drift.
Fundamental Rocks, polished, grooved, and scratched.
Abrading and striating Action of Glaciers.
Moraines, Erratic Blocks, and "Roches Moutonnees."
Alpine Blocks on the Jura.
Continental Ice of Greenland.
Ancient Centres of the Dispersion of Erratics.
Transportation of Drift by floating Icebergs.
Bed of the Sea furrowed and polished by the running aground of floating Ice-
islands.
CHAPTER XII.
POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS, CONCLUDED.
Glaciation of Scandinavia and Russia.
Glaciation of Scotland.
Mammoth in Scotch Till.
Marine Shells in Scotch Glacial Drift.
Their Arctic Character.
Rarity of Organic Remains in Glacial Deposits.
Contorted Strata in Drift.
Glaciation of Wales, England, and Ireland.
Marine Shells of Moel Tryfaen.
Erratics near Chichester.
Glacial Formations of North America.
Many Species of Testacea and Quadrupeds survived the Glacial Cold.
Connection of the Predominance of Lakes with Glacial Action.
Action of Ice in preventing the silting up of Lake-basins.
Absence of Lakes in the Caucasus.
Equatorial Lakes of Africa.
CHAPTER XIII.
PLIOCENE PERIOD.
Glacial Formations of Pliocene Age.
Bridlington Beds.
Glacial Drifts of Ireland.
Drift of Norfolk Cliffs.
Cromer Forest-bed.
Aldeby and Chillesford Beds.
Norwich Crag.
Older Pliocene Strata.
Red Crag of Suffolk.
Coprolitic Bed of Red Crag.
White or Coralline Crag.
Relative Age, Origin, and Climate of the Crag Deposits.
Antwerp Crag.
Newer Pliocene Strata of Sicily.
Newer Pliocene Strata of the Upper Val d'Arno.
Older Pliocene of Italy.
Subapennine Strata.
Older Pliocene Flora of Italy.
CHAPTER XIV.
MIOCENE PERIOD.-- UPPER MIOCENE.
Upper Miocene Strata of France.
faluns of Touraine.
Tropical Climate implied by Testacea.
Proportion of recent Species of Shells.
faluns more ancient than the Suffolk Crag.
Upper Miocene of Bordeaux and the South of France.
Upper Miocene of Oeningen, in Switzerland.
Plants of the Upper Fresh-water Molasse.
Fossil Fruit and Flowers as well as Leaves.
Insects of the Upper Molasse.
Middle or Marine Molasse of Switzerland.
Upper Miocene Beds of the Bolderberg, in Belgium.
Vienna Basin.
Upper Miocene of Italy and Greece.
Upper Miocene of India; Siwalik Hills.
Older Pliocene and Miocene of the United States.
CHAPTER XV.
LOWER MIOCENE.
Lower Miocene Strata of France.
Line between Miocene and Eocene.
Lacustrine Strata of Auvergne.
Fossil Mammalia of the Limagne d'Auvergne.
Lower Molasse of Switzerland.
Dense Conglomerates and Proofs of Subsidence.
Flora of the Lower Molasse.
American Character of the Flora.
Theory of a Miocene Atlantis.
Lower Miocene of Belgium.
Rupelian Clay of Hermsdorf near Berlin.
Mayence Basin.
Lower Miocene of Croatia.
Oligocene Strata of Beyrich.
Lower Miocene of Italy.
Lower Miocene of England.
Hempstead Beds.
Bovey Tracey Lignites in Devonshire.
Isle of Mull Leaf-Beds.
Arctic Miocene Flora.
Disco Island.
Lower Miocene of United States.
Fossils of Nebraska.
CHAPTER XVI.
EOCENE FORMATIONS.
Eocene Areas of North of Europe.
Table of English and French Eocene Strata.
Upper Eocene of England.
Bembridge Beds.
Osborne or St. Helen's Beds.
Headon Series.
Fossils of the Barton Sands and Clays.
Middle Eocene of England.
Shells, Nummulites, Fish and Reptiles of the Bracklesham Beds and Bagshot Sands.
Plants of Alum Bay and Bournemouth.
Lower Eocene of England.
London Clay Fossils.
Woolwich and Reading Beds formerly called "Plastic Clay."
Fluviatile Beds underlying Deep-sea Strata.
Thanet Sands.
Upper Eocene Strata of France.
Gypseous Series of Montmartre and Extinct Quadrupeds.
Fossil Footprints in Paris Gypsum.
Imperfection of the Record.
Calcaire Silicieux.
Gres de Beauchamp.
Calcaire Grossier.
Miliolite Limestone.
Soissonnais Sands.
Lower Eocene of France.
Nummulitic Formations of Europe, Africa, and Asia.
Eocene Strata in the United States.
Gigantic Cetacean.
CHAPTER XVII.
UPPER CRETACEOUS GROUP.
Lapse of Time between Cretaceous and Eocene Periods.
Table of successive Cretaceous Formations.
Maestricht Beds.
Pisolitic Limestone of France.
Chalk of Faxoe.
Geographical Extent and Origin of the White Chalk.
Chalky Matter now forming in the Bed of the Atlantic.
Marked Difference between the Cretaceous and existing Fauna.
Chalk-flints.
Pot-stones of Horstead.
Vitreous Sponges in the Chalk.
Isolated Blocks of Foreign Rocks in the White Chalk supposed to be ice-borne.
Distinctness of Mineral Character in contemporaneous Rocks of the Cretaceous
Epoch.
Fossils of the White Chalk.
Lower White Chalk without Flints.
Chalk Marl and its Fossils.
Chloritic Series or Upper Greensand.
Coprolite Bed near Cambridge.
Fossils of the Chloritic Series.
Gault.
Connection between Upper and Lower Cretaceous Strata.
Blackdown Beds.
Flora of the Upper Cretaceous Period.
Hippurite Limestone.
Cretaceous Rocks in the United States.
CHAPTER XVIII.
LOWER CRETACEOUS OR NEOCOMIAN FORMATION.
Classification of marine and fresh-water Strata.
Upper Neocomian.
Folkestone and Hythe Beds.
Atherfield Clay.
Similarity of Conditions causing Reappearance of Species after short Intervals.
Upper Speeton Clay.
Middle Neocomian.
Tealby Series.
Middle Speeton Clay.
Lower Neocomian.
Lower Speeton Clay.
Wealden Formation.
Fresh-water Character of the Wealden.
Weald Clay.
Hastings Sands.
Punfield Beds of Purbeck, Dorsetshire.
Fossil Shells and Fish of the Wealden.
Area of the Wealden.
Flora of the Wealden.
CHAPTER XIX.
JURASSIC GROUP.-- PURBECK BEDS AND OOLITE.
The Purbeck Beds a Member of the Jurassic Group.
Subdivisions of that Group.
Physical Geography of the Oolite in England and France.
Upper Oolite.
Purbeck Beds.
New Genera of fossil Mammalia in the Middle Purbeck of Dorsetshire.
Dirt-bed or ancient Soil.
Fossils of the Purbeck Beds.
Portland Stone and Fossils.
Kimmeridge Clay.
Lithographic Stone of Solenhofen.
Archaeopteryx.
Middle Oolite.
Coral Rag.
Nerinaea Limestone.
Oxford Clay, Ammonites and Belemnites.
Kelloway Rock.
Lower, or Bath, Oolite.
Great Plants of the Oolite.
Oolite and Bradford Clay.
Stonesfield Slate.
Fossil Mammalia.
Fuller's Earth.
Inferior Oolite and Fossils.
Northamptonshire Slates.
Yorkshire Oolitic Coal-field.
Brora Coal.
Palaeontological Relations of the several Subdivisions of the Oolitic group.
CHAPTER XX.
JURASSIC GROUP-- CONTINUED.-- LIAS.
Mineral Character of Lias.
Numerous successive Zones in the Lias, marked by distinct Fossils, without
Unconformity in the Stratification, or Change in the Mineral Character of the
Deposits.
Gryphite Limestone.
Shells of the Lias.
Fish of the Lias.
Reptiles of the Lias.
Ichthyosaur and Plesiosaur.
Marine Reptile of the Galapagos Islands.
Sudden Destruction and Burial of Fossil Animals in Lias.
Fluvio-marine Beds in Gloucestershire, and Insect Limestone.
Fossil Plants.
The origin of the Oolite and Lias, and of alternating Calcareous and
Argillaceous Formations.
CHAPTER XXI.
TRIAS, OR NEW RED SANDSTONE GROUP.
Beds of Passage between the Lias and Trias, Rhaetic Beds.
Triassic Mammifer.
Triple Division of the Trias.
Keuper, or Upper Trias of England.
Reptiles of the Upper Trias.
Foot-prints in the Bunter formation in England.
Dolomitic Conglomerate of Bristol.
Origin of Red Sandstone and Rock-salt.
Precipitation of Salt from inland Lakes and Lagoons.
Trias of Germany.
Keuper.
St. Cassian and Hallstadt Beds.
Peculiarity of their Fauna.
Muschelkalk and its Fossils.
Trias of the United States.
Fossil Foot-prints of Birds and Reptiles in the Valley of the Connecticut.
Triassic Mammifer of North Carolina.
Triassic Coal-field of Richmond, Virginia.
Low Grade of early Mammals favourable to the Theory of Progressive Development.
CHAPTER XXII.
PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Line of Separation between Mesozoic and Palaeozoic Rocks.
Distinctness of Triassic and Permian Fossils.
Term Permian.
Thickness of calcareous and sedimentary Rocks in North of England.
Upper, Middle, and Lower Permian.
Marine Shells and Corals of the English Magnesian Limestone.
Reptiles and Fish of Permian Marl-slate.
Foot-prints of Reptiles.
Angular Breccias in Lower Permian.
Permian Rocks of the Continent.
Zechstein and Rothliegendes of Thuringia.
Permian Flora.
Its generic Affinity to the Carboniferous.
CHAPTER XXIII.
THE COAL OR CARBONIFEROUS GROUP.
Principal Subdivisions of the Carboniferous Group.
Different Thickness of the sedimentary and calcareous Members in Scotland and
the South of England.
Coal-measures.
Terrestrial Nature of the Growth of Coal.
Erect fossil Trees.
Uniting of many Coal-seams into one thick Bed.
Purity of the Coal explained.
Conversion of Coal into Anthracite.
Origin of Clay-ironstone.
Marine and brackish-water Strata in Coal.
Fossil Insects.
Batrachian Reptiles.
Labyrinthodont Foot-prints in Coal-measures.
Nova Scotia Coal-measures with successive Growths of erect fossil Trees.
Similarity of American and European Coal.
Air-breathers of the American Coal.
Changes of Condition of Land and Sea indicated by the Carboniferous Strata of
Nova Scotia.
CHAPTER XXIV.
FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD.
Vegetation of the Coal Period.
Ferns, Lycopodiaceae, Equisetaceae, Sigillariae, Stigmariae, Coniferae.
Angiosperms.
Climate of the Coal Period.
Mountain Limestone.
Marine Fauna of the Carboniferous Period.
Corals.
Bryozoa, Crinoidea.
Mollusca.
Great Number of fossil Fish.
Foraminifera.
CHAPTER XXV.
DEVONIAN OR OLD RED SANDSTONE GROUP.
Classification of the Old Red Sandstone in Scotland and in Devonshire.
Upper Old Red Sandstone in Scotland, with Fish and Plants.
Middle Old Red Sandstone.
Classification of the Ichthyolites of the Old Red, and their Relation to Living
Types.
Lower Old Red Sandstone, with Cephalaspis and Pterygotus.
Marine or Devonian Type of Old Red Sandstone.
Table of Devonian Series.
Upper Devonian Rocks and Fossils.
Middle.
Lower.
Eifel Limestone of Germany.
Devonian of Russia.
Devonian Strata of the United States and Canada.
Devonian Plants and Insects of Canada.
CHAPTER XXVI.
SILURIAN GROUP.
Classification of the Silurian Rocks.
Ludlow Formation and Fossils.
Bone-bed of the Upper Ludlow.
Lower Ludlow Shales with Pentamerus.
Oldest known Remains of fossil Fish.
Table of the progressive Discovery of Vertebrata in older Rocks.
Wenlock Formation, Corals, Cystideans and Trilobites.
Llandovery Group or Beds of Passage.
Lower Silurian Rocks.
Caradoc and Bala Beds.
Brachiopoda.
Trilobites.
Cystideae.
Graptolites.
Llandeilo Flags.
Arenig or Stiper-stones Group.
Foreign Silurian Equivalents in Europe.
Silurian Strata of the United States.
Canadian Equivalents.
Amount of specific Agreement of Fossils with those of Europe.
CHAPTER XXVII.
CAMBRIAN AND LAURENTIAN GROUPS.
Classification of the Cambrian Group, and its Equivalent in Bohemia.
Upper Cambrian Rocks.
Tremadoc Slates and their Fossils.
Lingula Flags.
Lower Cambrian Rocks.
Menevian Beds.
Longmynd Group.
Harlech Grits with large Trilobites.
Llanberis Slates.
Cambrian Rocks of Bohemia.
Primordial Zone of Barrande.
Metamorphosis of Trilobites.
Cambrian Rocks of Sweden and Norway.
Cambrian Rocks of the United States and Canada.
Potsdam Sandstone.
Huronian Series.
Laurentian Group, upper and lower.
Eozoon Canadense, oldest known Fossil.
Fundamental Gneiss of Scotland.
CHAPTER XXVIII.
VOLCANIC ROCKS.
External Form, Structure, and Origin of Volcanic Mountains.
Cones and Craters.
Hypothesis of "Elevation Craters" considered.
Trap Rocks.
Name whence derived.
Minerals most abundant in Volcanic Rocks.
Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks.
Similar Minerals in Meteorites.
Theory of Isomorphism.
Basaltic Rocks.
Trachytic Rocks.
Special Forms of Structure.
The columnar and globular Forms.
Trap Dikes and Veins.
Alteration of Rocks by volcanic Dikes.
Conversion of Chalk into Marble.
Intrusion of Trap between Strata.
Relation of trappean Rocks to the Products of active Volcanoes.
CHAPTER XXIX.
ON THE AGES OF VOLCANIC ROCKS.
Tests of relative Age of Volcanic Rocks.
Why ancient and modern Rocks can not be identical.
Tests by Superposition and intrusion.
Test by Alteration of Rocks in Contact.
Test by Organic Remains.
Test of Age by Mineral Character.
Test by Included Fragments.
Recent and Post-pliocene volcanic Rocks.
Vesuvius, Auvergne, Puy de Come, and Puy de Pariou.
Newer Pliocene volcanic Rocks.
Cyclopean Isles, Etna, Dikes of Palagonia, Madeira.
Older Pliocene volcanic Rocks.
Italy.
Pliocene Volcanoes of the Eifel.
Trass.
CHAPTER XXX.
AGE OF VOLCANIC ROCKS-- CONTINUED.
Volcanic Rocks of the Upper Miocene Period.
Madeira.
Grand Canary.
Azores.
Lower Miocene Volcanic Rocks.
Isle of Mull.
Staffa and Antrim.
The Eifel.
Upper and Lower Miocene Volcanic Rocks of Auvergne.
Hill of Gergovia.
Eocene Volcanic Rocks of Monte Bolca.
Trap of Cretaceous Period.
Oolitic Period.
Triassic Period.
Permian Period.
Carboniferous Period.
Erect Trees buried in Volcanic Ash in the Island of Arran.
Old Red Sandstone Period.
Silurian Period.
Cambrian Period.
Laurentian Volcanic Rocks.
CHAPTER XXXI.
PLUTONIC ROCKS.
General Aspect of Plutonic Rocks.
Granite and its Varieties.
Decomposing into Spherical Masses.
Rude columnar Structure.
Graphic Granite.
Mutual Penetration of Crystals of Quartz and Feldspar.
Glass Cavities in Quartz of Granite.
Porphyritic, talcose, and syenitic Granite.
Schorlrock and Eurite.
Syenite.
Connection of the Granites and Syenites with the Volcanic Rocks.
Analogy in Composition of Trachyte and Granite.
Granite Veins in Glen Tilt, Cape of Good Hope, and Cornwall.
Metalliferous Veins in Strata near their Junction with Granite.
Quartz Veins.
Exposure of Plutonic Rocks at the surface due to Denudation.
CHAPTER XXXII.
ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.
Difficulty in ascertaining the precise Age of a Plutonic Rock.
Test of Age by Relative Position.
Test by Intrusion and Alteration.
Test by Mineral Composition.
Test by included Fragments.
Recent and Pliocene Plutonic Rocks, why invisible.
Miocene Syenite of the Isle of Skye.
Eocene Plutonic Rocks in the Andes.
Granite altering Cretaceous Rocks.
Granite altering Lias in the Alps and in Skye.
Granite of Dartmoor altering Carboniferous Strata.
Granite of the Old Red Sandstone Period.
Syenite altering Silurian Strata in Norway.
Blending of the same with Gneiss.
Most ancient Plutonic Rocks.
Granite protruded in a solid Form.
CHAPTER XXXIII.
METAMORPHIC ROCKS.
General Character of Metamorphic Rocks.
Gneiss.
Hornblende-schist.
Serpentine.
Mica-schist.
Clay-slate.
Quartzite.
Chlorite-schist.
Metamorphic Limestone.
Origin of the metamorphic Strata.
Their Stratification.
Fossiliferous Strata near intrusive Masses of Granite converted into Rocks
identical with different Members of the metamorphic Series.
Arguments hence derived as to the Nature of Plutonic Action.
Hydrothermal Action, or the Influence of Steam and Gases in producing
Metamorphism.
Objections to the metamorphic Theory considered.
CHAPTER XXXIV.
METAMORPHIC ROCKS-- CONTINUED.
Definition of slaty Cleavage and Joints.
Supposed Causes of these Structures.
Crystalline Theory of Cleavage.
Mechanical Theory of Cleavage.
Condensation and Elongation of slate Rocks by lateral Pressure.
Lamination of some volcanic Rocks due to Motion.
Whether the Foliation of the crystalline Schists be usually parallel with the
original Planes of Stratification.
Examples in Norway and Scotland.
Causes of Irregularity in the Planes of Foliation.
CHAPTER XXXV.
ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.
Difficulty of ascertaining the Age of metamorphic Strata.
Metamorphic Strata of Eocene date in the Alps of Switzerland and Savoy.
Limestone and Shale of Carrara.
Metamorphic Strata of older date than the Silurian and Cambrian Rocks.
Order of Succession in metamorphic Rocks.
Uniformity of mineral Character.
Supposed Azoic Period.
Connection between the Absence of Organic Remains and the Scarcity of calcareous
Matter in metamorphic Rocks.
CHAPTER XXXVI.
MINERAL VEINS.
Different Kinds of mineral Veins.
Ordinary metalliferous Veins or Lodes.
Their frequent Coincidence with Faults.
Proofs that they originated in Fissures in solid Rock.
Veins shifting other Veins.
Polishing of their Walls or "Slicken sides."
Shells and Pebbles in Lodes.
Evidence of the successive Enlargement and Reopening of veins.
Examples in Cornwall and in Auvergne.
Dimensions of Veins.
Why some alternately swell out and contract.
Filling of Lodes by Sublimation from below.
Supposed relative Age of the precious Metals.
Copper and lead Veins in Ireland older than Cornish Tin.
Lead Vein in Lias, Glamorganshire.
Gold in Russia, California, and Australia.
Connection of hot Springs and mineral Veins.
INDEX.
...
STUDENT'S ELEMENTS OF GEOLOGY.
CHAPTER I.
ON THE DIFFERENT CLASSES OF ROCKS.
Geology defined.
Successive Formation of the Earth's Crust.
Classification of Rocks according to their Origin and Age.
Aqueous Rocks.
Their Stratification and imbedded Fossils.
Volcanic Rocks, with and without Cones and Craters.
Plutonic Rocks, and their Relation to the Volcanic.
Metamorphic Rocks, and their probable Origin.
The term Primitive, why erroneously applied to the Crystalline Formations.
Leading Division of the Work.
Of what materials is the earth composed, and in what manner are these materials
arranged? These are the first inquiries with which Geology is occupied, a
science which derives its name from the Greek ge, the earth, and logos, a
discourse. Previously to experience we might have imagined that investigations
of this kind would relate exclusively to the mineral kingdom, and to the various
rocks, soils, and metals, which occur upon the surface of the earth, or at
various depths beneath it. But, in pursuing such researches, we soon find
ourselves led on to consider the successive changes which have taken place in
the former state of the earth's surface and interior, and the causes which have
given rise to these changes; and, what is still more singular and unexpected, we
soon become engaged in researches into the history of the animate creation, or
of the various tribes of animals and plants which have, at different periods of
the past, inhabited the globe.
All are aware that the solid parts of the earth consist of distinct substances,
such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but
previously to observation it is commonly imagined that all these had remained
from the first in the state in which we now see them-- that they were created in
their present form, and in their present position. The geologist soon comes to a
different conclusion, discovering proofs that the external parts of the earth
were not all produced in the beginning of things in the state in which we now
behold them, nor in an instant of time. On the contrary, he can show that they
have acquired their actual configuration and condition gradually, under a great
variety of circumstances, and at successive periods, during each of which
distinct races of living beings have flourished on the land and in the waters,
the remains of these creatures still lying buried in the crust of the earth.
By the "earth's crust," is meant that small portion of the exterior of our
planet which is accessible to human observation. It comprises not merely all of
which the structure is laid open in mountain precipices, or in cliffs
overhanging a river or the sea, or whatever the miner may reveal in artificial
excavations; but the whole of that outer covering of the planet on which we are
enabled to reason by observations made at or near the surface. These reasonings
may extend to a depth of several miles, perhaps ten miles; and even then it may
be said, that such a thickness is no more than 1/400 part of the distance from
the surface to the centre. The remark is just: but although the dimensions of
such a crust are, in truth, insignificant when compared to the entire globe, yet
they are vast, and of magnificent extent in relation to man, and to the organic
beings which people our globe. Referring to this standard of magnitude, the
geologist may admire the ample limits of his domain, and admit, at the same
time, that not only the exterior of the planet, but the entire earth, is but an
atom in the midst of the countless worlds surveyed by the astronomer.
The materials of this crust are not thrown together confusedly; but distinct
mineral masses, called rocks, are found to occupy definite spaces, and to
exhibit a certain order of arrangement. The term ROCK is applied indifferently
by geologists to all these substances, whether they be soft or stony, for clay
and sand are included in the term, and some have even brought peat under this
denomination. Our old writers endeavoured to avoid offering such violence to our
language, by speaking of the component materials of the earth as consisting of
rocks and SOILS. But there is often so insensible a passage from a soft and
incoherent state to that of stone, that geologists of all countries have found
it indispensable to have one technical term to include both, and in this sense
we find ROCHE applied in French, ROCCA in Italian, and FELSART in German. The
beginner, however, must constantly bear in mind that the term rock by no means
implies that a mineral mass is in an indurated or stony condition.
The most natural and convenient mode of classifying the various rocks which
compose the earth's crust, is to refer, in the first place, to their origin, and
in the second to their relative age. I shall therefore begin by endeavouring
briefly to explain to the student how all rocks may be divided into four great
classes by reference to their different origin, or, in other words, by reference
to the different circumstances and causes by which they have been produced.
The first two divisions, which will at once be understood as natural, are the
aqueous and volcanic, or the products of watery and those of igneous action at
or near the surface.
AQUEOUS ROCKS.
The aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a
larger part of the earth's surface than any others. They consist chiefly of
mechanical deposits (pebbles, sand, and mud), but are partly of chemical and
some of them of organic origin, especially the limestones. These rocks are
STRATIFIED, or divided into distinct layers, or strata. The term STRATUM means
simply a bed, or any thing spread out or STREWED over a given surface; and we
infer that these strata have been generally spread out by the action of water,
from what we daily see taking place near the mouths of rivers, or on the land
during temporary inundations. For, whenever a running stream charged with mud or
sand, has its velocity checked, as when it enters a lake or sea, or overflows a
plain, the sediment, previously held in suspension by the motion of the water,
sinks, by its own gravity to the bottom. In this manner layers of mud and sand
are thrown down one upon another.
If we drain a lake which has been fed by a small stream, we frequently find at
the bottom a series of deposits, disposed with considerable regularity, one
above the other; the uppermost, perhaps, may be a stratum of peat, next below a
more dense and solid variety of the same material; still lower a bed of shell-
marl, alternating with peat or sand, and then other beds of marl, divided by
layers of clay. Now, if a second pit be sunk through the same continuous
lacustrine FORMATION at some distance from the first, nearly the same series of
beds is commonly met with, yet with slight variations; some, for example, of the
layers of sand, clay, or marl, may be wanting, one or more of them having
thinned out and given place to others, or sometimes one of the masses first
examined is observed to increase in thickness to the exclusion of other beds.
The term "FORMATION," which I have used in the above explanation, expresses in
geology any assemblage of rocks which have some character in common, whether of
origin, age, or composition. Thus we speak of stratified and unstratified,
fresh-water and marine, aqueous and volcanic, ancient and modern, metalliferous
and non-metalliferous formations.
In the estuaries of large rivers, such as the Ganges and the Mississippi, we may
observe, at low water, phenomena analogous to those of the drained lakes above
mentioned, but on a grander scale, and extending over areas several hundred
miles in length and breadth. When the periodical inundations subside, the river
hollows out a channel to the depth of many yards through horizontal beds of clay
and sand, the ends of which are seen exposed in perpendicular cliffs. These beds
vary in their mineral composition, or colour, or in the fineness or coarseness
of their particles, and some of them are occasionally characterised by
containing drift-wood. At the junction of the river and the sea, especially in
lagoons nearly separated by sand-bars from the ocean, deposits are often formed
in which brackish and salt-water shells are included.
In Egypt, where the Nile is always adding to its delta by filling up part of the
Mediterranean with mud, the newly deposited sediment is STRATIFIED, the thin
layer thrown down in one season differing slightly in colour from that of a
previous year, and being separable from it, as has been observed in excavations
at Cairo and other places. (See "Principles of Geology" by the Author Index
"Nile" "Rivers" etc.)
When beds of sand, clay, and marl, containing shells and vegetable matter, are
found arranged in a similar manner in the interior of the earth, we ascribe to
them a similar origin; and the more we examine their characters in minute
detail, the more exact do we find the resemblance. Thus, for example, at various
heights and depths in the earth, and often far from seas, lakes, and rivers, we
meet with layers of rounded pebbles composed of flint, limestone, granite, or
other rocks, resembling the shingles of a sea-beach or the gravel in a torrent's
bed. Such layers of pebbles frequently alternate with others formed of sand or
fine sediment, just as we may see in the channel of a river descending from
hills bordering a coast, where the current sweeps down at one season coarse sand
and gravel, while at another, when the waters are low and less rapid, fine mud
and sand alone are carried seaward. (See Figure 7 Chapter 2.)
If a stratified arrangement, and the rounded form of pebbles, are alone
sufficient to lead us to the conclusion that certain rocks originated under
water, this opinion is farther confirmed by the distinct and independent
evidence of FOSSILS, so abundantly included in the earth's crust. By a FOSSIL is
meant any body, or the traces of the existence of any body, whether animal or
vegetable, which has been buried in the earth by natural causes. Now the remains
of animals, especially of aquatic species, are found almost everywhere imbedded
in stratified rocks, and sometimes, in the case of limestone, they are in such
abundance as to constitute the entire mass of the rock itself. Shells and corals
are the most frequent, and with them are often associated the bones and teeth of
fishes, fragments of wood, impressions of leaves, and other organic substances.
Fossil shells, of forms such as now abound in the sea, are met with far inland,
both near the surface, and at great depths below it. They occur at all heights
above the level of the ocean, having been observed at elevations of more than
8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above
18,000 feet in the Himalaya. (Colonel R.J. Strachey found oolitic fossils 18,400
feet high in the Himalaya.)
These shells belong mostly to marine testacea, but in some places exclusively to
forms characteristic of lakes and rivers. Hence it is concluded that some
ancient strata were deposited at the bottom of the sea, and others in lakes and
estuaries.
We have now pointed out one great class of rocks, which, however they may vary
in mineral composition, colour, grain, or other characters, external and
internal, may nevertheless be grouped together as having a common origin. They
have all been formed under water, in the same manner as modern accumulations of
sand, mud, shingle, banks of shells, reefs of coral, and the like, and are all
characterised by stratification or fossils, or by both.
VOLCANIC ROCKS.
The division of rocks which we may next consider are the volcanic, or those
which have been produced at or near the surface whether in ancient or modern
times, not by water, but by the action of fire or subterranean heat. These rocks
are for the most part unstratified, and are devoid of fossils. They are more
partially distributed than aqueous formations, at least in respect to horizontal
extension. Among those parts of Europe where they exhibit characters not to be
mistaken, I may mention not only Sicily and the country round Naples, but
Auvergne, Velay, and Vivarais, now the departments of Puy de Dome, Haute Loire,
and Ardeche, towards the centre and south of France, in which are several
hundred conical hills having the forms of modern volcanoes, with craters more or
less perfect on many of their summits. These cones are composed moreover of
lava, sand, and ashes, similar to those of active volcanoes. Streams of lava may
sometimes be traced from the cones into the adjoining valleys, where they have
choked up the ancient channels of rivers with solid rock, in the same manner as
some modern flows of lava in Iceland have been known to do, the rivers either
flowing beneath or cutting out a narrow passage on one side of the lava.
Although none of these French volcanoes have been in activity within the period
of history or tradition, their forms are often very perfect. Some, however, have
been compared to the mere skeletons of volcanoes, the rains and torrents having
washed their sides, and removed all the loose sand and scoriae, leaving only the
harder and more solid materials. By this erosion, and by earthquakes, their
internal structure has occasionally been laid open to view, in fissures and
ravines; and we then behold not only many successive beds and masses of porous
lava, sand, and scoriae, but also perpendicular walls, or DIKES, as they are
called, of volcanic rock, which have burst through the other materials. Such
dikes are also observed in the structure of Vesuvius, Etna, and other active
volcanoes. They have been formed by the pouring of melted matter, whether from
above or below, into open fissures, and they commonly traverse deposits of
VOLCANIC TUFF, a substance produced by the showering down from the air, or
incumbent waters, of sand and cinders, first shot up from the interior of the
earth by the explosions of volcanic gases.
Besides the parts of France above alluded to, there are other countries, as the
north of Spain, the south of Sicily, the Tuscan territory of Italy, the lower
Rhenish provinces, and Hungary, where spent volcanoes may be seen, still
preserving in many cases a conical form, and having craters and often lava-
streams connected with them.
There are also other rocks in England, Scotland, Ireland, and almost every
country in Europe, which we infer to be of igneous origin, although they do not
form hills with cones and craters. Thus, for example, we feel assured that the
rock of Staffa, and that of the Giant's Causeway, called basalt, is volcanic,
because it agrees in its columnar structure and mineral composition with streams
of lava which we know to have flowed from the craters of volcanoes. We find also
similar basaltic and other igneous rocks associated with beds of TUFF in various
parts of the British Isles, and forming DIKES, such as have been spoken of; and
some of the strata through which these dikes cut are occasionally altered at the
point of contact, as if they had been exposed to the intense heat of melted
matter.
The absence of cones and craters, and long narrow streams of superficial lava,
in England and many other countries, is principally to be attributed to the
eruptions having been submarine, just as a considerable proportion of volcanoes
in our own times burst out beneath the sea. But this question must be enlarged
upon more fully in the chapters on Igneous Rocks, in which it will also be
shown, that as different sedimentary formations, containing each their
characteristic fossils, have been deposited at successive periods, so also
volcanic sand and scoriae have been thrown out, and lavas have flowed over the
land or bed of the sea, at many different epochs, or have been injected into
fissures; so that the igneous as well as the aqueous rocks may be classed as a
chronological series of monuments, throwing light on a succession of events in
the history of the earth.
PLUTONIC ROCKS (GRANITE ETC).
We have now pointed out the existence of two distinct orders of mineral masses,
the aqueous and the volcanic: but if we examine a large portion of a continent,
especially if it contain within it a lofty mountain range, we rarely fail to
discover two other classes of rocks, very distinct from either of those above
alluded to, and which we can neither assimilate to deposits such as are now
accumulated in lakes or seas, nor to those generated by ordinary volcanic
action. The members of both these divisions of rocks agree in being highly
crystalline and destitute of organic remains. The rocks of one division have
been called Plutonic, comprehending all the granites and certain porphyries,
which are nearly allied in some of their characters to volcanic formations. The
members of the other class are stratified and often slaty, and have been called
by some the CRYSTALLINE SCHISTS, in which group are included gneiss, micaceous-
schist (or mica-slate), hornblende-schist, statuary marble, the finer kinds of
roofing slate, and other rocks afterwards to be described.
As it is admitted that nothing strictly analogous to these crystalline
productions can now be seen in the progress of formation on the earth's surface,
it will naturally be asked, on what data we can find a place for them in a
system of classification founded on the origin of rocks. I can not, in reply to
this question, pretend to give the student, in a few words, an intelligible
account of the long chain of facts and reasonings from which geologists have
been led to infer the nature of the rocks in question. The result, however, may
be briefly stated. All the various kinds of granites which constitute the
Plutonic family are supposed to be of igneous or aqueo-igneous origin, and to
have been formed under great pressure, at a considerable depth in the earth, or
sometimes, perhaps, under a certain weight of incumbent ocean. Like the lava of
volcanoes, they have been melted, and afterwards cooled and crystallised, but
with extreme slowness, and under conditions very different from those of bodies
cooling in the open air. Hence they differ from the volcanic rocks, not only by
their more crystalline texture, but also by the absence of tuffs and breccias,
which are the products of eruptions at the earth's surface, or beneath seas of
inconsiderable depth. They differ also by the absence of pores or cellular
cavities, to which the expansion of the entangled gases gives rise in ordinary
lava.
METAMORPHIC, OR STRATIFIED CRYSTALLINE ROCKS.
The fourth and last great division of rocks are the crystalline strata and
slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist,
marble, and the like, the origin of which is more doubtful than that of the
other three classes. They contain no pebbles, or sand, or scoriae, or angular
pieces of imbedded stone, and no traces of organic bodies, and they are often as
crystalline as granite, yet are divided into beds, corresponding in form and
arrangement to those of sedimentary formations, and are therefore said to be
stratified. The beds sometimes consist of an alternation of substances varying
in colour, composition, and thickness, precisely as we see in stratified
fossiliferous deposits. According to the Huttonian theory, which I adopt as the
most probable, and which will be afterwards more fully explained, the materials
of these strata were originally deposited from water in the usual form of
sediment, but they were subsequently so altered by subterranean heat, as to
assume a new texture. It is demonstrable, in some cases at least, that such a
complete conversion has actually taken place, fossiliferous strata having
exchanged an earthy for a highly crystalline texture for a distance of a quarter
of a mile from their contact with granite. In some cases, dark limestones,
replete with shells and corals, have been turned into white statuary marble; and
hard clays, containing vegetable or other remains, into slates called mica-
schist or hornblende-schist, every vestige of the organic bodies having been
obliterated.
Although we are in a great degree ignorant of the precise nature of the
influence exerted in these cases, yet it evidently bears some analogy to that
which volcanic heat and gases are known to produce; and the action may be
conveniently called Plutonic, because it appears to have been developed in those
regions where Plutonic rocks are generated, and under similar circumstances of
pressure and depth in the earth. Intensely heated water or steam permeating
stratified masses under great pressure have no doubt played their part in
producing the crystalline texture and other changes, and it is clear that the
transforming influence has often pervaded entire mountain masses of strata.
In accordance with the hypothesis above alluded to, I proposed in the first
edition of the Principles of Geology (1833) the term "Metamorphic" for the
altered strata, a term derived from meta, trans, and morphe, forma.
Hence there are four great classes of rocks considered in reference to their
origin-- the aqueous, the volcanic, the Plutonic, and the metamorphic. In the
course of this work it will be shown that portions of each of these four
distinct classes have originated at many successive periods. They have all been
produced contemporaneously, and may even now be in the progress of formation on
a large scale. It is not true, as was formerly supposed, that all granites,
together with the crystalline or metamorphic strata, were first formed, and
therefore entitled to be called "primitive," and that the aqueous and volcanic
rocks were afterwards superimposed, and should, therefore, rank as secondary in
the order of time. This idea was adopted in the infancy of the science, when all
formations, whether stratified or unstratified, earthy or crystalline, with or
without fossils, were alike regarded as of aqueous origin. At that period it was
naturally argued that the foundation must be older than the superstructure; but
it was afterwards discovered that this opinion was by no means in every instance
a legitimate deduction from facts; for the inferior parts of the earth's crust
have often been modified, and even entirely changed, by the influence of
volcanic and other subterranean causes, while superimposed formations have not
been in the slightest degree altered. In other words, the destroying and
renovating processes have given birth to new rocks below, while those above,
whether crystalline or fossiliferous, have remained in their ancient condition.
Even in cities, such as Venice and Amsterdam, it cannot be laid down as
universally true that the upper parts of each edifice, whether of brick or
marble, are more modern than the foundations on which they rest, for these often
consist of wooden piles, which may have rotted and been replaced one after the
other, without the least injury to the buildings above; meanwhile, these may
have required scarcely any repair, and may have been constantly inhabited. So it
is with the habitable surface of our globe, in its relation to large masses of
rock immediately below; it may continue the same for ages, while subjacent
materials, at a great depth, are passing from a solid to a fluid state, and then
reconsolidating, so as to acquire a new texture.
As all the crystalline rocks may, in some respects, be viewed as belonging to
one great family, whether they be stratified or unstratified, metamorphic or
Plutonic, it will often be convenient to speak of them by one common name. It
being now ascertained, as above stated, that they are of very different ages,
sometimes newer than the strata called secondary, the terms primitive and
primary which were formerly used for the whole must be abandoned, as they would
imply a manifest contradiction. It is indispensable, therefore, to find a new
name, one which must not be of chronological import, and must express, on the
one hand, some peculiarity equally attributable to granite and gneiss (to the
Plutonic as well as the ALTERED rocks), and, on the other, must have reference
to characters in which those rocks differ, both from the volcanic and from the
UNALTERED sedimentary strata. I proposed in the Principles of Geology (first
edition volume 3) the term "hypogene" for this purpose, derived from upo, under,
and ginomai, to be, or to be born; a word implying the theory that granite,
gneiss, and the other crystalline formations are alike NETHERFORMED rocks, or
rocks which have not assumed their present form and structure at the surface.
They occupy the lowest place in the order of superposition. Even in regions such
as the Alps, where some masses of granite and gneiss can be shown to be of
comparatively modern date, belonging, for example, to the period hereafter to be
described as tertiary, they are still UNDERLYING rocks. They never repose on the
volcanic or trappean formations, nor on strata containing organic remains. They
are HYPOGENE, as "being under" all the rest.
From what has now been said, the reader will understand that each of the four
great classes of rocks may be studied under two distinct points of view; first,
they may be studied simply as mineral masses deriving their origin from
particular causes, and having a certain composition, form, and position in the
earth's crust, or other characters both positive and negative, such as the
presence or absence of organic remains. In the second place, the rocks of each
class may be viewed as a grand chronological series of monuments, attesting a
succession of events in the former history of the globe and its living
inhabitants.
I shall accordingly proceed to treat of each family of rocks; first, in
reference to those characters which are not chronological, and then in
particular relation to the several periods when they were formed.
CHAPTER II.
AQUEOUS ROCKS.-- THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral Composition of Strata.
Siliceous Rocks.
Argillaceous.
Calcareous.
Gypsum.
Forms of Stratification.
Original Horizontality.
Thinning out.
Diagonal Arrangement.
Ripple-mark.
In pursuance of the arrangement explained in the last chapter, we shall begin by
examining the aqueous or sedimentary rocks, which are for the most part
distinctly stratified, and contain fossils. We may first study them with
reference to their mineral composition, external appearance, position, mode of
origin, organic contents, and other characters which belong to them as aqueous
formations, independently of their age, and we may afterwards consider them
chronologically or with reference to the successive geological periods when they
originated.
I have already given an outline of the data which led to the belief that the
stratified and fossiliferous rocks were originally deposited under water; but,
before entering into a more detailed investigation, it will be desirable to say
something of the ordinary materials of which such strata are composed. These may
be said to belong principally to three divisions, the siliceous, the
argillaceous, and the calcareous, which are formed respectively of flint, clay,
and carbonate of lime. Of these, the siliceous are chiefly made up of sand or
flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter
with a certain proportion, about a fourth in weight, of aluminous earth; and,
lastly, the calcareous rocks, or limestones, of carbonic acid and lime.
SILICEOUS AND ARENACEOUS ROCKS.
To speak first of the sandy division: beds of loose sand are frequently met
with, of which the grains consist entirely of silex, which term comprehends all
purely siliceous minerals, as quartz and common flint. Quartz is silex in its
purest form. Flint usually contains some admixture of alumina and oxide of iron.
The siliceous grains in sand are usually rounded, as if by the action of running
water. Sandstone is an aggregate of such grains, which often cohere together
without any visible cement, but more commonly are bound together by a slight
quantity of siliceous or calcareous matter, or by oxide of iron or clay.
Pure siliceous rocks may be known by not effervescing when a drop of nitric,
sulphuric or other acid is applied to them, or by the grains not being readily
scratched or broken by ordinary pressure. In nature there is every intermediate
gradation, from perfectly loose sand to the hardest sandstone. In MICACEOUS
SANDSTONES mica is very abundant; and the thin silvery plates into which that
mineral divides are often arranged in layers parallel to the planes of
stratification, giving a slaty or laminated texture to the rock.
When sandstone is coarse-grained, it is usually called GRIT. If the grains are
rounded, and large enough to be called pebbles, it becomes a CONGLOMERATE or
PUDDING-STONE, which may consist of pieces of one or of many different kinds of
rock. A conglomerate, therefore, is simply gravel bound together by cement.
ARGILLACEOUS ROCKS.
Clay, strictly speaking, is a mixture of silex or flint with a large proportion,
usually about one fourth, of alumina, or argil; but in common language, any
earth which possesses sufficient ductility, when kneaded up with water, to be
fashioned like paste by the hand, or by the potter's lathe, is called a CLAY;
and such clays vary greatly in their composition, and are, in general, nothing
more than mud derived from the decomposition or wearing down of rocks. The
purest clay found in nature is porcelain clay, or kaolin, which results from the
decomposition of a rock composed of feldspar and quartz, and it is almost always
mixed with quartz. The kaolin of China consists of 71.15 parts of silex, 15.86
of alumine, 1.92 of lime, and 6.73 of water (W. Phillips Mineralogy page 33.);
but other porcelain clays differ materially, that of Cornwall being composed,
according to Boase, of nearly equal parts of silica and alumine, with 1 per cent
of magnesia. (Phil. Mag. volume 10 1837.) SHALE has also the property, like
clay, of becoming plastic in water: it is a more solid form of clay, or
argillaceous matter, condensed by pressure. It always divides into laminae more
or less regular.
One general character of all argillaceous rocks is to give out a peculiar,
earthy odour when breathed upon, which is a test of the presence of alumine,
although it does not belong to pure alumine, but, apparently, to the combination
of that substance with oxide of iron. (See W. Phillips Mineralogy "Alumine.")
CALCAREOUS ROCKS.
This division comprehends those rocks which, like chalk, are composed chiefly of
lime and carbonic acid. Shells and corals are also formed of the same elements,
with the addition of animal matter. To obtain pure lime it is necessary to
calcine these calcareous substances, that is to say, to expose them to heat of
sufficient intensity to drive off the carbonic acid, and other volatile matter.
White chalk is sometimes pure carbonate of lime; and this rock, although usually
in a soft and earthy state, is occasionally sufficiently solid to be used for
building, and even passes into a COMPACT stone, or a stone of which the separate
parts are so minute as not to be distinguishable from each other by the naked
eye.
Many limestones are made up entirely of minute fragments of shells and coral, or
of calcareous sand cemented together. These last might be called "calcareous
sandstones;" but that term is more properly applied to a rock in which the
grains are partly calcareous and partly siliceous, or to quartzose sandstones,
having a cement of carbonate of lime.
The variety of limestone called OOLITE is composed of numerous small egg-like
grains, resembling the roe of a fish, each of which has usually a small fragment
of sand as a nucleus, around which concentric layers of calcareous matter have
accumulated.
Any limestone which is sufficiently hard to take a fine polish is called MARBLE.
Many of these are fossiliferous; but statuary marble, which is also called
saccharoid limestone, as having a texture resembling that of loaf-sugar, is
devoid of fossils, and is in many cases a member of the metamorphic series.
SILICEOUS LIMESTONE is an intimate mixture of carbonate of lime and flint, and
is harder in proportion as the flinty matter predominates.
The presence of carbonate of lime in a rock may be ascertained by applying to
the surface a small drop of diluted sulphuric, nitric, or muriatic acid, or
strong vinegar; for the lime, having a greater chemical affinity for any one of
these acids than for the carbonic, unites immediately with them to form new
compounds, thereby becoming a sulphate, nitrate or muriate of lime. The carbonic
acid, when thus liberated from its union with the lime, escapes in a gaseous
form, and froths up or effervesces as it makes its way in small bubbles through
the drop of liquid. This effervescence is brisk or feeble in proportion as the
limestone is pure or impure, or, in other words, according to the quantity of
foreign matter mixed with the carbonate of lime. Without the aid of this test,
the most experienced eye can not always detect the presence of carbonate of lime
in rocks.
The above-mentioned three classes of rocks, the siliceous, argillaceous, and
calcareous, pass continually into each other, and rarely occur in a perfectly
separate and pure form. Thus it is an exception to the general rule to meet with
a limestone as pure as ordinary white chalk, or with clay as aluminous as that
used in Cornwall for porcelain, or with sand so entirely composed of siliceous
grains as the white sand of Alum Bay, in the Isle of Wight, employed in the
manufacture of glass, or sandstone so pure as the grit of Fontainebleau, used
for pavement in France. More commonly we find sand and clay, or clay and marl,
intermixed in the same mass. When the sand and clay are each in considerable
quantity, the mixture is called LOAM. If there is much calcareous matter in clay
it is called MARL; but this term has unfortunately been used so vaguely, as
often to be very ambiguous. It has been applied to substances in which there is
no lime; as, to that red loam usually called red marl in certain parts of
England. Agriculturists were in the habit of calling any soil a marl which, like
true marl, fell to pieces readily on exposure to the air. Hence arose the
confusion of using this name for soils which, consisting of loam, were easily
worked by the plough, though devoid of lime.
MARL SLATE bears the same relation to marl which shale bears to clay, being a
calcareous shale. It is very abundant in some countries, as in the Swiss Alps.
Argillaceous or marly limestone is also of common occurrence.
There are few other kinds of rock which enter so largely into the composition of
sedimentary strata as to make it necessary to dwell here on their characters. I
may, however, mention two others-- magnesian limestone or dolomite, and gypsum.
MAGNESIAN LIMESTONE is composed of carbonate of lime and carbonate of magnesia;
the proportion of the latter amounting in some cases to nearly one half. It
effervesces much more slowly and feebly with acids than common limestone. In
England this rock is generally of a yellowish colour; but it varies greatly in
mineralogical character, passing from an earthy state to a white compact stone
of great hardness. DOLOMITE, so common in many parts of Germany and France, is
also a variety of magnesian limestone, usually of a granular texture.
Gypsum is a rock composed of sulphuric acid, lime, and water. It is usually a
soft whitish-yellow rock, with a texture resembling that of loaf-sugar, but
sometimes it is entirely composed of lenticular crystals. It is insoluble in
acids, and does not effervesce like chalk and dolomite, because it does not
contain carbonic acid gas, or fixed air, the lime being already combined with
sulphuric acid, for which it has a stronger affinity than for any other.
Anhydrous gypsum is a rare variety, into which water does not enter as a
component part. GYPSEOUS MARL is a mixture of gypsum and marl. ALABASTER is a
granular and compact variety of gypsum found in masses large enough to be used
in sculpture and architecture. It is sometimes a pure snow-white substance, as
that of Volterra in Tuscany, well known as being carved for works of art in
Florence and Leghorn. It is a softer stone than marble, and more easily wrought.
FORMS OF STRATIFICATION.
A series of strata sometimes consists of one of the above rocks, sometimes of
two or more in alternating beds.
Thus, in the coal districts of England, for example, we often pass through
several beds of sandstone, some of finer, others of coarser grain, some white,
others of a dark colour, and below these, layers of shale and sandstone or beds
of shale, divisible into leaf-like laminae, and containing beautiful impressions
of plants. Then again we meet with beds of pure and impure coal, alternating
with shales and sandstones, and underneath the whole, perhaps, are calcareous
strata, or beds of limestone, filled with corals and marine shells, each bed
distinguishable from another by certain fossils, or by the abundance of
particular species of shells or zoophytes.
This alternation of different kinds of rock produces the most distinct
stratification; and we often find beds of limestone and marl, conglomerate and
sandstone, sand and clay, recurring again and again, in nearly regular order,
throughout a series of many hundred strata. The causes which may produce these
phenomena are various, and have been fully discussed in my treatise on the
modern changes of the earth's surface. (Consult Index to Principles of Geology,
"Stratification" "Currents" "Deltas" "Water" etc.) It is there seen that rivers
flowing into lakes and seas are charged with sediment, varying in quantity,
composition, colour, and grain according to the seasons; the waters are
sometimes flooded and rapid, at other periods low and feeble; different
tributaries, also, draining peculiar countries and soils, and therefore charged
with peculiar sediment, are swollen at distinct periods. It was also shown that
the waves of the sea and currents undermine the cliffs during wintry storms, and
sweep away the materials into the deep, after which a season of tranquillity
succeeds, when nothing but the finest mud is spread by the movements of the
ocean over the same submarine area.
It is not the object of the present work to give a description of these
operations, repeated as they are, year after year, and century after century;
but I may suggest an explanation of the manner in which some micaceous
sandstones have originated, namely, those in which we see innumerable thin
layers of mica dividing layers of fine quartzose sand. I observed the same
arrangement of materials in recent mud deposited in the estuary of Laroche St.
Bernard in Brittany, at the mouth of the Loire. The surrounding rocks are of
gneiss, which, by its waste, supplies the mud: when this dries at low water, it
is found to consist of brown laminated clay, divided by thin seams of mica. The
separation of the mica in this case, or in that of micaceous sandstones, may be
thus understood. If we take a handful of quartzose sand, mixed with mica, and
throw it into a clear running stream, we see the materials immediately sorted by
the water, the grains of quartz falling almost directly to the bottom, while the
plates of mica take a much longer time to reach the bottom, and are carried
farther down the stream. At the first instant the water is turbid, but
immediately after the flat surfaces of the plates of mica are seen all alone,
reflecting a silvery light, as they descend slowly, to form a distinct micaceous
lamina. The mica is the heavier mineral of the two; but it remains a longer time
suspended in the fluid, owing to its greater extent of surface. It is easy,
therefore, to perceive that where such mud is acted upon by a river or tidal
current, the thin plates of mica will be carried farther, and not deposited in
the same places as the grains of quartz; and since the force and velocity of the
stream varies from time to time, layers of mica or of sand will be thrown down
successively on the same area.
ORIGINAL HORIZONTALITY.
It is said generally that the upper and under surfaces of strata, or the "planes
of stratification," are parallel. Although this is not strictly true, they make
an approach to parallelism, for the same reason that sediment is usually
deposited at first in nearly horizontal layers. Such an arrangement can by no
means be attributed to an original evenness or horizontality in the bed of the
sea: for it is ascertained that in those places where no matter has been
recently deposited, the bottom of the ocean is often as uneven as that of the
dry land, having in like manner its hills, valleys, and ravines. Yet if the sea
should go down, or be removed from near the mouth of a large river where a delta
has been forming, we should see extensive plains of mud and sand laid dry,
which, to the eye, would appear perfectly level, although, in reality, they
would slope gently from the land towards the sea.
This tendency in newly-formed strata to assume a horizontal position arises
principally from the motion of the water, which forces along particles of sand
or mud at the bottom, and causes them to settle in hollows or depressions where
they are less exposed to the force of a current than when they are resting on
elevated points. The velocity of the current and the motion of the superficial
waves diminish from the surface downward, and are least in those depressions
where the water is deepest.
(FIGURE 1. Layers of sand and ashes on uneven ground.)
A good illustration of the principle here alluded to may be sometimes seen in
the neighbourhood of a volcano, when a section, whether natural or artificial,
has laid open to view a succession of various-coloured layers of sand and ashes,
which have fallen in showers upon uneven ground. Thus let A B (Figure 1) be two
ridges, with an intervening valley. These original inequalities of the surface
have been gradually effaced by beds of sand and ashes c, d, e, the surface at e
being quite level. It will be seen that, although the materials of the first
layers have accommodated themselves in a great degree to the shape of the ground
A B, yet each bed is thickest at the bottom. At first a great many particles
would be carried by their own gravity down the steep sides of A and B, and
others would afterwards be blown by the wind as they fell off the ridges, and
would settle in the hollow, which would thus become more and more effaced as the
strata accumulated from c to e. Now, water in motion can exert this levelling
power on similar materials more easily than air, for almost all stones lose in
water more than a third of the weight which they have in air, the specific
gravity of rocks being in general as 2 1/2 when compared to that of water, which
is estimated at 1. But the buoyancy of sand or mud would be still greater in the
sea, as the density of salt-water exceeds that of fresh.
(FIGURE 2. Section of strata of sandstone, grit, and conglomerate.)
Yet, however uniform and horizontal may be the surface of new deposits in
general, there are still many disturbing causes, such as eddies in the water,
and currents moving first in one and then in another direction, which frequently
cause irregularities. We may sometimes follow a bed of limestone, shale, or
sandstone, for a distance of many hundred yards continuously; but we generally
find at length that each individual stratum thins out, and allows the beds which
were previously above and below it to meet. If the materials are coarse, as in
grits and conglomerates, the same beds can rarely be traced many yards without
varying in size, and often coming to an end abruptly. (See Figure 2.)
DIAGONAL OR CROSS STRATIFICATION.
(FIGURE 3. Section of sand at Sandy Hill, near Biggleswade, Bedfordshire. Height
20 feet. (Green-sand formation.))
(FIGURE 4. Layers of sediment on a bank.)
(FIGURE 5. Nearly horizontal layers of sediment over sloping strata.)
(FIGURE 6. Cliff between mismer and Dunwich.)
There is also another phenomenon of frequent occurrence. We find a series of
larger strata, each of which is composed of a number of minor layers placed
obliquely to the general planes of stratification. To this diagonal arrangement
the name of "false or cross bedding" has been given. Thus in the section (Figure
3) we see seven or eight large beds of loose sand, yellow and brown, and the
lines a, b, c mark some of the principal planes of stratification, which are
nearly horizontal. But the greater part of the subordinate laminae do not
conform to these planes, but have often a steep slope, the inclination being
sometimes towards opposite points of the compass. When the sand is loose and
incoherent, as in the case here represented, the deviation from parallelism of
the slanting laminae can not possibly be accounted for by any rearrangement of
the particles acquired during the consolidation of the rock. In what manner,
then, can such irregularities be due to original deposition? We must suppose
that at the bottom of the sea, as well as in the beds of rivers, the motions of
waves, currents, and eddies often cause mud, sand, and gravel to be thrown down
in heaps on particular spots, instead of being spread out uniformly over a wide
area. Sometimes, when banks are thus formed, currents may cut passages through
them, just as a river forms its bed. Suppose the bank A (Figure 4) to be thus
formed with a steep sloping side, and, the water being in a tranquil state, the
layer of sediment No. 1 is thrown down upon it, conforming nearly to its
surface. Afterwards the other layers, 2, 3, 4, may be deposited in succession,
so that the bank B C D is formed. If the current then increases in velocity, it
may cut away the upper portion of this mass down to the dotted line e, and
deposit the materials thus removed farther on, so as to form the layers 5, 6, 7,
8. We have now the bank B, C, D, E (Figure 5), of which the surface is almost
level, and on which the nearly horizontal layers, 9, 10, 11, may then
accumulate. It was shown in Figure 3 that the diagonal layers of successive
strata may sometimes have an opposite slope. This is well seen in some cliffs of
loose sand on the Suffolk coast. A portion of one of these is represented in
Figure 6, where the layers, of which there are about six in the thickness of an
inch, are composed of quartzose grains. This arrangement may have been due to
the altered direction of the tides and currents in the same place.
(FIGURE 7. Section from Monte Calvo to the sea by the valley of the Magnan, near
Nice.
A. Dolomite and sandstone. (Green-sand formation?)
a, b, d. Beds of gravel and sand.
c. Fine marl and sand of Ste. Madeleine, with marine (Pliocene) shells.)
The description above given of the slanting position of the minor layers
constituting a single stratum is in certain cases applicable on a much grander
scale to masses several hundred feet thick, and many miles in extent. A fine
example may be seen at the base of the Maritime Alps near Nice. The mountains
here terminate abruptly in the sea, so that a depth of one hundred fathoms is
often found within a stone's throw of the beach, and sometimes a depth of 3000
feet within half a mile. But at certain points, strata of sand, marl, or
conglomerate intervene between the shore and the mountains, as in the section
(Figure 7), where a vast succession of slanting beds of gravel and sand may be
traced from the sea to Monte Calvo, a distance of no less than nine miles in a
straight line. The dip of these beds is remarkably uniform, being always
southward or towards the Mediterranean, at an angle of about 25 degrees. They
are exposed to view in nearly vertical precipices, varying from 200 to 600 feet
in height, which bound the valley through which the river Magnan flows.
Although, in a general view, the strata appear to be parallel and uniform, they
are nevertheless found, when examined closely, to be wedge-shaped, and to thin
out when followed for a few hundred feet or yards, so that we may suppose them
to have been thrown down originally upon the side of a steep bank where a river
or Alpine torrent discharged itself into a deep and tranquil sea, and formed a
delta, which advanced gradually from the base of Monte Calvo to a distance of
nine miles from the original shore. If subsequently this part of the Alps and
bed of the sea were raised 700 feet, the delta may have emerged, a deep channel
may then have been cut through it by the river, and the coast may at the same
time have acquired its present configuration.
(FIGURE 8. Slab of ripple-marked (New Red) sandstone from Cheshire.)
It is well known that the torrents and streams which now descend from the Alpine
declivities to the shore, bring down annually, when the snow melts, vast
quantities of shingle and sand, and then, as they subside, fine mud, while in
summer they are nearly or entirely dry; so that it may be safely assumed that
deposits like those of the valley of the Magnan, consisting of coarse gravel
alternating with fine sediment, are still in progress at many points, as, for
instance, at the mouth of the Var. They must advance upon the Mediterranean in
the form of great shoals terminating in a steep talus; such being the original
mode of accumulation of all coarse materials conveyed into deep water,
especially where they are composed in great part of pebbles, which can not be
transported to indefinite distances by currents of moderate velocity. By
inattention to facts and inferences of this kind, a very exaggerated estimate
has sometimes been made of the supposed depth of the ancient ocean. There can be
no doubt, for example, that the strata a, Figure 7, or those nearest to Monte
Calvo, are older than those indicated by b, and these again were formed before
c; but the vertical depth of gravel and sand in any one place can not be proved
to amount even to 1000 feet, although it may perhaps be much greater, yet
probably never exceeding at any point 3000 or 4000 feet. But were we to assume
that all the strata were once horizontal, and that their present dip or
inclination was due to subsequent movements, we should then be forced to
conclude that a sea several miles deep had been filled up with alternate layers
of mud and pebbles thrown down one upon another.
In the locality now under consideration, situated a few miles to the west of
Nice, there are many geological data, the details of which can not be given in
this place, all leading to the opinion that, when the deposit of the Magnan was
formed, the shape and outline of the Alpine declivities and the shore greatly
resembled what we now behold at many points in the neighbourhood. That the beds
a, b, c, d are of comparatively modern date is proved by this fact, that in
seams of loamy marl intervening between the pebbly beds are fossil shells, half
of which belong to species now living in the Mediterranean.
RIPPLE-MARK.
The ripple-mark, so common on the surface of sandstones of all ages (see Figure
8), and which is so often seen on the sea-shore at low tide, seems to originate
in the drifting of materials along the bottom of the water, in a manner very
similar to that which may explain the inclined layers above described. This
ripple is not entirely confined to the beach between high and low water mark,
but is also produced on sands which are constantly covered by water. Similar
undulating ridges and furrows may also be sometimes seen on the surface of drift
snow and blown sand.
The ripple-mark is usually an indication of a sea-beach, or of water from six to
ten feet deep, for the agitation caused by waves even during storms extends to a
very slight depth. To this rule, however, there are some exceptions, and recent
ripple-marks have been observed at the depth of 60 or 70 feet. It has also been
ascertained that currents or large bodies of water in motion may disturb mud and
sand at the depth of 300 or even 450 feet. (Darwin Volcanic Islands page 134.)
Beach ripple, however, may usually be distinguished from current ripple by
frequent changes in its direction. In a slab of sandstone, not more than an inch
thick, the furrows or ridges of an ancient ripple may often be seen in several
successive laminae to run towards different points of the compass.
CHAPTER III.
ARRANGEMENT OF FOSSILS IN STRATA.-- FRESH-WATER AND MARINE FOSSILS.
Successive Deposition indicated by Fossils.
Limestones formed of Corals and Shells.
Proofs of gradual Increase of Strata derived from Fossils.
Serpula attached to Spatangus.
Wood bored by Teredina.
Tripoli formed of Infusoria.
Chalk derived principally from Organic Bodies.
Distinction of Fresh-water from Marine Formations.
Genera of Fresh-water and Land Shells.
Rules for recognising Marine Testacea.
Gyrogonite and Chara.
Fresh-water Fishes.
Alternation of Marine and Fresh-water Deposits.
Lym-Fiord.
Having in the last chapter considered the forms of stratification so far as they
are determined by the arrangement of inorganic matter, we may now turn our
attention to the manner in which organic remains are distributed through
stratified deposits. We should often be unable to detect any signs of
stratification or of successive deposition, if particular kinds of fossils did
not occur here and there at certain depths in the mass. At one level, for
example, univalve shells of some one or more species predominate; at another,
bivalve shells; and at a third, corals; while in some formations we find layers
of vegetable matter, commonly derived from land plants, separating strata.
It may appear inconceivable to a beginner how mountains, several thousand feet
thick, can have become full of fossils from top to bottom; but the difficulty is
removed, when he reflects on the origin of stratification, as explained in the
last chapter, and allows sufficient time for the accumulation of sediment. He
must never lose sight of the fact that, during the process of deposition, each
separate layer was once the uppermost, and immediately in contact with the water
in which aquatic animals lived. Each stratum, in fact, however far it may now
lie beneath the surface, was once in the state of shingle, or loose sand or soft
mud at the bottom of the sea, in which shells and other bodies easily became
enveloped.
RATE OF DEPOSITION INDICATED BY FOSSILS.
By attending to the nature of these remains, we are often enabled to determine
whether the deposition was slow or rapid, whether it took place in a deep or
shallow sea, near the shore or far from land, and whether the water was salt,
brackish, or fresh. Some limestones consist almost exclusively of corals, and in
many cases it is evident that the present position of each fossil zoophyte has
been determined by the manner in which it grew originally. The axis of the
coral, for example, if its natural growth is erect, still remains at right
angles to the plane of stratification. If the stratum be now horizontal, the
round spherical heads of certain species continue uppermost, and their points of
attachment are directed downward. This arrangement is sometimes repeated
throughout a great succession of strata. From what we know of the growth of
similar zoophytes in modern reefs, we infer that the rate of increase was
extremely slow, and some of the fossils must have flourished for ages like
forest-trees, before they attained so large a size. During these ages, the water
must have been clear and transparent, for such corals can not live in turbid
water.
(FIGURE 9. Fossil Gryphaea, covered both on the outside and inside with fossil
Serpulae.)
In like manner, when we see thousands of full-grown shells dispersed everywhere
throughout a long series of strata, we can not doubt that time was required for
the multiplication of successive generations; and the evidence of slow
accumulation is rendered more striking from the proofs, so often discovered, of
fossil bodies having lain for a time on the floor of the ocean after death
before they were imbedded in sediment. Nothing, for example, is more common than
to see fossil oysters in clay, with Serpulae, or barnacles (acorn-shells), or
corals, and other creatures, attached to the inside of the valves, so that the
mollusk was certainly not buried in argillaceous mud the moment it died. There
must have been an interval during which it was still surrounded with clear
water, when the creatures whose remains now adhere to it grew from an embryonic
to a mature state. Attached shells which are merely external, like some of the
Serpulae (a) in Figure 9, may often have grown upon an oyster or other shell
while the animal within was still living; but if they are found on the inside,
it could only happen after the death of the inhabitant of the shell which
affords the support. Thus, in Figure 9, it will be seen that two Serpulae have
grown on the interior, one of them exactly on the place where the adductor
muscle of the Gryphaea (a kind of oyster) was fixed.
(FIGURE 10. Serpula attached to a fossil Micraster from the Chalk.)
(FIGURE 11. Recent Spatangus with the spines removed from one side.
b. Spine and tubercles, natural size.
a. The same magnified.)
Some fossil shells, even if simply attached to the OUTSIDE of others, bear full
testimony to the conclusion above alluded to, namely, that an interval elapsed
between the death of the creature to whose shell they adhere, and the burial of
the same in mud or sand. The sea-urchins, or Echini, so abundant in white chalk,
afford a good illustration. It is well known that these animals, when living,
are invariably covered with spines supported by rows of tubercles. These last
are only seen after the death of the sea-urchin, when the spines have dropped
off. In Figure 11 a living species of Spatangus, common on our coast, is
represented with one half of its shell stripped of the spines. In Figure 10 a
fossil of a similar and allied genus from the white chalk of England shows the
naked surface which the individuals of this family exhibit when denuded of their
bristles. The full-grown Serpula, therefore, which now adheres externally, could
not have begun to grow till the Micraster had died, and the spines became
detached.
(FIGURE 12.
a. Ananchytes from the chalk with lower valve of Crania attached.
b. Upper valve of Crania detached.)
Now the series of events here attested by a single fossil may be carried a step
farther. Thus, for example, we often meet with a sea-urchin (Ananchytes) in the
chalk (see Figure 12) which has fixed to it the lower valve of a Crania, a genus
of bivalve mollusca. The upper valve (b, Figure 12) is almost invariably
wanting, though occasionally found in a perfect state of preservation in white
chalk at some distance. In this case, we see clearly that the sea-urchin first
lived from youth to age, then died and lost its spines, which were carried away.
Then the young Crania adhered to the bared shell, grew and perished in its turn;
after which the upper valve was separated from the lower before the Ananchytes
became enveloped in chalky mud.
(FIGURES 13 AND 14. Fossil and recent wood drilled by perforating Mollusca.
(FIGURE 13.
a. Fossil wood from London Clay, bored by Teredina.
b. Shell and tube of Teredina personata, the right-hand figure the ventral, the
left the dorsal view.)
(FIGURE 14.
e. Recent wood bored by Toredo.
d. Shell and tube of Teredo navalis, from the same.
c. Anterior and posterior view of the valves of same detached from the tube.))
It may be well to mention one more illustration of the manner in which single
fossils may sometimes throw light on a former state of things, both in the bed
of the ocean and on some adjoining land. We meet with many fragments of wood
bored by ship-worms at various depths in the clay on which London is built.
Entire branches and stems of trees, several feet in length, are sometimes found
drilled all over by the holes of these borers, the tubes and shells of the
mollusk still remaining in the cylindrical hollows. In Figure 14, e, a
representation is given of a piece of recent wood pierced by the Teredo navalis,
or common ship-worm, which destroys wooden piles and ships. When the cylindrical
tube d has been extracted from the wood, the valves are seen at the larger or
anterior extremity, as shown at c. In like manner, a piece of fossil wood (a,
Figure 13) has been perforated by a kindred but extinct genus, the Teredina of
Lamarck. The calcareous tube of this mollusk was united and, as it were,
soldered on to the valves of the shell (b), which therefore can not be detached
from the tube, like the valves of the recent Teredo. The wood in this fossil
specimen is now converted into a stony mass, a mixture of clay and lime; but it
must once have been buoyant and floating in the sea, when the Teredinae lived
upon, and perforated it. Again, before the infant colony settled upon the drift
wood, part of a tree must have been floated down to the sea by a river,
uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind:
and thus our thoughts are carried back to a prior period, when the tree grew for
years on dry land, enjoying a fit soil and climate.
STRATA OF ORGANIC ORIGIN.
(FIGURE 15. Gaillonella ferruginea, Ehb.)
(FIGURE 16. Gaillonella distans, Ehb.)
(FIGURE 17. Bacillaria paradoxa.
a. Front view.
b. Side view.)
It has been already remarked that there are rocks in the interior of continents,
at various depths in the earth, and at great heights above the sea, almost
entirely made up of the remains of zoophytes and testacea. Such masses may be
compared to modern oyster-beds and coral-reefs; and, like them, the rate of
increase must have been extremely gradual. But there are a variety of stone
deposits in the earth's crust, now proved to have been derived from plants and
animals of which the organic origin was not suspected until of late years, even
by naturalists. Great surprise was therefore created some years since by the
discovery of Professor Ehrenberg, of Berlin, that a certain kind of siliceous
stone, called tripoli, was entirely composed of millions of the remains of
organic beings, which were formerly referred to microscopic Infusoria, but which
are now admitted to be plants. They abound in rivulets, lakes, and ponds in
England and other countries, and are termed Diatomaceae by those naturalists who
believe in their vegetable origin. The subject alluded to has long been well-
known in the arts, under the name of infusorial earth or mountain meal, and is
used in the form of powder for polishing stones and metals. It has been
procured, among other places, from the mud of a lake at Dolgelly, in North
Wales, and from Bilin, in Bohemia, in which latter place a single stratum,
extending over a wide area, is no less than fourteen feet thick. This stone,
when examined with a powerful microscope, is found to consist of the siliceous
plates or frustules of the above-figured Diatomaceae, united together without
any visible cement. It is difficult to convey an idea of their extreme
minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000
millions of individuals of the Gaillonella distans (see Figure 16) in every
cubic inch (which weighs about 220 grains), or about 187 millions in a single
grain. At every stroke, therefore, that we make with this polishing powder,
several millions, perhaps tens of millions, of perfect fossils are crushed to
atoms.
A well-known substance, called bog-iron ore, often met with in peat-mosses, has
often been shown by Ehrenberg to consist of innumerable articulated threads, of
a yellow ochre colour, composed of silica, argillaceous matter, and peroxide of
iron. These threads are the cases of a minute microscopic body, called
Gaillonella ferruginea (Figure 15), associated with the siliceous frustules of
other fresh-water algae. Layers of this iron ore occurring in Scotch peat bogs
are often called "the pan," and are sometimes of economical value.
It is clear much time must have been required for the accumulation of strata to
which countless generations of Diatomaceae have contributed their remains; and
these discoveries lead us naturally to suspect that other deposits, of which the
materials have been supposed to be inorganic, may in reality be composed chiefly
of microscopic organic bodies. That this is the case with the white chalk, has
often been imagined, and is now proved to be the fact. It has, moreover, been
lately discovered that the chambers into which these Foraminifera are divided
are actually often filled with thousands of well-preserved organic bodies, which
abound in every minute grain of chalk, and are especially apparent in the white
coating of flints, often accompanied by innumerable needle-shaped spiculae of
sponges (see Chapter 17.).
"The dust we tread upon was once alive!"-- Byron.
How faint an idea does this exclamation of the poet convey of the real wonders
of nature! for here we discover proofs that the calcareous and siliceous dust of
which hills are composed has not only been once alive, but almost every
particle, albeit invisible to the naked eye, still retains the organic structure
which, at periods of time incalculably remote, was impressed upon it by the
powers of life.
FRESH-WATER AND MARINE FOSSILS.
Strata, whether deposited in salt or fresh water, have the same forms; but the
imbedded fossils are very different in the two cases, because the aquatic
animals which frequent lakes and rivers are distinct from those inhabiting the
sea. In the northern part of the Isle of Wight formations of marl and limestone,
more than 50 feet thick occur, in which the shells are of extinct species. Yet
we recognise their fresh-water origin, because they are of the same genera as
those now abounding in ponds, lakes, and rivers, either in our own country or in
warmer latitudes.
In many parts of France-- in Auvergne, for example-- strata occur of limestone,
marl, and sandstone hundreds of feet thick, which contain exclusively fresh-
water and land shells, together with the remains of terrestrial quadrupeds. The
number of land-shells scattered through some of these fresh-water deposits is
exceedingly great; and there are districts in Germany where the rocks scarcely
contain any other fossils except snail-shells (helices); as, for instance, the
limestone on the left bank of the Rhine, between Mayence and Worms, at
Oppenheim, Findheim, Budenheim, and other places. In order to account for this
phenomenon, the geologist has only to examine the small deltas of torrents which
enter the Swiss lakes when the waters are low, such as the newly-formed plain
where the Kander enters the Lake of Thun. He there sees sand and mud strewn over
with innumerable dead land-shells, which have been brought down from the valleys
in the Alps in the preceding spring, during the melting of the snows. Again, if
we search the sands on the borders of the Rhine, in the lower part of its
course, we find countless land-shells mixed with others of species belonging to
lakes, stagnant pools, and marshes. These individuals have been washed away from
the alluvial plains of the great river and its tributaries, some from
mountainous regions, others from the low country.
Although fresh-water formations are often of great thickness, yet they are
usually very limited in area when compared to marine deposits, just as lakes and
estuaries are of small dimensions in comparison with seas.
The absence of many fossil forms usually met with in marine strata, affords a
useful negative indication of the fresh-water origin of a formation. For
example, there are no sea-urchins, no corals, no chambered shells, such as the
nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In
distinguishing the latter from formations accumulated in the sea, we are chiefly
guided by the forms of the mollusca. In a fresh-water deposit, the number of
individual shells is often as great as in a marine stratum, if not greater; but
there is a smaller variety of species and genera. This might be anticipated from
the fact that the genera and species of recent fresh-water and land shells are
few when contrasted with the marine. Thus, the genera of true mollusca according
to Woodward's system, excluding those altogether extinct and those without
shells, amount to 446 in number, of which the terrestrial and fresh-water genera
scarcely form more than a fifth. (See Woodward's Manual of Mollusca 1856.)
(FIGURE 18. Cyrena obovata, Sowerby; fossil. Hants.)
(FIGURE 19. Cyrena (Corbicella) fluminalis, Moll.; fossil. Grays, Essex.)
(FIGURE 20. Anodonta Cordierii; D'Orbigny; fossil. Paris.)
(FIGURE 21. Anodonta latimarginata; recent. Bahia.)
(FIGURE 22. Unio littoralis. Lamarck; recent. Auvergne.)
(FIGURE 23. Gryphaea incurva, Sowerby; (G. arcuata, Lamarck) upper valve. Lias.)
Almost all bivalve shells, or those of acephalous mollusca, are marine, about
sixteen only out of 140 genera being fresh-water. Among these last, the four
most common forms, both recent and fossil, are Cyclas, Cyrena, Unio, and
Anodonta (see Figures 18-22); the two first and two last of which are so nearly
allied as to pass into each other.
Lamarck divided the bivalve mollusca into the Dimyary, or those having two large
muscular impressions in each valve, as a, b in the Cyclas, Figure 18, and Unio,
Figure 22, and the Monomyary, such as the oyster and scallop, in which there is
only one of these impressions, as is seen in Figure 23. Now, as none of these
last, or the unimuscular bivalves, are fresh-water, we may at once presume a
deposit containing any of them to be marine. (The fresh-water Mulleria, when
young, forms a single exception to the rule, as it then has two muscular
impressions, but it has only one in the adult state.)
(FIGURE 24. Planorbis euomphalus, Sowerby; fossil. Isle of Wight.)
(FIGURE 25. Limnaea longiscala, Brongniart; fossil. Isle of Wight.)
(FIGURE 26. Paludina lenta, Brand.; fossil. Isle of Wight.)
(FIGURE 27. Succinea amphibia, Drap. (S. putris, L.); fossil. Loess, Rhine.)
(FIGURE 28. Ancylus velletia (A. elegans), Sowerby; fossil. Isle of Wight.)
(FIGURE 29. Valvata piscinalis, Mull.; fossil. Grays, Essex.)
(FIGURE 30. Physa hypnorum, Linne; recent. Isle of Wight.)
(FIGURE 31. Auricula; recent. Ava.)
(FIGURE 32. Melania inquinata, Def. Paris basin.)
(FIGURE 33. Physa columnaris, Desh. Paris basin.)
(FIGURE 34. Melanopsis buccinoidea, Ferr.; recent. Asia.)
The univalve shells most characteristic of fresh-water deposits are, Planorbis,
Limnaea, and Paludina. (See Figures 24-26.) But to these are occasionally added
Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides, and Neritina
(see Figures 27-34), the four last being usually found in estuaries.
(FIGURE 35. Neritina globulus, Def. Paris basin.)
(FIGURE 36. Nerita granulosa, Desh. Paris basin.)
Some naturalists include Neritina (Figure 35) and the marine Nerita (Figure 36)
in the same genus, it being scarcely possible to distinguish the two by good
generic characters. But, as a general rule, the fluviatile species are smaller,
smoother, and more globular than the marine; and they have never, like the
Neritae, the inner margin of the outer lip toothed or crenulated. (See Figure
36.)
(FIGURE 37. Potamides cinctus, Sowerby. Paris basin.)
The Potamides inhabit the mouths of rivers in warm latitudes, and are
distinguishable from the marine Cerithia by their orbicular and multispiral
opercula. The genus Auricula (Figure 31) is amphibious, frequenting swamps and
marshes within the influence of the tide.
(FIGURE 38. Helix Turonensis, Desh.; faluns, Touraine.)
(FIGURE 39. Cyclostoma elegans, Mull.; Loess.)
(FIGURE 40. Pupa tridens, Drap.; Loess.)
(FIGURE 41. Clausilia bidens, Drap.; Loess.)
(FIGURE 42. Bulimus lubricus, Mull.; Loess, Rhine.)
The terrestrial shells are all univalves. The most important genera among these,
both in a recent and fossil state, are Helix (Figure 38), Cyclostoma (Figure
39), Pupa (Figure 40), Clausilia (Figure 41) Bulimus (Figure 42), Glandina and
Achatina.
(FIGURE 43. Ampullaria glauca, from the Jumna.)
Ampullaria (Figure 43) is another genus of shells inhabiting rivers and ponds in
hot countries. Many fossil species formerly referred to this genus, and which
have been met with chiefly in marine formations, are now considered by
conchologists to belong to Natica and other marine genera.
(FIGURE 44. Pleurotoma exorta, Brand. Upper and Middle Eocene. Barton and
Bracklesham.)
(FIGURE 45. Ancillaria subulata, Sowerby. Barton clay. Eocene.)
All univalve shells of land and fresh-water species, with the exception of
Melanopsis (Figure 34), and Achatina, which has a slight indentation, have
entire mouths; and this circumstance may often serve as a convenient rule for
distinguishing fresh-water from marine strata; since, if any univalves occur of
which the mouths are not entire, we may presume that the formation is marine.
The aperture is said to be entire in such shells as the fresh-water Ampullaria
and the land-shells (Figures 38-42), when its outline is not interrupted by an
indentation or notch, such as that seen at b in Ancillaria (Figure 45); or is
not prolonged into a canal, as that seen at a in Pleurotoma (Figure 44).
The mouths of a large proportion of the marine univalves have these notches or
canals, and almost all species are carnivorous; whereas nearly all testacea
having entire mouths are plant-eaters, whether the species be marine, fresh-
water, or terrestrial.
There is, however, one genus which affords an occasional exception to one of the
above rules. The Potamides (Figure 37), a subgenus of Cerithium, although
provided with a short canal, comprises some species which inhabit salt, others
brackish, and others fresh-water, and they are said to be all plant-eaters.
Among the fossils very common in fresh-water deposits are the shells of Cypris,
a minute bivalve crustaceous animal. (For figures of fossil species of Purbeck
see below, Chapter 19.) Many minute living species of this genus swarm in lakes
and stagnant pools in Great Britain; but their shells are not, if considered
separately, conclusive as to the fresh-water origin of a deposit, because the
majority of species in another kindred genus of the same order, the Cytherina of
Lamarck, inhabit salt-water; and, although the animal differs slightly, the
shell is scarcely distinguishable from that of the Cypris.
FRESH-WATER FOSSIL PLANTS.
(FIGURE 46. Chara medicaginula; fossil. Upper Eocene, Isle of Wight.)
The seed-vessels and stems of Chara, a genus of aquatic plants, are very
frequent in fresh-water strata. These seed-vessels were called, before their
true nature was known, gyrogonites, and were supposed to be foraminiferous
shells. (See Figure 46, a.)
(FIGURE 47. Chara elastica; recent, Italy.
a. Sessile seed-vessel between the divisions of the leaves of the female plant.
b. Magnified transverse section of a branch, with five seed-vessels, seen from
below upward.)
The Charae inhabit the bottom of lakes and ponds, and flourish mostly where the
water is charged with carbonate of lime. Their seed-vessels are covered with a
very tough integument, capable of resisting decomposition; to which circumstance
we may attribute their abundance in a fossil state. Figure 47 represents a
branch of one of many new species found by Professor Amici in the lakes of
Northern Italy. The seed-vessel in this plant is more globular than in the
British Charae, and therefore more nearly resembles in form the extinct fossil
species found in England, France, and other countries. The stems, as well as the
seed-vessels, of these plants occur both in modern shell-marl and in ancient
fresh-water formations. They are generally composed of a large central tube
surrounded by smaller ones; the whole stem being divided at certain intervals by
transverse partitions or joints. (See b, Figure 46.)
It is not uncommon to meet with layers of vegetable matter, impressions of
leaves, and branches of trees, in strata containing fresh-water shells; and we
also find occasionally the teeth and bones of land quadrupeds, of species now
unknown. The manner in which such remains are occasionally carried by rivers
into lakes, especially during floods, has been fully treated of in the
"Principles of Geology."
FRESH-WATER AND MARINE FISH.
The remains of fish are occasionally useful in determining the fresh-water
origin of strata. Certain genera, such as carp, perch, pike, and loach
(Cyprinus, Perca, Esox, and Cobitis), as also Lebias, being peculiar to fresh-
water. Other genera contain some fresh-water and some marine species, as Cottus,
Mugil, and Anguilla, or eel. The rest are either common to rivers and the sea,
as the salmon; or are exclusively characteristic of salt-water. The above
observations respecting fossil fishes are applicable only to the more modern or
tertiary deposits; for in the more ancient rocks the forms depart so widely from
those of existing fishes, that it is very difficult, at least in the present
state of science, to derive any positive information from ichthyolites
respecting the element in which strata were deposited.
The alternation of marine and fresh-water formations, both on a small and large
scale, are facts well ascertained in geology. When it occurs on a small scale,
it may have arisen from the alternate occupation of certain spaces by river-
water and the sea; for in the flood season the river forces back the ocean and
freshens it over a large area, depositing at the same time its sediment; after
which the salt-water again returns, and, on resuming its former place, brings
with it sand, mud, and marine shells.
There are also lagoons at the mouth of many rivers, as the Nile and Mississippi,
which are divided off by bars of sand from the sea, and which are filled with
salt and fresh water by turns. They often communicate exclusively with the river
for months, years, or even centuries; and then a breach being made in the bar of
sand, they are for long periods filled with salt-water.
LYM-FIORD.
The Lym-Fiord in Jutland offers an excellent illustration of analogous changes;
for, in the course of the last thousand years, the western extremity of this
long frith, which is 120 miles in length, including its windings, has been four
times fresh and four times salt, a bar of sand between it and the ocean having
been often formed and removed. The last irruption of salt water happened in
1824, when the North Sea entered, killing all the fresh-water shells, fish, and
plants; and from that time to the present, the sea-weed Fucus vesiculosus,
together with oysters and other marine mollusca, have succeeded the Cyclas,
Lymnaea, Paludina, and Charae. (See Principles Index "Lym-Fiord.")
But changes like these in the Lym-Fiord, and those before mentioned as occurring
at the mouths of great rivers, will only account for some cases of marine
deposits of partial extent resting on fresh-water strata. When we find, as in
the south-east of England (Chapter 18), a great series of fresh-water beds, 1000
feet in thickness, resting upon marine formations and again covered by other
rocks, such as the Cretaceous, more than 1000 feet thick, and of deep-sea
origin, we shall find it necessary to seek for a different explanation of the
phenomena.
CHAPTER IV.
CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.
Chemical and Mechanical Deposits.
Cementing together of Particles.
Hardening by Exposure to Air.
Concretionary Nodules.
Consolidating Effects of Pressure.
Mineralization of Organic Remains.
Impressions and Casts: how formed.
Fossil Wood.
Goppert's Experiments.
Precipitation of Stony Matter most rapid where Putrefaction is going on.
Sources of Lime and Silex in Solution.
Having spoken in the preceding chapters of the characters of sedimentary
formations, both as dependent on the deposition of inorganic matter and the
distribution of fossils, I may next treat of the consolidation of stratified
rocks, and the petrifaction of imbedded organic remains.
CHEMICAL AND MECHANICAL DEPOSITS.
A distinction has been made by geologists between deposits of a mechanical, and
those of a chemical, origin. By the name mechanical are designated beds of mud,
sand, or pebbles produced by the action of running water, also accumulations of
stones and scoriae thrown out by a volcano, which have fallen into their present
place by the force of gravitation. But the matter which forms a chemical deposit
has not been mechanically suspended in water, but in a state of solution until
separated by chemical action. In this manner carbonate of lime is occasionally
precipitated upon the bottom of lakes in a solid form, as may be well seen in
many parts of Italy, where mineral springs abound, and where the calcareous
stone, called travertin, is deposited. In these springs the lime is usually held
in solution by an excess of carbonic acid, or by heat if it be a hot spring,
until the water, on issuing from the earth, cools or loses part of its acid. The
calcareous matter then falls down in a solid state, incrusting shells, fragments
of wood and leaves, and binding them together.
That similar travertin is formed at some points in the bed of the sea where
calcareous springs issue can not be doubted, but as a general rule the quantity
of lime, according to Bischoff, spread through the waters of the ocean is very
small, the free carbonic acid gas in the same waters being five times as much as
is necessary to keep the lime in a fluid state. Carbonate of lime, therefore,
can rarely be precipitated at the bottom of the sea by chemical action alone,
but must be produced by vital agency as in the case of coral reefs.
In such reefs, large masses of limestone are formed by the stony skeletons of
zoophytes; and these, together with shells, become cemented together by
carbonate of lime, part of which is probably furnished to the sea-water by the
decomposition of dead corals. Even shells, of which the animals are still living
on these reefs, are very commonly found to be incrusted over with a hard coating
of limestone.
If sand and pebbles are carried by a river into the sea, and these are bound
together immediately by carbonate of lime, the deposit may be described as of a
mixed origin, partly chemical, and partly mechanical.
Now, the remarks already made in Chapter 2 on the original horizontality of
strata are strictly applicable to mechanical deposits, and only partially to
those of a mixed nature. Such as are purely chemical may be formed on a very
steep slope, or may even incrust the vertical walls of a fissure, and be of
equal thickness throughout; but such deposits are of small extent, and for the
most part confined to vein-stones.
CONSOLIDATION OF STRATA.
It is chiefly in the case of calcareous rocks that solidification takes place at
the time of deposition. But there are many deposits in which a cementing process
comes into operation long afterwards. We may sometimes observe, where the water
of ferruginous or calcareous springs has flowed through a bed of sand or gravel,
that iron or carbonate of lime has been deposited in the interstices between the
grains or pebbles, so that in certain places the whole has been bound together
into a stone, the same set of strata remaining in other parts loose and
incoherent.
Proofs of a similar cementing action are seen in a rock at Kelloway, in
Wiltshire. A peculiar band of sandy strata belonging to the group called Oolite
by geologists may be traced through several counties, the sand being for the
most part loose and unconsolidated, but becoming stony near Kelloway. In this
district there are numerous fossil shells which have decomposed, having for the
most part left only their casts. The calcareous matter hence derived has
evidently served, at some former period, as a cement to the siliceous grains of
sand, and thus a solid sandstone has been produced. If we take fragments of many
other argillaceous grits, retaining the casts of shells, and plunge them into
dilute muriatic or other acid, we see them immediately changed into common sand
and mud; the cement of lime, derived from the shells, having been dissolved by
the acid.
Traces of impressions and casts are often extremely faint. In some loose sands
of recent date we meet with shells in so advanced a stage of decomposition as to
crumble into powder when touched. It is clear that water percolating such strata
may soon remove the calcareous matter of the shell; and unless circumstances
cause the carbonate of lime to be again deposited, the grains of sand will not
be cemented together; in which case no memorial of the fossil will remain.
In what manner silex and carbonate of lime may become widely diffused in small
quantities through the waters which permeate the earth's crust will be spoken of
presently, when the petrifaction of fossil bodies is considered; but I may
remark here that such waters are always passing in the case of thermal springs
from hotter to colder parts of the interior of the earth; and, as often as the
temperature of the solvent is lowered, mineral matter has a tendency to separate
from it and solidify. Thus a stony cement is often supplied to sand, pebbles, or
any fragmentary mixture. In some conglomerates, like the pudding-stone of
Hertfordshire (a Lower Eocene deposit), pebbles of flint and grains of sand are
united by a siliceous cement so firmly, that if a block be fractured, the rent
passes as readily through the pebbles as through the cement.
It is probable that many strata became solid at the time when they emerged from
the waters in which they were deposited, and when they first formed a part of
the dry land. A well-known fact seems to confirm this idea: by far the greater
number of the stones used for building and road-making are much softer when
first taken from the quarry than after they have been long exposed to the air;
and these, when once dried, may afterwards be immersed for any length of time in
water without becoming soft again. Hence it is found desirable to shape the
stones which are to be used in architecture while they are yet soft and wet, and
while they contain their "quarry-water," as it is called; also to break up stone
intended for roads when soft, and then leave it to dry in the air for months
that it may harden. Such induration may perhaps be accounted for by supposing
the water, which penetrates the minutest pores of rocks, to deposit, on
evaporation, carbonate of lime, iron, silex, and other minerals previously held
in solution, and thereby to fill up the pores partially. These particles, on
crystallising, would not only be themselves deprived of freedom of motion, but
would also bind together other portions of the rock which before were loosely
aggregated. On the same principle wet sand and mud become as hard as stone when
frozen; because one ingredient of the mass, namely, the water, has crystallised,
so as to hold firmly together all the separate particles of which the loose mud
and sand were composed.
Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like dough
when first found; and some simple minerals, which are rigid and as hard as glass
in our cabinets, are often flexible and soft in their native beds: this is the
case with asbestos, sahlite, tremolite, and chalcedony, and it is reported also
to happen in the case of the beryl. (Dr. MacCulloch System of Geology volume 1
page 123.)
The marl recently deposited at the bottom of Lake Superior, in North America, is
soft, and often filled with fresh-water shells; but if a piece be taken up and
dried, it becomes so hard that it can only be broken by a smart blow of the
hammer. If the lake, therefore, was drained, such a deposit would be found to
consist of strata of marlstone, like that observed in many ancient European
formations, and, like them, containing fresh-water shells.
CONCRETIONARY STRUCTURE.
(FIGURE 48. Calcareous nodules in Lias.)
It is probable that some of the heterogeneous materials which rivers transport
to the sea may at once set under water, like the artificial mixture called
pozzolana, which consists of fine volcanic sand charged with about twenty per
cent of oxide of iron, and the addition of a small quantity of lime. This
substance hardens, and becomes a solid stone in water, and was used by the
Romans in constructing the foundations of buildings in the sea. Consolidation in
such cases is brought about by the action of chemical affinity on finely
comminuted matter previously suspended in water. After deposition similar
particles seem often to exert a mutual attraction on each other, and congregate
together in particular spots, forming lumps, nodules, and concretions. Thus in
many argillaceous deposits there are calcareous balls, or spherical concretions,
ranged in layers parallel to the general stratification; an arrangement which
took place after the shale or marl had been thrown down in successive laminae;
for these laminae are often traceable through the concretions, remaining
parallel to those of the surrounding unconsolidated rock. (See Figure 48.) Such
nodules of limestone have often a shell or other foreign body in the centre.
(FIGURE 49. Spheroidal concretions in magnesian limestone.)
Among the most remarkable examples of concretionary structure are those
described by Professor Sedgwick as abounding in the magnesian limestone of the
north of England. The spherical balls are of various sizes, from that of a pea
to a diameter of several feet, and they have both a concentric and radiated
structure, while at the same time the laminae of original deposition pass
uninterruptedly through them. In some cliffs this limestone resembles a great
irregular pile of cannon-balls. Some of the globular masses have their centre in
one stratum, while a portion of their exterior passes through to the stratum
above or below. Thus the larger spheroid in the section (Figure 49) passes from
the stratum b upward into a. In this instance we must suppose the deposition of
a series of minor layers, first forming the stratum b, and afterwards the
incumbent stratum a; then a movement of the particles took place, and the
carbonates of lime and magnesia separated from the more impure and mixed matter
forming the still unconsolidated parts of the stratum. Crystallisation,
beginning at the centre, must have gone on forming concentric coats around the
original nucleus without interfering with the laminated structure of the rock.
(FIGURE 50. Section through strata of grit.)
When the particles of rocks have been thus rearranged by chemical forces, it is
sometimes difficult or impossible to ascertain whether certain lines of division
are due to original deposition or to the subsequent aggregation of several
particles. Thus suppose three strata of grit, A, B, C, are charged unequally
with calcareous matter, and that B is the most calcareous. If consolidation
takes place in B, the concretionary action may spread upward into a part of A,
where the carbonate of lime is more abundant than in the rest; so that a mass, d
e f, forming a portion of the superior stratum, becomes united with B into one
solid mass of stone. The original line of division, d e, being thus effaced, the
line d f would generally be considered as the surface of the bed B, though not
strictly a true plane of stratification. (Figure 50.)
PRESSURE AND HEAT.
When sand and mud sink to the bottom of a deep sea, the particles are not
pressed down by the enormous weight of the incumbent ocean; for the water, which
becomes mingled with the sand and mud, resists pressure with a force equal to
that of the column of fluid above. The same happens in regard to organic remains
which are filled with water under great pressure as they sink, otherwise they
would be immediately crushed to pieces and flattened. Nevertheless, if the
materials of a stratum remain in a yielding state, and do not set or solidify,
they will be gradually squeezed down by the weight of other materials
successively heaped upon them, just as soft clay or loose sand on which a house
is built may give way. By such downward pressure particles of clay, sand, and
marl may become packed into a smaller space, and be made to cohere together
permanently.
Analogous effects of condensation may arise when the solid parts of the earth's
crust are forced in various directions by those mechanical movements hereafter
to be described, by which strata have been bent, broken, and raised above the
level of the sea. Rocks of more yielding materials must often have been forced
against others previously consolidated, and may thus by compression have
acquired a new structure. A recent discovery may help us to comprehend how fine
sediment derived from the detritus of rocks may be solidified by mere pressure.
The graphite or "black lead" of commerce having become very scarce, Mr.
Brockedon contrived a method by which the dust of the purer portions of the
mineral found in Borrowdale might be recomposed into a mass as dense and compact
as native graphite. The powder of graphite is first carefully prepared and freed
from air, and placed under a powerful press on a strong steel die, with air-
tight fittings. It is then struck several blows, each of a power of 1000 tons;
after which operation the powder is so perfectly solidified that it can be cut
for pencils, and exhibits when broken the same texture as native graphite.
But the action of heat at various depths in the earth is probably the most
powerful of all causes in hardening sedimentary strata. To this subject I shall
refer again when treating of the metamorphic rocks, and of the slaty and jointed
structure.
MINERALISATION OF ORGANIC REMAINS.
(FIGURE 51. Phasianella Heddingtonensis, and cast of the same. Coral Rag.)
(FIGURE 52. Pleurotomaria Anglica, and cast. Lias.)
The changes which fossil organic bodies have undergone since they were first
imbedded in rocks, throw much light on the consolidation of strata. Fossil
shells in some modern deposits have been scarcely altered in the course of
centuries, having simply lost a part of their animal matter. But in other cases
the shell has disappeared, and left an impression only of its exterior, or,
secondly, a cast of its interior form, or, thirdly, a cast of the shell itself,
the original matter of which has been removed. These different forms of
fossilisation may easily be understood if we examine the mud recently thrown out
from a pond or canal in which there are shells. If the mud be argillaceous, it
acquires consistency on drying, and on breaking open a portion of it we find
that each shell has left impressions of its external form. If we then remove the
shell itself, we find within a solid nucleus of clay, having the form of the
interior of the shell. This form is often very different from that of the outer
shell. Thus a cast such as a, Figure 51, commonly called a fossil screw, would
never be suspected by an inexperienced conchologist to be the internal shape of
the fossil univalve, b, Figure 51. Nor should we have imagined at first sight
that the shell a and the cast b, Figure 52, belong to one and the same fossil.
The reader will observe, in the last-mentioned figure (b, Figure 52), that an
empty space shaded dark, which the SHELL ITSELF once occupied, now intervenes
between the enveloping stone and the cast of the smooth interior of the whorls.
In such cases the shell has been dissolved and the component particles removed
by water percolating the rock. If the nucleus were taken out, a hollow mould
would remain, on which the external form of the shell with its tubercles and
striae, as seen in a, Figure 52, would be seen embossed. Now if the space
alluded to between the nucleus and the impression, instead of being left empty,
has been filled up with calcareous spar, flint, pyrites, or other mineral, we
then obtain from the mould an exact cast both of the external and internal form
of the original shell. In this manner silicified casts of shells have been
formed; and if the mud or sand of the nucleus happen to be incoherent, or
soluble in acid, we can then procure in flint an empty shell, which in shape is
the exact counterpart of the original. This cast may be compared to a bronze
statue, representing merely the superficial form, and not the internal
organisation; but there is another description of petrifaction by no means
uncommon, and of a much more wonderful kind, which may be compared to certain
anatomical models in wax, where not only the outward forms and features, but the
nerves, blood-vessels, and other internal organs are also shown. Thus we find
corals, originally calcareous, in which not only the general shape, but also the
minute and complicated internal organisation is retained in flint.
(FIGURE 53. Section of a tree from the coal-measures, magnified (Witham),
showing texture of wood.)
Such a process of petrifaction is still more remarkably exhibited in fossil
wood, in which we often perceive not only the rings of annual growth, but all
the minute vessels and medullary rays. Many of the minute cells and fibres of
plants, and even those spiral vessels which in the living vegetable can only be
discovered by the microscope, are preserved. Among many instances, I may mention
a fossil tree, seventy-two feet in length, found at Gosforth, near Newcastle, in
sandstone strata associated with coal. By cutting a transverse slice so thin as
to transmit light, and magnifying it about fifty-five times, the texture, as
seen in Figure 53, is exhibited. A texture equally minute and complicated has
been observed in the wood of large trunks of fossil trees found in the
Craigleith quarry near Edinburgh, where the stone was not in the slightest
degree siliceous, but consisted chiefly of carbonate of lime, with oxide of
iron, alumina, and carbon. The parallel rows of vessels here seen are the rings
of annual growth, but in one part they are imperfectly preserved, the wood
having probably decayed before the mineralising matter had penetrated to that
portion of the tree.
In attempting to explain the process of petrifaction in such cases, we may first
assume that strata are very generally permeated by water charged with minute
portions of calcareous, siliceous, and other earths in solution. In what manner
they become so impregnated will be afterwards considered. If an organic
substance is exposed in the open air to the action of the sun and rain, it will
in time putrefy, or be dissolved into its component elements, consisting usually
of oxygen, hydrogen, nitrogen, and carbon. These will readily be absorbed by the
atmosphere or be washed away by rain, so that all vestiges of the dead animal or
plant disappear. But if the same substances be submerged in water, they
decompose more gradually; and if buried in earth, still more slowly; as in the
familiar example of wooden piles or other buried timber. Now, if as fast as each
particle is set free by putrefaction in a fluid or gaseous state, a particle
equally minute of carbonate of lime, flint, or other mineral, is at hand ready
to be precipitated, we may imagine this inorganic matter to take the place just
before left unoccupied by the organic molecule. In this manner a cast of the
interior of certain vessels may first be taken, and afterwards the more solid
walls of the same may decay and suffer a like transmutation. Yet when the whole
is lapidified, it may not form one homogeneous mass of stone or metal. Some of
the original ligneous, osseous, or other organic elements may remain mingled in
certain parts, or the lapidifying substance itself may be differently coloured
at different times, or so crystallised as to reflect light differently, and thus
the texture of the original body may be faithfully exhibited.
The student may perhaps ask whether, on chemical principles, we have any ground
to expect that mineral matter will be thrown down precisely in those spots where
organic decomposition is in progress? The following curious experiments may
serve to illustrate this point: Professor Goppert of Breslau, with a view of
imitating the natural process of petrifaction, steeped a variety of animal and
vegetable substances in waters, some holding siliceous, others calcareous,
others metallic matter in solution. He found that in the period of a few weeks,
or sometimes even days, the organic bodies thus immersed were mineralised to a
certain extent. Thus, for example, thin vertical slices of deal, taken from the
Scotch fir (Pinus sylvestris), were immersed in a moderately strong solution of
sulphate of iron. When they had been thoroughly soaked in the liquid for several
days they were dried and exposed to a red-heat until the vegetable matter was
burnt up and nothing remained but an oxide of iron, which was found to have
taken the form of the deal so exactly that casts even of the dotted vessels
peculiar to this family of plants were distinctly visible under the microscope.
The late Dr. Turner observes, that when mineral matter is in a "nascent state,"
that is to say, just liberated from a previous state of chemical combination, it
is most ready to unite with other matter, and form a new chemical compound.
Probably the particles or atoms just set free are of extreme minuteness, and
therefore move more freely, and are more ready to obey any impulse of chemical
affinity. Whatever be the cause, it clearly follows, as before stated, that
where organic matter newly imbedded in sediment is decomposing, there will
chemical changes take place most actively.
An analysis was lately made of the water which was flowing off from the rich mud
deposited by the Hooghly River in the Delta of the Ganges after the annual
inundation. This water was found to be highly charged with carbonic acid holding
lime in solution. (Piddington Asiatic Researches volume 18 page 226.) Now if
newly-deposited mud is thus proved to be permeated by mineral matter in a state
of solution, it is not difficult to perceive that decomposing organic bodies,
naturally imbedded in sediment, may as readily become petrified as the
substances artificially immersed by Professor Goppert in various fluid mixtures.
It is well known that the waters of all springs are more or less charged with
earthy, alkaline, or metallic ingredients derived from the rocks and mineral
veins through which they percolate. Silex is especially abundant in hot springs,
and carbonate of lime is almost always present in greater or less quantity. The
materials for the petrifaction of organic remains are, therefore, usually at
hand in a state of chemical solution wherever organic remains are imbedded in
new strata.
CHAPTER V.
ELEVATION OF STRATA ABOVE THE SEA.-- HORIZONTAL AND INCLINED STRATIFICATION.
Why the Position of Marine Strata, above the Level of the Sea, should be
referred to the rising up of the Land, not to the going down of the Sea.
Strata of Deep-sea and Shallow-water Origin alternate.
Also Marine and Fresh-water Beds and old Land Surfaces.
Vertical, inclined, and folded Strata.
Anticlinal and Synclinal Curves.
Theories to explain Lateral Movements.
Creeps in Coal-mines.
Dip and Strike.
Structure of the Jura.
Various Forms of Outcrop.
Synclinal Strata forming Ridges.
Connection of Fracture and Flexure of Rocks.
Inverted Strata.
Faults described.
Superficial Signs of the same obliterated by Denudation.
Great Faults the Result of repeated Movements.
Arrangement and Direction of parallel Folds of Strata.
Unconformability.
Overlapping Strata.
LAND HAS BEEN RAISED, NOT THE SEA LOWERED.
It has been already stated that the aqueous rocks containing marine fossils
extend over wide continental tracts, and are seen in mountain chains rising to
great heights above the level of the sea (Chapter 1). Hence it follows, that
what is now dry land was once under water. But if we admit this conclusion, we
must imagine, either that there has been a general lowering of the waters of the
ocean, or that the solid rocks, once covered by water, have been raised up
bodily out of the sea, and have thus become dry land. The earlier geologists,
finding themselves reduced to this alternative, embraced the former opinion,
assuming that the ocean was originally universal, and had gradually sunk down to
its actual level, so that the present islands and continents were left dry. It
seemed to them far easier to conceive that the water had gone down, than that
solid land had risen upward into its present position. It was, however,
impossible to invent any satisfactory hypothesis to explain the disappearance of
so enormous a body of water throughout the globe, it being necessary to infer
that the ocean had once stood at whatever height marine shells might be
detected. It moreover appeared clear, as the science of geology advanced, that
certain spaces on the globe had been alternately sea, then land, then estuary,
then sea again, and, lastly, once more habitable land, having remained in each
of these states for considerable periods. In order to account for such phenomena
without admitting any movement of the land itself, we are required to imagine
several retreats and returns of the ocean; and even then our theory applies
merely to cases where the marine strata composing the dry land are horizontal,
leaving unexplained those more common instances where strata are inclined,
curved, or placed on their edges, and evidently not in the position in which
they were first deposited.
Geologists, therefore, were at last compelled to have recourse to the doctrine
that the solid land has been repeatedly moved upward or downward, so as
permanently to change its position relatively to the sea. There are several
distinct grounds for preferring this conclusion. First, it will account equally
for the position of those elevated masses of marine origin in which the
stratification remains horizontal, and for those in which the strata are
disturbed, broken, inclined, or vertical. Secondly, it is consistent with human
experience that land should rise gradually in some places and be depressed in
others. Such changes have actually occurred in our own days, and are now in
progress, having been accompanied in some cases by violent convulsions, while in
others they have proceeded so insensibly as to have been ascertainable only by
the most careful scientific observations, made at considerable intervals of
time. On the other hand, there is no evidence from human experience of a rising
or lowering of the sea's level in any region, and the ocean can not be raised or
depressed in one place without its level being changed all over the globe.
These preliminary remarks will prepare the reader to understand the great
theoretical interest attached to all facts connected with the position of
strata, whether horizontal or inclined, curved or vertical.
Now the first and most simple appearance is where strata of marine origin occur
above the level of the sea in horizontal position. Such are the strata which we
meet with in the south of Sicily, filled with shells for the most part of the
same species as those now living in the Mediterranean. Some of these rocks rise
to the height of more than 2000 feet above the sea. Other mountain masses might
be mentioned, composed of horizontal strata of high antiquity, which contain
fossil remains of animals wholly dissimilar from any now known to exist. In the
south of Sweden, for example, near Lake Wener, the beds of some of the oldest
fossiliferous deposits, called Silurian and Cambrian by geologists, occur in as
level a position as if they had recently formed part of the delta of a great
river, and been left dry on the retiring of the annual floods. Aqueous rocks of
equal antiquity extend for hundreds of miles over the lake-district of North
America, and exhibit in like manner a stratification nearly undisturbed. The
Table Mountain at the Cape of Good Hope is another example of highly elevated
yet perfectly horizontal strata, no less than 3500 feet in thickness, and
consisting of sandstone of very ancient date.
Instead of imagining that such fossiliferous rocks were always at their present
level, and that the sea was once high enough to cover them, we suppose them to
have constituted the ancient bed of the ocean, and to have been afterwards
uplifted to their present height. This idea, however startling it may at first
appear, is quite in accordance, as before stated, with the analogy of changes
now going on in certain regions of the globe. Thus, in parts of Sweden, and the
shores and islands of the Gulf of Bothnia, proofs have been obtained that the
land is experiencing, and has experienced for centuries, a slow upheaving
movement. (See "Principles of Geology" 1867 page 314.)
It appears from the observations of Mr. Darwin and others, that very extensive
regions of the continent of South America have been undergoing slow and gradual
upheaval, by which the level plains of Patagonia, covered with recent marine
shells, and the Pampas of Buenos Ayres, have been raised above the level of the
sea. On the other hand, the gradual sinking of the west coast of Greenland, for
the space of more than 600 miles from north to south, during the last four
centuries, has been established by the observations of a Danish naturalist, Dr.
Pingel. And while these proofs of continental elevation and subsidence, by slow
and insensible movements, have been recently brought to light, the evidence has
been daily strengthened of continued changes of level effected by violent
convulsions in countries where earthquakes are frequent. There the rocks are
rent from time to time, and heaved up or thrown down several feet at once, and
disturbed in such a manner as to show how entirely the original position of
strata may be modified in the course of centuries.
Mr. Darwin has also inferred that, in those seas where circular coral islands
and barrier reefs abound, there is a slow and continued sinking of the submarine
mountains on which the masses of coral are based; while there are other areas of
the South Sea where the land is on the rise, and where coral has been upheaved
far above the sea-level.
ALTERNATIONS OF MARINE AND FRESH-WATER STRATA.
It has been shown in the third chapter that there is such a difference between
land, fresh-water, and marine fossils as to enable the geologist to determine
whether particular groups of strata were formed at the bottom of the ocean or in
estuaries, rivers, or lakes. If surprise was at first created by the discovery
of marine corals and shells at the height of several miles above the sea-level,
the imagination was afterwards not less startled by observing that in the
successive strata composing the earth's crust, especially if their total
thickness amounted to thousands of feet, they comprised in some parts formations
of shallow-sea as well as of deep-sea origin; also beds of brackish or even of
purely fresh-water formation, as well as vegetable matter or coal accumulated on
ancient land. In these cases we as frequently find fresh-water beds below a
marine set or shallow-water under those of deep-sea origin as the reverse. Thus,
if we bore an artesian well below London, we pass through a marine clay, and
there reach, at the depth of several hundred feet, a shallow-water and
fluviatile sand, beneath which comes the white chalk originally formed in a deep
sea. Or if we bore vertically through the chalk of the North Downs, we come,
after traversing marine chalky strata, upon a fresh-water formation many
hundreds of feet thick, called the Wealden, such as is seen in Kent and Surrey,
which is known in its turn to rest on purely marine beds. In like manner, in
various parts of Great Britain we sink vertical shafts through marine deposits
of great thickness, and come upon coal which was formed by the growth of plants
on an ancient land-surface sometimes hundreds of square miles in extent.
VERTICAL, INCLINED, AND CURVED STRATA.
(FIGURE 54. Vertical conglomerate and sandstone.)
It has been stated that marine strata of different ages are sometimes found at a
considerable height above the sea, yet retaining their original horizontality;
but this state of things is quite exceptional. As a general rule, strata are
inclined or bent in such a manner as to imply that their original position has
been altered.
(FIGURE 55. Section of Forfarshire, from N.W. to S.E., from the foot of the
Grampians to the sea at Arbroath (volcanic or trap rocks omitted). Length of
section twenty miles.
From S.E. (left) Sea: Whiteness, Arbroath: Strata a, 2, 3: Leys Mill: Strata 4:
Sidlaw Hills. Viney R.: Strata B: Pitmuies: Strata 4: Position and nature of the
rocks below No. 4 unknown: Turin: Findhaven: Strata 3, 2, A: Valley of
Strathmore: Strata 1, 2, 3: W. Ogle: Strata 4 and Clay-Slate: to N.W. (right).)
The most unequivocal evidence of such a change is afforded by their standing up
vertically, showing their edges, which is by no means a rare phenomenon,
especially in mountainous countries. Thus we find in Scotland, on the southern
skirts of the Grampians, beds of pudding-stone alternating with thin layers of
fine sand, all placed vertically to the horizon. When Saussure first observed
certain conglomerates in a similar position in the Swiss Alps, he remarked that
the pebbles, being for the most part of an oval shape, had their longer axes
parallel to the planes of stratification (see Figure 54). From this he inferred
that such strata must, at first, have been horizontal, each oval pebble having
settled at the bottom of the water, with its flatter side parallel to the
horizon, for the same reason that an egg will not stand on either end if
unsupported. Some few, indeed, of the rounded stones in a conglomerate
occasionally afford an exception to the above rule, for the same reason that in
a river's bed, or on a shingle beach, some pebbles rest on their ends or edges;
these having been shoved against or between other stones by a wave or current,
so as to assume this position.
ANTICLINAL AND SYNCLINAL CURVES.
Vertical strata, when they can be traced continuously upward or downward for
some depth, are almost invariably seen to be parts of great curves, which may
have a diameter of a few yards, or of several miles. I shall first describe two
curves of considerable regularity, which occur in Forfarshire, extending over a
country twenty miles in breadth, from the foot of the Grampians to the sea near
Arbroath.
The mass of strata here shown may be 2000 feet in thickness, consisting of red
and white sandstone, and various coloured shales, the beds being distinguishable
into four principal groups, namely, No. 1, red marl or shale; No. 2, red
sandstone, used for building; No. 3, conglomerate; and No. 4, grey paving-stone,
and tile-stone, with green and reddish shale, containing peculiar organic
remains. A glance at the section (Figure 55.) will show that each of the
formations 2, 3, 4 are repeated thrice at the surface, twice with a southerly,
and once with a northerly inclination or DIP, and the beds in No. 1, which are
nearly horizontal, are still brought up twice by a slight curvature to the
surface, once on each side of A. Beginning at the north-west extremity, the
tile-stones and conglomerates, No. 4 and No. 3, are vertical, and they generally
form a ridge parallel to the southern skirts of the Grampians. The superior
strata, Nos. 2 and 1, become less and less inclined on descending to the valley
of Strathmore, where the strata, having a concave bend, are said by geologists
to lie in a "trough" or "basin." Through the centre of this valley runs an
imaginary line A, called technically a "synclinal line," where the beds, which
are tilted in opposite directions, may be supposed to meet. It is most important
for the observer to mark such lines, for he will perceive by the diagram that,
in travelling from the north to the centre of the basin, he is always passing
from older to newer beds; whereas, after crossing the line A, and pursuing his
course in the same southerly direction, he is continually leaving the newer, and
advancing upon older strata. All the deposits which he had before examined begin
then to recur in reversed order, until he arrives at the central axis of the
Sidlaw hills, where the strata are seen to form an arch, or SADDLE, having an
ANTICLINAL line, B, in the centre. On passing this line, and continuing towards
the S.E., the formations 4, 3, and 2, are again repeated, in the same relative
order of superposition, but with a southerly dip. At Whiteness (see Figure 55)
it will be seen that the inclined strata are covered by a newer deposit, a, in
horizontal beds. These are composed of red conglomerate and sand, and are newer
than any of the groups, 1, 2, 3, 4, before described, and rest UNCONFORMABLY
upon strata of the sandstone group, No. 2.
An example of curved strata, in which the bends or convolutions of the rock are
sharper and far more numerous within an equal space, has been well described by
Sir James Hall. (Edinburgh Transactions volume 7 plate 3.) It occurs near St.
Abb's Head, on the east coast of Scotland, where the rocks consist principally
of a bluish slate, having frequently a ripple-marked surface. The undulations of
the beds reach from the top to the bottom of cliffs from 200 to 300 feet in
height, and there are sixteen distinct bendings in the course of about six
miles, the curvatures being alternately concave and convex upward.
FOLDING BY LATERAL MOVEMENT.
(FIGURE 56. Curved strata of slate near St. Abb's Head, Berwickshire. (Sir J.
Hall.)
(FIGURE 57. Curved strata in line of cliff.)
(FIGURE 58. Folded cloths imitating bent strata.)
An experiment was made by Sir James Hall, with a view of illustrating the manner
in which such strata, assuming them to have been originally horizontal, may have
been forced into their present position. A set of layers of clay were placed
under a weight, and their opposite ends pressed towards each other with such
force as to cause them to approach more nearly together. On the removal of the
weight, the layers of clay were found to be curved and folded, so as to bear a
miniature resemblance to the strata in the cliffs. We must, however, bear in
mind that in the natural section or sea-cliff we only see the foldings
imperfectly, one part being invisible beneath the sea, and the other, or upper
portion, being supposed to have been carried away by DENUDATION, or that action
of water which will be explained in the next chapter. The dark lines in the plan
(Figure 57) represent what is actually seen of the strata in the line of cliff
alluded to; the fainter lines, that portion which is concealed beneath the sea-
level, as also that which is supposed to have once existed above the present
surface.
We may still more easily illustrate the effects which a lateral thrust might
produce on flexible strata, by placing several pieces of differently coloured
cloths upon a table, and when they are spread out horizontally, cover them with
a book. Then apply other books to each end, and force them towards each other.
The folding of the cloths (see Figure 58) will imitate those of the bent strata;
the incumbent book being slightly lifted up, and no longer touching the two
volumes on which it rested before, because it is supported by the tops of the
anticlinal ridges formed by the curved cloths. In like manner there can be no
doubt that the squeezed strata, although laterally condensed and more closely
packed, are yet elongated and made to rise upward, in a direction perpendicular
to the pressure.
Whether the analogous flexures in stratified rocks have really been due to
similar sideway movements is a question which we can not decide by reference to
our own observation. Our inability to explain the nature of the process is,
perhaps, not simply owing to the inaccessibility of the subterranean regions
where the mechanical force is exerted, but to the extreme slowness of the
movement. The changes may sometimes be due to variation in the temperature of
mountain masses of rock causing them, while still solid, to expand or contract;
or melting them, and then again cooling them and allowing them to crystallise.
If such be the case, we have scarcely more reason to expect to witness the
operation of the process within the limited periods of our scientific
observation than to see the swelling of the roots of a tree, by which, in the
course of years, a wall of solid masonry may be lifted up, rent or thrown down.
In both instances the force may be irresistible, but though adequate, it need
not be visible by us, provided the time required for its development be very
great. The lateral pressure arising from the unequal expansion of rocks by heat
may cause one mass lying in the same horizontal plane gradually to occupy a
larger space, so as to press upon another rock, which, if flexible, may be
squeezed into a bent and folded form. It will also appear, when the volcanic and
granitic rocks are described, that some of them have, when melted in the
interior of the earth's crust, been injected forcibly into fissures, and after
the solidification of such intruded matter, other sets of rents, crossing the
first, have been formed and in their turn filled by melted rock. Such repeated
injections imply a stretching, and often upheaval, of the whole mass.
We also know, especially by the study of regions liable to earthquakes, that
there are causes at work in the interior of the earth capable of producing a
sinking in of the ground, sometimes very local, but often extending over a wide
area. The continuance of such a downward movement, especially if partial and
confined to linear areas, may produce regular folds in the strata.
CREEPS IN COAL-MINES.
The "creeps," as they are called in coal-mines, afford an excellent illustration
of this fact.-- First, it may be stated generally, that the excavation of coal
at a considerable depth causes the mass of overlying strata to sink down bodily,
even when props are left to support the roof of the mine. "In Yorkshire," says
Mr. Buddle, "three distinct subsidences were perceptible at the surface, after
the clearing out of three seams of coal below, and innumerable vertical cracks
were caused in the incumbent mass of sandstone and shale which thus settled
down." (Proceedings of Geological Society volume 3 page 148.) The exact amount
of depression in these cases can only be accurately measured where water
accumulates on the surface, or a railway traverses a coal-field.
(FIGURE 59. Section of carboniferous strata at Wallsend, Newcastle, showing
"creeps." (J. Buddle, Esq.)
Horizontal length of section 174 feet. The upper seam, or main coal, here worked
out, was 630 feet below the surface.
Section through, from top to bottom:
Siliceous sandstone.
Shale.
1. Main coal, 6 feet 6 inches, with creeps a, b, c, d.
Shale eighteen yards thick.
2. Metal coal, 3 feet, with fractures e, f, g, h.)
When a bed of coal is worked out, pillars or rectangular masses of coal are left
at intervals as props to support the roof, and protect the colliers. Thus in
Figure 59, representing a section at Wallsend, Newcastle, the galleries which
have been excavated are represented by the white spaces a, b, while the
adjoining dark portions are parts of the original coal seam left as props, beds
of sandy clay or shale constituting the floor of the mine. When the props have
been reduced in size, they are pressed down by the weight of overlying rocks (no
less than 630 feet thick) upon the shale below, which is thereby squeezed and
forced up into the open spaces.
Now it might have been expected that, instead of the floor rising up, the
ceiling would sink down, and this effect, called a "thrust," does, in fact, take
place where the pavement is more solid than the roof. But it usually happens, in
coalmines, that the roof is composed of hard shale, or occasionally of
sandstone, more unyielding than the foundation, which often consists of clay.
Even where the argillaceous substrata are hard at first, they soon become
softened and reduced to a plastic state when exposed to the contact of air and
water in the floor of a mine.
The first symptom of a "creep," says Mr. Buddle, is a slight curvature at the
bottom of each gallery, as at a, Figure 59: then the pavement, continuing to
rise, begins to open with a longitudinal crack, as at b; then the points of the
fractured ridge reach the roof, as at c; and, lastly, the upraised beds close up
the whole gallery, and the broken portions of the ridge are reunited and
flattened at the top, exhibiting the flexure seen at d. Meanwhile the coal in
the props has become crushed and cracked by pressure. It is also found that
below the creeps a, b, c, d, an inferior stratum, called the "metal coal," which
is 3 feet thick, has been fractured at the points e, f, g, h, and has risen, so
as to prove that the upward movement, caused by the working out of the "main
coal," has been propagated through a thickness of 54 feet of argillaceous beds,
which intervene between the two coal-seams. This same displacement has also been
traced downward more than 150 feet below the metal coal, but it grows
continually less and less until it becomes imperceptible.
No part of the process above described is more deserving of our notice than the
slowness with which the change in the arrangement of the beds is brought about.
Days, months, or even years, will sometimes elapse between the first bending of
the pavement and the time of its reaching the roof. Where the movement has been
most rapid, the curvature of the beds is most regular, and the reunion of the
fractured ends most complete; whereas the signs of displacement or violence are
greatest in those creeps which have required months or years for their entire
accomplishment. Hence we may conclude that similar changes may have been wrought
on a larger scale in the earth's crust by partial and gradual subsidences,
especially where the ground has been undermined throughout long periods of time;
and we must be on our guard against inferring sudden violence, simply because
the distortion of the beds is excessive.
Engineers are familiar with the fact that when they raise the level of a railway
by heaping stone or gravel on a foundation of marsh, quicksand, or other
yielding formation, the new mound often sinks for a time as fast as they attempt
to elevate it; when they have persevered so as to overcome this difficulty, they
frequently find that some of the adjoining flexible ground has risen up in one
or more parallel arches or folds, showing that the vertical pressure of the
sinking materials has given rise to a lateral folding movement.
In like manner, in the interior of the earth, the solid parts of the earth's
crust may sometimes, as before mentioned, be made to expand by heat, or may be
pressed by the force of steam against flexible strata loaded with a great weight
of incumbent rocks. In this case the yielding mass, squeezed, but unable to
overcome the resistance which it meets with in a vertical direction, may be
gradually relieved by lateral folding.
DIP AND STRIKE.
(FIGURE 60. Series of inclined strata dipping to the north at an angle of 45
degrees.)
In describing the manner in which strata depart from their original
horizontality, some technical terms, such as "dip" and "strike," "anticlinal"
and "synclinal" line or axis, are used by geologists. I shall now proceed to
explain some of these to the student. If a stratum or bed of rock, instead of
being quite level, be inclined to one side, it is said to DIP; the point of the
compass to which it is inclined is called the POINT OF DIP, and the degree of
deviation from a level or horizontal line is called THE AMOUNT OF DIP, or THE
ANGLE OF DIP. Thus, in the diagram (Figure 60), a series of strata are inclined,
and they dip to the north at an angle of forty-five degrees. The STRIKE, or LINE
OF BEARING, is the prolongation or extension of the strata in a direction AT
RIGHT ANGLES to the dip; and hence it is sometimes called the DIRECTION of the
strata. Thus, in the above instance of strata dipping to the north, their strike
must necessarily be east and west. We have borrowed the word from the German
geologists, streichen signifying to extend, to have a certain direction. Dip and
strike may be aptly illustrated by a row of houses running east and west, the
long ridge of the roof representing the strike of the stratum of slates, which
dip on one side to the north, and on the other to the south.
A stratum which is horizontal, or quite level in all directions, has neither dip
nor strike.
It is always important for the geologist, who is endeavouring to comprehend the
structure of a country, to learn how the beds dip in every part of the district;
but it requires some practice to avoid being occasionally deceived, both as to
the point of dip and the amount of it.
(FIGURE 61. Apparent horizontality of inclined strata.)
If the upper surface of a hard stony stratum be uncovered, whether artificially
in a quarry, or by waves at the foot of a cliff, it is easy to determine towards
what point of the compass the slope is steepest, or in what direction water
would flow if poured upon it. This is the true dip. But the edges of highly
inclined strata may give rise to perfectly horizontal lines in the face of a
vertical cliff, if the observer see the strata in the line of the strike, the
dip being inward from the face of the cliff. If, however, we come to a break in
the cliff, which exhibits a section exactly at right angles to the line of the
strike, we are then able to ascertain the true dip. In the drawing (Figure 61),
we may suppose a headland, one side of which faces to the north, where the beds
would appear perfectly horizontal to a person in the boat; while in the other
side facing the west, the true dip would be seen by the person on shore to be at
an angle of 40 degrees. If, therefore, our observations are confined to a
vertical precipice facing in one direction, we must endeavour to find a ledge or
portion of the plane of one of the beds projecting beyond the others, in order
to ascertain the true dip.
(FIGURE 62. Two hands used to determine the inclination of strata.)
If not provided with a clinometer, a most useful instrument, when it is of
consequence to determine with precision the inclination of the strata, the
observer may measure the angle within a few degrees by standing exactly opposite
to a cliff where the true dip is exhibited, holding the hands immediately before
the eyes, and placing the fingers of one in a perpendicular, and of the other in
a horizontal position, as in Figure 62. It is thus easy to discover whether the
lines of the inclined beds bisect the angle of 90 degrees, formed by the meeting
of the hands, so as to give an angle of 45 degrees, or whether it would divide
the space into two equal or unequal portions. You have only to change hands to
get the line of dip on the upper side of the horizontal hand.
(FIGURE 63. Section illustrating the structure of the Swiss Jura.)
It has been already seen, in describing the curved strata on the east coast of
Scotland, in Forfarshire and Berwickshire, that a series of concave and convex
bendings are occasionally repeated several times. These usually form part of a
series of parallel waves of strata, which are prolonged in the same direction,
throughout a considerable extent of country. Thus, for example, in the Swiss
Jura, that lofty chain of mountains has been proved to consist of many parallel
ridges, with intervening longitudinal valleys, as in Figure 63, the ridges being
formed by curved fossiliferous strata, of which the nature and dip are
occasionally displayed in deep transverse gorges, called "cluses," caused by
fractures at right angles to the direction of the chain. (Thurmann "Essai sur
les Soulevemens Jurassiques de Porrentruy" Paris 1832.) Now let us suppose these
ridges and parallel valleys to run north and south, we should then say that the
STRIKE of the beds is north and south, and the DIP east and west. Lines drawn
along the summits of the ridges, A, B, would be anticlinal lines, and one
following the bottom of the adjoining valleys a synclinal line.
OUTCROP OF STRATA.
(FIGURE 64. Ground-plan of the denuded ridge C, Figure 63.)
(FIGURE 65. Transverse section of the denuded ridge C, Figure 63..)
It will be observed that some of these ridges, A, B, are unbroken on the summit,
whereas one of them, C, has been fractured along the line of strike, and a
portion of it carried away by denudation, so that the ridges of the beds in the
formations a, b, c come out to the day, or, as the miners say, CROP OUT, on the
sides of a valley. The ground-plan of such a denuded ridge as C, as given in a
geological map, may be expressed by the diagram, Figure 64, and the cross-
section of the same by Figure 65. The line D E, Figure 64, is the anticlinal
line, on each side of which the dip is in opposite directions, as expressed by
the arrows. The emergence of strata at the surface is called by miners their
OUTCROP, or BASSET.
If, instead of being folded into parallel ridges, the beds form a boss or dome-
shaped protuberance, and if we suppose the summit of the dome carried off, the
ground-plan would exhibit the edges of the strata forming a succession of
circles, or ellipses, round a common centre. These circles are the lines of
strike, and the dip being always at right angles is inclined in the course of
the circuit to every point of the compass, constituting what is termed a qua-
quaversal dip-- that is, turning every way.
There are endless variations in the figures described by the basset-edges of the
strata, according to the different inclination of the beds, and the mode in
which they happen to have been denuded. One of the simplest rules, with which
every geologist should be acquainted, relates to the V-like form of the beds as
they crop out in an ordinary valley. First, if the strata be horizontal, the V-
like form will be also on a level, and the newest strata will appear at the
greatest heights.
(FIGURE 66. Slope of valley 40 degrees, dip of strata 20 degrees.)
Secondly, if the beds be inclined and intersected by a valley sloping in the
same direction, and the dip of the beds be less steep than the slope of the
valley, then the V's, as they are often termed by miners, will point upward (see
Figure 66), those formed by the newer beds appearing in a superior position, and
extending highest up the valley, as A is seen above B.
(FIGURE 67. Slope of valley 20 degrees, dip of strata 50 degrees.)
Thirdly, if the dip of the beds be steeper than the slope of the valley, then
the V's will point downward (see Figure 67), and those formed of the older beds
will now appear uppermost, as B appears above A.
(FIGURE 68. Slope of valley 20 degrees, dip of strata 20 degrees, in opposite
directions.)
Fourthly, in every case where the strata dip in a contrary direction to the
slope of the valley, whatever be the angle of inclination, the newer beds will
appear the highest, as in the first and second cases. This is shown by the
drawing (Figure 68), which exhibits strata rising at an angle of 20 degrees, and
crossed by a valley, which declines in an opposite direction at 20 degrees.
These rules may often be of great practical utility; for the different degrees
of dip occurring in the two cases represented in Figures 66 and 67 may
occasionally be encountered in following the same line of flexure at points a
few miles distant from each other. A miner unacquainted with the rule, who had
first explored the valley Figure 66, may have sunk a vertical shaft below the
coal-seam A, until he reached the inferior bed, B. He might then pass to the
valley, Figure 67, and discovering there also the outcrop of two coal-seams,
might begin his workings in the uppermost in the expectation of coming down to
the other bed A, which would be observed cropping out lower down the valley. But
a glance at the section will demonstrate the futility of such hopes. (I am
indebted to the kindness of T. Sopwith, Esq., for three models which I have
copied in the above diagrams; but the beginner may find it by no means easy to
understand such copies, although, if he were to examine and handle the
originals, turning them about in different ways, he would at once comprehend
their meaning, as well as the import of others far more complicated, which the
same engineer has constructed to illustrate FAULTS.)
SYNCLINAL STRATA FORMING RIDGES.
(FIGURE 69. Section of carboniferous rocks of Lancashire. (E. Hull. (Edward
Hull, Quarterly Geological Journal volume 24 page 324. 1868.))
a. Synclinal. Grits and shales.
c. Anticlinal. Mountain limestone.
b. Synclinal. Grits and shales.)
Although in many cases an anticlinal axis forms a ridge, and a synclinal axis a
valley, as in A B, Figure 63, yet this can by no means be laid down as a general
rule, as the beds very often slope inward from either side of a mountain, as at
a, b, Figure 69, while in the intervening valley, c, they slope upward, forming
an arch.
It would be natural to expect the fracture of solid rocks to take place chiefly
where the bending of the strata has been sharpest, and such rending may produce
ravines giving access to running water and exposing the surface to atmospheric
waste. The entire absence, however, of such cracks at points where the strain
must have been greatest, as at a, Figure 63, is often very remarkable, and not
always easy of explanation. We must imagine that many strata of limestone,
chert, and other rocks which are now brittle, were pliant when bent into their
present position. They may have owed their flexibility in part to the fluid
matter which they contained in their minute pores, as before described, and in
part to the permeation of sea-water while they were yet submerged.
(FIGURE 70. Strata of chert, grit, and marl, near St. Jean de Luz.)
At the western extremity of the Pyrenees, great curvatures of the strata are
seen in the sea-cliffs, where the rocks consist of marl, grit, and chert. At
certain points, as at a, Figure 70, some of the bendings of the flinty chert are
so sharp that specimens might be broken off well fitted to serve as ridge-tiles
on the roof of a house. Although this chert could not have been brittle as now,
when first folded into this shape, it presents, nevertheless, here and there, at
the points of greatest flexure, small cracks, which show that it was solid, and
not wholly incapable of breaking at the period of its displacement. The numerous
rents alluded to are not empty, but filled with chalcedony and quartz.
(FIGURE 71. Bent and undulating gypseous marl.
g. Gypsum. m. Marl.)
Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous
marls occur, with here and there thin beds of solid gypsum interstratified.
Sometimes these solid layers have been broken into detached fragments, still
preserving their sharp edges (g, g, Figure 71), while the continuity of the more
pliable and ductile marls, m, m, has not been interrupted.
(FIGURE 72. Folded strata.)
(FIGURE 73. Folded strata.)
We have already explained, Figure 69, that stratified rocks have usually their
strata bent into parallel folds forming anticlinal and synclinal axes, a group
of several of these folds having often been subjected to a common movement, and
having acquired a uniform strike or direction. In some disturbed regions these
folds have been doubled back upon themselves in such a manner that it is often
difficult for an experienced geologist to determine correctly the relative age
of the beds by superposition. Thus, if we meet with the strata seen in the
section, Figure 72, we should naturally suppose that there were twelve distinct
beds, or sets of beds, No. 1 being the newest, and No. 12 the oldest of the
series. But this section may perhaps exhibit merely six beds, which have been
folded in the manner seen in Figure 73, so that each of them is twice repeated,
the position of one half being reversed, and part of No. 1, originally the
uppermost, having now become the lowest of the series.
These phenomena are observable on a magnificent scale in certain regions in
Switzerland, in precipices often more than 2000 feet in perpendicular height,
and there are flexures not inferior in dimensions in the Pyrenees. The upper
part of the curves seen in this diagram, Figure 73, and expressed in fainter
lines, has been removed by what is called denudation, to be afterwards
explained.
FRACTURES OF THE STRATA AND FAULTS.
Numerous rents may often be seen in rocks which appear to have been simply
broken, the fractured parts still remaining in contact; but we often find a
fissure, several inches or yards wide, intervening between the disunited
portions. These fissures are usually filled with fine earth and sand, or with
angular fragments of stone, evidently derived from the fracture of the
contiguous rocks.
The face of each wall of the fissure is often beautifully polished, as if
glazed, striated, or scored with parallel furrows and ridges, such as would be
produced by the continued rubbing together of surfaces of unequal hardness.
These polished surfaces are called by miners "slickensides." It is supposed that
the lines of the striae indicate the direction in which the rocks were moved.
During one of the minor earthquakes in Chili, in 1840, the brick walls of a
building were rent vertically in several places, and made to vibrate for several
minutes during each shock, after which they remained uninjured, and without any
opening, although the line of each crack was still visible. When all movement
had ceased, there were seen on the floor of the house, at the bottom of each
rent, small heaps of fine brick-dust, evidently produced by trituration.
(FIGURE 74. Faults. A B perpendicular, C D oblique to the horizon.)
(FIGURE 75. E F, fault or fissure filled with rubbish, on each side of which the
shifted strata are not parallel.)
It is not uncommon to find the mass of rock on one side of a fissure thrown up
above or down below the mass with which it was once in contact on the other
side. "This mode of displacement is called a fault, shift, slip, or throw." "The
miner," says Playfair, describing a fault, "is often perplexed, in his
subterranean journey, by a derangement in the strata, which changes at once all
those lines and bearings which had hitherto directed his course. When his mine
reaches a certain plane, which is sometimes perpendicular, as in A B, Figure 74,
sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock
broken asunder, those on the one side of the plane having changed their place,
by sliding in a particular direction along the face of the others. In this
motion they have sometimes preserved their parallelism, as in Figure 74, so that
the strata on each side of faults A B, C D, continue parallel to one another; in
other cases, the strata on each side are inclined, as in a, b, c, d (Figure 75),
though their identity is still to be recognised by their possessing the same
thickness and the same internal characters." (Playfair, Illustration of Hutt.
Theory paragraph 42.)
In Coalbrook Dale, says Mr. Prestwich (Geological Transactions second series
volume 5 page 452.), deposits of sandstone, shale, and coal, several thousand
feet thick, and occupying an area of many miles, have been shivered into
fragments, and the broken remnants have been placed in very discordant
positions, often at levels differing several hundred feet from each other. The
sides of the faults, when perpendicular, are commonly several yards apart, and
are sometimes as much as 50 yards asunder, the interval being filled with broken
debris of the strata. In following the course of the same fault it is sometimes
found to produce in different places very unequal changes of level, the amount
of shift being in one place 300, and in another 700 feet, which arises from the
union of two or more faults. In other words, the disjointed strata have in
certain districts been subjected to renewed movements, which they have not
suffered elsewhere.
We may occasionally see exact counterparts of these slips, on a small scale, in
pits of loose sand and gravel, many of which have doubtless been caused by the
drying and shrinking of argillaceous and other beds, slight subsidences having
taken place from failure of support. Sometimes, however, even these small slips
may have been produced during earthquakes; for land has been moved, and its
level, relatively to the sea, considerably altered, within the period when much
of the alluvial sand and gravel now covering the surface of continents was
deposited.
I have already stated that a geologist must be on his guard, in a region of
disturbed strata, against inferring repeated alternations of rocks, when, in
fact, the same strata, once continuous, have been bent round so as to recur in
the same section, and with the same dip. A similar mistake has often been
occasioned by a series of faults.
(FIGURE 76. Apparent alternations of strata caused by vertical faults.)
If, for example, the dark line A H (Figure 76) represent the surface of a
country on which the strata a, b, c frequently crop out, an observer who is
proceeding from H to A might at first imagine that at every step he was
approaching new strata, whereas the repetition of the same beds has been caused
by vertical faults, or downthrows. Thus, suppose the original mass, A, B, C, D,
to have been a set of uniformly inclined strata, and that the different masses
under E F, F G, and G D sank down successively, so as to leave vacant the spaces
marked in the diagram by dotted lines, and to occupy those marked by the
continuous lines, then let denudation take place along the line A H, so that the
protruding masses indicated by the fainter lines are swept away-- a miner, who
has not discovered the faults, finding the mass a, which we will suppose to be a
bed of coal four times repeated, might hope to find four beds, workable to an
indefinite depth, but first, on arriving at the fault G, he is stopped suddenly
in his workings, for he comes partly upon the shale b, and partly on the
sandstone c; the same result awaits him at the fault F, and on reaching E he is
again stopped by a wall composed of the rock d.
The very different levels at which the separated parts of the same strata are
found on the different sides of the fissure, in some faults, is truly
astonishing. One of the most celebrated in England is that called the "ninety-
fathom dike," in the coal-field of Newcastle. This name has been given to it,
because the same beds are ninety fathoms (540 feet) lower on the northern than
they are on the southern side. The fissure has been filled by a body of sand,
which is now in the state of sandstone, and is called the dike, which is
sometimes very narrow, but in other places more than twenty yards wide.
(Conybeare and Phillips Outlines, etc. page 376.) The walls of the fissure are
scored by grooves, such as would have been produced if the broken ends of the
rock had been rubbed along the plane of the fault. (Phillips Geology Lardner's
Cyclop. page 41.) In the Tynedale and Craven faults, in the north of England,
the vertical displacement is still greater, and the fracture has extended in a
horizontal direction for a distance of thirty miles or more.
GREAT FAULTS THE RESULT OF REPEATED MOVEMENTS.
It must not, however, be supposed that faults generally consist of single linear
rents; there are usually a number of faults springing off from the main one, and
sometimes a long strip of country seems broken up into fragments by sets of
parallel and connecting transverse faults. Oftentimes a great line of fault has
been repeated, or the movements have been continued through successive periods,
so that, newer deposits having covered the old line of displacement, the strata
both newer and older have given way along the old line of fracture. Some
geologists have considered it necessary to imagine that the upward or downward
movement in these cases was accomplished at a single stroke, and not by a series
of sudden but interrupted movements. They appear to have derived this idea from
a notion that the grooved walls have merely been rubbed in one direction, which
is far from being a constant phenomenon. Not only are some sets of striae not
parallel to others, but the clay and rubbish between the walls, when squeezed or
rubbed, have been streaked in different directions, the grooves which the harder
minerals have impressed on the softer being frequently curved and irregular.
(FIGURE 77. Faults and denuded coal-strata, Ashby de la Zouch. (Mammatt.))
The usual absence of protruding masses of rock forming precipices or ridges
along the lines of great faults has already been alluded to in explaining Figure
76, and the same remarkable fact is well exemplified in every coal-field which
has been extensively worked. It is in such districts that the former relation of
the beds which have been shifted is determinable with great accuracy. Thus in
the coal-field of Ashby de la Zouch, in Leicestershire (see Figure 77), a fault
occurs, on one side of which the coal-beds a, b, c, d must once have risen to
the height of 500 feet above the corresponding beds on the other side. But the
uplifted strata do not stand up 500 feet above the general surface; on the
contrary, the outline of the country, as expressed by the line z z, is uniformly
undulating, without any break, and the mass indicated by the dotted outline must
have been washed away. (See Mammatt's Geological Facts etc. page 90 and plate.)
The student may refer to Mr. Hull's measurement of faults, observed in the
Lancashire coal-field, where the vertical displacement has amounted to thousands
of feet, and yet where all the superficial inequalities which must have resulted
from such movements have been obliterated by subsequent denudation. In the same
memoir proofs are afforded of there having been two periods of vertical movement
in the same fault-- one, for example, before, and another after, the Triassic
epoch. (Hull Quarterly Geological Journal volume 24 page 318. 1868.)
The shifting of the beds by faults is often intimately connected with those same
foldings which constitute the anticlinal and synclinal axes before alluded to,
and there is no doubt that the subterranean causes of both forms of disturbance
are to a great extent the same. A fault in Virginia, believed to imply a
displacement of several thousand feet, has been traced for more than eighty
miles in the same direction as the foldings of the Appalachian chain. (H.D.
Rogers Geology of Pennsylvania page 897.) An hypothesis which attributes such a
change of position to a succession of movements, is far preferable to any theory
which assumes each fault to have been accomplished by a single upcast or
downthrow of several thousand feet. For we know that there are operations now in
progress, at great depths in the interior of the earth, by which both large and
small tracts of ground are made to rise above and sink below their former level,
some slowly and insensibly, others suddenly and by starts, a few feet or yards
at a time; whereas there are no grounds for believing that, during the last 3000
years at least, any regions have been either upheaved or depressed, at a single
stroke, to the amount of several hundred, much less several thousand feet.
It is certainly not easy to understand how in the subterranean regions one mass
of solid rock should have been folded up by a continued series of movements,
while another mass in contact, or only separated by a line of fissure, has
remained stationary or has perhaps subsided. But every volcano, by the
intermittent action of the steam, gases, and lava evolved during an eruption,
helps us to form some idea of the manner in which such operations take place.
For eruptions are repeated at uncertain intervals throughout the whole or a
large part of a geological period, some of the surrounding and contiguous
districts remaining quite undisturbed. And in most of the instances with which
we are best acquainted the emission of lava, scoria, and steam is accompanied by
the uplifting of the solid crust. Thus in Vesuvius, Etna, the Madeiras, the
Canary Islands, and the Azores there is evidence of marine deposits of recent
and tertiary date having been elevated to the height of a thousand feet, and
sometimes more, since the commencement of the volcanic explosions. There is,
moreover, a general tendency in contemporaneous volcanic vents to affect a
linear arrangement, extending in some instances, as in the Andes or the Indian
Archipelago, to distances equalling half the circumference of the globe. Where
volcanic heat, therefore, operates at such a depth as not to obtain vent at the
surface, in the form of an eruption, it may nevertheless be conceived to give
rise to upheavals, foldings, and faults in certain linear tracts. And marine
denudation, to be treated of in the next chapter, will help us to understand why
that which should be the protruding portion of the faulted rocks is missing at
the surface.
ARRANGEMENT AND DIRECTION OF PARALLEL FOLDS OF STRATA.
The possible causes of the folding of strata by lateral movements have been
considered in a former part of this chapter. No European chain of mountains
affords so remarkable an illustration of the persistency of such flexures for a
great distance as the Appalachians before alluded to, and none has been studied
and described by many good observers with more accuracy. The chain extends from
north to south, or rather N.N.E. to S.S.W., for nearly 1500 miles, with a
breadth of 50 miles, throughout which the Palaeozoic strata have been so bent as
to form a series of parallel anticlinal and synclinal ridges and troughs,
comprising usually three or four principal and many smaller plications, some of
them forming broad and gentle arches, others narrower and steeper ones, while
some, where the bending has been greatest, have the position of their beds
inverted, as before shown in Figure 73.
The strike of the parallel ridges, after continuing in a straight line for many
hundred miles, is then found to vary for a more limited distance as much as 30
degrees, the folds wheeling round together in the new direction and continuing
to be parallel, as if they had all obeyed the same movement. The date of the
movements by which the great flexures were brought about must, of course, be
subsequent to the formation of the uppermost part of the coal or the newest of
the bent rocks, but the disturbance must have ceased before the Triassic strata
were deposited on the denuded edges of the folded beds.
The manner in which the numerous parallel folds, all simultaneously formed,
assume a new direction common to the whole of them, and sometimes varying at an
angle of 30 degrees from the normal strike of the chain, shows what deviation
from an otherwise uniform strike of the beds may be experienced when the
geographical area through which they are traced is on so vast a scale.
The disturbances in the case here adverted to occurred between the Carboniferous
period and that of the Trias, and this interval is so vast that they may have
occupied a great lapse of time, during which their parallelism was always
preserved. But, as a rule, wherever after a long geological interval the
recurrence of lateral movements gives rise to a new set of folds, the strike of
these last is different. Thus, for example, Mr. Hull has pointed out that three
principal lines of disturbance, all later than the Carboniferous period, have
affected the stratified rocks of Lancashire. The first of these, having an
E.N.E. direction, took place at the close of the Carboniferous period. The next,
running north and south, at the close of the Permian, and the third, having a
N.N.W. direction, at the close of the Jurassic period. (Edward Hull Quarterly
Geological Journal volume 24 page 323.)
UNCONFORMABILITY OF STRATA.
(FIGURE 78. Unconformable junction of old red sandstone and Silurian schist at
the Siccar Point, near St. Abb's Head, Berwickshire.)
Strata are said to be unconformable when one series is so placed over another
that the planes of the superior repose on the edges of the inferior (see Figure
78.) In this case it is evident that a period had elapsed between the production
of the two sets of strata, and that, during this interval, the older series had
been tilted and disturbed. Afterwards the upper series was thrown down in
horizontal strata upon it. If these superior beds, d d Figure 78, are also
inclined, it is plain that the lower strata a a, have been twice displaced;
first, before the deposition of the newer beds, d d, and a second time when
these same strata were upraised out of the sea, and thrown slightly out of the
horizontal position.
(FIGURE 79. Junction of unconformable strata near Mons, in Belgium.)
It often happens that in the interval between the deposition of two sets of
unconformable strata, the inferior rock has not only been denuded, but drilled
by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons,
beds of an ancient (primary or palaeozoic) limestone, highly inclined, and often
bent, are covered with horizontal strata of greenish and whitish marls of the
Cretaceous formation. The lowest, and therefore the oldest, bed of the
horizontal series is usually the sand and conglomerate, a, in which are rounded
fragments of stone, from an inch to two feet in diameter. These fragments have
often adhering shells attached to them, and have been bored by perforating
mollusca. The solid surface of the inferior limestone has also been bored, so as
to exhibit cylindrical and pear-shaped cavities, as at c, the work of saxicavous
mollusca; and many rents, as at b, which descend several feet or yards into the
limestone, have been filled with sand and shells, similar to those in the
stratum a.
OVERLAPPING STRATA.
Strata are said to overlap when an upper bed extends beyond the limits of a
lower one. This may be produced in various ways; as, for example, when
alterations of physical geography cause the arms of a river or channels of
discharge to vary, so that sediment brought down is deposited over a wider area
than before, or when the sea-bottom has been raised up and again depressed
without disturbing the horizontal position of the strata. In this case the newer
strata may rest for the most part conformably on the older, but, extending
farther, pass over their edges. Every intermediate state between unconformable
and over-lapping beds may occur, because there may be every gradation between a
slight derangement of position, and a considerable disturbance and denudation of
the older formation before the newer beds come on.
CHAPTER VI.
DENUDATION.
Denudation defined.
Its Amount more than equal to the entire Mass of Stratified Deposits in the
Earth's Crust.
Subaerial Denudation.
Action of the Wind.
Action of Running Water.
Alluvium defined.
Different Ages of Alluvium.
Denuding Power of Rivers affected by Rise or Fall of Land.
Littoral Denudation.
Inland Sea-Cliffs.
Escarpments.
Submarine Denudation.
Dogger-bank.
Newfoundland Bank.
Denuding Power of the Ocean during Emergence of Land.
Denudation, which has been occasionally spoken of in the preceding chapters, is
the removal of solid matter by water in motion, whether of rivers or of the
waves and currents of the sea, and the consequent laying bare of some inferior
rock. This operation has exerted an influence on the structure of the earth's
crust as universal and important as sedimentary deposition itself; for
denudation is the necessary antecedent of the production of all new strata of
mechanical origin. The formation of every new deposit by the transport of
sediment and pebbles necessarily implies that there has been, somewhere else, a
grinding down of rock into rounded fragments, sand, or mud, equal in quantity to
the new strata. All deposition, therefore, except in the case of a shower of
volcanic ashes, and the outflow of lava, and the growth of certain organic
formations, is the sign of superficial waste going on contemporaneously, and to
an equal amount, elsewhere. The gain at one point is no more than sufficient to
balance the loss at some other. Here a lake has grown shallower, there a ravine
has been deepened. Here the depth of the sea has been augmented by the removal
of a sandbank during a storm, there its bottom has been raised and shallowed by
the accumulation in its bed of the same sand transported from the bank.
When we see a stone building, we know that somewhere, far or near, a quarry has
been opened. The courses of stone in the building may be compared to successive
strata, the quarry to a ravine or valley which has suffered denudation. As the
strata, like the courses of hewn stone, have been laid one upon another
gradually, so the excavation both of the valley and quarry have been gradual. To
pursue the comparison still farther, the superficial heaps of mud, sand, and
gravel, usually called alluvium, may be likened to the rubbish of a quarry which
has been rejected as useless by the workmen, or has fallen upon the road between
the quarry and the building, so as to lie scattered at random over the ground.
But we occasionally find in a conglomerate large rounded pebbles of an older
conglomerate, which had previously been derived from a variety of different
rocks. In such cases we are reminded that, the same materials having been used
over and over again, it is not enough to affirm that the entire mass of
stratified deposits in the earth's crust affords a monument and measure of the
denudation which has taken place, for in truth the quantity of matter now extant
in the form of stratified rock represents but a fraction of the material removed
by water and redeposited in past ages.
SUBAERIAL DENUDATION.
Denudation may be divided into subaerial, or the action of wind, rain, and
rivers; and submarine, or that effected by the waves of the sea, and its tides
and currents. With the operation of the first of these we are best acquainted,
and it may be well to give it our first attention.
ACTION OF THE WIND.
In desert regions where no rain falls, or where, as in parts of the Sahara, the
soil is so salt as to be without any covering of vegetation, clouds of dust and
sand attest the power of the wind to cause the shifting of the unconsolidated or
disintegrated rock.
In examining volcanic countries I have been much struck with the great
superficial changes brought about by this power in the course of centuries. The
highest peak of Madeira is about 6050 feet above the sea, and consists of the
skeleton of a volcanic cone now 250 feet high, the beds of which once dipped
from a centre in all directions at an angle of more than 30 degrees. The summit
is formed of a dike of basalt with much olivine, fifteen feet wide, apparently
the remains of a column of lava which once rose to the crater. Nearly all the
scoriae of the upper part of the cone have been swept away, those portions only
remaining which were hardened by the contact or proximity of the dike. While I
was myself on this peak on January 25, 1854, I saw the wind, though it was not
stormy weather, removing sand and dust derived from the decomposing scoriae.
There had been frost in the night, and some ice was still seen in the crevices
of the rock.
On the highest platform of the Grand Canary, at an elevation of 6000 feet, there
is a cylindrical column of hard lava, from which the softer matter has been
carried away; and other similar remnants of the dikes of cones of eruption
attest the denuding power of the wind at points where running water could never
have exerted any influence. The waste effected by wind aided by frost and snow,
may not be trifling, even in a single winter, and when multiplied by centuries
may become indefinitely great.
ACTION OF RUNNING WATER.
(FIGURE 80. Section through several eroded formations.
a. Older alluvium or drift.
b. Modern alluvium.)
There are different classes of phenomena which attest in a most striking manner
the vast spaces left vacant by the erosive power of water. I may allude, first,
to those valleys on both sides of which the same strata are seen following each
other in the same order, and having the same mineral composition and fossil
contents. We may observe, for example, several formations, as Nos. 1, 2, 3, 4,
in the diagram (Figure 80): No. 1, conglomerate, No. 2, clay, No. 3, grit, and
No. 4, limestone, each repeated in a series of hills separated by valleys
varying in depth. When we examine the subordinate parts of these four
formations, we find, in like manner, distinct beds in each, corresponding, on
the opposite sides of the valleys, both in composition and order of position. No
one can doubt that the strata were originally continuous, and that some cause
has swept away the portions which once connected the whole series. A torrent on
the side of a mountain produces similar interruptions; and when we make
artificial cuts in lowering roads, we expose, in like manner, corresponding beds
on either side. But in nature, these appearances occur in mountains several
thousand feet high, and separated by intervals of many miles or leagues in
extent.
In the "Memoirs of the Geological Survey of Great Britain" (volume 1), Professor
Ramsay has shown that the missing beds, removed from the summit of the Mendips,
must have been nearly a mile in thickness; and he has pointed out considerable
areas in South Wales and some of the adjacent counties of England, where a
series of primary (or palaeozoic) strata, no less than 11,000 feet in thickness,
have been stripped off. All these materials have of course been transported to
new regions, and have entered into the composition of more modern formations. On
the other hand, it is shown by observations in the same "Survey," that the
Palaeozoic strata are from 20,000 to 30,000 feet thick. It is clear that such
rocks, formed of mud and sand, now for the most part consolidated, are the
monuments of denuding operations, which took place on a grand scale at a very
remote period in the earth's history. For, whatever has been given to one area
must always have been borrowed from another; a truth which, obvious as it may
seem when thus stated, must be repeatedly impressed on the student's mind,
because in many geological speculations it is taken for granted that the
external crust of the earth has been always growing thicker in consequence of
the accumulation, period after period, of sedimentary matter, as if the new
strata were not always produced at the expense of pre-existing rocks, stratified
or unstratified. By duly reflecting on the fact that all deposits of mechanical
origin imply the transportation from some other region, whether contiguous or
remote, of an equal amount of solid matter, we perceive that the stony exterior
of the planet must always have grown thinner in one place, whenever, by
accessions of new strata, it was acquiring thickness in another.
It is well known that generally at the mouths of large rivers, deltas are
forming and the land is encroaching upon the sea; these deltas are monuments of
recent denudation and deposition; and it is obvious that if the mud, sand, and
gravel were taken from them and restored to the continents they would fill up a
large part of the gullies and valleys which are due to the excavating and
transporting power of torrents and rivers.
ALLUVIUM.
Between the superficial covering of vegetable mould and the subjacent rock there
usually intervenes in every district a deposit of loose gravel, sand, and mud,
to which when it occurs in valleys the name of alluvium has been popularly
applied. The term is derived from alluvio, an inundation, or alluo, to wash,
because the pebbles and sand commonly resemble those of a river's bed or the mud
and gravel washed over low lands by a flood.
In the course of those changes in physical geography which may take place during
the gradual emergence of the bottom of the sea and its conversion into dry land,
any spot may either have been a sunken reef, or a bay, or estuary, or sea-shore,
or the bed of a river. The drainage, moreover, may have been deranged again and
again by earthquakes, during which temporary lakes are caused by landslips, and
partial deluges occasioned by the bursting of the barriers of such lakes. For
this reason it would be unreasonable to hope that we should ever be able to
account for all the alluvial phenomena of each particular country, seeing that
the causes of their origin are so various. Besides, the last operations of water
have a tendency to disturb and confound together all pre-existing alluviums.
Hence we are always in danger of regarding as the work of a single era, and the
effect of one cause, what has in reality been the result of a variety of
distinct agents, during a long succession of geological epochs. Much useful
instruction may therefore be gained from the exploration of a country like
Auvergne, where the superficial gravel of very different eras happens to have
been preserved and kept separate by sheets of lava, which were poured out one
after the other at periods when the denudation, and probably the upheaval, of
rocks were in progress. That region had already acquired in some degree its
present configuration before any volcanoes were in activity, and before any
igneous matter was superimposed upon the granitic and fossiliferous formations.
The pebbles therefore in the older gravels are exclusively constituted of
granite and other aboriginal rocks; and afterwards, when volcanic vents burst
forth into eruption, those earlier alluviums were covered by streams of lava,
which protected them from intermixture with gravel of subsequent date. In the
course of ages, a new system of valleys was excavated, so that the rivers ran at
lower levels than those at which the first alluviums and sheets of lava were
formed. When, therefore, fresh eruptions gave rise to new lava, the melted
matter was poured out over lower grounds; and the gravel of these plains
differed from the first or upland alluvium, by containing in it rounded
fragments of various volcanic rocks, and often fossil bones belonging to species
of land animals different from those which had previously flourished in the same
country and been buried in older gravels.
(FIGURE 81. Lavas of Auvergne resting on alluviums of different ages.)
Figure 81 will explain the different heights at which beds of lava and gravel,
each distinct from the other in composition and age, are observed, some on the
flat tops of hills, 700 or 800 feet high, others on the slope of the same hills,
and the newest of all in the channel of the existing river where there is
usually gravel alone, although in some cases a narrow strip of solid lava shares
the bottom of the valley with the river.
The proportion of extinct species of quadrupeds is more numerous in the fossil
remains of the gravel No. 1 than in that indicated as No. 2; and in No. 3 they
agree more closely, sometimes entirely, with those of the existing fauna. The
usual absence or rarity of organic remains in beds of loose gravel and sand is
partly owing to the friction which originally ground down the rocks into small
fragments, and partly to the porous nature of alluvium, which allows the free
percolation through it of rain-water, and promotes the decomposition and removal
of fossil remains.
The loose transported matter on the surface of a large part of the land now
existing in the temperate and arctic regions of the northern hemisphere, must be
regarded as being in a somewhat exceptional state, in consequence of the
important part which ice has played in comparatively modern geological times.
This subject will be more specially alluded to when we describe, in the eleventh
chapter, the deposits called "glacial."
DENUDING POWER OF RIVERS AFFECTED BY RISE OR FALL OF LAND.
It has long been a matter of common observation that most rivers are now cutting
their channels through alluvial deposits of greater depth and extent than could
ever have been formed by the present streams. From this fact it has been
inferred that rivers in general have grown smaller, or become less liable to be
flooded than formerly. It may be true that in the history of almost every
country the rivers have been both larger and smaller than they are at the
present moment. For the rainfall in particular regions varies according to
climate and physical geography, and is especially governed by the elevation of
the land above the sea, or its distance from it and other conditions equally
fluctuating in the course of time. But the phenomenon alluded to may sometimes
be accounted for by oscillations in the level of the land, experienced since the
existing valleys originated, even where no marked diminution in the quantity of
rain and in the size of the rivers has occurred.
We know that many large areas of land are rising and others sinking, and unless
it could be assumed that both the upward and downward movements are everywhere
uniform, many of the existing hydrographical basins ought to have the appearance
of having been temporary lakes first filled with fluviatile strata and then
partially re-excavated.
Suppose, for example, part of a continent, comprising within it a large
hydrographical basin like that of the Mississippi, to subside several inches or
feet in a century, as the west coast of Greenland, extending 600 miles north and
south, has been sinking for three or four centuries, between the latitudes 60
and 69 degrees N. (Principles of Geology 7th edition page 506; 10th edition
volume 2 page 196.) It will rarely happen that the rate of subsidence will be
everywhere equal, and in many cases the amount of depression in the interior
will regularly exceed that of the region nearer the sea. Whenever this happens,
the fall of the waters flowing from the upland country will be diminished, and
each tributary stream will have less power to carry its sand and sediment into
the main river, and the main river less power to convey its annual burden of
transported matter to the sea. All the rivers, therefore, will proceed to fill
up partially their ancient channels, and, during frequent inundations, will
raise their alluvial plains by new deposits. If then the same area of land be
again upheaved to its former height, the fall, and consequently the velocity, of
every river will begin to augment. Each of them will be less given to overflow
its alluvial plain; and their power of carrying earthy matter seaward, and of
scouring out and deepening their channels, will be sustained till, after a lapse
of many thousand years, each of them has eroded a new channel or valley through
a fluviatile formation of comparatively modern date. The surface of what was
once the river-plain at the period of greatest depression, will then remain
fringing the valley-sides in the form of a terrace apparently flat, but in
reality sloping down with the general inclination of the river. Everywhere this
terrace will present cliffs of gravel and sand, facing the river. That such a
series of movements has actually taken place in the main valley of the
Mississippi and in its tributary valleys during oscillations of level, I have
endeavoured to show in my description of that country (Second Visit to the
United States volume 1 chapter 34.); and the fresh-water shells of existing
species and bones of land quadrupeds, partly of extinct races, preserved in the
terraces of fluviatile origin, attest the exclusion of the sea during the whole
process of filling up and partial re-excavation.
LITTORAL DENUDATION.
Part of the action of the waves between high and low watermark must be included
in subaerial denudation, more especially as the undermining of cliffs by the
waves is facilitated by land-springs, and these often lead to the sliding down
of great masses of land into the sea. Along our coasts we find numerous
submerged forests, only visible at low water, having the trunks of the trees
erect and their roots attached to them and still spreading through the ancient
soil as when they were living. They occur in too many places, and sometimes at
too great a depth, to be explained by a mere change in the level of the tides,
although as the coasts waste away and alter in shape, the height to which the
tides rise and fall is always varying, and the level of high tide at any given
point may, in the course of many ages, differ by several feet or even fathoms.
It is this fluctuation in the height of the tides, and the erosion and
destruction of the sea-coast by the waves, that makes it exceedingly difficult
for us in a few centuries, or even perhaps in a few thousand years, to determine
whether there is a change by subterranean movement in the relative level of sea
and land.
We often behold, as on the coasts of Devonshire and Pembrokeshire, facts which
appear to lead to opposite conclusions. In one place a raised beach with marine
littoral shells, and in another immediately adjoining a submerged forest. These
phenomena indicate oscillations of level, and as the movements are very gradual,
they must give repeated opportunities to the breakers to denude the land which
is thus again and again exposed to their fury, although it is evident that the
submergence is sometimes effected in such a manner as to allow the trees which
border the coast not to be carried away.
INLAND SEA-CLIFFS.
In countries where hard limestone rocks abound, inland cliffs have often
retained faithfully for ages the characters which they acquired when they
constituted the boundary of land and sea. Thus, in the Morea, no less than three
or even four ranges of cliffs are well-preserved, rising one above the other at
different distances from the actual shore, the summit of the highest and oldest
occasionally attaining 1000 feet in elevation. A consolidated beach with marine
shells is usually found at the base of each cliff, and a line of littoral
caverns. These ranges of cliff probably imply pauses in the process of upheaval
when the waves and currents had time to undermine and clear away considerable
masses of rock.
But the beginner should be warned not to expect to find evidence of the former
sojourn of the sea on all those lands which we are nevertheless sure have been
submerged at periods comparatively modern; for notwithstanding the enduring
nature of the marks left by littoral action on some rocks, especially
limestones, we can by no means detect sea-beaches and inland cliffs everywhere.
On the contrary, they are, upon the whole, extremely partial, and are often
entirely wanting in districts composed of argillaceous and sandy formations,
which must, nevertheless, have been upheaved at the same time, and by the same
intermittent movements, as the adjoining harder rocks.
ESCARPMENTS.
Besides the inland cliffs above alluded to which mark the ancient limits of the
sea, there are other abrupt terminations of rocks of various kinds which
resemble sea-cliffs, but which have in reality been due to subaerial denudation.
These have been called "escarpments," a term which it is useful to confine to
the outcrop of particular formations having a scarped outline, as distinct from
cliffs due to marine action.
I formerly supposed that the steep line of cliff-like slopes seen along the
outcrop of the chalk, when we follow the edge of the North or South Downs, was
due to marine action; but Professor Ramsay has shown (Physical Geography and
Geology of Great Britain page 78 1864.) that the present outline of the physical
geography is more in favour of the idea of the escarpments having been due to
gradual waste since the rocks were exposed in the atmosphere to the action of
rain and rivers.
Mr. Whittaker has given a good summary of the grounds for ascribing these
apparent sea-cliffs to waste in the open air. 1. There is an absence of all
signs of ancient sea-beaches or littoral deposits at the base of the escarpment.
2. Great inequality is observed in the level of the base line. 3. The
escarpments do not intersect, like sea-cliffs, a series of distinct rocks, but
are always confined to the boundary-line of the same formation. 4. There are
sometimes different contiguous and parallel escarpments-- those, for example, of
the greensand and chalk-- which are so near each other, and occasionally so
similar in altitude, that we can not imagine any existing archipelago if
converted into dry land to present a like outline.
The above theory is by no means inconsistent with the opinion that the limits of
the outcrop of the chalk and greensand which the escarpments now follow, were
originally determined by marine denudation. When the south-east of England last
emerged from beneath the level of the sea, it was acted upon, no doubt, by the
tide, waves, and currents, and the chalk would form from the first a mass
projecting above the more destructible clay called Gault. Still the present
escarpments so much resembling sea-cliffs have no doubt, for reasons above
stated, derived their most characteristic features subsequently to emergence
from subaerial waste by rain and rivers.
SUBMARINE DENUDATION.
When we attempt to estimate the amount of submarine denudation, we become
sensible of the disadvantage under which we labour from our habitual incapacity
of observing the action of marine currents on the bed of the sea. We know that
the agitation of the waves, even during storms, diminishes at a rapid rate, so
as to become very insignificant at the depth of a few fathoms, and is quite
imperceptible at the depth of about sixteen fathoms; but when large bodies of
water are transferred by a current from one part of the ocean to another, they
are known to maintain at great depths such a velocity as must enable them to
remove the finer, and sometimes even the coarser, materials of the rocks over
which they flow. As the Mississippi when more than 150 feet deep can keep open
its channel and even carry down gravel and sand to its delta, the surface
velocity being not more than two or three miles an hour, so a gigantic current,
like the Gulf Stream, equal in volume to many hundred Mississippis, and having
in parts a surface velocity of more than three miles, may act as a propelling
and abrading power at still greater depths. But the efficacy of the sea as a
denuding agent, geologically considered, is not dependent on the power of
currents to preserve at great depths a velocity sufficient to remove sand and
mud, because, even where the deposition or removal of sediment is not in
progress, the depth of water does not remain constant throughout geological
time. Every page of the geological record proves to us that the relative levels
of land and sea, and the position of the ocean and of continents and islands,
has been always varying, and we may feel sure that some portions of the
submarine area are now rising and others sinking. The force of tidal and other
currents and of the waves during storms is sufficient to prevent the emergence
of many lands, even though they may be undergoing continual upheaval. It is not
an uncommon error to imagine that the waste of sea-cliffs affords the measure of
the amount of marine denudation of which it probably constitutes an
insignificant portion.
DOGGER-BANK.
That great shoal called the Dogger-bank, about sixty miles east of the coast of
Northumberland, and occupying an area about as large as Wales, has nowhere a
depth of more than ninety feet, and in its shallower parts is less than forty
feet under water. It might contribute towards the safety of the navigation of
our seas to form an artificial island, and to erect a light-house on this bank;
but no engineer would be rash enough to attempt it, as he would feel sure that
the ocean in the first heavy gale would sweep it away as readily as it does
every temporary shoal that accumulates from time to time around a sunk vessel on
the same bank. (Principles 10th edition volume 1 page 569.)
No observed geographical changes in historical times entitle us to assume that
where upheaval may be in progress it proceeds at a rapid rate. Three or four
feet rather than as many yards in a century may probably be as much as we can
reckon upon in our speculations; and if such be the case, the continuance of the
upward movement might easily be counteracted by the denuding force of such
currents aided by such waves as, during a gale, are known to prevail in the
German Ocean. What parts of the bed of the ocean are stationary at present, and
what areas may be rising or sinking, is a matter of which we are very ignorant,
as the taking of accurate soundings is but of recent date.
NEWFOUNDLAND BANK.
The great bank of Newfoundland may be compared in size to the whole of England.
This part of the bottom of the Atlantic is surrounded on three sides by a
rapidly deepening ocean, the bank itself being from twenty to fifty fathoms (or
from 120 to 300 feet) under water. We are unable to determine by the comparison
of different charts made at distant periods, whether it is undergoing any change
of level, but if it be gradually rising we can not anticipate on that account
that it will become land, because the breakers in an open sea would exercise a
prodigious force even on solid rock brought up to within a few yards of the
surface. We know, for example, that when a new volcanic island rose in the
Mediterranean in 1831, the waves were capable in a few years of reducing it to a
sunken rock.
In the same way currents which flow over the Newfoundland bank a great part of
the year at the rate of two miles an hour, and are known to retain a
considerable velocity to near the bottom, may carry away all loose sand and mud,
and make the emergence of the shoal impossible, in spite of the accessions of
mud, sand, and boulders derived occasionally from melting icebergs which, coming
from the northern glaciers, are frequently stranded on various parts of the
bank. They must often leave at the bottom large erratic blocks which the marine
currents may be incapable of moving, but the same rocky fragments may be made to
sink by the undermining of beds consisting of finer matter on which the blocks
and gravel repose. In this way gravel and boulders may continue to overspread a
submarine bottom after the latter has been lowered for hundreds of feet, the
surface never having been able to emerge and become land. It is by no means
improbable that the annual removal of an average thickness of half an inch of
rock might counteract the ordinary upheaval which large submarine areas are
undergoing; and the real enigma which the geologist has to solve is not the
extensive denudation of the white chalk or of our tertiary sands and clays, but
the fact that such incoherent materials have ever succeeded in lifting up their
heads above water in an open sea. Why were they not swept away during storms
into some adjoining abysses, the highest parts of each shoal being always planed
off down to the depth of a few fathoms? The hardness and toughness of some rocks
already exposed to windward and acting as breakwaters may perhaps have assisted;
nor must we forget the protection afforded by a dense and unbroken covering of
barnacles, limpets, and other creatures which flourish most between high and low
water and shelter some newly risen coasts from the waves.
CHAPTER VII.
JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH'S
CRUST.
How we obtain an Insight at the Surface, of the Arrangement of Rocks at great
Depths.
Why the Height of the successive Strata in a given Region is so disproportionate
to their Thickness.
Computation of the average annual Amount of subaerial Denudation.
Antagonism of Volcanic Force to the Levelling Power of running Water.
How far the Transfer of Sediment from the Land to a neighbouring Sea-bottom may
affect Subterranean Movements.
Permanence of Continental and Oceanic Areas.
HOW WE OBTAIN AN INSIGHT AT THE SURFACE, OF THE ARRANGEMENT OF ROCKS AT GREAT
DEPTHS.
The reader has been already informed that, in the structure of the earth's
crust, we often find proofs of the direct superposition of marine to fresh-water
strata, and also evidence of the alternation of deep-sea and shallow-water
formations. In order to explain how such a series of rocks could be made to form
our present continents and islands, we have not only to assume that there have
been alternate upward and downward movements of great vertical extent, but that
the upheaval in the areas which we at present inhabit has, in later geological
times, sufficiently predominated over subsidence to cause these portions of the
earth's crust to be land instead of sea. The sinking down of a delta beneath the
sea-level may cause strata of fluviatile or even terrestrial origin, such as
peat with trees proper to marshes, to be covered by deposits of deep-sea origin.
There is also no end to the thickness of mud and sand which may accumulate in
shallow water, provided that fresh sediment is brought down from the wasting
land at a rate corresponding to that of the sinking of the bed of the sea. The
latter, again, may sometimes sink so fast that the earthy matter, being
intercepted in some new landward depression, may never reach its former resting-
place, where, the water becoming clear may favour the growth of shells and
corals, and calcareous rocks of organic origin may thus be superimposed on
mechanical deposits.
The succession of strata here alluded to would be consistent with the occurrence
of gradual downward and upward movements of the land and bed of the sea without
any disturbance of the horizontality of the several formations. But the
arrangement of rocks composing the earth's crust differs materially from that
which would result from a mere series of vertical movements. Had the volcanic
forces been confined to such movements, and had the stratified rocks been first
formed beneath the sea and then raised above it, without any lateral
compression, the geologist would never have obtained an insight into the
monuments of various ages, some of extremely remote antiquity.
What we have said in Chapter 5 of dip and strike, of the folding and inversion
of strata, of anticlinal and synclinal flexures, and in Chapter 6 of denudation
at different periods, whether subaerial or submarine, must be understood before
the student can comprehend what may at first seem to him an anomaly, but which
it is his business particularly to understand. I allude to the small height
above the level of the sea attained by strata often many miles in thickness, and
about the chronological succession of which, in one and the same region, there
is no doubt whatever. Had stratified rocks in general remained horizontal, the
waves of the sea would have been enabled during oscillations of level to plane
off entirely the uppermost beds as they rose or sank during the emergence or
submergence of the land. But the occurrence of a series of formations of widely
different ages, all remaining horizontal and in conformable stratification, is
exceptional, and for this reason the total annihilation of the uppermost strata
has rarely taken place. We owe, indeed, to the side way movements of LATERAL
COMPRESSION those anticlinal and synclinal curves of the beds already described
(Figure 55 Chapter 4), which, together with denudation, subaerial and submarine,
enable us to investigate the structure of the earth's crust many miles below
those points which the miner can reach. I have already shown in Figure 56
Chapter 4, how, at St. Abb's Head, a series of strata of indefinite thickness
may become vertical, and then denuded, so that the edges of the beds alone shall
be exposed to view, the altitude of the upheaved ridges being reduced to a
moderate height above the sea-level; and it may be observed that although the
incumbent strata of Old Red Sandstone are in that place nearly horizontal, yet
these same newer beds will in other places be found so folded as to present
vertical strata, the edges of which are abruptly cut off, as in 2, 3, 4 on the
right-hand side of the diagram, Figure 55 Chapter 4.
WHY THE HEIGHT OF THE SUCCESSIVE STRATA IN A GIVEN REGION IS SO DISPROPORTIONATE
TO THEIR THICKNESS.
We can not too distinctly bear in mind how dependent we are on the joint action
of the volcanic and aqueous forces, the one in disturbing the original position
of rocks, and the other in destroying large portions of them, for our power of
consulting the different pages and volumes of those stony records of which the
crust of the globe is composed. Why, it may be asked, if the ancient bed of the
sea has been in many regions uplifted to the height of two or three miles, and
sometimes twice that altitude, and if it can be proved that some single
formations are of themselves two or three miles thick, do we so often find
several important groups resting one upon the other, yet attaining only the
height of a few hundred feet above the level of the sea?
The American geologists, after carefully studying the Allegheny or Appalachian
mountains, have ascertained that the older fossiliferous rocks of that chain
(from the Silurian to the Carboniferous inclusive) are not less than 42,000 feet
thick, and if they were now superimposed on each other in the order in which
they were thrown down, they ought to equal in height the Himalayas with the Alps
piled upon them. Yet they rarely reach an altitude of 5000 feet, and their
loftiest peaks are no more than 7000 feet high. The Carboniferous strata forming
the highest member of the series, and containing beds of coal, can be shown to
be of shallow-water origin, or even sometimes to have originated in swamps in
the open air. But what is more surprising, the lowest part of this great
Palaeozoic series, instead of having been thrown down at the bottom of an abyss
more than 40,000 feet deep, consists of sediment (the Potsdam sandstone),
evidently spread out on the bottom of a shallow sea, on which ripple-marked
sands were occasionally formed. This vast thickness of 40,000 feet is not
obtained by adding together the maximum density attained by each formation in
distant parts of the chain, but by measuring the successive groups as they are
exposed in a very limited area, and where the denuded edges of the vertical
strata forming the parallel folds alluded to in Chapter 5 "crop out" at the
surface. Our attention has been called by Mr. James Hall, Palaeontologist of New
York, to the fact that these Palaeozoic rocks of the Appalachian chain, which
are of such enormous density, where they are almost entirely of mechanical
origin, thin out gradually as they are traced to the westward, where evidently
the contemporaneous seas allowed organic rocks to be formed by corals,
echinoderms, and encrinites in clearer water, and where, although the same
successive periods are represented, the total mass of strata from the Silurian
to the Carboniferous, instead of being 40,000 is only 4000 feet thick.
A like phenomenon is exhibited in every mountainous country, as, for example, in
the European Alps; but we need not go farther than the north of England for its
illustration. Thus in Lancashire and central England the thickness of the
Carboniferous formation, including the Millstone Grit and Yoredale beds, is
computed to be more than 18,000 feet; to this we may add the Mountain Limestone,
at least 2000 feet in thickness, and the overlying Permian and Triassic
formations, 3000 or 4000 feet thick. How then does it happen that the loftiest
hills of Yorkshire and Lancashire, instead of being 24,000 feet high, never rise
above 3000 feet? For here, as before pointed out in the Alleghenies, all the
great thicknesses are sometimes found in close approximation and in a region
only a few miles in diameter. It is true that these same sets of strata do not
preserve their full force when followed for indefinite distances. Thus the
18,000 feet of Carboniferous grits and shales in Lancashire, before alluded to,
gradually thin out, as Mr. Hull has shown, as they extend southward, by
attenuation or original deficiency of sediment, and not in consequence of
subsequent denudation, so that when we have followed them for about 100 miles
into Leicestershire, they have dwindled away to a thickness of only 3000 feet.
In the same region the Carboniferous limestone attains so unusual a thickness--
namely, more than 4000 feet-- as to appear to compensate in some measure for the
deficiency of contemporaneous sedimentary rock. (Hull Quarterly Geological
Journal volume 24 page 322 1868.)
(FIGURE 82. Unconformable Palaeozoic strata, Sutherlandshire (Murchison).
Queenaig (2673 feet).
1. Laurentian gneiss.
2. Cambrian conglomerate and sandstone.
3, 3'. Quartzose Lower Silurian, with annelid burrows.)
It is admitted that when two formations are unconformable their fossil remains
almost always differ considerably. The break in the continuity of the organic
forms seems connected with a great lapse of time, and the same interval has
allowed extensive disturbance of the strata, and removal of parts of them by
denudation, to take place. The more we extend our investigations the more
numerous do the proofs of these breaks become, and they extend to the most
ancient rocks yet discovered. The oldest examples yet brought to light in the
British Isles are on the borders of Rosshire and Sutherlandshire, and have been
well described by Sir Roderick Murchison, by whom their chronological relations
were admirably worked out, and proved to be very different from those which
previous observers had imagined them to be. I had an opportunity in the autumn
of 1869 of verifying the splendid section given in Figure 82 by climbing in a
few hours from the banks of Loch Assynt to the summit of the mountain called
Queenaig, 2673 feet high.
The formations 1, 2, 3, the Laurentian, Cambrian, and Silurian, to be explained
in Chapters 25 and 26, not only occur in succession in this one mountain, but
their unconformable junctions are distinctly exposed to view.
To begin with the oldest set of rocks, No. 1; they consist chiefly of
hornblendic gneiss, and in the neighbouring Hebrides form whole islands,
attaining a thickness of thousands of feet, although they have suffered such
contortions and denudation that they seldom rise more than a few hundred feet
above the sea-level. In discordant stratification upon the edges of this gneiss
reposes No. 2, a group of conglomerate and purple sandstone referable to the
Cambrian (or Longmynd) formation, which can elsewhere be shown to be
characterised by its peculiar organic remains. On this again rests No. 3, a
lower member of the important group called Silurian, an outlier of which, 3',
caps the summit of Queenaig, attesting the removal by denudation of rocks of the
same age, which once extended from the great mass 3 to 3'. Although this rock
now consists of solid quartz, it is clear that in its original state it was
formed of fine sand, perforated by numerous lob-worms or annelids, which left
their burrows in the shape of tubular hollows (Chapter 26, Figure 563 of
Arenicolites), hundreds, nay thousands, of which I saw as I ascended the
mountain.
(FIGURE 83. Diagrammatic section of the same groups near Queenaig (Murchison)
through west (left), Suilvein, Assynt and Ben More, east (right).
1. Laurentian gneiss.
2. Cambrian conglomerate and sandstone.
3, 3'. Quartzose Lower Silurian, with annelid burrows.
3a. Fossiliferous Silurian limestone.
3b. Quartzose, micaceous and gneissose rocks (altered Silurian).)
In Queenaig we only behold this single quartzose member of the Silurian series,
but in the neighbouring country (see Figure 83) it is seen to the eastward to be
followed by limestones, 3a, and schists, 3b, presenting numerous folds, and
becoming more and more metamorphic and crystalline, until at length, although
very different in age and strike, they much resemble in appearance the group No.
1. It is very seldom that in the same country one continuous formation, such as
the Silurian, is, as in this case, more fossiliferous and less altered by
volcanic heat in its older than in its newer strata, and still more rare to find
an underlying and unconformable group like the Cambrian retaining its original
condition of a conglomerate and sandstone more perfectly than the overlying
formation. Here also we may remark in regard to the origin of these Cambrian
rocks that they were evidently produced at the expense of the underlying
Laurentian, for the rounded pebbles occurring in them are identical in
composition and texture with that crystalline gneiss which constitutes the
contorted beds of the inferior formation No. 1. When the reader has studied the
chapter on metamorphism, and has become aware how much modification by heat,
pressure, and chemical action is required before the conversion of sedimentary
into crystalline strata can be brought about, he will appreciate the insight
which we thus gain into the date of the changes which had already been effected
in the Laurentian rocks long before the Cambrian pebbles of quartz and gneiss
were derived from them. The Laurentian is estimated by Sir William Logan to
amount in Canada to 30,000 feet in thickness. As to the Cambrian, it is supposed
by Sir Roderick Murchison that the fragment left in Sutherlandshire is about
3500 feet thick, and in Wales and the borders of Shropshire this formation may
equal 10,000 feet, while the Silurian strata No. 3, difficult as it may be to
measure them in their various foldings to the eastward, where they have been
invaded by intrusive masses of granite, are supposed many times to surpass the
Cambrian in volume and density.
But although we are dealing here with stratified rocks, each of which would be
several miles in thickness, if they were fully represented, the whole of them do
not attain the elevation of a single mile above the level of the sea.
COMPUTATION OF THE AVERAGE ANNUAL AMOUNT OF SUBAERIAL DENUDATION.
The geology of the district above alluded to may assist our imagination in
conceiving the extent to which groups of ancient rocks, each of which may in
their turn have formed continents and oceanic basins, have been disturbed,
folded, and denuded even in the course of a few out of many of those geological
periods to which our imperfect records relate. It is not easy for us to
overestimate the effects which causes in every day action must produce when the
multiplying power of time is taken into account.
Attempts were made by Manfredi in 1736, and afterwards by Playfair in 1802, to
calculate the time which it would require to enable the rivers to deliver over
the whole of the land into the basin of the ocean. The data were at first too
imperfect and vague to allow them even to approximate to safe conclusions. But
in our own time similar investigations have been renewed with more prospect of
success, the amount brought down by many large rivers to the sea having been
more accurately ascertained. Mr. Alfred Tylor, in 1850, inferred that the
quantity of detritus now being distributed over the sea-bottom would, at the end
of 10,000 years, cause an elevation of the sea-level to the extent of at least
three inches. (Tylor Philosophical Magazine 4th series page 268 1850.)
Subsequently Mr. Croll, in 1867, and again, with more exactness, in 1868,
deduced from the latest measurement of the sediment transported by European and
American rivers the rate of subaerial denudation to which the surface of large
continents is exposed, taking especially the hydrographical basin of the
Mississippi as affording the best available measure of the average waste of the
land. The conclusion arrived at in his able memoir was that the whole
terrestrial surface is denuded at the rate of one foot in 6000 years (Croll
Philosophical Magazine 1868 page 381.), and this opinion was simultaneously
enforced by his fellow-labourer, Mr. Geikie, who, being jointly engaged in the
same line of inquiry, published a luminous essay on the subject in 1868.
The student, by referring to my "Principles of Geology" (Volume 1 page 442
1867.) may see that Messrs. Humphrey and Abbot, during their survey of the
Mississippi, attempted to make accurate measurements of the proportion of
sediment carried down annually to the sea by that river, including not only the
mud held in suspension, but also the sand and gravel forced along the bottom.
It is evident that when we know the dimensions of the area which is drained, and
the annual quantity of earthy matter taken from it and borne into the sea, we
can affirm how much on an average has been removed from the general surface in
one year, and there seems no danger of our overrating the mean rate of waste by
selecting the Mississippi as our example, for that river drains a country equal
to more than half the continent of Europe, extends through twenty degrees of
latitude, and therefore through regions enjoying a great variety of climate, and
some of its tributaries descend from mountains of great height. The Mississippi
is also more likely to afford us a fair test of ordinary denudation, because,
unlike the St. Lawrence and its tributaries, there are no great lakes in which
the fluviatile sediment is thrown down and arrested in its way to the sea. In
striking a general average we have to remember that there are large deserts in
which there is scarcely any rainfall, and tracts which are as rainless as parts
of Peru, and these must not be neglected as counterbalancing others, in the
tropics, where the quantity of rain is in excess. If then, argues Mr. Geikie, we
assume that the Mississippi is lowering the surface of the great basin which it
drains at the rate of one foot in 6000 years, 10 feet in 60,000 years, 100 feet
in 600,000 years, and 1000 feet in 6,000,000 years, it would not require more
than about 4,500,000 years to wear away the whole of the North American
continent if its mean height is correctly estimated by Humboldt at 748 feet. And
if the mean height of all the land now above the sea throughout the globe is
1000 feet, as some geographers believe, it would only require six million years
to subject a mass of rock equal in volume to the whole of the land to the action
of subaerial denudation. It may be objected that the annual waste is partial,
and not equally derived from the general surface of the country, inasmuch as
plains, water-sheds, and level ground at all heights remain comparatively
unaltered; but this, as Mr. Geikie has well pointed out, does not affect our
estimate of the sum total of denudation. The amount remains the same, and if we
allow too little for the loss from the surface of table-lands we only increase
the proportion of the loss sustained by the sides and bottoms of the valleys,
and vice versa. (Transactions of the Geological Society Glasgow volume 3 page
169.)
ANTAGONISM OF VOLCANIC FORCE TO THE LEVELLING POWER OF RUNNING WATER.
In all these estimates it is assumed that the entire quantity of land above the
sea-level remains on an average undiminished in spite of annual waste. Were it
otherwise the subaerial denudation would be continually lessened by the
diminution of the height and dimensions of the land exposed to waste.
Unfortunately we have as yet no accurate data enabling us to measure the action
of that force by which the inequalities of the surface of the earth's crust may
be restored, and the height of the continents and depth of the seas made to
continue unimpaired. I stated in 1830 in the "Principles of Geology" (1st
edition chapter 10 page 167 1830; see also 10th edition volume 1 chapter 15 page
327 1867.), that running water and volcanic action are two antagonistic forces;
the one labouring continually to reduce the whole of the land to the level of
the sea, the other to restore and maintain the inequalities of the crust on
which the very existence of islands and continents depends. I stated, however,
that when we endeavour to form some idea of the relation of these destroying and
renovating forces, we must always bear in mind that it is not simply by upheaval
that subterranean movements can counteract the levelling force of running water.
For whereas the transportation of sediment from the land to the ocean would
raise the general sea-level, the subsidence of the sea-bottom, by increasing its
capacity, would check this rise and prevent the submergence of the land. I have,
indeed, endeavoured to show that unless we assume that there is, on the whole,
more subsidence than upheaval, we must suppose the diameter of the planet to be
always increasing, by that quantity of volcanic matter which is annually poured
out in the shape of lava or ashes, whether on the land or in the bed of the sea,
and which is derived from the interior of the earth. The abstraction of this
matter causes, no doubt, subterranean vacuities and a corresponding giving way
of the surface; if it were not so, the average density of parts of the interior
would be always lessening and the size of the planet increasing. (Principles
volume 2 page 237; also 1st edition page 447 1830.)
Our inability to estimate the amount or direction of the movements due to
volcanic power by no means renders its efficacy as a land-preserving force in
past times a mere matter of conjecture. The student will see in Chapter 24 that
we have proofs of Carboniferous forests hundreds of miles in extent which grew
on the lowlands or deltas near the sea, and which subsided and gave place to
other forests, until in some regions fluviatile and shallow-water strata with
occasional seams of coal were piled one over the other, till they attained a
thickness of many thousand feet. Such accumulations, observed in Great Britain
and America on opposite sides of the Atlantic, imply the long-continued
existence of land vegetation, and of rivers draining a former continent placed
where there is now deep sea.
It will be also seen in Chapter 25 that we have evidence of a rich terrestrial
flora, the Devonian, even more ancient than the Carboniferous; while on the
other hand, the later Triassic, Oolitic, Cretaceous, and successive Tertiary
periods have all supplied us with fossil plants, insects, or terrestrial
mammalia; showing that, in spite of great oscillations of level and continued
changes in the position of land and sea, the volcanic forces have maintained a
due proportion of dry land. We may appeal also to fresh-water formations, such
as the Purbeck and Wealden, to prove that in the Oolitic and Neocomian eras
there were rivers draining ancient lands in Europe in times when we know that
other spaces, now above water, were submerged.
HOW FAR THE TRANSFER OF SEDIMENT FROM THE LAND TO A NEIGHBOURING SEA-BOTTOM MAY
AFFECT SUBTERRANEAN MOVEMENTS.
Little as we understand at present the laws which govern the distribution of
volcanic heat in the interior and crust of the globe, by which mountain chains,
high table-lands, and the abysses of the ocean are formed, it seems clear that
this heat is the prime mover on which all the grander features in the external
configuration of the planet depend.
It has been suggested that the stripping off by denudation of dense masses from
one part of a continent and the delivery of the same into the bed of the ocean
must have a decided effect in causing changes of temperature in the earth's
crust below, or, in other words, in causing the subterranean isothermals to
shift their position. If this be so, one part of the crust may be made to rise,
and another to sink, by the expansion and contraction of the rocks, of which the
temperature is altered.
I can not, at present, discuss this subject, of which I have treated more fully
elsewhere (Principles volume 2 page 229 1868.), but may state here that I
believe this transfer of sediment to play a very subordinate part in modifying
those movements on which the configuration of the earth's crust depends. In
order that strata of shallow-water origin should be able to attain a thickness
of several thousand feet, and so come to exert a considerable downward pressure,
there must have been first some independent and antecedent causes at work which
have given rise to the incipient shallow receptacle in which the sediment began
to accumulate. The same causes there continuing to depress the sea-bottom, room
would be made for fresh accessions of sediment, and it would only be by a long
repetition of the depositing process that the new matter could acquire weight
enough to affect the temperature of the rocks far below, so as to increase or
diminish their volume.
PERMANENCE OF CONTINENTAL AND OCEANIC AREAS.
If the thickness of more than 40,000 feet of sedimentary strata before alluded
to in the Appalachians proves a preponderance of downward movements in
Palaeozoic times in a district now forming the eastern border of North America,
it also proves, as before hinted, the continued existence and waste of some
neighbouring continent, probably formed of Laurentian rocks, and situated where
the Atlantic now prevails. Such an hypothesis would be in perfect harmony with
the conclusions forced upon us by the study of the present configuration of our
continents, and the relation of their height to the depth of the oceanic basins;
also to the considerable elevation and extent sometimes reached by drift
containing shells of recent species, and still more by the fact of sedimentary
strata, several thousand feet thick, as those of central Sicily, or such as
flank the Alps and Apennines, containing fossil Mollusca sometimes almost wholly
identical with species still living.
I have remarked elsewhere (Principles volume 1 page 265 1867.) that upward and
downward movements of 1000 feet or more would turn much land into sea and sea
into land in the continental areas and their borders, whereas oscillations of
equal magnitude would have no corresponding effect in the bed of the ocean
generally, believed as it is to have a mean depth of 15,000 feet, and which,
whether this estimate be correct or not, is certainly of great profundity.
Subaerial denudation would not of itself lessen the area of the land, but would
tend to fill up with sediment seas of moderate depth adjoining the coast. The
coarser matter falls to the bottom near the shore in the first still water which
it reaches, and whenever the sea-bottom on which this matter has been thrown is
slightly elevated, it becomes land, and an upheaval of a thousand feet causes it
to attain the mean elevation of continents in general.
Suppose, therefore, we had ascertained that the triturating power of subaerial
denudation might in a given time-- in three, or six, or a greater number of
millions of years-- pulverise a volume of rock equal in dimensions to all the
present land, we might yet find, could we revisit the earth at the end of such a
period, that the continents occupied very much the same position which they held
before; we should find the rivers employed in carrying down to the sea the very
same mud, sand, and pebbles with which they had been charged in our own time,
the superficial alluvial matter as well as a great thickness of sedimentary
strata would inclose shells, all or a great part of which we should recognise as
specifically identical with those already known to us as living. Every geologist
is aware that great as have been the geographical changes in the northern
hemisphere since the commencement of the Glacial Period, there having been
submergence and re-emergence of land to the extent of 1000 feet vertically, and
in the temperate latitudes great vicissitudes of climate, the marine mollusca
have not changed, and the same drift which had been carried down to the sea at
the beginning of the period is now undergoing a second transportation in the
same direction.
As when we have measured a fraction of time in an hour-glass we have only to
reverse the position of our chronometer and we make the same sand measure over
again the duration of a second equal period, so when the volcanic force has
remoulded the form of a continent and the adjoining sea-bottom, the same
materials are made to do duty a second time. It is true that at each oscillation
of level the solid rocks composing the original continent suffer some fresh
denudation, and do not remain unimpaired like the wooden and glass framework of
the hour-glass, still the wear and tear suffered by the larger area exposed to
subaerial denudation consists either of loose drift or of sedimentary strata,
which were thrown down in seas near the land, and subsequently upraised, the
same continents and oceanic basins remaining in existence all the while.
From all that we know of the extreme slowness of the upward and downward
movements which bring about even slight geographical changes, we may infer that
it would require a long succession of geological periods to cause the submarine
and supramarine areas to change places, even if the ascending movements in the
one region and the descending in the other were continuously in one direction.
But we have only to appeal to the structure of the Alps, where there are so many
shallow and deep water formations of various ages crowded into a limited area,
to convince ourselves that mountain chains are the result of great oscillations
of level. High land is not produced simply by uniform upheaval, but by a
predominance of elevatory over subsiding movements. Where the ocean is extremely
deep it is because the sinking of the bottom has been in excess, in spite of
interruptions by upheaval.
Yet persistent as may be the leading features of land and sea on the globe, they
are not immutable. Some of the finest mud is doubtless carried to indefinite
distances from the coast by marine currents, and we are taught by deep-sea
dredgings that in clear water at depths equalling the height of the Alps organic
beings may flourish, and their spoils slowly accumulate on the bottom. We also
occasionally obtain evidence that submarine volcanoes are pouring out ashes and
streams of lava in mid-ocean as well as on land (see Principles volume 2 page
64), and that wherever mountains like Etna, Vesuvius, and the Canary Islands are
now the site of eruptions, there are signs of accompanying upheaval, by which
beds of ashes full of recent marine shells have been uplifted many hundred feet.
We need not be surprised, therefore, if we learn from geology that the
continents and oceans were not always placed where they now are, although the
imagination may well be overpowered when it endeavours to contemplate the
quantity of time required for such revolutions.
We shall have gained a great step if we can approximate to the number of
millions of years in which the average aqueous denudation going on upon the land
would convey seaward a quantity of matter equal to the average volume of our
continents, and this might give us a gauge of the minimum of volcanic force
necessary to counteract such levelling power of running water; but to discover a
relation between these great agencies and the rate at which species of organic
beings vary, is at present wholly beyond the reach of our computation, though
perhaps it may not prove eventually to transcend the powers of man.
CHAPTER VIII.
CHRONOLOGICAL CLASSIFICATION OF ROCKS.
Aqueous, Plutonic, volcanic, and metamorphic Rocks considered chronologically.
Terms Primary, Secondary, and Tertiary; Palaeozoic, Mesozoic, and Cainozoic
explained.
On the different Ages of the aqueous Rocks.
Three principal Tests of relative Age: Superposition, Mineral Character, and
Fossils.
Change of Mineral Character and Fossils in the same continuous Formation.
Proofs that distinct Species of Animals and Plants have lived at successive
Periods.
Distinct Provinces of indigenous Species.
Great Extent of single Provinces.
Similar Laws prevailed at successive Geological Periods.
Relative Importance of mineral and palaeontological Characters.
Test of Age by included Fragments.
Frequent Absence of Strata of intervening Periods.
Tabular Views of fossiliferous Strata.
CHRONOLOGY OF ROCKS.
In the first chapter it was stated that the four great classes of rocks, the
aqueous, the volcanic, the Plutonic, and the metamorphic, would each be
considered not only in reference to their mineral characters, and mode of
origin, but also to their relative age. In regard to the aqueous rocks, we have
already seen that they are stratified, that some are calcareous, others
argillaceous or siliceous, some made up of sand, others of pebbles; that some
contain fresh-water, others marine fossils, and so forth; but the student has
still to learn which rocks, exhibiting some or all of these characters, have
originated at one period of the earth's history, and which at another.
To determine this point in reference to the fossiliferous formations is more
easy than in any other class, and it is therefore the most convenient and
natural method to begin by establishing a chronology for these strata, and then
to refer as far as possible to the same divisions, the several groups of
Plutonic, volcanic, and metamorphic rocks. Such a system of classification is
not only recommended by its greater clearness and facility of application, but
is also best fitted to strike the imagination by bringing into one view the
contemporaneous revolutions of the inorganic and organic creations of former
times. For the sedimentary formations are most readily distinguished by the
different species of fossil animals and plants which they inclose, and of which
one assemblage after another has flourished and then disappeared from the earth
in succession.
In the present work, therefore, the four great classes of rocks, the aqueous,
Plutonic, volcanic, and metamorphic, will form four parallel, or nearly
parallel, columns in one chronological table. They will be considered as four
sets of monuments relating to four contemporaneous, or nearly contemporaneous,
series of events. I shall endeavour, in a subsequent chapter on the Plutonic
rocks, to explain the manner in which certain masses belonging to each of the
four classes of rocks may have originated simultaneously at every geological
period, and how the earth's crust may have been continually remodelled, above
and below, by aqueous and igneous causes, from times indefinitely remote. In the
same manner as aqueous and fossiliferous strata are now formed in certain seas
or lakes, while in other places volcanic rocks break out at the surface, and are
connected with reservoirs of melted matter at vast depths in the bowels of the
earth, so, at every era of the past, fossiliferous deposits and superficial
igneous rocks were in progress contemporaneously with others of subterranean and
Plutonic origin, and some sedimentary strata were exposed to heat, and made to
assume a crystalline or metamorphic structure.
It can by no means be taken for granted, that during all these changes the solid
crust of the earth has been increasing in thickness. It has been shown, that so
far as aqueous action is concerned, the gain by fresh deposits, and the loss by
denudation, must at each period have been equal (see Chapter 6); and in like
manner, in the inferior portion of the earth's crust, the acquisition of new
crystalline rocks, at each successive era, may merely have counterbalanced the
loss sustained by the melting of materials previously consolidated. As to the
relative antiquity of the crystalline foundations of the earth's crust, when
compared to the fossiliferous and volcanic rocks which they support, I have
already stated, in the first chapter, that to pronounce an opinion on this
matter is as difficult as at once to decide which of the two, whether the
foundations or superstructure of an ancient city built on wooden piles may be
the oldest. We have seen that, to answer this question, we must first be
prepared to say whether the work of decay and restoration had gone on most
rapidly above or below; whether the average duration of the piles has exceeded
that of the buildings, or the contrary. So also in regard to the relative age of
the superior and inferior portions of the earth's crust; we can not hazard even
a conjecture on this point, until we know whether, upon an average, the power of
water above, or that of heat below, is most efficacious in giving new forms to
solid matter.
The early geologists gave to all the crystalline and non-fossiliferous rocks the
name of Primitive or Primary, under the idea that they were formed anterior to
the appearance of life upon the earth, while the aqueous or fossiliferous strata
were termed Secondary, and alluviums or other superficial deposits, Tertiary.
The meaning of these terms, has, however, been gradually modified with advancing
knowledge, and they are now used to designate three great chronological
divisions under which all geological formations can be classed, each of them
being characterised by the presence of distinctive groups of organic remains
rather than by any mechanical peculiarities of the strata themselves. If,
therefore, we retain the term "primary," it must not be held to designate a set
of crystalline rocks some of which have been proved to be even of Tertiary age,
but must be applied to all rocks older than the secondary formations. Some
geologists, to avoid misapprehension, have introduced the term Palaeozoic for
primary, from palaion, "ancient," and zoon, "an organic being," still retaining
the terms secondary and tertiary; Mr. Phillips, for the sake of uniformity, has
proposed Mesozoic, for secondary, from mesos, "middle," etc.; and Cainozoic, for
tertiary, from kainos, "recent," etc.; but the terms primary, secondary, and
tertiary have the claim of priority in their favour, and are of corresponding
value.
It may perhaps be suggested that some metamorphic strata, and some granites, may
be anterior in date to the oldest of the primary fossiliferous rocks. This
opinion is doubtless true, and will be discussed in future chapters; but I may
here observe, that when we arrange the four classes of rocks in four parallel
columns in one table of chronology, it is by no means assumed that these columns
are all of equal length; one may begin at an earlier period than the rest, and
another may come down to a later point of time, and we may not be yet acquainted
with the most ancient of the primary fossiliferous beds, or with the newest of
the hypogene.
For reasons already stated, I proceed first to treat of the aqueous or
fossiliferous formations considered in chronological order or in relation to the
different periods at which they have been deposited.
There are three principal tests by which we determine the age of a given set of
strata; first, superposition; secondly, mineral character; and, thirdly, organic
remains. Some aid can occasionally be derived from a fourth kind of proof,
namely, the fact of one deposit including in it fragments of a pre-existing
rock, by which the relative ages of the two may, even in the absence of all
other evidence, be determined.
SUPERPOSITION.
The first and principal test of the age of one aqueous deposit, as compared to
another, is relative position. It has been already stated, that, where strata
are horizontal, the bed which lies uppermost is the newest of the whole, and
that which lies at the bottom the most ancient. So, of a series of sedimentary
formations, they are like volumes of history, in which each writer has recorded
the annals of his own times, and then laid down the book, with the last written
page uppermost, upon the volume in which the events of the era immediately
preceding were commemorated. In this manner a lofty pile of chronicles is at
length accumulated; and they are so arranged as to indicate, by their position
alone, the order in which the events recorded in them have occurred.
In regard to the crust of the earth, however, there are some regions where, as
the student has already been informed, the beds have been disturbed, and
sometimes extensively thrown over and turned upside down. (See Chapter 5.) But
an experienced geologist can rarely be deceived by these exceptional cases. When
he finds that the strata are fractured, curved, inclined, or vertical, he knows
that the original order of superposition must be doubtful, and he then
endeavours to find sections in some neighbouring district where the strata are
horizontal, or only slightly inclined. Here, the true order of sequence of the
entire series of deposits being ascertained, a key is furnished for settling the
chronology of those strata where the displacement is extreme.
MINERAL CHARACTER.
The same rocks may often be observed to retain for miles, or even hundreds of
miles, the same mineral peculiarities, if we follow the planes of
stratification, or trace the beds, if they be undisturbed, in a horizontal
direction. But if we pursue them vertically, or in any direction transverse to
the planes of stratification, this uniformity ceases almost immediately. In that
case we can scarcely ever penetrate a stratified mass for a few hundred yards
without beholding a succession of extremely dissimilar rocks, some of fine,
others of coarse grain, some of mechanical, others of chemical origin; some
calcareous, others argillaceous, and others siliceous. These phenomena lead to
the conclusion that rivers and currents have dispersed the same sediment over
wide areas at one period, but at successive periods have been charged, in the
same region, with very different kinds of matter. The first observers were so
astonished at the vast spaces over which they were able to follow the same
homogeneous rocks in a horizontal direction, that they came hastily to the
opinion, that the whole globe had been environed by a succession of distinct
aqueous formations, disposed round the nucleus of the planet, like the
concentric coats of an onion. But, although, in fact, some formations may be
continuous over districts as large as half of Europe, or even more, yet most of
them either terminate wholly within narrower limits, or soon change their
lithological character. Sometimes they thin out gradually, as if the supply of
sediment had failed in that direction, or they come abruptly to an end, as if we
had arrived at the borders of the ancient sea or lake which served as their
receptacle. It no less frequently happens that they vary in mineral aspect and
composition, as we pursue them horizontally. For example, we trace a limestone
for a hundred miles, until it becomes more arenaceous, and finally passes into
sand, or sandstone. We may then follow this sandstone, already proved by its
continuity to be of the same age, throughout another district a hundred miles or
more in length.
ORGANIC REMAINS.
This character must be used as a criterion of the age of a formation, or of the
contemporaneous origin of two deposits in distant places, under very much the
same restrictions as the test of mineral composition.
First, the same fossils may be traced over wide regions, if we examine strata in
the direction of their planes, although by no means for indefinite distances.
Secondly, while the same fossils prevail in a particular set of strata for
hundreds of miles in a horizontal direction, we seldom meet with the same
remains for many fathoms, and very rarely for several hundred yards, in a
vertical line, or a line transverse to the strata. This fact has now been
verified in almost all parts of the globe, and has led to a conviction that at
successive periods of the past, the same area of land and water has been
inhabited by species of animals and plants even more distinct than those which
now people the antipodes, or which now co-exist in the arctic, temperate, and
tropical zones. It appears that from the remotest periods there has been ever a
coming in of new organic forms, and an extinction of those which pre-existed on
the earth; some species having endured for a longer, others for a shorter, time;
while none have ever reappeared after once dying out. The law which has governed
the succession of species, whether we adopt or reject the theory of
transmutation, seems to be expressed in the verse of the poet:--
Natura il fece, e poi ruppe la stampa. Ariosto.
Nature made him, and then broke the die.
And this circumstance it is, which confers on fossils their highest value as
chronological tests, giving to each of them, in the eyes of the geologist, that
authority which belongs to contemporary medals in history.
The same can not be said of each peculiar variety of rock; for some of these, as
red marl and red sandstone, for example, may occur at once at the top, bottom,
and middle of the entire sedimentary series; exhibiting in each position so
perfect an identity of mineral aspect as to be undistinguishable. Such exact
repetitions, however, of the same mixtures of sediment have not often been
produced, at distant periods, in precisely the same parts of the globe; and even
where this has happened, we are seldom in any danger of confounding together the
monuments of remote eras, when we have studied their imbedded fossils and their
relative position.
ZOOLOGICAL PROVINCES.
It was remarked that the same species of organic remains can not be traced
horizontally, or in the direction of the planes of stratifications for
indefinite distances. This might have been expected from analogy; for when we
inquire into the present distribution of living beings, we find that the
habitable surface of the sea and land may be divided into a considerable number
of distinct provinces, each peopled by a peculiar assemblage of animals and
plants. In the "Principles of Geology," I have endeavoured to point out the
extent and probable origin of these separate divisions; and it was shown that
climate is only one of many causes on which they depend, and that difference of
longitude as well as latitude is generally accompanied by a dissimilarity of
indigenous species.
As different seas, therefore, and lakes are inhabited, at the same period, by
different aquatic animals and plants, and as the lands adjoining these may be
peopled by distinct terrestrial species, it follows that distinct fossils will
be imbedded in contemporaneous deposits. If it were otherwise-- if the same
species abounded in every climate, or in every part of the globe where, so far
as we can discover, a corresponding temperature and other conditions favourable
to their existence are found-- the identification of mineral masses of the same
age, by means of their included organic contents, would be a matter of still
greater certainty.
Nevertheless, the extent of some single zoological provinces, especially those
of marine animals, is very great; and our geological researches have proved that
the same laws prevailed at remote periods; for the fossils are often identical
throughout wide spaces, and in detached deposits, consisting of rocks varying
entirely in their mineral nature.
The doctrine here laid down will be more readily understood, if we reflect on
what is now going on in the Mediterranean. That entire sea may be considered as
one zoological province; for although certain species of testacea and zoophytes
may be very local, and each region has probably some species peculiar to it,
still a considerable number are common to the whole Mediterranean. If,
therefore, at some future period, the bed of this inland sea should be converted
into land, the geologist might be enabled, by reference to organic remains, to
prove the contemporaneous origin of various mineral masses scattered over a
space equal in area to half of Europe.
Deposits, for example, are well known to be now in progress in this sea in the
deltas of the Po, Rhone, Nile, and other rivers, which differ as greatly from
each other in the nature of their sediment as does the composition of the
mountains which their drain. There are also other quarters of the Mediterranean,
as off the coast of Campania, or near the base of Etna, in Sicily, or in the
Grecian Archipelago, where another class of rocks is now forming; where showers
of volcanic ashes occasionally fall into the sea, and streams of lava overflow
its bottom; and where, in the intervals between volcanic eruptions, beds of sand
and clay are frequently derived from the waste of cliffs, or the turbid waters
of rivers. Limestones, moreover, such as the Italian travertins, are here and
there precipitated from the waters of mineral springs, some of which rise up
from the bottom of the sea. In all these detached formations, so diversified in
their lithological characters, the remains of the same shells, corals,
crustacea, and fish are becoming inclosed; or, at least, a sufficient number
must be common to the different localities to enable the zoologist to refer them
all to one contemporaneous assemblage of species.
There are, however, certain combinations of geographical circumstances which
cause distinct provinces of animals and plants to be separated from each other
by very narrow limits; and hence it must happen that strata will be sometimes
formed in contiguous regions, differing widely both in mineral contents and
organic remains. Thus, for example, the testacea, zoophytes, and fish of the Red
Sea are, as a group, extremely distinct from those inhabiting the adjoining
parts of the Mediterranean, although the two seas are separated only by the
narrow isthmus of Suez. Calcareous formations have accumulated on a great scale
in the Red Sea in modern times, and fossil shells of existing species are well
preserved therein; and we know that at the mouth of the Nile large deposits of
mud are amassed, including the remains of Mediterranean species. It follows,
therefore, that if at some future period the bed of the Red Sea should be laid
dry, the geologist might experience great difficulties in endeavouring to
ascertain the relative age of these formations, which, although dissimilar both
in organic and mineral characters, were of synchronous origin.
But, on the other hand, we must not forget that the north-western shores of the
Arabian Gulf, the plains of Egypt, and the Isthmus of Suez, are all parts of one
province of TERRESTRIAL species. Small streams, therefore, occasional land-
floods, and those winds which drift clouds of sand along the deserts, might
carry down into the Red Sea the same shells of fluviatile and land testacea
which the Nile is sweeping into its delta, together with some remains of
terrestrial plants and the bones of quadrupeds, whereby the groups of strata
before alluded to might, notwithstanding the discrepancy of their mineral
composition and MARINE organic fossils, be shown to have belonged to the same
epoch.
Yet, while rivers may thus carry down the same fluviatile and terrestrial spoils
into two or more seas inhabited by different marine species, it will much more
frequently happen that the coexistence of terrestrial species of distinct
zoological and botanical provinces will be proved by the identity of the marine
beings which inhabited the intervening space. Thus, for example, the land
quadrupeds and shells of the valley of the Mississippi, of central America, and
of the West India islands differ very considerably, yet their remains are all
washed down by rivers flowing from these three zoological provinces into the
Gulf of Mexico.
In some parts of the globe, at the present period, the line of demarkation
between distinct provinces of animals and plants is not very strongly marked,
especially where the change is determined by temperature, as it is in seas
extending from the temperate to the tropical zone, or from the temperate to the
arctic regions. Here a gradual passage takes place from one set of species to
another. In like manner the geologist, in studying particular formations of
remote periods, has sometimes been able to trace the gradation from one ancient
province to another, by observing carefully the fossils of all the intermediate
places. His success in thus acquiring a knowledge of the zoological or botanical
geography of very distant eras has been mainly owing to this circumstance, that
the mineral character has no tendency to be affected by climate. A large river
may convey yellow or red mud into some part of the ocean, where it may be
dispersed by a current over an area several hundred leagues in length, so as to
pass from the tropics into the temperate zone. If the bottom of the sea be
afterwards upraised, the organic remains imbedded in such yellow or red strata
may indicate the different animals or plants which once inhabited at the same
time the temperate and equatorial regions.
It may be true, as a general rule, that groups of the same species of animals
and plants may extend over wider areas than deposits of homogeneous composition;
and if so, palaeontological characters will be of more importance in geological
classification than the test of mineral composition; but it is idle to discuss
the relative value of these tests, as the aid of both is indispensable, and it
fortunately happens, that where the one criterion fails, we can often avail
ourselves of the other.
TEST BY INCLUDED FRAGMENTS OF OLDER ROCKS.
It was stated, that proof may sometimes be obtained of the relative date of two
formations by fragments of an older rock being included in a newer one. This
evidence may sometimes be of great use, where a geologist is at a loss to
determine the relative age of two formations from want of clear sections
exhibiting their true order of position, or because the strata of each group are
vertical. In such cases we sometimes discover that the more modern rock has been
in part derived from the degradation of the older. Thus, for example, we may
find chalk in one part of a country, and in another strata of clay, sand, and
pebbles. If some of these pebbles consist of that peculiar flint, of which
layers more or less continuous are characteristic of the chalk, and which
include fossil shells, sponges, and foraminifera of cretaceous species, we may
confidently infer that the chalk was the oldest of the two formations.
CHRONOLOGICAL GROUPS.
The number of groups into which the fossiliferous strata may be separated are
more or less numerous, according to the views of classification which different
geologists entertain; but when we have adopted a certain system of arrangement,
we immediately find that a few only of the entire series of groups occur one
upon the other in any single section or district.
(FIGURE 84. Seven fossiliferous groups.)
The thinning out of individual strata was before described (Chapter 2). But let
the diagram (Figure 84) represent seven fossiliferous groups, instead of as many
strata. It will then be seen that in the middle all the superimposed formations
are present; but in consequence of some of them thinning out, No. 2 and No. 5
are absent at one extremity of the section, and No. 4 at the other.
(FIGURE 85. Section South of Bristol (A.C. Ramsay.)
Dundry Hill.
Length of section 4 miles.
a-b. Level of the sea.
1. Inferior Oolite.
2. Lias.
3. New Red Sandstone.
4. Dolomitic or magnesian conglomerate.
5. Upper coal-measures (shales, etc.)
6. Pennant rock (sandstone.)
7. Lower coal-measures (shales, etc.)
8. Carboniferous or mountain limestone.
9. Old Red Sandstone.)
In another diagram (Figure 85), a real section of the geological formations in
the neighbourhood of Bristol and the Mendip Hills is presented to the reader, as
laid down on a true scale by Professor Ramsay, where the newer groups 1, 2, 3, 4
rest unconformably on the formations 5, 6, 7 and 8. At the southern end of the
line of section we meet with the beds No. 3 (the New Red Sandstone) resting
immediately on Nos. 7 and 8, while farther north as at Dundry Hill in
Somersetshire, we behold eight groups superimposed one upon the other,
comprising all the strata from the inferior Oolite, No. 1, to the coal and
carboniferous limestone. The limited horizontal extension of the groups 1 and 2
is owing to denudation, as these formations end abruptly, and have left outlying
patches to attest the fact of their having originally covered a much wider area.
In order, therefore, to establish a chronological succession of fossiliferous
groups, a geologist must begin with a single section in which several sets of
strata lie one upon the other. He must then trace these formations, by attention
to their mineral character and fossils, continuously, as far as possible, from
the starting-point. As often as he meets with new groups, he must ascertain by
superposition their age relatively to those first examined, and thus learn how
to intercalate them in a tabular arrangement of the whole.
By this means the German, French, and English geologists have determined the
succession of strata throughout a great part of Europe, and have adopted pretty
generally the following groups, almost all of which have their representatives
in the British Islands.
ABRIDGED GENERAL TABLE OF FOSSILIFEROUS STRATA.
1. RECENT.-- POST-TERTIARY.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
2. POST-PLIOCENE.-- POST-TERTIARY.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
3. NEWER-PLIOCENE.-- PLIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
4. OLDER PLIOCENE.-- PLIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
5. UPPER MIOCENE.-- MIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
6. LOWER MIOCENE.-- MIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
7. UPPER EOCENE.-- EOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
8. MIDDLE EOCENE.-- EOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
9. LOWER EOCENE.-- EOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC.
10. MAESTRICHT BEDS.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
11. WHITE CHALK.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
12. CHLORITIC SERIES.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
13. GAULT.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
14. NEOCOMIAN.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
15. WEALDEN.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
16. PURBECK BEDS.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
17. PORTLAND STONE.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
18. KIMMERIDGE CLAY.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
19. CORAL RAG.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
20. OXFORD CLAY.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
21. GREAT or BATH OOLITE.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
22. INFERIOR OOLITE.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
23. LIAS.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
24. UPPER TRIAS.-- TRIASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
25. MIDDLE TRIAS.-- TRIASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
26. LOWER TRIAS.-- TRIASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC.
27. PERMIAN.-- PERMIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
28. COAL-MEASURES.-- CARBONIFEROUS.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
29. CARBONIFEROUS LIMESTONE.-- CARBONIFEROUS.-- -- PRIMARY OR PALAEOZOIC.--
PALAEOZOIC.
30. UPPER DEVONIAN.-- DEVONIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
31. MIDDLE DEVONIAN.-- DEVONIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
32. LOWER DEVONIAN.-- DEVONIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
33. UPPER SILURIAN.-- SILURIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
34. LOWER SILURIAN.-- SILURIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
35. UPPER CAMBRIAN.-- CAMBRIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
36. LOWER CAMBRIAN.-- CAMBRIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
37. UPPER LAURENTIAN.-- LAURENTIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
38. LOWER LAURENTIAN.-- LAURENTIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC.
TABULAR VIEW OF THE FOSSILIFEROUS STRATA,
SHOWING THE ORDER OF SUPERPOSITION OR CHRONOLOGICAL SUCCESSION OF THE PRINCIPAL
GROUPS DESCRIBED IN THIS WORK (CITING EXAMPLES).
POST-TERTIARY.
1. RECENT. Shells and mammalia, all of living species.
BRITISH.
Clyde marine strata, with canoes (Chapter 10.)
FOREIGN.
Danish kitchen middens (Chapter 10.)
Lacustrine mud, with remains of Swiss lake-dwellings (Chapter 10.)
Marine strata inclosing Temple of Serapis, at Puzzuoli (Chapter 10.)
2. POST-PLIOCENE. Shells, recent mammalia in part extinct.
BRITISH.
Loam of Brixham cave, with flint implements and bones of extinct and living
quadrupeds. (Chapter 10.)
Drift near Salisbury, with bones of mammoth, Spermophilus, and stone implements.
(Chapter 10.)
Glacial drift of Scotland, with marine shells and remains of mammoth. (Chapter
11.)
Erratics of Pagham and Selsey Bill. (Chapter 11.)
Glacial drift of Wales, with marine fossil shells, about 1400 feet high, on Moel
Tryfaen. (Chapter 11.)
FOREIGN.
Dordogne caves of the reindeer period. (Chapter 10.)
Older valley-gravels of Amiens, with flint implements and bones of extinct
mammalia. (Chapter 10.)
Loess of Rhine. (Chapter 10.)
Ancient Nile-mud forming river-terraces. (Chapter 10.)
Loam and breccia of Liege caverns, with human remains. (Chapter 10.)
Australian cave breccias, with bones of extinct marsupials. (Chapter 10.)
Glacial drift of Northern Europe. (Chapters 11 and 12.)
TERTIARY OR CAINOZOIC.
PLIOCENE.
3. NEWER PLIOCENE. The shells almost all of living species.
BRITISH.
Bridlington beds, marine Arctic fauna. (Chapter 13.)
Glacial boulder formation of Norfolk cliffs. (Chapter 13.)
Forest-bed of Norfolk cliffs, with bones of Elephas meridionalis, etc. (Chapter
13.)
Chillesford and Aldeby beds, with marine shells, chiefly Arctic. (Chapter 13.)
Norwich Crag. (Chapter 13.)
FOREIGN.
Eastern base of Mount Etna, with marine shells. (Chapter 13.)
Sicilian calcareous and tufaceous strata. (Chapter 13.)
Lacustrine strata of Upper Val d'Arno. (Chapter 13.)
Madeira leaf-bed and land-shells. (Chapter 29.)
4. OLDER PLIOCENE. Extinct species of shells forming a large minority.
BRITISH.
Red crag of Suffolk, marine shells, some of northern forms. (Chapter 13.)
White or coralline crag of Suffolk. (Chapter 13.)
FOREIGN.
Antwerp crag. (Chapter 13.)
Subapennine marls and sands. (Chapter 13.)
MIOCENE.
5. UPPER MIOCENE. Majority of the shells extinct.
BRITISH.
Wanting.
FOREIGN.
faluns of Touraine (Chapter 14.)
faluns, proper, of Bordeaux. (Chapter 14.)
Fresh-water strata of Gers. (Chapter 14.)
Swiss Oeningen beds, rich in plants and insects. (Chapter 14.)
Marine Molasse, Switzerland. (Chapter 14.)
Bolderberg beds of Belgium. (Chapter 14.)
Vienna basin. (Chapter 14.)
Beds of the Superga, near Turin. (Chapter 14.)
Deposit at Pikerme, near Athens. (Chapter 14.)
Strata of the Siwalik hills, India. (Chapter 14.)
Marine strata of the Atlantic border in the United States. (Chapter 14.)
Volcanic tuff and limestone of Madeira, the Canaries, and the Azores. (Chapter
30.)
6. LOWER MIOCENE. Nearly all the shells extinct.
BRITISH.
Hempstead beds, marine and fresh-water strata. (Chapter 15.)
Lignites and clays of Bovey Tracey. (Chapter 15.)
Isle of Mull leaf-bed, volcanic tuff. (Chapter 15.)
FOREIGN.
Calcaire de la Beauce, etc. (Chapter 15.)
Gres de Fontainebleau. (Chapter 15.)
Lacustrine strata of the Limagne d'Auvergne, and the Cantal. (Chapter 15.)
Mayence basin. (Chapter 15.)
Radaboj beds of Croatia. (Chapter 15.)
Brown coal of Germany. (Chapter 15.)
Lower Molasse of Switzerland, fresh-water and brackish. (Chapter 15.)
Rupelmonde, Kleynspawen, and Tongrian beds of Belgium. (Chapter 15.)
Nebraska beds, United States. (Chapter 15.)
Lower Miocene beds of Italy. (Chapter 15.)
Miocene flora of North Greenland. (Chapter 15.)
7. UPPER EOCENE.
BRITISH.
Bembridge fluvio-marine strata. (Chapter 16.)
Osborne or St. Helen's series. (Chapter 16.)
Headon series, with marine and fresh-water shells. (Chapter 16.)
Barton sands and clays (Chapter 16.)
FOREIGN.
Gypsum of Montmartre, fresh-water with Palaeotherium. (Chapter 16.)
Calcaire silicieux, or Travertin inferieur. (Chapter 16.)
Gres de Beauchamp, or Sables moyens. (Chapter 16.)
8. MIDDLE EOCENE.
BRITISH.
Bracklesham beds and Bagshot sands. (Chapter 16.)
White clays of Alum Bay and Bournemouth. (Chapter 16.)
FOREIGN.
Calcaire grossier, miliolitic limestone. (Chapter 16.)
Soissonnais sands, or Lits coquilliers, with Nummulites planulata. (Chapter 16.)
Claiborne beds of the United States, with Orbitoides and Zeuglodon. (Chapter
16.)
9. LOWER EOCENE.
Nummulitic formation of Europe, Asia, etc. (Chapter 16.)
BRITISH.
London Clay proper. (Chapter 16.)
Woolwich and Reading series, fluvio-marine. (Chapter 16.)
Thanet sands. (Chapter 16.)
FOREIGN.
Argile de Londres, near Dunkirk. (Chapter 16.)
Argile plastique. (Chapter 16.)
Sables de Bracheux. (Chapter 16.)
SECONDARY OR MESOZOIC.
CRETACEOUS.
10. UPPER CRETACEOUS.
BRITISH.
Upper white chalk, with flints. (Chapter 17.)
Lower white chalk, without flints. (Chapter 17.)
Chalk marl. (Chapter 17.)
Chloritic series (or Upper Greensand), fire-stone of Surrey. (Chapter 17.)
Gault. (Chapter 17.)
Blackdown beds. (Chapter 17.)
FOREIGN.
Maestricht beds and Faxoe chalk. (Chapter 17.)
Pisolitic limestone of France. (Chapter 17.)
White chalk of France, Sweden, and Russia. (Chapter 17.)
Planer-kalk of Saxony. (Chapter 17.)
Sands and clays of Aix-la-Chapelle. (Chapter 17.)
Hippurite limestone of South of France. (Chapter 17.)
New Jersey, U.S., sands and marls. (Chapter 17.)
11. LOWER CRETACEOUS OR NEOCOMIAN.
BRITISH.
Sands of Folkestone, Sandgate, and Hythe. (Chapter 18.)
Atherfield clay, with Perna mulleti. (Chapter 18.)
Punfield marine beds, with Vicarya lujana. (Chapter 18.)
Speeton clay of Flamborough Head and Tealby. (Chapter 18.)
Weald clay of Surrey, Kent, and Sussex, fresh-water, with Cypris. (Chapter 18.)
Hastings sands.
FOREIGN.
Neocomian of Neufchatel, and Hils conglomerate of North Germany. (Chapter 18.)
Wealden beds of Hanover. (Chapter 18.)
OOLITE.
12. UPPER OOLITE.
BRITISH.
Upper Purbeck beds, fresh-water. (Chapter 19.)
Middle Purbeck, with numerous marsupial quadrupeds, etc. (Chapter 19.)
Lower Purbeck, fresh-water, with intercalated dirt-bed. (Chapter 19.)
Portland stone and sand. (Chapter 19.)
Kimmeridge clay. (Chapter 19.)
FOREIGN.
Marnes a gryphees virgules of Argonne. (Chapter 19.)
Lithographic-stone of Solenhofen, with Archaeopteryx. (Chapter 19.)
13. MIDDLE OOLITE.
BRITISH.
Coral rag of Berkshire, Wilts, and Yorkshire. (Chapter 19.)
Oxford clay, with belemnites and Ammonite. (Chapter 19.)
Kelloway rock of Wilts and Yorkshire. (Chapter 19.)
FOREIGN.
Nerinaean limestone of the Jura.
14. LOWER OOLITE.
BRITISH.
Cornbrash and forest marble. (Chapter 19.)
Great or Bath oolite of Bradford. (Chapter 19.)
Stonesfield slate, with marsupials and Araucaria. (Chapter 19.)
Fuller's earth of Bath. (Chapter 19.)
Inferior oolite. (Chapter 19.)
LIAS.
15. LIAS.
Upper Lias, argillaceous, with Ammonites striatulus. (Chapter 20.)
Shale and limestone, with Ammonites bifrons. (Chapter 20.)
Middle Lias or Marlstone series, with zones containing characteristic Ammonites.
(Chapter 20.)
Lower Lias, also with zones characterised by peculiar Ammonites. (Chapter 20.)
TRIAS.
16. UPPER TRIAS.
BRITISH.
Rhaetic, Penarth or Avicula contorta beds (beds of passage). (Chapter 21.)
Keuper or Upper New Red sandstone, etc. (Chapter 21.)
Red shales of Cheshire and Lancashire, with rock-salt. (Chapter 21.)
Dolomite conglomerate of Bristol (Chapter 21.)
FOREIGN.
Keuper beds of Germany. (Chapter 21.)
St. Cassian or Hallstadt beds, with rich marine fauna. (Chapter 21.)
Coal-field of Richmond, Virginia. (Chapter 21.)
Chatham coal-field, North Carolina. (Chapter 21.)
17. MIDDLE TRIAS.
BRITISH.
Wanting.
FOREIGN.
Muschelkalk of Germany. (Chapter 21.)
18. LOWER TRIAS.
BRITISH.
Bunter or Lower New Red sandstone of Lancashire and Cheshire. (Chapter 21.)
FOREIGN.
Bunter-sandstein of Germany. (Chapter 21.)
Red sandstone of Connecticut Valley, with footprints of birds and reptiles.
(Chapter 21.)
PRIMARY OR PALAEOZOIC.
PERMIAN.
19. PERMIAN.
BRITISH.
Upper Permian of St. Bees' Head, Cumberland. (Chapter 22.)
Middle Permian, magnesian limestone, and marl-slate of Durham and Yorkshire,
with Protosaurus. (Chapter 22.)
Lower Permian sandstones and breccias of Penrith and Dumfriesshire,
intercalated. (Chapter 22.)
FOREIGN.
Dark-coloured shales of Thuringia. (Chapter 22.)
Zechstein or Dolomitic limestone. (Chapter 22.)
Mergel-schiefer or Kupfer-schiefer. (Chapter 22.)
Rothliegendes of Thuringia, with Psaronius. (Chapter 22.)
Magnesian limestones, etc., of Russia. (Chapter 22.)
CARBONIFEROUS.
20. UPPER CARBONIFEROUS.
BRITISH.
Coal-measures of South Wales, with underclays inclosing Stigmaria. (Chapter 23.)
Coal-measures of north and central England. (Chapter 23.)
Millstone grit. (Chapter 23.)
Yoredale series of Yorkshire. (Chapter 23.)
Coal-field of Kilkenny with Labyrinthodont. (Chapter 23.)
FOREIGN.
Coal-field of Saarbruck, with Archegosaurus. (Chapter 23.)
Carboniferous strata of South Joggins, Nova Scotia. (Chapter 23.)
Pennsylvania coal-field. (Chapter 23.)
21. LOWER CARBONIFEROUS.
BRITISH.
Mountain limestone of Wales and South of England. (Chapter 24.)
Same in Ireland. (Chapter 24.)
Carboniferous limestone of Scotland alternating with coal-bearing sandstones.
(Chapter 23.)
Erect trees in volcanic ash in the Island of Arran. (Chapter 30.)
FOREIGN.
Mountain limestone of Belgium. (Chapter 24.)
DEVONIAN OR OLD RED SANDSTONE.
22. UPPER DEVONIAN.
BRITISH.
Yellow sandstone of Dura Den, with Holoptychius, etc. (Chapter 25.); and of
Ireland with Anodon Jukesii. (Chapter 25.)
Sandstones of Forfarshire and Perthshire, with Holoptychius, etc. (Chapter 25.)
Pilton group of North Devon. (Chapter 25.)
Petherwyn group of Cornwall, with Clymenia and Cypridina. (Chapter 25.)
FOREIGN.
Clymenien-kalk and Cypridinen-schiefer of Germany. (Chapter 25.)
23. MIDDLE DEVONIAN.
BRITISH.
Bituminous schists of Gamrie, Caithness, etc., with numerous fish. (Chapter 25.)
Ilfracombe beds with peculiar trilobites and corals. (Chapter 25.)
Limestones of Torquay, with broad-winged Spirifers. (Chapter 25.)
FOREIGN. (Chapter 25.)
Eifel limestone, with underlying schists containing Calceola. (Chapter 25.)
Devonian strata of Russia. (Chapter 25.)
24. LOWER DEVONIAN.
BRITISH.
Arbroath paving-stones, with Cephalaspis and Pterygotus. (Chapter 25.)
Lower sandstones of Forfarshire, with Pterygotus. (Chapter 25.)
Sandstones and slates of the Foreland and Linton. (Chapter 25.)
FOREIGN.
Oriskany sandstone of Western Canada and New York. (Chapter 25.)
Sandstones of Gaspe, with Cephalaspis. (Chapter 25.)
SILURIAN.
25. UPPER SILURIAN.
BRITISH.
Upper Ludlow formation, Downton sandstone, with bone-bed. (Chapter 26.)
Lower Ludlow formation, with oldest known fish remains. (Chapter 26.)
Wenlock limestone and shale. (Chapter 26.)
Woolhope limestone and grit. (Chapter 26.)
Tarannon shales. (Chapter 26.)
Beds of passage between Upper and Lower Silurian:
Upper Llandovery, or May-hill sandstone, with Pentamerus oblongus, etc. (Chapter
26.)
Lower Llandovery slates. (Chapter 26.)
FOREIGN.
Niagara limestone, with Calymene, Homalonotus, etc. (Chapter 26.)
Clinton group of America, with Pentamerus oblongus, etc. (Chapter 26.)
Silurian strata of Russia, with Pentamerus. (Chapter 26.)
26. LOWER SILURIAN.
BRITISH.
Bala and Caradoc beds. (Chapter 26.)
Llandeilo flags. (Chapter 26.)
Arenig or Stiper-stones group (Lower Llandeilo of Murchison.) (Chapter 26.)
FOREIGN.
Ungulite or Obolus grit of Russia. (Chapter 26.)
Trenton limestone, and other Lower Silurian groups of North America. (Chapter
26.)
Lower Silurian of Sweden. (Chapter 26.)
CAMBRIAN.
27. UPPER CAMBRIAN.
BRITISH.
Tremadoc slates. (Chapter 27.)
Lingula flags, with Lingula Davisii. (Chapter 27.)
FOREIGN.
"Primordial" zone of Bohemia in part, with trilobites of the genera Paradoxides,
etc. (Chapter 27.)
Alum schists of Sweden and Norway. (Chapter 27.)
Potsdam sandstone, with Dikelocephalus and Obolella. (Chapter 27.)
28. LOWER CAMBRIAN.
BRITISH.
Menevian beds of Wales, with Paradoxides Davidis, etc. (Chapter 27.)
Longmynd group, comprising the Harlech grits and Llanberis slates. (Chapter 27.)
FOREIGN.
Lower portion of Barrande's "Primordial" zone in Bohemia. (Chapter 27.)
Fucoid sandstones of Sweden. (Chapter 27.)
Huronian series of Canada? (Chapter 27.)
LAURENTIAN.
29. UPPER LAURENTIAN.
BRITISH.
Fundamental gneiss of the Hebrides? (Chapter 27.)
Hypersthene rocks of Skye? (Chapter 27.)
FOREIGN.
Labradorite series north of the river St. Lawrence in Canada. (Chapter 27.)
Adirondack mountains of New York. (Chapter 27.)
30. LOWER LAURENTIAN.
BRITISH.
Wanting?
FOREIGN.
Beds of gneiss and quartzite, with interstratified limestones, in one of which,
1000 feet thick, occurs a foraminifer, Eozoon Canadense, the oldest known
fossil. (Chapter 27.)
CHAPTER IX.
CLASSIFICATION OF TERTIARY FORMATIONS.
Order of Succession of Sedimentary Formations.
Frequent Unconformability of Strata.
Imperfection of the Record.
Defectiveness of the Monuments greater in Proportion to their Antiquity.
Reasons for studying the newer Groups first.
Nomenclature of Formations.
Detached Tertiary Formations scattered over Europe.
Value of the Shell-bearing Mollusca in Classification.
Classification of Tertiary Strata.
Eocene, Miocene, and Pliocene Terms explained.
By reference to the tables given at the end of the last chapter the reader will
see that when the fossiliferous rocks are arranged chronologically, we have
first to consider the Post-tertiary and then the Tertiary or Cainozoic
formations, and afterwards to pass on to those of older date.
ORDER OF SUPERPOSITION.
(FIGURE 86. Section through Primary (left), Secondary, Tertiary and Post-
tertiary (right) Strata.
1. Laurentian.
2. Cambrian.
3. Silurian.
4. Devonian.
5. Carboniferous.
6. Permian.
7. Triassic.
8. Jurassic.
9. Cretaceous.
10. Eocene.
11. Miocene.
12. Pliocene.
13. Post-pliocene.
14. Recent.
Sea.)
The diagram (Figure 86.) will show the order of superposition of these deposits,
assuming them all to be visible in one continuous section. In nature, as before
hinted (Chapter 6), we have never an opportunity of seeing the whole of them so
displayed in a single region; first, because sedimentary deposition is confined,
during any one geological period, to limited areas; and secondly, because
strata, after they have been formed, are liable to be utterly annihilated over
wide areas by denudation. But wherever certain members of the series are
present, they overlie one another in the order indicated in the diagram, though
not always in the exact manner there represented, because some of them repose
occasionally in unconformable stratification on others. This mode of
superposition has been already explained (Chapters 5 and 7), where I pointed out
that the discordance which implies a considerable lapse of time between two
formations in juxtaposition is almost invariably accompanied by a great
dissimilarity in the species of organic remains.
FREQUENT UNCONFORMABILITY OF STRATA.
Where the widest gaps appear in the sequence of the fossil forms, as between the
Permian and Triassic rocks, or between the Cretaceous and Eocene, examples of
such unconformability are very frequent. But they are also met with in some part
or other of the world at the junction of almost all the other principal
formations, and sometimes the subordinate divisions of any one of the leading
groups may be found lying unconformably on another subordinate member of the
same-- the Upper, for example, on the Lower Silurian, or the superior division
of the Old Red Sandstone on a lower member of the same, and so forth. Instances
of such irregularities in the mode of succession of the strata are the more
intelligible the more we extend our survey of the fossiliferous formations, for
we are continually bringing to light deposits of intermediate date, which have
to be intercalated between those previously known, and which reveal to us a long
series of events, of which antecedently to such discoveries we had no knowledge.
But while unconformability invariably bears testimony to a lapse of
unrepresented time, the conformability of two sets of strata in contact by no
means implies that the newer formation immediately succeeded the older one. It
simply implies that the ancient rocks were subjected to no movements of such a
nature as to tilt, bend, or break them before the more modern formation was
superimposed. It does not show that the earth's crust was motionless in the
region in question, for there may have been a gradual sinking or rising,
extending uniformly over a large surface, and yet, during such movement, the
stratified rocks may have retained their original horizontality of position.
There may have been a conversion of a wide area from sea into land and from land
into sea, and during these changes of level some strata may have been slowly
removed by aqueous action, and after this new strata may be superimposed,
differing perhaps in date by thousands of years or centuries, and yet resting
conformably on the older set. There may even be a blending of the materials
constituting the older deposit with those of the newer, so as to give rise to a
passage in the mineral character of the one rock into the other as if there had
been no break or interruption in the depositing process.
IMPERFECTION OF THE RECORD.
Although by the frequent discovery of new sets of intermediate strata the
transition from one type of organic remains to another is becoming less and less
abrupt, yet the entire series of records appears to the geologists now living
far more fragmentary and defective than it seemed to their predecessors half a
century ago. The earlier inquirers, as often as they encountered a break in the
regular sequence of formations, connected it theoretically with a sudden and
violent catastrophe, which had put an end to the regular course of events that
had been going on uninterruptedly for ages, annihilating at the same time all or
nearly all the organic beings which had previously flourished, after which,
order being re-established, a new series of events was initiated. In proportion
as our faith in these views grows weaker, and the phenomena of the organic or
inorganic world presented to us by geology seem explicable on the hypothesis of
gradual and insensible changes, varied only by occasional convulsions, on a
scale comparable to that witnessed in historical times; and in proportion as it
is thought possible that former fluctuations in the organic world may be due to
the indefinite modifiability of species without the necessity of assuming new
and independent acts of creation, the number and magnitude of the gaps which
still remain, or the extreme imperfection of the record, become more and more
striking, and what we possess of the ancient annals of the earth's history
appears as nothing when contrasted with that which has been lost.
When we examine a large area such as Europe, the average as well as the extreme
height above the sea attained by the older formations is usually found to exceed
that reached by the more modern ones, the primary or palaeozoic rising higher
than the secondary, and these in their turn than the tertiary; while in
reference to the three divisions of the tertiary, the lowest or Eocene group
attains a higher summit-level than the Miocene, and these again a greater height
than the Pliocene formations. Lastly, the post-tertiary deposits, such, at
least, as are of marine origin, are most commonly restricted to much more
moderate elevations above the sea-level than the tertiary strata.
It is also observed that strata, in proportion as they are of newer date, bear
the nearest resemblance in mineral character to those which are now in the
progress of formation in seas or lakes, the newest of all consisting principally
of soft mud or loose sand, in some places full of shells, corals, or other
organic bodies, animal or vegetable, in others wholly devoid of such remains.
The farther we recede from the present time, and the higher the antiquity of the
formations which we examine, the greater are the changes which the sedimentary
deposits have undergone. Time, as I have explained in Chapters 5, 6, and 7, has
multiplied the effects of condensation by pressure and cementation, and the
modification produced by heat, fracture, contortion, upheaval, and denudation.
The organic remains also have sometimes been obliterated entirely, or the
mineral matter of which they were composed has been removed and replaced by
other substances.
WHY NEWER GROUPS SHOULD BE STUDIED FIRST.
We likewise observe that the older the rocks the more widely do their organic
remains depart from the types of the living creation. First, we find in the
newer tertiary rocks a few species which no longer exist, mixed with many living
ones, and then, as we go farther back, many genera and families at present
unknown make their appearance, until we come to strata in which the fossil
relics of existing species are nowhere to be detected, except a few of the
lowest forms of invertebrate, while some orders of animals and plants wholly
unrepresented in the living world begin to be conspicuous.
When we study, therefore, the geological records of the earth and its
inhabitants, we find, as in human history, the defectiveness and obscurity of
the monuments always increasing the remoter the era to which we refer, and the
difficulty of determining the true chronological relations of rocks is more and
more enhanced, especially when we are comparing those which were formed
simultaneously in very distant regions of the globe. Hence we advance with
securer steps when we begin with the study of the geological records of later
times, proceeding from the newer to the older, or from the more to the less
known.
In thus inverting what might at first seem to be the more natural order of
historical research, we must bear in mind that each of the periods above
enumerated, even the shortest, such as the Post-tertiary, or the Pliocene,
Miocene, or Eocene, embrace a succession of events of vast extent, so that to
give a satisfactory account of what we already know of any one of them would
require many volumes. When, therefore, we approach one of the newer groups
before endeavouring to decipher the monuments of an older one, it is like
endeavouring to master the history of our own country and that of some
contemporary nations, before we enter upon Roman History, or like investigating
the annals of Ancient Italy and Greece before we approach those of Egypt and
Assyria.
NOMENCLATURE.
The origin of the terms Primary and Secondary, and the synonymous terms
Palaeozoic, and Mesozoic, were explained in Chapter 8.
The Tertiary or Cainozoic strata (see Chapter 8) were so called because they
were all posterior in date to the Secondary series, of which last the Chalk of
Cretaceous, No. 9, Figure 86, constitutes the newest group. The whole of them
were at first confounded with the superficial alluviums of Europe; and it was
long before their real extent and thickness, and the various ages to which they
belong, were fully recognised. They were observed to occur in patches, some of
fresh-water, others of marine origin, their geographical area being usually
small as compared to the secondary formations, and their position often
suggesting the idea of their having been deposited in different bays, lakes,
estuaries, or inland seas, after a large portion of the space now occupied by
Europe had already been converted into dry land.
The first deposits of this class, of which the characters were accurately
determined, were those occurring in the neighbourhood of Paris, described in
1810 by MM. Cuvier and Brongniart. They were ascertained to consist of
successive sets of strata, some of marine, others of fresh-water origin, lying
one upon the other. The fossil shells and corals were perceived to be almost all
of unknown species, and to have in general a near affinity to those now
inhabiting warmer seas. The bones and skeletons of land animals, some of them of
large size, and belonging to more than forty distinct species, were examined by
Cuvier, and declared by him not to agree specifically, nor most of them even
generically, with any hitherto observed in the living creation.
Strata were soon afterwards brought to light in the vicinity of London, and in
Hampshire, which, although dissimilar in mineral composition, were justly
inferred by Mr. T. Webster to be of the same age as those of Paris, because the
greater number of the fossil shells were specifically identical. For the same
reason, rocks found on the Gironde, in the South of France, and at certain
points in the North of Italy, were suspected to be of contemporaneous origin.
Another important discovery was soon afterwards made by Brocchi in Italy, who
investigated the argillaceous and sandy deposits, replete with shells, which
form a low range of hills, flanking the Apennines on both sides, from the plains
of the Po to Calabria. These lower hills were called by him the Subapennines,
and were formed of strata chiefly marine, and newer than those of Paris and
London.
Another tertiary group occurring in the neighbourhood of Bordeaux and Dax, in
the South of France, was examined by M. de Basterot in 1825, who described and
figured several hundred species of shells, which differed for the most part both
from the Parisian series and those of the Subapennine hills. It was soon,
therefore, suspected that this fauna might belong to a period intermediate
between that of the Parisian and Subapennine strata, and it was not long before
the evidence of superposition was brought to bear in support of this opinion;
for other strata, contemporaneous with those of Bordeaux, were observed in one
district (the Valley of the Loire), to overlie the Parisian formation, and in
another (in Piedmont) to underlie the Subapennine beds. The first example of
these was pointed out in 1829 by M. Desnoyers, who ascertained that the sand and
marl of marine origin called faluns, near Tours, in the basin of the Loire, full
of sea-shells and corals, rested upon a lacustrine formation, which constitutes
the uppermost subdivision of the Parisian group, extending continuously
throughout a great table-land intervening between the basin of the Seine and
that of the Loire. The other example occurs in Italy, where strata containing
many fossils similar to those of Bordeaux were observed by Bonelli and others in
the environs of Turin, subjacent to strata belonging to the Subapennine group of
Brocchi.
VALUE OF TESTACEAN FOSSILS IN CLASSIFICATION.
It will be observed that in the foregoing allusions to organic remains, the
testacea or the shell-bearing mollusca are selected as the most useful and
convenient class for the purposes of general classification. In the first place,
they are more universally distributed through strata of every age than any other
organic bodies. Those families of fossils which are of rare and casual
occurrence are absolutely of no avail in establishing a chronological
arrangement. If we have plants alone in one group of strata and the bones of
mammalia in another, we can draw no conclusion respecting the affinity or
discordance of the organic beings of the two epochs compared; and the same may
be said if we have plants and vertebrated animals in one series and only shells
in another. Although corals are more abundant, in a fossil state, than plants,
reptiles, or fish, they are still rare when contrasted with shells, because they
are more dependent for their well-being on the constant clearness of the water,
and are, therefore, less likely to be included in rocks which endure in
consequence of their thickness and the copiousness of sediment which prevailed
when they originated. The utility of the testacea is, moreover, enhanced by the
circumstance that some forms are proper to the sea, others to the land, and
others to fresh water. Rivers scarcely ever fail to carry down into their deltas
some land-shells, together with species which are at once fluviatile and
lacustrine. By this means we learn what terrestrial, fresh-water, and marine
species coexisted at particular eras of the past: and having thus identified
strata formed in seas with others which originated contemporaneously in inland
lakes, we are then enabled to advance a step farther, and show that certain
quadrupeds or aquatic plants, found fossil in lacustrine formations, inhabited
the globe at the same period when certain fish, reptiles, and zoophytes lived in
the ocean.
Among other characters of the molluscous animals, which render them extremely
valuable in settling chronological questions in geology, may be mentioned,
first, the wide geographical range of many species; and, secondly, what is
probably a consequence of the former, the great duration of species in this
class, for they appear to have surpassed in longevity the greater number of the
mammalia and fish. Had each species inhabited a very limited space, it could
never, when imbedded in strata, have enabled the geologist to identify deposits
at distant points; or had they each lasted but for a brief period, they could
have thrown no light on the connection of rocks placed far from each other in
the chronological, or, as it is often termed, vertical series.
CLASSIFICATION OF TERTIARY STRATA.
Many authors have divided the European Tertiary strata into three groups--
lower, middle, and upper; the lower comprising the oldest formations of Paris
and London before mentioned; the middle those of Bordeaux and Touraine; and the
upper all those newer than the middle group.
In the first edition of the Principles of Geology, I divided the whole of the
Tertiary formations into four groups, characterised by the percentage of recent
shells which they contained. The lower tertiary strata of London and Paris were
thought by M. Deshayes to contain only 3 1/2 per cent of recent species, and
were termed Eocene. The middle tertiary of the Loire and Gironde had, according
to the specific determinations of the same conchologist, 17 per cent, and formed
the Miocene division. The Subapennine beds contained 35 to 50 per cent, and were
termed Older Pliocene, while still more recent beds in Sicily, which had from 90
to 95 per cent of species identical with those now living, were called Newer
Pliocene. The first of the above terms, Eocene, is derived from eos, dawn, and
cainos, recent, because the fossil shells of this period contain an extremely
small proportion of living species, which may be looked upon as indicating the
dawn of the existing state of the testaceous fauna, no recent species having
been detected in the older or secondary rocks.
The term Miocene (from meion, less, and cainos, recent) is intended to express a
minor proportion of recent species (of testacea), the term Pliocene (from
pleion, more, and cainos, recent) a comparative plurality of the same. It may
assist the memory of students to remind them, that the MI-ocene contain a MI-nor
proportion, and PL-iocene a comparative PL-urality of recent species; and that
the greater number of recent species always implies the more modern origin of
the strata.
It has sometimes been objected to this nomenclature that certain species of
infusoria found in the chalk are still existing, and, on the other hand, the
Miocene and Older Pliocene deposits often contain the remains of mammalia,
reptiles, and fish, exclusively of extinct species. But the reader must bear in
mind that the terms Eocene, Miocene, and Pliocene were originally invented with
reference purely to conchological data, and in that sense have always been and
are still used by me.
Since the year 1830 the number of known shells, both recent and fossil, has
largely increased, and their identification has been more accurately determined.
Hence some modifications have been required in the classifications founded on
less perfect materials. The Eocene, Miocene, and Pliocene periods have been made
to comprehend certain sets of strata of which the fossils do not always conform
strictly in the proportion of recent to extinct species with the definitions
first given by me, or which are implied in the etymology of those terms.
CHAPTER X.
RECENT AND POST-PLIOCENE PERIODS.
Recent and Post-pliocene Periods.
Terms defined.
Formations of the Recent Period.
Modern littoral Deposits containing Works of Art near Naples.
Danish Peat and Shell-mounds.
Swiss Lake-dwellings.
Periods of Stone, Bronze, and Iron.
Post-pliocene Formations.
Coexistence of Man with extinct Mammalia.
Reindeer Period of South of France.
Alluvial Deposits of Paleolithic Age.
Higher and Lower-level Valley-gravels.
Loess or Inundation-mud of the Nile, Rhine, etc.
Origin of Caverns.
Remains of Man and extinct Quadrupeds in Cavern Deposits.
Cave of Kirkdale.
Australian Cave-breccias.
Geographical Relationship of the Provinces of living Vertebrata and those of
extinct Post-pliocene Species.
Extinct struthious Birds of New Zealand.
Climate of the Post-pliocene Period.
Comparative Longevity of Species in the Mammalia and Testacea.
Teeth of Recent and Post-pliocene Mammalia.
We have seen in the last chapter that the uppermost or newest strata are called
Post-tertiary, as being more modern than the Tertiary. It will also be observed
that the Post-tertiary formations are divided into two subordinate groups: the
Recent, and Post-pliocene. In the former, or the Recent, the mammalia as well as
the shells are identical with species now living: whereas in the Post-pliocene,
the shells being all of living forms, a part, and often a considerable part, of
the mammalia belonged to extinct species. To this nomenclature it may be
objected that the term Post-pliocene should in strictness include all geological
monuments posterior in date to the Pliocene; but when I have occasion to speak
of the whole collectively, I shall call them Post-tertiary, and reserve the term
Post-pliocene for the older Post-tertiary formations, while the Upper or newer
ones will be called "Recent."
Cases will occur where it may be scarcely possible to draw the boundary line
between the Recent and Post-pliocene deposits; and we must expect these
difficulties to increase rather than diminish with every advance in our
knowledge, and in proportion as gaps are filled up in the series of records.
RECENT PERIOD.
It was stated in the sixth chapter, when I treated of denudation, that the dry
land, or that part of the earth's surface which is not covered by the waters of
lakes or seas, is generally wasting away by the incessant action of rain and
rivers, and in some cases by the undermining and removing power of waves and
tides on the sea-coast. But the rate of waste is very unequal, since the level
and gently sloping lands, where they are protected by a continuous covering of
vegetation, escape nearly all wear and tear, so that they may remain for ages in
a stationary condition, while the removal of matter is constantly widening and
deepening the intervening ravines and valleys.
The materials, both fine and coarse, carried down annually by rivers from the
higher regions to the lower, and deposited in successive strata in the basins of
seas and lakes, must be of enormous volume. We are always liable to underrate
their magnitude, because the accumulation of strata is going on out of sight.
There are, however, causes at work which, in the course of centuries, tend to
render visible these modern formations, whether of marine or lacustrine origin.
For a large portion of the earth's crust is always undergoing a change of level,
some areas rising and others sinking at the rate of a few inches, or a few feet,
perhaps sometimes yards, in a century; so that spaces which were once subaqueous
are gradually converted into land, and others which were high and dry become
submerged. In consequence of such movements we find in certain regions, as in
Cashmere, for example, where the mountains are often shaken by earthquakes,
deposits which were formed in lakes in the historical period, but through which
rivers have now cut deep and wide channels. In lacustrine strata thus
intersected, works of art and fresh-water shells are seen. In other districts on
the borders of the sea, usually at very moderate elevations above its level,
raised beaches occur, or marine littoral deposits, such as those in which, on
the borders of the Bay of Baiae, near Naples, the well-known temple of Serapis
was imbedded. In that case the date of the monument buried in the marine strata
is ascertainable, but in many other instances the exact age of the remains of
human workmanship is uncertain, as in the estuary of the Clyde at Glasgow, where
many canoes have been exhumed, with other works of art, all assignable to some
part of the Recent Period.
DANISH PEAT AND SHELL-MOUNDS OR KITCHEN-MIDDENS.
Sometimes we obtain evidence, without the aid of a change of level, of events
which took place in pre-historic times. The combined labours, for example, of
the antiquary, zoologist, and botanist have brought to light many monuments of
the early inhabitants buried in peat-mosses in Denmark. Their geological age is
determined by the fact that, not only the contemporaneous fresh-water and land
shells, but all the quadrupeds, found in the peat, agree specifically with those
now inhabiting the same districts, or which are known to have been indigenous in
Denmark within the memory of man. In the lower beds of peat (a deposit varying
from 20 to 30 feet in thickness), weapons of stone accompany trunks of the
Scotch fir, Pinus sylvestris. This peat may be referred to that part of the
stone period for which Sir John Lubbock proposed the name of "Neolithic" in
contradistinction to a still older era, termed by him "Paleolithic," and which
will be described in the sequel. (Sir John Lubbock Pre-historic Times page 3
1865.) In the higher portions of the same Danish bogs, bronze implements are
associated with trunks and acorns of the common oak. It appears that the pine
has never been a native of Denmark in historical times, and it seems to have
given place to the oak about the time when articles and instruments of bronze
superseded those of stone. It also appears that, at a still later period, the
oak itself became scarce, and was nearly supplanted by the beech, a tree which
now flourishes luxuriantly in Denmark. Again, at the still later epoch when the
beech-tree abounded, tools of iron were introduced, and were gradually
substituted for those of bronze.
On the coasts of the Danish islands in the Baltic, certain mounds, called in
those countries "Kjokken-modding," or "kitchen-middens," occur, consisting
chiefly of the castaway shells of the oyster, cockle, periwinkle, and other
eatable kinds of molluscs. The mounds are from three to ten feet high, and from
100 to 1000 feet in their longest diameter. They greatly resemble heaps of
shells formed by the Red Indians of North America along the eastern shores of
the United States. In the old refuse-heaps, recently studied by the Danish
antiquaries and naturalists with great skill and diligence, no implements of
metal have ever been detected. All the knives, hatchets, and other tools, are of
stone, horn, bone, or wood. With them are often intermixed fragments of rude
pottery, charcoal and cinders, and the bones of quadrupeds on which the rude
people fed. These bones belong to wild species still living in Europe, though
some of them, like the beaver, have long been extirpated in Denmark. The only
animal which they seem to have domesticated was the dog.
As there is an entire absence of metallic tools, these refuse-heaps are referred
to the Neolithic division of the age of stone, which immediately preceded in
Denmark the age of bronze. It appears that a race more advanced in civilisation,
armed with weapons of that mixed metal, invaded Scandinavia, and ousted the
aborigines.
LACUSTRINE HABITATIONS OF SWITZERLAND.
In Switzerland a different class of monuments, illustrating the successive ages
of stone, bronze, and iron, has been of late years investigated with great
success, and especially since 1854, in which year Dr. F. Keller explored near
the shore at Meilen, in the bottom of the lake of Zurich, the ruins of an old
village, originally built on numerous wooden piles, driven, at some unknown
period, into the muddy bed of the lake. Since then a great many other
localities, more than a hundred and fifty in all, have been detected of similar
pile-dwellings, situated near the borders of the Swiss lakes, at points where
the depth of water does not exceed 15 feet. (Bulletin de la Societie Vaudoise
des Sciences Nat. tome 6 Lausanne 1860; and Antiquity of Man by the author
chapter 2.) The superficial mud in such cases is filled with various articles,
many hundreds of them being often dredged up from a very limited area. Thousands
of piles, decayed at their upper extremities, are often met with still firmly
fixed in the mud.
As the ages of stone, bronze, and iron merely indicate successive stages of
civilisation, they may all have coexisted at once in different parts of the
globe, and even in contiguous regions, among nations having little intercourse
with each other. To make out, therefore, a distinct chronological series of
monuments is only possible when our observations are confined to a limited
district, such as Switzerland.
The relative antiquity of the pile-dwellings, which belong respectively to the
ages of stone and bronze, is clearly illustrated by the associations of the
tools with certain groups of animal remains. Where the tools are of stone, the
castaway bones which served for the food of the ancient people are those of
deer, the wild boar, and wild ox, which abounded when society was in the hunter
state. But the bones of the later or bronze epoch were chiefly those of the
domestic ox, goat, and pig, indicating progress in civilisation. Some villages
of the stone age are of later date than others, and exhibit signs of an improved
state of the arts. Among their relics are discovered carbonised grains of wheat
and barley, and pieces of bread, proving that the cultivation of cereals had
begun. In the same settlements, also, cloth, made of woven flax and straw, has
been detected.
The pottery of the bronze age in Switzerland is of a finer texture, and more
elegant in form, than that of the age of stone. At Nidau, on the lake of Bienne,
articles of iron have also been discovered, so that this settlement was
evidently not abandoned till that metal had come into use.
At La Thene, in the northern angle of the lake of Neufchatel, a great many
articles of iron have been obtained, which in form and ornamentation are
entirely different both from those of the bronze period and from those used by
the Romans. Gaulish and Celtic coins have also been found there by MM. Schwab
and Desor. They agree in character with remains, including many iron swords,
which have been found at Tiefenau, near Berne, in ground supposed to have been a
battle-field; and their date appears to have been anterior to the great Roman
invasion of Northern Europe, though perhaps not long before that event. (Sir J.
Lubbock's Lecture, Royal Institution February 27, 1863.) Coins, which sometimes
occur in deposits of the age of iron, have never yet been found in formations of
the ages of bronze or stone.
The period of bronze must have been one of foreign commerce, as tin, which
enters into this metallic mixture in the proportion of about ten per cent to the
copper, was obtained by the ancients chiefly from Cornwall. (Diodorus 5, 21, 22
and Sir H. James Note on Block of Tin dredged up in Falmouth Harbour. Royal
Institution of Cornwall 1863.) Very few human bones of the bronze period have
been met with in the Danish peat, or in the Swiss lake-dwellings, and this
scarcity is generally attributed by archaeologists to the custom of burning the
dead, which prevailed in the age of bronze.
POST-PLIOCENE PERIOD.
From the foregoing observations we may infer that the ages of iron and bronze in
Northern and Central Europe were preceded by a stone age, the Neolithic,
referable to that division of the post-tertiary epoch which I have called
Recent, when the mammalia as well as the other organic remains accompanying the
stone implements were of living species. But memorials have of late been brought
to light of a still older age of stone, for which, as above stated, the name
Paleolithic has been proposed, when man was contemporary in Europe with the
elephant and rhinoceros, and various other animals, of which many of the most
conspicuous have long since died out.
REINDEER PERIOD IN SOUTH OF FRANCE.
In the larger number of the caves of Europe, as for example in those of England,
Belgium, Germany, and many parts of France, the animal remains agree
specifically with the fauna of this oldest division of the age of stone, or that
to which belongs the drift of Amiens and Abbeville presently to be mentioned,
containing flint implements of a very antique type. But there are some caves in
the departments of Dordogne, Aude, and other parts of the south of France, which
are believed by M. Lartet to be of intermediate date between the Paleolithic and
Neolithic periods. To this intermediate era M. Lartet gave, in 1863, the name of
the "reindeer period," because vast quantities of the bones and horns of that
deer have been met with in the French caverns. In some cases separate plates of
molars of the mammoth, and several teeth of the great Irish deer, Cervus
megaceros, and of the cave-lion, Felis spelaea, have been found mixed up with
cut and carved bones of reindeer. On one of these sculptured bones in the cave
of Perigord, a rude representation of the mammoth, with its long curved tusks
and covering of wool, occurs, which is regarded by M. Lartet as placing beyond
all doubt the fact that the early inhabitants of these caves must have seen this
species of elephant still living in France. The presence of the marmot, as well
as the reindeer and some other northern animals, in these caverns seems to imply
a colder climate than that of the Swiss lake-dwellings, in which no remains of
reindeer have as yet been discovered. The absence of this last in the old
lacustrine habitations of Switzerland is the more significant, because in a cave
in the neighbourhood of the lake of Geneva, namely, that of Mont Saleve, bones
of the reindeer occur with flint implements similar to those of the caverns of
Dordogne and Perigord.
The state of the arts, as exemplified by the instruments found in these caverns
of the reindeer period, is somewhat more advanced than that which characterises
the tools of the Amiens drift, but is nevertheless more rude than that of the
Swiss lake-dwellings. No metallic articles occur, and the stone hatchets are not
ground after the fashion of celts; the needles of bone are shaped in a
workmanlike style, having their eyes drilled with consummate skill.
The formations above alluded to, which are as yet but imperfectly known, may be
classed as belonging to the close of the Paleolithic era, of the monuments of
which I am now about to treat.
ALLUVIAL DEPOSITS OF THE PALEOLITHIC AGE.
(FIGURE 87. Recent and Post-pliocene alluvial deposits.
1. Peat of the recent period.
2. Gravel of modern river.
2'. Loam of brick-earth (loess) of same age as 2, formed by inundations of the
river.
3. Lower-level valley-gravel with extinct mammalia (Post-pliocene).
3'. Loam of same age.
4. Higher-level valley-gravel (Post-pliocene).
4'. Loam of same age.
5. Upland gravel of various kinds and periods, consisting in some places of
unstratified boulder clay or glacial drift.
6. Older rocks.)
The alluvial and marine deposits of the Paleolithic age, the earliest to which
any vestiges of man have yet been traced back, belong to a time when the
physical geography of Europe differed in a marked degree from that now
prevailing. In the Neolithic period, the valleys and rivers coincided almost
entirely with those by which the present drainage of the land is effected, and
the peat-mosses were the same as those now growing. The situation of the shell-
mounds and lake-dwellings above alluded to is such as to imply that the
topography of the districts where they are observed has not subsequently
undergone any material alteration. Whereas we no sooner examine the Post-
pliocene formations, in which the remains of so many extinct mammalia are found,
than we at once perceive a more decided discrepancy between the former and
present outline of the surface. Since those deposits originated, changes of
considerable magnitude have been effected in the depth and width of many
valleys, as also in the direction of the superficial and subterranean drainage,
and, as is manifest near the sea-coast, in the relative position of land and
water. In Figure 87 an ideal section is given, illustrating the different
position which the Recent and Post-pliocene alluvial deposits occupy in many
European valleys.
The peat, No. 1, has been formed in a low part of the modern alluvial plain, in
parts of which gravel No. 2 of the recent period is seen. Over this gravel the
loam or fine sediment 2' has in many places been deposited by the river during
floods which covered nearly the whole alluvial plain.
No. 3 represents an older alluvium, composed of sand and gravel, formed before
the valley had been excavated to its present depth. It contains the remains of
fluviatile shells of living species associated with the bones of mammalia, in
part of recent, and in part of extinct species. Among the latter, the mammoth
(E. primigenius) and the Siberian rhinoceros (R. tichorhinus) are the most
common in Europe. No. 3' is a remnant of the loam or brick-earth by which No. 3
was overspread. No. 4 is a still older and more elevated terrace, similar in its
composition and organic remains to No. 3, and covered in like manner with its
inundation-mud, 4'. Sometimes the valley-gravels of older date are entirely
missing, or there is only one, and occasionally there are more than two, marking
as many successive stages in the excavation of the valley. They usually occur at
heights varying from 10 to 100 feet, sometimes on the right and sometimes on the
left side of the existing river-plain, but rarely in great strength on exactly
opposite sides of the valley.
Among the genera of extinct quadrupeds most frequently met with in England,
France, Germany, and other parts of Europe, are the elephant, rhinoceros,
hippopotamus, horse, great Irish deer, bear, tiger, and hyaena. In the peat, No.
1 (Figure 87), and in the more modern gravel and silt (No. 2), works of art of
the ages of iron and bronze, and of the later or Neolithic stone period, already
described, are met with. In the more ancient or Paleolithic gravels, 3 and 4,
there have been found of late years in several valleys in France and England--
as, for example, in those of the Seine and Somme, and of the Thames and Ouse,
near Bedford-- stone implements of a rude type, showing that man coexisted in
those districts with the mammoth and other extinct quadrupeds of the genera
above enumerated. In 1847, M. Boucher de Perthes observed in an ancient alluvium
at Abbeville, in Picardy, the bones of extinct mammalia associated in such a
manner with flint implements of a rude type as to lead him to infer that both
the organic remains and the works of art were referable to one and the same
period. This inference was soon after confirmed by Mr. Prestwich, who found in
1859 a flint tool in situ in the same stratum at Amiens that contained the
remains of extinct mammalia.
The flint implements found at Abbeville and Amiens are most of them considered
to be hatchets and spear-heads, and are different from those commonly called
"celts." These celts, so often found in the recent formations, have a more
regular oblong shape, the result of grinding, by which also a sharp edge has
been given to them. The Abbeville tools found in gravel at different levels, as
in Nos. 3 and 4, Figure 87, in which bones of the elephant, rhinoceros, and
other extinct mammalia occur, are always unground, having evidently been brought
into their present form simply by the chipping off of fragments of flint by
repeated blows, such as could be given by a stone hammer.
Some of them are oval, others of a spear-headed form, no two exactly alike, and
yet the greater number of each kind are obviously fashioned after the same
general pattern. Their outer surface is often white, the original black flint
having been discoloured and bleached by exposure to the air, or by the action of
acids, as they lay in the gravel. They are most commonly stained of the same
ochreous colour as the flints of the gravel in which they are imbedded.
Occasionally their antiquity is indicated not only by their colour but by
superficial incrustations of carbonate of lime, or by dendrites formed of oxide
of iron and manganese. The edges also of most of them are worn, sometimes by
having been used as tools, or sometimes by having been rolled in the old river's
bed. They are met with not only in the lower-level gravels, as in No. 3, Figure
87, but also in No. 4, or the higher gravels, as at St. Acheul, in the suburbs
of Amiens, where the old alluvium lies at an elevation of about 100 feet above
the level of the river Somme. At both levels fluviatile and land-shells are met
with in the loam as well as in the gravel, but there are no marine shells
associated, except at Abbeville, in the lowest part of the gravel, near the sea,
and a few feet only above the present high-water mark. Here with fossil shells
of living species are mingled the bones of Elephas primigenius and E. antiquus,
Rhinoceros tichorhinus, Hippopotamus, Felis spelaea, Hyaena spelaea, reindeer,
and many others, the bones accompanying the flint implements in such a manner as
to show that both were buried in the old alluvium at the same period.
Nearly the entire skeleton of a rhinoceros was found at one point, namely, in
the Menchecourt drift at Abbeville, the bones being in such juxtaposition as to
show that the cartilage must have held them together at the time of their
inhumation.
The general absence here and elsewhere of human bones from gravel and sand in
which flint tools are discovered, may in some degree be due to the present
limited extent of our researches. But it may also be presumed that when a hunter
population, always scanty in numbers, ranged over this region, they were too
wary to allow themselves to be overtaken by the floods which swept away many
herbivorous animals from the low river-plains where they may have been pasturing
or sleeping. Beasts of prey prowling about the same alluvial flats in search of
food may also have been surprised more readily than the human tenant of the same
region, to whom the signs of a coming tempest were better known.
INUNDATION-MUD OF RIVERS.-- BRICK-EARTH.-- FLUVIATILE LOAM, OR LOESS.
As a general rule, the fluviatile alluvia of different ages (Nos. 2, 3, 4,
Figure 87) are severally made up of coarse materials in their lower portions,
and of fine silt or loam in their upper parts. For rivers are constantly
shifting their position in the valley-plain, encroaching gradually on one bank,
near which there is deep water, and deserting the other or opposite side, where
the channel is growing shallower, being destined eventually to be converted into
land. Where the current runs strongest, coarse gravel is swept along, and where
its velocity is slackened, first sand, and then only the finest mud, is thrown
down. A thin film of this fine sediment is spread, during floods, over a wide
area, on one, or sometimes on both sides, of the main stream, often reaching as
far as the base of the bluffs or higher grounds which bound the valley. Of such
a description are the well-known annual deposits of the Nile, to which Egypt
owes its fertility. So thin are they, that the aggregate amount accumulated in a
century is said rarely to exceed five inches, although in the course of
thousands of years it has attained a vast thickness, the bottom not having been
reached by borings extending to a depth of 60 feet towards the central parts of
the valley. Everywhere it consists of the same homogeneous mud, destitute of
stratification-- the only signs of successive accumulation being where the Nile
has silted up its channel, or where the blown sands of the Libyan desert have
invaded the plain, and given rise to alternate layers of sand and mud.
In European river-loams we occasionally observe isolated pebbles and angular
pieces of stone which have been floated by ice to the places where they now
occur; but no such coarse materials are met with in the plains of Egypt.
In some parts of the valley of the Rhine the accumulation of similar loam,
called in Germany "loess," has taken place on an enormous scale. Its colour is
yellowish-grey, and very homogeneous; and Professor Bischoff has ascertained, by
analysis, that it agrees in composition with the mud of the Nile. Although for
the most part unstratified, it betrays in some places marks of stratification,
especially where it contains calcareous concretions, or in its lower part where
it rests on subjacent gravel and sand which alternate with each other near the
junction. About a sixth part of the whole mass is composed of carbonate of lime,
and there is usually an intermixture of fine quartzose and micaceous sand.
(FIGURE 88. Succinea elongata.)
Although this loam of the Rhine is unsolidified, it usually terminates where it
has been undermined by running water in a vertical cliff, from the face of which
shells of terrestrial, fresh-water and amphibious mollusks project in relief.
These shells do not imply the permanent sojourn of a body of fresh water on the
spot, for the most aquatic of them, the Succinea, inhabits marshes and wet
grassy meadows. The Succinea elongata (or S. oblongata), Figure 88, is very
characteristic both of the loess of the Rhine and of some other European river-
loams.
(FIGURE 89. Pupa muscorum (Linn.).)
(FIGURE 90. Helix hispida (Linn.) (plebeia).)
Among the land-shells of the Rhenish loess, Helix hispida, Figure 90, and Pupa
muscorum, Figure 89, are very common. Both the terrestrial and aquatic shells
are of most fragile and delicate structure, and yet they are almost invariably
perfect and uninjured. They must have been broken to pieces had they been swept
along by a violent inundation. Even the colour of some of the land-shells, as
that of Helix nemoralis, is occasionally preserved.
In parts of the valley of the Rhine, between Bingen and Basle, the fluviatile
loam or loess now under consideration is several hundred feet thick, and
contains here and there throughout that thickness land and amphibious shells. As
it is seen in masses fringing both sides of the great plain, and as occasionally
remnants of it occur in the centre of the valley, forming hills several hundred
feet in height, it seems necessary to suppose, first, a time when it slowly
accumulated; and secondly, a later period, when large portions of it were
removed, or when the original valley, which had been partially filled up with
it, was re-excavated.
Such changes may have been brought about by a great movement of oscillation,
consisting first of a general depression of the land, and then of a gradual re-
elevation of the same. The amount of continental depression which first took
place in the interior, must be imagined to have exceeded that of the region near
the sea, in which case the higher part of the great valley would have its
alluvial plain gradually raised by an accumulation of sediment, which would only
cease when the subsidence of the land was at an end. If the direction of the
movement was then reversed, and, during the re-elevation of the continent, the
inland region nearest the mountains should rise more rapidly than that near the
coast, the river would acquire a denuding power sufficient to enable it to sweep
away gradually nearly all the loam and gravel with which parts of its basin had
been filled up. Terraces and hillocks of mud and sand would then alone remain to
attest the various levels at which the river had thrown down and afterwards
removed alluvial matter.
CAVERN DEPOSITS CONTAINING HUMAN REMAINS AND BONES OF EXTINCT ANIMALS.
In England, and in almost all countries where limestone rocks abound, caverns
are found, usually consisting of cavities of large dimensions, connected
together by low, narrow, and sometimes torturous galleries or tunnels. These
subterranean vaults are usually filled in part with mud, pebbles, and breccia,
in which bones occur belonging to the same assemblage of animals as those
characterising the Post-pliocene alluvia above described. Some of these bones
are referable to extinct and others to living species, and they are occasionally
intermingled, as in the valley-gravels, with implements of one or other of the
great divisions of the stone age, and these are not unfrequently accompanied by
human bones, which are much more common in cavern deposits than in valley-
alluvium.
Each suite of caverns, and the passages by which they communicate the one with
the other, afford memorials to the geologist of successive phases through which
they must have passed. First, there was a period when the carbonate of lime was
carried out gradually by springs; secondly, an era when engulfed rivers or
occasional floods swept organic and inorganic debris into the subterranean
hollows previously formed; and thirdly, there were such changes in the
configuration of the region as caused the engulfed rivers to be turned into new
channels, and springs to be dried up, after which the cave-mud, breccia, gravel,
and fossil bones would bear the same kind of relation to the existing drainage
of the country as the older valley-drifts with their extinct mammalian remains
and works of art bear to the present rivers and alluvial plains.
The quarrying away of large masses of Carboniferous and Devonian limestone, near
Liege, in Belgium, has afforded the geologist magnificent sections of some of
these caverns, and the former communication of cavities in the interior of the
rocks with the old surface of the country by means of vertical or oblique
fissures, has been demonstrated in places where it would not otherwise have been
suspected, so completely have the upper extremities of these fissures been
concealed by superficial drift, while their lower ends, which extended into the
roofs of the caves, are masked by stalactitic incrustations.
The origin of the stalactite is thus explained by the eminent chemist Liebig.
Mould or humus, being acted on by moisture and air, evolves carbonic acid, which
is dissolved by rain. The rain-water, thus impregnated, permeates the porous
limestone, dissolves a portion of it, and afterwards, when the excess of
carbonic acid evaporates in the caverns, parts with the calcareous matter, and
forms stalactite. Even while caverns are still liable to be occasionally flooded
such calcareous incrustations accumulate, but it is generally when they are no
longer in the line of drainage that a solid floor of hard stalagmite is formed
on the bottom.
The late Dr. Schmerling examined forty caves near Liege, and found in all of
them the remains of the same fauna, comprising the mammoth, tichorhine
rhinoceros, cave-bear, cave-hyaena, cave-lion, and many others, some of extinct
and some of living species, and in all of them flint implements. In four or five
caves only parts of human skeletons were met with, comprising sometimes skulls
with a few other bones, sometimes nearly every part of the skeleton except the
skull. In one of the caves, that of Engihoul, where Schmerling had found the
remains of at least three human individuals, they were mingled in such a manner
with bones of extinct mammalia, as to leave no doubt on his mind (in 1833) of
man having co-existed with them.
In 1860, Professor Malaise, of Liege, explored with me this same cave of
Engihoul, and beneath a hard floor of stalagmite we found mud full of bones of
extinct and recent animals, such as Schmerling had described, and my companion,
persevering in his researches after I had returned to England, extracted from
the same deposit two human lower jaw-bones retaining their teeth. The skulls
from these Belgian caverns display no marked deviation from the normal European
type of the present day.
The careful investigations carried on by Dr. Falconer, Mr. Pengelly, and others,
in the Brixham cave near Torquay, in 1858, demonstrated that flint knives were
there imbedded in such a manner in loam underlying a floor of stalagmite as to
prove that man had been an inhabitant of that region when the cave-bear and
other members of the ancient post-pliocene fauna were also in existence.
The absence of gnawed bones had led Dr. Schmerling to infer that none of the
Belgian caves which he explored had served as the dens of wild beasts; but there
are many caves in Germany and England which have certainly been so inhabited,
especially by the extinct hyaena and bear.
A fine example of a hyaena's den was afforded by the cave of Kirkdale, so well
described by the late Dr. Buckland in his Reliquiae Diluvianae. In that cave,
above twenty-five miles north-north-east of York, the remains of about 300
hyaenas, belonging to individuals of every age, were detected. The species
(Hyaena spelaea) has been considered by palaeontologists as extinct; it was
larger than the fierce Hyaena crocuta of South Africa, which it closely
resembled, and of which it is regarded by Mr. Boyd Dawkins as a variety. Dr.
Buckland, after carefully examining the spot, proved that the hyaenas must have
lived there; a fact attested by the quantity of their dung, which, as in the
case of the living hyaena, is of nearly the same composition as bone, and almost
as durable. In the cave were found the remains of the ox, young elephant,
hippopotamus, rhinoceros, horse, bear, wolf, hare, water-rat, and several birds.
All the bones have the appearance of having been broken and gnawed by the teeth
of the hyaenas; and they occur confusedly mixed in loam or mud, or dispersed
through a crust of stalagmite which covers it. In these and many other cases it
is supposed that portions of herbivorous quadrupeds have been dragged into
caverns by beasts of prey, and have served as their food-- an opinion quite
consistent with the known habits of the living hyaena.
AUSTRALIAN CAVE-BRECCIAS.
Ossiferous breccias are not confined to Europe, but occur in all parts of the
globe; and those discovered in fissures and caverns in Australia correspond
closely in character with what has been called the bony breccia of the
Mediterranean, in which the fragments of bone and rock are firmly bound together
by a red ochreous cement.
Some of these caves were examined by the late Sir T. Mitchell in the Wellington
Valley, about 210 miles west of Sidney, on the river Bell, one of the principal
sources of the Macquarie, and on the Macquarie itself. The caverns often branch
off in different directions through the rock, widening and contracting their
dimensions, and the roofs and floors are covered with stalactite. The bones are
often broken, but do not seem to be water-worn. In some places they lie imbedded
in loose earth, but they are usually included in a breccia.
The remains belong to marsupial animals. Among the most abundant are those of
the kangaroo, of which there are four species, while others belong to the genera
Phascolomys, the wombat; Dasyurus, the ursine opossum; Phalangista, the vulpine
opossum; and Hypsiprymnus, the kangaroo-rat.
(FIGURE 91. Part of lower jaw of Macropus atlas. Owen. A young individual of an
extinct species.
a. Permanent false molar, in the alveolus.)
(FIGURE 92. Lower jaw of largest living species of kangaroo. (Macropus major.))
In the fossils above enumerated, several species are larger than the largest
living ones of the same genera now known in Australia. Figure 91 of the right
side of a lower jaw of a kangaroo (Macropus atlas, Owen) will at once be seen to
exceed in magnitude the corresponding part of the largest living kangaroo, which
is represented in Figure 92. In both these specimens part of the substance of
the jaw has been broken open, so as to show the permanent false molar (a, Figure
91), concealed in the socket. From the fact of this molar not having been cut,
we learn that the individual was young, and had not shed its first teeth.
The reader will observe that all these extinct quadrupeds of Australia belong to
the marsupial family, or, in other words, that they are referable to the same
peculiar type of organisation which now distinguishes the Australian mammalia
from those of other parts of the globe. This fact is one of many pointing to a
general law deducible from the fossil vertebrate and invertebrate animals of
times immediately antecedent to our own, namely, that the present geographical
distribution of organic FORMS dates back to a period anterior to the origin of
existing SPECIES; in other words, the limitation of particular genera or
families of quadrupeds, mollusca, etc., to certain existing provinces of land
and sea, began before the larger part of the species now contemporary with man
had been introduced into the earth.
Professor Owen, in his excellent "History of British Fossil Mammals," has called
attention to this law, remarking that the fossil quadrupeds of Europe and Asia
differ from those of Australia or South America. We do not find, for example, in
the Europaeo-Asiatic province fossil kangaroos, or armadillos, but the elephant,
rhinoceros, horse, bear, hyaena, beaver, hare, mole, and others, which still
characterise the same continent.
In like manner, in the Pampas of South America the skeletons of Megatherium,
Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct forms,
are analogous to the living sloth, armadillo, cavy, capybara, and llama. The
fossil quadrumana, also associated with some of these forms in the Brazilian
caves, belong to the Platyrrhine family of monkeys, now peculiar to South
America. That the extinct fauna of Buenos Ayres and Brazil was very modern has
been shown by its relation to deposits of marine shells, agreeing with those now
inhabiting the Atlantic.
The law of geographical relationship above alluded to, between the living
vertebrata of every great zoological province and the fossils of the period
immediately antecedent, even where the fossil species are extinct, is by no
means confined to the mammalia. New Zealand, when first examined by Europeans,
was found to contain no indigenous land quadrupeds, no kangaroos, or opossums,
like Australia; but a wingless bird abounded there, the smallest living
representative of the ostrich family, called the Kiwi by the natives (Apteryx).
In the fossils of the Post-pliocene period in this same island, there is the
like absence of kangaroos, opossums, wombats, and the rest; but in their place a
prodigious number of well-preserved specimens of gigantic birds of the
struthious order, called by Owen Dinornis and Palapteryx, which are entombed in
superficial deposits. These genera comprehended many species, some of which were
four, some seven, others nine, and others eleven feet in height! It seems
doubtful whether any contemporary mammalia shared the land with this population
of gigantic feathered bipeds.
Mr. Darwin, when describing the recent and fossil mammalia of South America, has
dwelt much on the wonderful relationship of the extinct to the living types in
that part of the world, inferring from such geographical phenomena that the
existing species are all related to the extinct ones which preceded them by a
bond of common descent.
CLIMATE OF THE POST-PLIOCENE PERIOD.
The evidence as to the climate of Europe during this epoch is somewhat
conflicting. The fluviatile and land-shells are all of existing species, but
their geographical range has not always been the same as at present. Some, for
example, which then lived in Britain are now only found in Norway and Finland,
probably implying that the Post-pliocene climate of Britain was colder,
especially in the winter. So also the reindeer and the musk-ox (Ovibos
moschatus), now inhabitants of the Arctic regions, occur fossil in the valleys
of the Thames and Avon, and also in France and Germany, accompanied in most
places by the mammoth and the woolly rhinoceros. At Grays in Essex, on the other
hand, another species both of elephant and rhinoceros occurs, together with a
hippopotamus and the Cyrena fluminalis, a shell now extinct in Europe but still
an inhabitant of the Nile and some Asiatic rivers. With it occurs the Unio
littoralis, now living in the Seine and Loire. In the valley of the Somme flint
tools have been found associated with Hippopotamus major and Cyrena fluminalis
in the lower-level Post-pliocene gravels; while in the higher-level (and more
ancient) gravels similar tools are more abundant, and are associated with the
bones of the mammoth and other Post-pliocene quadrupeds indicative of a colder
climate.
It is possible that we may here have evidence of summer and winter migrations
rather than of a general change of temperature. Instead of imagining that the
hippopotamus lived all the year round with the musk-ox and lemming, we may
rather suppose that the apparently conflicting evidence may be due to the place
of our observations being near the boundary line of a northern and southern
fauna, either of which may have advanced or receded during comparatively slight
and temporary fluctuations of climate. There may then have been a continuous
land communication between England and the North of Siberia, as well as in an
opposite direction with Africa, then united to Southern Europe.
In drift at Fisherton, near Salisbury, thirty feet above the river Wiley, the
Greenland lemming and a new species of the Arctic genus Spermophilus have been
found, along with the mammoth, reindeer, cave-hyaena, and other mammalia suited
to a cold climate. A flint implement was taken out from beneath the bones of the
mammoth. In a higher and older deposit in the vicinity, flint tools like those
of Amiens have been discovered. Nearly all the known Post-pliocene quadrupeds
have now been found accompanying flint knives or hatchets in such a way as to
imply their coexistence with man; and we have thus the concurrent testimony of
several classes of geological facts to the vast antiquity of the human race. In
the first place, the disappearance of a great variety of species of wild animals
from every part of a wide continent must have required a vast period for its
accomplishment; yet this took place while man existed upon the earth, and was
completed before that early period when the Danish shell-mounds were formed or
the oldest of the Swiss lake-dwellings constructed. Secondly, the deepening and
widening of valleys, indicated by the position of the river gravels at various
heights, implies an amount of change of which that which has occurred during the
historical period forms a scarcely perceptible part. Thirdly, the change in the
course of rivers which once flowed through caves now removed from any line of
drainage, and the formation of solid floors of stalagmite, must have required a
great lapse of time. Lastly, ages must have been required to change the climate
of wide regions to such an extent as completely to alter the geographical
distribution of many mammalia as well as land and fresh-water shells. The 3000
or 4000 years of the historical period does not furnish us with any appreciable
measure for calculating the number of centuries which would suffice for such a
series of changes, which are by no means of a local character, but have operated
over a considerable part of Europe.
RELATIVE LONGEVITY OF SPECIES IN THE MAMMALIA AND TESTACEA.
I called attention in 1830 to the fact, which had not at that time attracted
notice, that the association in the Post-pliocene deposits of shells,
exclusively of living species, with many extinct quadrupeds betokened a
longevity of species in the testacea far exceeding that in the mammalia.
(Principles of Geology 1st edition volume 3 page 140.) Subsequent researches
seem to show that this greater duration of the same specific forms in the class
mollusca is dependent on a still more general law, namely, that the lower the
grade of animals, or the greater the simplicity of their structure, the more
persistent are they in general in their specific characters throughout vast
periods of time. Not only have the invertebrata, as shown by geological data,
altered at a less rapid rate than the vertebrata, but if we take one of the
classes of the former, as for example the mollusca, we find those of more simple
structure to have varied at a slower rate than those of a higher and more
complex organisation; the Brachiopoda, for example, more slowly than the
lamellibranchiate bivalves, while the latter have been more persistent than the
univalves, whether gasteropoda or cephalopoda. In like manner the specific
identity of the characters of the foraminifera, which are among the lowest types
of the invertebrata, has outlasted that of the mollusca in an equally decided
manner.
TEETH OF POST-PLIOCENE MAMMALIA.
To those who have never studied comparative anatomy, it may seem scarcely
credible that a single bone taken from any part of the skeleton may enable a
skilful osteologist to distinguish, in many cases, the genus, and sometimes the
species, of quadrupeds to which it belonged. Although few geologists can aspire
to such knowledge, which must be the result of long practice and study, they
will nevertheless derive great advantage from learning, what is comparatively an
easy task, to distinguish the principal divisions of the mammalia by the forms
and characters of their teeth.
Figures 93 through 105 represent the teeth of some of the more common species
and genera found in alluvial and cavern deposits.
(FIGURE 93. Elephas primigenius (or Mammoth ); molar of upper jaw, right side;
one-third of natural size. Post-pliocene.
a. Grinding surface.
b. Side view.)
(FIGURE 94. Elephas antiquus, Falconer. Penultimate molar, one-third of natural
size. Post-pliocene and Pliocene.)
(FIGURE 95. Elephas meridionalis, Nesti. Penultimate molar, one-third of natural
size. Post-pliocene and Pliocene.)
(FIGURE 96. Rhinoceros leptorhinus, Cuvier-- Rhin. megarhinus, Christol; fossil
from fresh-water beds of Grays, Essex; penultimate molar, lower jaw, left side;
two-thirds of natural size. Post-pliocene and Newer Pliocene.)
(FIGURE 97. Rhinoceros tichorhinus; penultimate molar, lower jaw, left side;
two-thirds of natural size. Post-pliocene.)
(FIGURE 98. Hippopotamus; from cave near Palermo; molar tooth; two-thirds of
natural size. Post-pliocene.)
(FIGURE 99. Horse. Equus caballus, L. (common horse); from the shell-marl,
Forfarshire; second molar, lower jaw. Recent.
a. Grinding surface, two-thirds natural size.
b. Side view of same, half natural size.)
(FIGURE 100. Deer.
Moose (Cervus alces, L.); recent; molar of upper jaw.
a. Grinding surface.
b. Side view, two-thirds of natural size.)
(FIGURE 101. Ox.
Ox, common, from shell-marl, Forfarshire; true molar, upper jaw; two-thirds
natural size. Recent.
c. Grinding surface.
d. Side view, fangs uppermost.)
(FIGURE 102. Bear.
a. Canine tooth or tusk of bear (Ursus spelaeus); from cave near Liege.
b. Molar of left side, upper jaw; one-third of natural size. Post-pliocene.)
(FIGURE 103. Tiger.
c. Canine tooth of tiger (Felis tigris); recent.
d. Outside view of posterior molar, lower jaw: one-third of natural size.
Recent.)
(FIGURE 104. Hyaena spelaea, Goldf. (variety of H. crocuta); lower jaw.
Kent's Hole, Torquay, Devonshire; one-third natural size. Post-pliocene.)
(FIGURE 105. Teeth of a new species of Arvicola, field-mouse; from the Norwich
Crag. Newer Pliocene.
a. Grinding surface.
b. Side view of the same.
c. Natural size of a and b.)
On comparing the grinding surfaces of the corresponding molars of the three
species of elephants, Figures 93, 94, 95 it will be seen that the folds of
enamel are most numerous in the mammoth, fewer and wider, or more open, in E.
antiquus; and most open and fewest in E. meridionalis. It will be also seen that
the enamel in the molar of the Rhinoceros tichorhinus (Figure 97), is much
thicker than in that of the Rhinoceros leptorhinus (Figure 96).
CHAPTER XI.
POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS. (As to the former excess
of cold, whether brought about by modifications in the height and distribution
of the land or by altered astronomical conditions, see Principles volume 1 10th
edition 1867 chapters 12 and 13 "Vicissitudes of Climate.")
Geographical Distribution, Form, and Characters of Glacial Drift.
Fundamental Rocks, polished, grooved, and scratched.
Abrading and striating Action of Glaciers.
Moraines, Erratic Blocks, and "Roches Moutonnees."
Alpine Blocks on the Jura.
Continental Ice of Greenland.
Ancient Centres of the Dispersion of Erratics.
Transportation of Drift by floating Icebergs.
Bed of the Sea furrowed and polished by the running aground of floating Ice-
islands.
CHARACTER AND DISTRIBUTION OF GLACIAL DRIFT.
In speaking of the loose transported matter commonly found on the surface of the
land in all parts of the globe, I alluded to the exceptional character of what
has been called the boulder formation in the temperate and Arctic latitudes of
the northern hemisphere. The peculiarity of its form in Europe north of the
50th, and in North America north of the 40th parallel of latitude, is now
universally attributed to the action of ice, and the difference of opinion
respecting it is now chiefly restricted to the question whether land-ice or
floating icebergs have played the chief part in its distribution. It is wanting
in the warmer and equatorial regions, and reappears when we examine the lands
which lie south of the 40th and 50th parallels in the southern hemisphere, as,
for example, in Patagonia, Tierra del Fuego, and New Zealand. It consists of
sand and clay, sometimes stratified, but often wholly devoid of stratification
for a depth of 50, 100, or even a greater number of feet. To this unstratified
form of the deposit the name of TILL has long been applied in Scotland. It
generally contains a mixture of angular and rounded fragments of rock, some of
large size, having occasionally one or more of their sides flattened and
smoothed, or even highly polished. The smoothed surfaces usually exhibit many
scratches parallel to each other, one set of which often crosses an older set.
The till is almost everywhere wholly devoid of organic remains, except those
washed into it from older formations, though in some places it contains marine
shells, usually of northern or Arctic species, and frequently in a fragmentary
state. The bulk of the till has usually been derived from the grinding down into
mud of rocks in the immediate neighbourhood, so that it is red in a region of
Red Sandstone, as in Strathmore in Forfarshire; grey or black in a district of
coal and bituminous shale, as around Edinburgh; and white in a chalk country, as
in parts of Norfolk and Denmark. The stony fragments dispersed irregularly
through the till usually belong, especially in mountainous countries, to rocks
found in some part of the same hydrographical basin; but there are regions where
the whole of the boulder clay has come from a distance, and huge blocks, or
"erratics," as they have been called, many feet in diameter, have not
unfrequently travelled hundreds of miles from their point of departure, or from
the parent rocks from which they have evidently been detached. These are
commonly angular, and have often one or more of their sides polished and
furrowed.
The rock on which the boulder formation reposes, if it consists of granite,
gneiss, marble, or other hard stone, capable of permanently retaining any
superficial markings which may have been imprinted upon it, is usually smoothed
or polished, like the erratics above described, and exhibits parallel striae and
furrows having a determinate direction. This direction, both in Europe and North
America, agrees generally in a marked manner with the course taken by the
erratic blocks in the same district. The boulder clay, when it was first
studied, seemed in many of its characters so singular and anomalous, that
geologists despaired of ever being able to interpret the phenomena by reference
to causes now in action. In those exceptional cases where marine shells of the
same date as the boulder clay were found, nearly all of them were recognised as
living species-- a fact conspiring with the superficial position of the drift to
indicate a comparatively modern origin.
The term "diluvium" was for a time the most popular name of the boulder
formation, because it was referred by many to the deluge of Noah, while others
retained the name as expressive of their opinion that a series of diluvial waves
raised by hurricanes and storms, or by earthquakes, or by the sudden upheaval of
land from the bed of the sea, had swept over the continents, carrying with them
vast masses of mud and heavy stones, and forcing these stones over rocky
surfaces so as to polish and imprint upon them long furrows and striae. But
geologists were not long in seeing that the boulder formation was characteristic
of high latitudes, and that on the whole the size and number of erratic blocks
increases as we travel towards the Arctic regions. They could not fail to be
struck with the contrast which the countries bordering the Baltic presented when
compared with those surrounding the Mediterranean. The multitude of travelled
blocks and striated rocks in the one region, and the absence of such appearances
in the other, were too obvious to be overlooked. Even the great development of
the boulder formation, with large erratics so far south as the Alps, offered an
exception to the general rule favourable to the hypothesis that there was some
intimate connection between it and accumulations of snow and ice.
TRANSPORTING AND ABRADING POWER OF GLACIERS.
(FIGURE 106. Limestone, polished, furrowed, and scratched by the glacier of
Rosenlau in Switzerland. (Agassiz.)
a a. White streaks or scratches, caused by small grains of flint frozen into the
ice.
b b. Furrows.)
I have described elsewhere ("Principles" volume 1 chapter 16 1867) the manner in
which the snow of the Alpine heights is prevented from accumulating indefinitely
in thickness by the constant descent of a large portion of it by gravitation.
Becoming converted into ice it forms what are termed glaciers, which glide down
the principal valleys. On their surface are seen mounds of rubbish or large
heaps of sand and mud, with angular fragments of rock which fall from the steep
slopes or precipices bounding the glaciers. When a glacier, thus laden, descends
so far as to reach a region about 3500 feet above the level of the sea, the
warmth of the air is such that it melts rapidly in summer, and all the mud,
sand, and pieces of rock are slowly deposited at its lower end, forming a
confused heap of unstratified rubbish called a MORAINE, and resembling the TILL
before described.
Besides the blocks thus carried down on the top of the glacier, many fall
through fissures in the ice to the bottom, where some of them become firmly
frozen into the mass, and are pushed along the base of the glacier, abrading,
polishing, and grooving the rocky floor below, as a diamond cuts glass, or as
emery-powder polishes steel. The striae which are made, and the deep grooves
which are scooped out by this action, are rectilinear and parallel to an extent
never seen in those produced on loose stones or rocks, where shingle is hurried
along by a torrent, or by the waves on a sea-beach. In addition to these
polished, striated, and grooved surfaces of rock, another mark of the former
action of a glacier is the "roche moutonnee." Projecting eminences of rock so
called have been smoothed and worn into the shape of flattened domes by the
glacier as it passed over them. They have been traced in the Alps to great
heights above the present glaciers, and to great horizontal distances beyond
them.
ALPINE BLOCKS ON THE JURA.
The moraines, erratics, polished surfaces, domes, and striae, above described,
are observed in the great valley of Switzerland, fifty miles broad; and almost
everywhere on the Jura, a chain which lies to the north of this valley. The
average height of the Jura is about one-third that of the Alps, and it is now
entirely destitute of glaciers; yet it presents almost everywhere similar
moraines, and the same polished and grooved surfaces. The erratics, moreover,
which cover it, present a phenomenon which has astonished and perplexed the
geologist for more than half a century. No conclusion can be more incontestable
than that these angular blocks of granite, gneiss, and other crystalline
formations came from the Alps, and that they have been brought for a distance of
fifty miles and upward across one of the widest and deepest valleys in the
world; so that they are now lodged on a chain composed of limestone and other
formations, altogether distinct from those of the Alps. Their great size and
angularity, after a journey of so many leagues, has justly excited wonder; for
hundreds of them are as large as cottages; and one in particular, composed of
gneiss, celebrated under the name of Pierre a Bot, rests on the side of a hill
about 900 feet above the lake of Neufchatel, and is no less than 40 feet in
diameter.
In the year 1821, M. Venetz first announced his opinion that the Alpine glaciers
must formerly have extended far beyond their present limits, and the proofs
appealed to by him in confirmation of this doctrine were acknowledged by all
subsequent observers, and greatly strengthened by new observations and
arguments. M. Charpentier supposed that when the glaciers extended continuously
from the Alps to the Jura, the former mountains were 2000 or 3000 feet higher
than at present. Other writers, on the contrary, conjectured that the whole
country had been submerged, and the moraines and erratic blocks transported on
floating icebergs; but a careful study of the distribution of the travelled
masses, and the total absence of marine shells from the old glacial drift of
Switzerland, have entirely disproved this last hypothesis. In addition to the
many evidences of the action of ice in the northern parts of Europe which we
have already mentioned, there occur here and there in some of these countries,
what are wanting in Switzerland, deposits of marine fossil shells, which exhibit
so arctic a character that they must have led the geologist to infer the former
prevalence of a much colder climate, even had he not encountered so many
accompanying signs of ice-action. The same marine shells demonstrate the
submergence of large areas in Scandinavia and the British Isles, during the
glacial cold.
A characteristic feature of the deposits under consideration in all these
countries is the occurrence of large erratic blocks, and sometimes of moraine
matter, in situations remote from lofty mountains, and separated from the
nearest points where the parent rocks appear at the surface by great intervening
valleys, or arms of the sea. We also often observe striae and furrows, as in
Norway, Sweden, and Scotland, which deviate from the direction which they ought
to follow if they had been connected with the present line of drainage, and
they, therefore, imply the prevalence of a very distinct condition of things at
the time when the cold was most intense. The actual state of North Greenland
seems to afford the best explanation of such abnormal glacial markings.
GREENLAND CONTINENTAL ICE.
Greenland is a vast unexplored continent, buried under one continuous and
colossal mass of ice that is always moving seaward, a very small part of it in
an easterly direction, and all the rest westward, or towards Baffin's Bay. All
the minor ridges and valleys are levelled and concealed under a general covering
of snow, but here and there some steep mountains protrude abruptly from the icy
slope, and a few superficial lines of stones or moraines are visible at certain
seasons, when no snow has fallen for many months, and when evaporation, promoted
by the wind and sun, has caused much of the upper snow to disappear. The height
of this continent is unknown, but it must be very great, as the most elevated
lands of the outskirts, which are described as comparatively low, attain
altitudes of 4000 to 6000 feet. The icy slope gradually lowers itself towards
the outskirts, and then terminates abruptly in a mass about 2000 feet in
thickness, the great discharge of ice taking place through certain large friths,
which, at their upper ends, are usually about four miles across. Down these
friths the ice is protruded in huge masses, several miles wide, which continue
their course-- grating along the rocky bottom like ordinary glaciers long after
they have reached the salt water. When at last they arrive at parts of Baffin's
Bay deep enough to buoy up icebergs from 1000 to 1500 feet in vertical
thickness, broken masses of them float off, carrying with them on their surface
not only fine mud and sand but large stones. These fragments of rock are often
polished and scored on one or more sides, and as the ice melts, they drop down
to the bottom of the sea, where large quantities of mud are deposited, and this
muddy bottom is inhabited by many mollusca.
Although the direction of the ice-streams in Greenland may coincide in the main
with that which separate glaciers would take if there were no more ice than
there is now in the Swiss Alps, yet the striation of the surface of the rocks on
an ice-clad continent would, on the whole, vary considerably in its minor
details from that which would be imprinted on rocks constituting a region of
separate glaciers. For where there is a universal covering of ice there will be
a general outward movement from the higher and more central regions towards the
circumference and lower country, and this movement will be, to a certain extent,
independent of the minor inequalities of hill and valley, when these are all
reduced to one level by the snow. The moving ice may sometimes cross even at
right angles deep narrow ravines, or the crests of buried ridges, on which last
it may afterwards seem strange to detect glacial striae and polishing after the
liquefaction of the snow and ice has taken place.
Rink mentions that in North Greenland powerful springs of clayey water escape in
winter from under the ice, where it descends to "the outskirts," and where, as
already stated, it is often 2000 feet thick-- a fact showing how much grinding
action is going on upon the surface of the subjacent rocks. I also learn from
Dr. Torell that there are large areas in the outskirts, now no longer covered
with permanent snow or glaciers, which exhibit on their surface unmistakable
signs of ancient ice-action, so that, vast as is the power now exerted by ice in
Greenland, it must once have operated on a still grander scale. The land, though
now very elevated, may perhaps have been formerly much higher. It is well-known
that the south coast of Greenland, from latitude 60 degrees to about 70 degrees
north, has for the last four centuries been sinking at the rate of several feet
in a century. By this means a surface of rock, well scored and polished by ice,
is now slowly subsiding beneath the sea, and is becoming strewed over, as the
icebergs melt, with impalpable mud and smoothed and scratched stones. It is not
precisely known how far north this downward movement extends.
DRIFT CARRIED BY ICEBERGS.
An account was given so long ago as the year 1822, by Scoresby, of icebergs seen
by him in the Arctic seas drifting along in latitudes 69 and 70 degrees north,
which rose above the surface from 100 to 200 feet, and some of which measured a
mile in circumference. Many of them were loaded with beds of earth and rock, of
such thickness that the weight was conjectured to be from 50,000 to 100,000
tons. A similar transportation of rocks is known to be in progress in the
southern hemisphere, where boulders included in ice are far more frequent than
in the north. One of these icebergs was encountered in 1839, in mid-ocean, in
the antarctic regions, many hundred miles from any known land, sailing
northward, with a large erratic block firmly frozen into it. Many of them,
carefully measured by the officers of the French exploring expedition of the
Astrolabe, were between 100 and 225 feet high above water, and from two to five
miles in length. Captain d'Urville ascertained one of them which he saw floating
in the Southern Ocean to be 13 miles long and 100 feet high, with walls
perfectly vertical. The submerged portions of such islands must, according to
the weight of ice relatively to sea-water, be from six to eight times more
considerable than the part which is visible, so that when they are once fairly
set in motion, the mechanical force which they might exert against any obstacle
standing in their way would be prodigious.
We learn, therefore, from a study both of the arctic and antarctic regions, that
a great extent of land may be entirely covered throughout the whole year by snow
and ice, from the summits of the loftiest mountains to the sea-coast, and may
yet send down angular erratics to the ocean. We may also conclude that such land
will become in the course of ages almost everywhere scored and polished like the
rocks which underlie a glacier. The discharge of ice into the surrounding sea
will take place principally through the main valleys, although these are hidden
from our sight. Erratic blocks and moraine matter will be dispersed somewhat
irregularly after reaching the sea, for not only will prevailing winds and
marine currents govern the distribution of the drift, but the shape of the
submerged area will have its influence; inasmuch as floating ice, laden with
stones, will pass freely through deep water, while it will run a ground where
there are reefs and shallows. Some icebergs in Baffin's Bay have been seen
stranded on a bottom 1000 or even 1500 feet deep. In the course of ages such a
sea-bed may become densely covered with transported matter, from which some of
the adjoining greater depths may be free. If, as in West Greenland, the land is
slowly sinking, a large extent of the bottom of the ocean will consist of rock
polished and striated by land-ice, and then overspread by mud and boulders
detached from melting bergs.
The mud, sand, and boulders thus let fall in still water must be exactly like
the moraines of terrestrial glaciers, devoid of stratification and organic
remains. But occasionally, on the outer side of such packs of stranded bergs,
the waves and currents may cause the detached earthy and stony materials to be
sorted according to size and weight before they reach the bottom, and to acquire
a stratified arrangement.
I have already alluded to the large quantity of ice, containing great blocks of
stone, which is sometimes seen floating far from land, in the southern or
Antarctic seas. After the emergence, therefore, of such a submarine area, the
superficial detritus will have no necessary relation to the hills, valleys, and
river-plains over which it will be scattered. Many a water-shed may intervene
between the starting-point of each erratic or pebble and its final resting-
place, and the only means of discovering the country from which it took its
departure will consist in a careful comparison of its mineral or fossil contents
with those of the parent rocks.
CHAPTER XII.
POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS, CONCLUDED.
Glaciation of Scandinavia and Russia.
Glaciation of Scotland.
Mammoth in Scotch Till.
Marine Shells in Scotch Glacial Drift.
Their Arctic Character.
Rarity of Organic Remains in Glacial Deposits.
Contorted Strata in Drift.
Glaciation of Wales, England, and Ireland.
Marine Shells of Moel Tryfaen.
Erratics near Chichester.
Glacial Formations of North America.
Many Species of Testacea and Quadrupeds survived the Glacial Cold.
Connection of the Predominance of Lakes with Glacial Action.
Action of Ice in preventing the silting up of Lake-basins.
Absence of Lakes in the Caucasus.
Equatorial Lakes of Africa.
GLACIATION OF SCANDINAVIA AND RUSSIA.
In large tracts of Norway and Sweden, where there have been no glaciers in
historical times, the signs of ice-action have been traced as high as 6000 feet
above the level of the sea. These signs consist chiefly of polished and furrowed
rock-surfaces, of moraines and erratic blocks. The direction of the erratics,
like that of the furrows, has usually been conformable to the course of the
principal valleys; but the lines of both sometimes radiate outward in all
directions from the highest land, in a manner which is only explicable by the
hypothesis above alluded to of a general envelope of continental ice, like that
of Greenland (Chapter 11.) Some of the far-transported blocks have been carried
from the central parts of Scandinavia towards the Polar regions; others
southward to Denmark; some south-westward, to the coast of Norfolk in England;
others south-eastward, to Germany, Poland, and Russia.
In the immediate neighbourhood of Upsala, in Sweden, I had observed, in 1834, a
ridge of stratified sand and gravel, in the midst of which occurs a layer of
marl, evidently formed originally at the bottom of the Baltic, by the slow
growth of the mussel, cockle, and other marine shells of living species,
intermixed with some proper to fresh water. The marine shells are all of
dwarfish size, like those now inhabiting the brackish waters of the Baltic; and
the marl, in which many of them are imbedded, is now raised more than 100 feet
above the level of the Gulf of Bothnia. Upon the top of this ridge repose
several huge erratics, consisting of gneiss for the most part unrounded, from
nine to sixteen feet in diameter, and which must have been brought into their
present position since the time when the neighbouring gulf was already
characterised by its peculiar fauna. Here, therefore, we have proof that the
transport of erratics continued to take place, not merely when the sea was
inhabited by the existing testacea, but when the north of Europe had already
assumed that remarkable feature of its physical geography which separates the
Baltic from the North Sea, and causes the Gulf of Bothnia to have only one-
fourth of the saltness belonging to the ocean. In Denmark, also, recent shells
have been found in stratified beds, closely associated with the boulder clay.
GLACIATION OF SCOTLAND.
Mr. T.F. Jamieson, in 1858, adduced a great body of facts to prove that the
Grampians once sent down glaciers from the central regions in all directions
towards the sea. "The glacial grooves," he observed, "radiate outward from the
central heights towards all points of the compass, though they do not always
strictly conform to the actual shape and contour of the minor valleys and
ridges."
These facts and other characteristics of the Scotch drift lead us to the
inference that when the glacial cold first set in, Scotland stood higher above
the sea than at present, and was covered for the most part with snow and ice, as
Greenland is now. This sheet of land-ice sliding down to lower levels, ground
down and polished the subjacent rocks, sweeping off nearly all superficial
deposits of older date, and leaving only till and boulders in their place. To
this continental state succeeded a period of depression and partial submergence.
The sea advanced over the lower lands, and Scotland was converted into an
archipelago, some marine sand with shells being spread over the bottom of the
sea. On this sand a great mass of boulder clay usually quite devoid of fossils
was accumulated. Lastly, the land re-emerged from the water, and, reaching a
level somewhat above its present height, became connected with the continent of
Europe, glaciers being formed once more in the higher regions, though the ice
probably never regained its former extension. (Jamieson Quarterly Geological
Journal 1860 volume 16 page 370.) After all these changes, there were some minor
oscillations in the level of the land, on which, although they have had
important geographical consequences, separating Ireland from England, for
example, and England from the Continent, we need not here enlarge.
MAMMOTH IN SCOTCH TILL.
Almost all remains of the terrestrial fauna of the Continent which preceded the
period of submergence have been lost; but a few patches of estuarine and fresh-
water formations escaped denudation by submergence. To these belong the peaty
clay from which several mammoths' tusks and horns of reindeer were obtained at
Kilmaurs, in Ayrshire as long ago as 1816. Mr. Bryce in 1865 ascertained that
the fresh-water formation containing these fossils rests on carboniferous
sandstone, and is covered, first by a bed of marine sand with arctic shells, and
then with a great mass of till with glaciated boulders. (Bryce Quarterly
Geological Journal volume 21 page 217 1865.) Still more recent explorations in
the neighbourhood of Kilmaurs have shown that the fresh-water formation contains
the seed of the pond-weed Potamogeton and the aquatic Ranunculus; and Mr. Young
of the Glasgow Museum washed the mud adhering to the reindeer horns of Kilmaurs
and that which filled the cracks of the associated elephants' tusks, and
detected in these fossils (which had been in the Glasgow Museum for half a
century) abundance of the same seeds.
All doubts, therefore, as to the true position of the remains of the mammoth, a
fossil so rare in Scotland, have been set at rest, and it serves to prove that
part of the ancient continent sank beneath the sea at a period of great cold, as
the shells of the overlying sand attest. The incumbent till or boulder clay is
about 40 feet thick, but it often attains much greater thickness in the same
part of Scotland.
MARINE SHELLS OF SCOTCH DRIFT.
(FIGURE 107. Astarte borealis, Chem.; (A. arctica, Moll. A. compressa, Mont.)
(FIGURE 108. Leda lanceolata (oblonga), Sowerby.)
(FIGURE 109. Saxicava rugosa, Penn.)
(FIGURE 110. Pecten islandicus, Moll. Northern shell common in the drift of the
Clyde, in Scotland. )
(FIGURE 111. Natica clausa, Bred. Northern shell common in the drift of the
Clyde, in Scotland.)
(FIGURE 112. Trophon clathratum, Linne. Northern shell common in the drift of
the Clyde, in Scotland.)
(FIGURE 113. Leda truncata.
a. Exterior of left valve.
b. Interior of same.)
(FIGURE 114. Tellina calcarea, Chem. (Tellina proxima, Brown.)
a. Outside of left valve.
b. Interior of same.)
The greatest height to which marine shells have yet been traced in this boulder
clay is at Airdie, in Lanarkshire, ten miles east of Glasgow, 524 feet above the
level of the sea. At that spot they were found imbedded in stratified clays with
till above and below them. There appears no doubt that the overlying deposit was
true glacial till, as some boulders of granite were observed in it, which must
have come from distances of sixty miles at the least.
The shells figured in Figures 107 to 112 are only a few out of a large
assemblage of living species, which, taken as a whole, bear testimony to
conditions far more arctic than those now prevailing in the Scottish seas. But a
group of marine shells, indicating a still greater excess of cold, has been
brought to light since 1860 by the Reverend Thomas Brown, from glacial drift or
clay on the borders of the estuaries of the Forth and Tay. This clay occurs at
Elie, in Fife, and at Errol, in Perthshire; and has already afforded about 35
shells, all of living species, and now inhabitants of arctic regions, such as
Leda truncata, Tellina proxima (see Figures 113 and 114), Pecten Groenlandicus,
Crenella laevigata, Crenella nigra, and others, some of them first brought by
Captain Sir E. Parry from the coast of Melville Island, latitude 76 degrees
north. These were all identified in 1863 by Dr. Torell, who had just returned
from a survey of the seas around Spitzbergen, where he had collected no less
than 150 species of mollusca, living chiefly on a bottom of fine mud derived
from the moraines of melting glaciers which there protrude into the sea. He
informed me that the fossil fauna of this Scotch glacial deposit exhibits not
only the species but also the peculiar varieties of mollusca now characteristic
of very high latitudes. Their large size implies that they formerly enjoyed a
colder, or, what was to them a more genial climate, than that now prevailing in
the latitude where the fossils occur. Marine shells have also been found in the
glacial drift of Caithness and Aberdeenshire at heights of 250 feet, and in
Banff of 350 feet, and stratified drift continuous with the above ascends to
heights of 500 feet. Already 75 species are enumerated from Caithness, and the
same number from Aberdeenshire and Banff, and in both cases all but six are
arctic species.
I formerly suggested that the absence of all signs of organic life in the Scotch
drift might be connected with the severity of the cold, and also in some places
with the depth of the sea during the period of extreme submergence; but my faith
in such an hypothesis has been shaken by modern investigations, an exuberance of
life having been observed both in arctic and antarctic seas of great depth, and
where floating ice abounds. The difficulty, moreover, of accounting for the
entire dearth of marine shells in till is removed when once we have adopted the
theory of this boulder clay being the product of land-ice. For glaciers coming
down from a continental ice-sheet like that which covers Greenland may fill
friths many hundred feet below the sea-level, and even invade parts of a bay a
thousand feet deep, before they find water enough to float off their terminal
portions in the form of icebergs. In such a case till without marine shells may
first accumulate, and then, if the climate becomes warmer and the ice melts, a
marine deposit may be superimposed on the till without any change of level being
required.
Another curious phenomenon bearing on this subject was styled by the late Hugh
Miller the "striated pavements" of the boulder clay. Where portions of the till
have been removed by the sea on the shores of the Forth, or in the interior by
railway cuttings, the boulders imbedded in what remains of the drift are seen to
have been all subjected to a process of abrasion and striation, the striae and
furrows being parallel and persistent across them all, exactly as if a glacier
or iceberg had passed over them and scored them in a manner similar to that so
often undergone by the solid rocks below the glacial drift. It is possible, as
Mr. Geikie conjectures, that this second striation of the boulders may be
referable to floating ice. (Geikie Transactions of the Geological Society of
Glasgow volume 1 part 2 page 68 1863.)
CONTORTED STRATA IN DRIFT.
(FIGURE 115. Section of contorted drift overlying till, seen on left bank of
South Esk, near Cortachie, in 1840. Height of section seen, from a to d, about
50 feet.
a, b. Gravel and sand.
f, g. Contorted drift.
Till.)
In Scotland the till is often covered with stratified gravel, sand, and clay,
the beds of which are sometimes horizontal and sometimes contorted for a
thickness of several feet. Such contortions are not uncommon in Forfarshire,
where I observed them, among other places, in a vertical cutting made in 1840
near the left bank of the South Esk, east of the bridge of Cortachie. The
convolutions of the beds of fine and coarse sand, gravel, and loam, extend
through a thickness of no less than 25 feet vertical, or from b to c, Figure
115, the horizontal stratification being resumed very abruptly at a short
distance, as to the right of f, g. The overlying coarse gravel and sand, a, is
in some places horizontal, in others it exhibits cross bedding, and does not
partake of the disturbances which the strata b, c, have undergone. The
underlying till is exposed for a depth of about 20 feet; and we may infer from
sections in the neighbourhood that it is considerably thicker.
In some cases I have seen fragments of stratified clays and sands, bent in like
manner, in the middle of a great mass of till. Mr. Trimmer has suggested, in
explanation of such phenomena, the intercalation in the glacial period of large
irregular masses of snow or ice between layers of sand and gravel. Some of the
cliffs near Behring's Straits, in which the remains of elephants occur, consist
of ice mixed with mud and stones; and Middendorf describes the occurrence in
Siberia of masses of ice, found at various depths from the surface after digging
through drift. Whenever the intercalation of snow and ice with drift, whether
stratified or unstratified, has taken place, the melting of the ice will cause
such a failure of support as may give rise to flexures, and sometimes to the
most complicated foldings. But in many cases the strata may have been bent and
deranged by the mechanical pressure of an advancing glacier, or by the sideway
thrust of huge islands of ice running aground against sandbanks; in which case,
the position of the beds forming the foundation of the banks may not be at all
disturbed by the shock.
There are indeed many signs in Scotland of the action of floating ice, as might
have been expected where proofs of submergence in the Glacial Period are not
wanting. Among these are the occurrence of large erratic blocks, frequently in
clusters at or near the tops of hills or ridges, places which may have formed
islets or shallows in the sea where floating ice would mostly ground and
discharge its cargo on melting. Glaciers or land-ice would, on the contrary,
chiefly discharge their cargoes at the bottom of valleys. Traces of an earlier
and independent glaciation have also been observed in some regions where the
striation, apparently produced by ice proceeding from the north-west, is not
explicable by the radiation of land-ice from a central mountainous region.
(Milne Home Transactions of the Royal Society Edinburgh volume 25 1868-9.)
GLACIATION OF WALES AND ENGLAND.
The mountains of North Wales were recognised, in 1842, by Dr. Buckland, as
having been an independent centre of the dispersion of erratics-- great
glaciers, long since extinct, having radiated from the Snowdonian heights in
Carnarvonshire, through seven principal valleys towards as many points of the
compass, carrying with them large stony fragments, and grooving the subjacent
rocks in as many directions.
Besides this evidence of land-glaciers, Mr. Trimmer had previously, in 1831,
detected the signs of a great submergence in Wales in the Post-pliocene period.
He had observed stratified drift, from which he obtained about a dozen species
of marine shells, near the summit of Moel Tryfaen, a hill 1400 feet high, on the
south side of the Menai Straits. I had an opportunity of examining in the summer
of 1863, together with the Reverend W.S. Symonds, a long and deep cutting made
through this drift by the Alexandra Mining Company in search of slates. At the
top of the hill above-mentioned we saw a stratified mass of incoherent sand and
gravel 35 feet thick, from which no less than 54 species of mollusca, besides
three characteristic arctic varieties-- in all 57 forms-- have been obtained by
Mr. Darbishire. They belong without exception to species still living in British
or more northern seas; eleven of them being exclusively arctic, four common to
the arctic and British seas, and a large proportion of the remainder having a
northward range, or, if found at all in the southern seas of Britain, being
comparatively less abundant. In the lowest beds of the drift were large heavy
boulders of far-transported rocks, glacially polished and scratched on more than
one side. Underneath the whole we saw the edges of vertical slates exposed to
view, which here, like the rocks in other parts of Wales, both at greater and
less elevations, exhibit beneath the drift unequivocal marks of prolonged
glaciation. The whole deposit has much the appearance of an accumulation in
shallow water or on a beach, and it probably acquired its thickness during the
gradual subsidence of the coast-- an hypothesis which would require us to
ascribe to it a high antiquity, since we must allow time, first for its sinking,
and then for its re-elevation.
The height reached by these fossil shells on Moel Tryfaen is no less than 1300
feet-- a most important fact when we consider how very few instances we have on
record beyond the limits of Wales, whether in Europe or North America, of marine
shells having been found in glacial drift at half the height above indicated. A
marine molluscous fauna, however, agreeing in character with that of Moel
Tryfaen, and comprising as many species, has been found in drift at Macclesfield
and other places in central England, sometimes reaching an elevation of 1200
feet.
Professor Ramsay estimated the probable amount of submergence during some part
of the glacial period at about 2300 feet; for he was unable to distinguish the
superficial sands and gravel which reached that high elevation from the drift
which, at Moel Tryfaen and at lower points, contains shells of living species.
The evidence of the marine origin of the highest drift is no doubt inconclusive
in the absence of shells, so great is the resemblance of the gravel and sand of
a sea beach and of a river's bed, when organic remains are wanting; but, on the
other hand, when we consider the general rarity of shells in drift which we know
to be of marine origin, we can not suppose that, in the shelly sands of Moel
Tryfaen, we have hit upon the exact uppermost limit of marine deposition, or, in
other words, a precise measure of the submergence of the land beneath the sea
since the glacial period.
We are gradually obtaining proofs of the larger part of England, north of a line
drawn from the mouth of the Thames to the Bristol Channel, having been under the
sea and traversed by floating ice since the commencement of the glacial epoch.
Among recent observations illustrative of this point, I may allude to the
discovery, by Mr. J.F. Bateman, near Blackpool, in Lancashire, fifty miles from
the sea, and at the height of 568 feet above its level, of till containing
rounded and angular stones and marine shells, such as Turritella communis,
Purpura lapillus, Cardium edule, and others, among which Trophon clathratum
(=Fusus Bamffius), though still surviving in North British seas, indicates a
cold climate.
ERRATICS NEAR CHICHESTER.
The most southern memorials of ice-action and of a Post-pliocene fauna in Great
Britain is on the coast of the county of Sussex, about 25 miles west of
Brighton, and 15 south of Chichester. A marine deposit exposed between high and
low tide occurs on both sides of the promontory called Selsea Bill, in which Mr.
Godwin-Austen found thirty-eight species of shells, and the number has since
been raised to seventy.
This assemblage is interesting because on the whole, while all the species are
recent, they have a somewhat more southern aspect than those of the present
British Channel. It is true that about forty of them range from British to high
northern latitudes; but several of them, as, for example, Lutraria rugosa and
Pecten polymorphous, which are abundant, are not known at present to range
farther north than the coast of Portugal, and seem to indicate a warmer
temperature than now prevails on the coast where we find them fossil. What
renders this curious is the fact that the sandy loam in which they occur is
overlaid by yellow clayey gravel with large erratic blocks which must have been
drifted into their present position by ice when the climate had become much
colder. These transported fragments of granite, syenite, and greenstone, as well
as of Devonian and Silurian rocks, may have come from the coast of Normandy and
Brittany, and are many of them of such large size that we must suppose them to
have been drifted into their present site by coast-ice. I measured one of
granite, at Pagham, 21 feet in circumference. In the gravel of this drift with
erratics are a few littoral shells of living species, indicating an ancient
coast-line.
GLACIAL FORMATIONS OF NORTH AMERICA.
In the western hemisphere, both in Canada and as far south as the 40th and even
38th parallel of latitude in the United States, we meet with a repetition of all
the peculiarities which distinguish the European boulder formation. Fragments of
rock have travelled for great distances, especially from north to south: the
surface of the subjacent rock is smoothed, striated, and fluted; unstratified
mud or TILL containing boulders is associated with strata of loam, sand, and
clay, usually devoid of fossils. Where shells are present, they are of species
still living in northern seas, and not a few of them identical with those
belonging to European drift, including most of those already given in Figures
107 to 112. The fauna also of the glacial epoch in North America is less rich in
species than that now inhabiting the adjacent sea, whether in the Gulf of St.
Lawrence, or off the shores of Maine, or in the Bay of Massachusetts.
The extension on the American continent of the range of erratics during the
Post-pliocene period to lower latitudes than they reached in Europe, agrees well
with the present southward deflection of the isothermal lines, or rather the
lines of equal winter temperature. It seems that formerly, as now, a more
extreme climate and a more abundant supply of ice prevailed on the western side
of the Atlantic. Another resemblance between the distribution of the drift
fossils in Europe and North America has yet to be pointed out. In Canada and the
United States, as in Europe, the marine shells are generally confined to very
moderate elevations above the sea (between 100 and 700 feet), while the erratic
blocks and the grooved and polished surfaces of rock extend to elevations of
several thousand feet.
I have already mentioned that in Europe several quadrupeds of living, as well as
extinct, species were common to pre-glacial and post-glacial times. In like
manner there is reason to suppose that in North America much of the ancient
mammalian fauna, together with nearly all the invertebrata, lived through the
ages of intense cold. That in the United States the Mastodon giganteus was very
abundant after the drift period, is evident from the fact that entire skeletons
of this animal are met with in bogs and lacustrine deposits occupying hollows in
the glacial drift. They sometimes occur in the bottom even of small ponds
recently drained by the agriculturist for the sake of the shell-marl. In 1845 no
less than six skeletons of the same species of Mastodon were found in Warren
county, New Jersey, six feet below the surface, by a farmer who was digging out
the rich mud from a small pond which he had drained. Five of these skeletons
were lying together, and a large part of the bones crumbled to pieces as soon as
they were exposed to the air.
It would be rash, however, to infer from such data that these quadrupeds were
mired in MODERN times, unless we use that term strictly in a geological sense. I
have shown that there is a fluviatile deposit in the valley of the Niagara,
containing shells of the genera Melania, Lymnea, Planorbis, Velvata, Cyclaz,
Unio, Helix, etc., all of recent species, from which the bones of the great
Mastodon have been taken in a very perfect state. Yet the whole excavation of
the ravine, for many miles below the Falls, has been slowly effected since that
fluviatile deposit was thrown down. Other extinct animals accompany the Mastodon
giganteus in the post-glacial deposits of the United States, and this, taken
with the fact that so few of the mollusca, even of the commencement of the cold
period, differ from species now living is important, as refuting the hypothesis,
for which some have contended, that the intensity of the glacial cold
annihilated all the species in temperate and arctic latitudes.
CONNECTION OF THE PREDOMINANCE OF LAKES WITH GLACIAL ACTION.
It was first pointed out by Professor Ramsay in 1862, that lakes are exceedingly
numerous in those countries where erratics, striated blocks, and other signs of
ice-action abound; and that they are comparatively rare in tropical and sub-
tropical regions. Generally in countries where the winter cold is intense, such
as Canada, Scandinavia, and Finland, even the plains and lowlands are thickly
strewn with innumerable ponds and small lakes, together with some others of a
larger size; while in more temperate regions, such as Great Britain, Central and
Southern Europe, the United States, and New Zealand, lake districts occur in all
such mountainous tracts as can be proved to have been glaciated in times
comparatively modern or since the geographical configuration of the surface bore
a considerable resemblance to that now prevailing. In the same countries, beyond
the glaciated regions, lakes abruptly cease, and in warmer and tropical
countries are either entirely absent, or consist, as in equatorial Africa, of
large sheets of water unaccompanied so far as we yet know by numerous smaller
ponds and tarns.
The southern limits of the lake districts of the Northern Hemisphere are found
at about 40 degrees N. latitude on the American continent, and about 50 degrees
in Europe, or where the Alps intervene four degrees farther south. A large
proportion of the smaller lakes are dammed up by barriers of unstratified drift,
having the exact character of the moraines of glaciers, and are termed by
geologists "morainic," but some of them are true rock-basins, and would hold
water even if all the loose drift now resting on their margins were removed.
In a paper read before the Geological Society of London in 1862, Professor
Ramsay maintained that the first formation of most existing lakes took place
during the glacial epoch, and was due, not to elevation or subsidence, but to
actual erosion of their basins by glaciers. M. Mortillet in the same year
advanced the theory that after the Alpine lake-basins had been filled up with
loose fluviatile deposits, they were re-excavated by the great glaciers which
passed down the valleys at the time of the greatest cold, a doctrine which would
attribute to moving ice almost as great a capacity of erosion as that which
assumed that the original basins were scooped out of solid rock by glaciers. It
is impossible to deny that the mere geographical distribution of lakes points to
the intimate connection of their origin with the abundance of ice during a
former excess of cold, but how far the erosive action of moving ice has been the
sole or even the principal cause of lake-basins, is a question still open to
discussion.
The lakes of Switzerland and the north of Italy are some of them twenty and
thirty miles in length, and so deep that their bottoms are in some cases from
1000 to 2000 feet beneath the level of the sea. It is admitted on all hands that
they were once filled with ice, and as the existing glaciers polish and grind
down, as before stated, the surface of the rocks, we are prepared to find that
every lake-basin in countries once covered by ice should bear the marks of
superficial glaciation, and also that the ice during its advance and retreat
should have left behind it much transported matter as well as some evidence of
its having enlarged the pre-existing cavity. But much more than this is demanded
by the advocates of glacial erosion. They suggest that as the old extinct
glaciers were several thousand feet thick, they were able in some places
gradually to scoop out of the solid rock cavities twenty or thirty miles in
length, and as in the case of Lago Maggiore from a thousand to two thousand six
hundred feet below the previous level of the river-channel, and also that the
ice had the power to remove from the cavity formed by its grinding action all
the materials of the missing rocks. A constant supply, it is argued, of fine mud
issues from the termination of every glacier in the stream which is produced by
the melting of the ice, and this result of friction is exhibited both during
winter and summer, affording evidence of the continual deepening and widening of
the valleys through which glaciers pass. As the fine mud is carried away by a
river from the deep pool which is formed from the base of every cataract, so it
seems to be imagined that lake-basins may be gradually emptied of the mud formed
by abrasion during the glacial period.
I am by no means disposed to object to this theory on the ground of the
insufficiency of the time during which the extreme cold endured, but we must
carefully consider whether that same time is not so vast as to make it probable
that other forces, besides the motion of glaciers, must have cooperated in
converting some parts of the ancient valley courses into lake-basins. They who
have formed the most exalted conceptions of the erosive energy of moving ice do
not deny that during the period termed "Glacial" there have been movements of
the earth's crust sufficient to produce oscillations of level in Europe
amounting to 1000 feet or more in both directions. M. Charpentier, indeed,
attributed some of the principal changes of climate in Switzerland, during the
glacial period, to a depression of the central Alps to the extent of 3000 feet,
and Swiss geologists have long been accustomed to attribute their lake basins,
in part, to those convulsions by which the shape and course of the valleys may
have been modified. Our experience, in the lifetime of the present generation,
of the changes of level witnessed in New Zealand during great earthquakes is
entirely opposed to the notion that the movements, whether upward or downward,
are uniform in amount or direction throughout areas of indefinite extent. On the
contrary, the land has been permanently raised in one region several feet or
yards, and the rise has been found gradually to die out, so as to be
imperceptible at a distance of twenty miles, and in some areas is even exchanged
for a simultaneous downward movement of several feet.
But, it is asked, if such inequality of movement can have contributed towards
the production of lake basins, does it not leave unexplained the comparative
rarity of lakes in tropical and subtropical countries. In reply to this question
it may be observed that in our endeavour to estimate the effects of subterranean
movements in modifying the superficial geography of a country we must remember
that each convulsion effects a very slight change. If it interferes with the
drainage, whether by raising the lower or sinking the higher portion of a
hydrographical basin, the upheaval or depression will only amount to a few feet
at a time, and there may be an interval of years or centuries before any further
movement takes place in the same region. In the mean time an incipient lake if
produced may be filled up with sediment, and the recently-formed barrier will
then be cut through by the river, whereas in a country where glacial conditions
prevail no such obliteration of the temporary lake-basin would take place; for
however deep it became by repeated sinking of the upper or rising of the lower
extremity, being always filled with ice it might remain, throughout the greater
part of its extent, free from sediment or drift until the ice melted at the
close of the glacial period.
One of the most serious objections to the exclusive origin by ice-erosion of
wide and deep lake-basins arises from their capricious distribution, as for
example in Piedmont, both to the eastward and westward of Turin, where great
lakes are wanting (Antiquity of Man page 313.), although some of the largest
extinct glaciers descending from Mont Blanc and Monte Rosa came down from the
Alps, leaving their gigantic moraines in the low country. Here, therefore, we
might have expected to find lakes of the first magnitude rivalling the
contiguous Lago Maggiore in importance.
A still more striking illustration of the same absence of lakes where large
glaciers abound is afforded by the Caucasus, a chain more than 300 miles long,
and the loftiest peaks of which attain heights from 16,000 to 18,000 feet. This
greatest altitude is reached by Elbruz, a mountain in latitude 43 degrees north
three degrees south of Mont Blanc, but on the other hand 3000 feet higher. The
present Caucasian glaciers are equal or superior in dimensions to those of
Switzerland, and like them give rise occasionally to temporary lakes by
obstructing the course of rivers, and causing great floods when the icy barriers
give way. Mr. Freshfield, a careful observer, writing in 1869, says: "A total
absence of lakes on both sides of the chains is the most marked feature. Not
only are there no great subalpine sheets of water, like Como or Geneva, but
mountain tarns, such as the Dauben See on the Gemmi, or the Klonthal See near
Glarus, are equally wanting." (Travels in Central Caucasus 1869 page 452.) The
same author states on the authority of the eminent Swiss geologist, Mons. E.
Favre, who also explored the Caucasus in 1868, that moraines of great height and
huge erratics of granite and other rocks "justify the assertion that the present
glaciers of the Caucasus, like those of the Alps, are only the shadows of their
former selves."
It seems safe to assume that the chain of lakes, of which the Albert Nyanza
forms one in equatorial Africa, was due to causes other than glacial. Yet if we
could imagine a glacial period to visit that region filling the lakes with ice
and scoring the rocks which form their sides and bottoms, we should be unable to
decide how much the capacity of the basins had been enlarged and the surface
modified by glacial erosion. The same may be true of the Lago Maggiore and Lake
Superior, although the present basins of both of them afford abundant
superficial markings due to ice-action.
But to whatever combination of causes we attribute the great Alpine lakes one
thing is clear, namely, that they are, geologically speaking, of modern origin.
Every one must admit that the upper valley of the Rhone has been chiefly caused
by fluviatile denudation, and it is obvious that the quantity of matter removed
from that valley previous to the glacial period would have been amply sufficient
to fill up with sediment the basin of the Lake of Geneva, supposing it to have
been in existence, even if its capacity had been many times greater than it is
now. (See Principles volume 1 page 420 10th edition 1867.)
On the whole, it appears to me, in accordance with the views of Professor
Ramsay, M. Mortillet, Mr. Geikie, and others, that the abrading action of ice
has formed some mountain tarns and many morainic lakes; but when it is a
question of the origin of larger and deeper lakes, like those of Switzerland or
the north of Italy, or inland fresh-water seas, like those of Canada, it will
probably be found that ice has played a subordinate part in comparison with
those movements by which changes of level in the earth's crust are gradually
brought about.
TERTIARY OR CAINOZOIC PERIOD.
CHAPTER XIII.
PLIOCENE PERIOD.
Glacial Formations of Pliocene Age.
Bridlington Beds.
Glacial Drifts of Ireland.
Drift of Norfolk Cliffs.
Cromer Forest-bed.
Aldeby and Chillesford Beds.
Norwich Crag.
Older Pliocene Strata.
Red Crag of Suffolk.
Coprolitic Bed of Red Crag.
White or Coralline Crag.
Relative Age, Origin, and Climate of the Crag Deposits.
Antwerp Crag.
Newer Pliocene Strata of Sicily.
Newer Pliocene Strata of the Upper Val d'Arno.
Older Pliocene of Italy.
Subapennine Strata.
Older Pliocene Flora of Italy.
It will be seen in the description given in the last chapter of the Post-
pliocene formations of the British Isles that they comprise a large proportion
of those commonly termed glacial, characterised by shells which, although
referable to living species, usually indicate a colder climate than that now
belonging to the latitudes where they occur fossil. But in parts of England,
more especially in Yorkshire, Norfolk, and Suffolk, there are superficial
formations of clay with glaciated boulders, and of sand and pebbles, containing
occasional, though rare, patches of shells, in which the marine fauna begins to
depart from that now inhabiting the neighbouring sea, and comprises some species
of mollusca not yet known as living, as well as extinct varieties of others,
entitling us to class them as Newer Pliocene, although belonging to the close of
that period and chronologically on the verge of the later or Post-pliocene
epoch.
BRIDLINGTON DRIFT.
To this era belongs the well-known locality of Bridlington, near the mouth of
the Humber, in Yorkshire, where about seventy species or well-marked varieties
of shells have been found on the coast, near the sea-level, in a bed of sand
several feet thick resting on glacial clay with much chalk debris, and covered
by a deposit of purple clay with glaciated boulders. More than a third of the
species in this drift are now inhabitants of arctic regions, none of them
extending southward to the British seas; which is the more remarkable as
Bridlington is situated in latitude 54 degrees north. Fifteen species are
British and Arctic, a very few belong to those species which range south of our
British seas. Five species or well-marked varieties are not known living,
namely, the variety of Astarte borealis (called A. Withami); A. mutabilis; the
sinistral form of Tritonium carinatum, Cardita analis, and Tellina obliqua,
Figure 120. Mr. Searles Wood also inclines to consider Nucula Cobboldiae, Figure
119, now absent from the European seas and the Atlantic, as specifically
distinct from a closely-allied shell now living in the seas surrounding
Vancouver's Island, which some conchologists regard as a variety. Tellina
obliqua also approaches very near to a shell now living in Japan.
GLACIAL DRIFT OF IRELAND.
Marine drift containing the last-mentioned Nucula and other glacial shells
reaches a height of from 1000 to 1200 feet in the county of Wexford, south of
Dublin. More than eighty species have already been obtained from this formation,
of which two, Conovulus pyramidalis and Nassa monensis, are not known as living;
while Turritella incrassata and Cypraea lucida no longer inhabit the British
seas, but occur in the Mediterranean. The great elevation of these shells, and
the still greater height to which the surface of the rocks in the mountainous
regions of Ireland have been smoothed and striated by ice-action, has led
geologists to the opinion that that island, like the greater part of England and
Scotland, after having been united with the continent of Europe, from whence it
received the plants and animals now inhabiting it, was in great part submerged.
The conversion of this and other parts of Great Britain into an archipelago was
followed by a re-elevation of land and a second continental period. After all
these changes the final separation of Ireland from Great Britain took place, and
this event has been supposed to have preceded the opening of the straits of
Dover. (See Antiquity of Man chapter 14.)
DRIFT OF NORFOLK CLIFFS.
(FIGURE 116. Tellina balthica (T. solidula).)
There are deposits of boulder clay and till in the Norfolk cliffs principally
made up of the waste of white chalk and flints which, in the opinion of Mr.
Searles Wood, jun., and others, are older than the Bridlington drift, and
contain a larger proportion of shells common to the Norwich and Red Crag,
including a certain number of extinct forms, but also abounding in Tellina
balthica (T. solidula, Figure 116), which is found fossil at Bridlington, and
living in our British seas, but wanting in all the formations, even the newest,
afterwards to be described as Crag. As the greater part of these drifts are
barren of organic remains, their classification is at present a matter of great
uncertainty.
They can nowhere be so advantageously studied as on the coast between
Happisburgh and Cromer. Here we may see vertical cliffs, sometimes 300 feet and
more in height, exposed for a distance of fifty miles, at the base of which the
chalk with flints crops out in nearly horizontal strata. Beds of gravel and sand
repose on this undisturbed chalk. They are often strangely contorted, and
envelop huge masses or erratics of chalk with layers of vertical flint. I
measured one of these fragments in 1839 at Sherringham, and found it to be
eighty feet in its longest diameter. It has been since entirely removed by the
waves of the sea. In the floor of the chalk beneath it the layers of flint were
horizontal. Such erratics have evidently been moved bodily from their original
site, probably by the same glacial action which has polished and striated some
of the accompanying granitic and other boulders, occasionally six feet in
diameter, which are imbedded in the drift.
CROMER FOREST-BED.
Intervening between these glacial formations and the subjacent chalk lies what
has been called the Cromer Forest-bed. This buried forest has been traced from
Cromer to near Kessingland, a distance of more than forty miles, being exposed
at certain seasons between high and low water mark. It is the remains of an old
land and estuarine deposit, containing the submerged stumps of trees standing
erect with their roots in the ancient soil. Associated with the stumps and
overlying them, are lignite beds with fresh-water shells of recent species, and
laminated clay without fossils. Through the lignite and forest-bed are scattered
cones of the Scotch and spruce firs with the seeds of recent plants, and the
bones of at least twenty species of terrestrial mammalia. Among these are two
species of elephant, E. meridionalis, Nesti, and E. antiquus, the former found
in the Newer Pliocene beds of the Val d'Arno, near Florence. In the same bed
occur Hippopotamus major, Rhinoceros etruscus, both of them also Val d'Arno
species, many species of deer considered by Mr. Boyd Dawkins to be
characteristic of warmer countries, and also a horse, beaver, and field-mouse.
Half of these mammalia are extinct, and the rest still survive in Europe. The
vegetation taken alone does not imply a temperature higher than that now
prevailing in the British Isles. There must have been a subsidence of the forest
to the amount of 400 or 500 feet, and a re-elevation of the same to an equal
extent in order to allow the ancient surface of the chalk or covering of soil,
on which the forest grew, to be first covered with several hundred feet of
drift, and then upheaved so that the trees should reach their present level.
Although the relative antiquity of the forest-bed to the overlying glacial till
is clear, there is some difference of opinion as to its relation to the crag
presently to be described.
CHILLESFORD AND ALDEBY BEDS.
(FIGURE 117. Natica helicoides, Johnson.)
It is in the counties of Norfolk, Suffolk, and Essex, that we obtain our most
valuable information respecting the British Pliocene strata, whether newer or
older. They have obtained in those counties the provincial name of "Crag,"
applied particularly to masses of shelly sand which have long been used in
agriculture to fertilise soils deficient in calcareous matter. At Chillesford,
between Woodbridge and Aldborough in Suffolk, and Aldeby, near Beccles, in the
same county, there occur stratified deposits, apparently older than any of the
preceding drifts of Yorkshire, Norfolk, and Suffolk. They are composed at
Chillesford of yellow sands and clays, with much mica, forming horizontal beds
about twenty feet thick. Messrs. Prestwich and Searles Wood, senior, who first
described these beds, point out that the shells indicate on the whole a colder
climate than the Red Crag; two-thirds of them being characteristic of high
latitudes. Among these are Cardium Groenlandicum, Leda limatula, Tritonium
carinatum, and Scalaria Groenlandica. In the upper part of the laminated clays a
skeleton of a whale was found associated with casts of the characteristic
shells, Nucula Cobboldiae and Tellina obliqua, already referred to as no longer
inhabiting our seas, and as being extinct varieties if not species. The same
shells occur in a perfect state in the lower part of the formation. Natica
helicoides (Figure 117) is an example of a species formerly known only as
fossil, but which has now been found living in our seas.
At Aldeby, where beds occur decidedly similar in mineral character as well as
fossil remains, Messrs. Crowfoot and Dowson have now obtained sixty-six species
of mollusca, comprising the Chillesford species and some others. Of these about
nine-tenths are recent. They are in a perfect state, clearly indicating a cold
climate; as two-thirds of them are now met with in arctic regions. As a rule,
the lamellibranchiate molluscs have both valves united, and many of them, such
as Mya arenaria, stand with the siphonal end upward, as when in a living state.
Tellina balthica, before mentioned (Figure 116) as so characteristic of the
glacial beds, including the drift of Bridlington, has not yet been found in
deposits of Chillesford and Aldeby age, whether at Sudbourn, East Bavent,
Horstead, Coltishall, Burgh, or in the highest beds overlying the Norwich Crag
proper at Bramerton and Thorpe.
NORWICH OR FLUVIO-MARINE CRAG.
(FIGURE 118. Mastodon arvernensis, third milk molar, left side, upper jaw;
grinding surface, natural size. Norwich Crag, Postwick, also found in Red Crag,
see below.)
The beds above alluded to ought, perhaps, to be regarded as beds of passage
between the glacial formations and those called from a provincial name "Crag,"
the newest member of which has been commonly called the "Norwich Crag." It is
chiefly seen in the neighbourhood of Norwich, and consists of beds of incoherent
sand, loam, and gravel, which are exposed to view on both banks of the Yare, as
at Bramerton and Thorpe. As they contain a mixture of marine, land, and fresh-
water shells, with bones of fish and mammalia, it is clear that these beds have
been accumulated at the bottom of a sea near the mouth of a river. They form
patches rarely exceeding twenty feet in thickness, resting on white chalk. At
their junction with the chalk there invariably intervenes a bed called the
"Stone-bed," composed of unrolled chalk-flints, commonly of large size, mingled
with the remains of a land fauna comprising Mastodon arvernensis, Elephas
meridionalis, and an extinct species of deer. The mastodon, which is a species
characteristic of the Pliocene strata of Italy and France, is the most abundant
fossil, and one not found in the Cromer forest before mentioned. When these
flints, probably long exposed in the atmosphere, became submerged, they were
covered with barnacles, and the surface of the chalk became perforated by the
Pholas crispata, each fossil shell still remaining at the bottom of its
cylindrical cavity, now filled up with loose sand from the incumbent crag. This
species of Pholas still exists, and drills the rocks between high and low water
on the British coast. The name of "Fluvio-marine" has often been given to this
formation, as no less than twenty species of land and fresh-water shells have
been found in it. They are all of living species; at least only one univalve,
Paludina lenta, has any, and that a very doubtful, claim to be regarded as
extinct.
(FIGURE 119. Nucula Cobboldiae.)
(FIGURE 120. Tellina obliqua.)
Of the marine shells, 124 in number, about 18 per cent are extinct, according to
the latest estimate given me by Mr. Searles Wood; but, for reasons presently to
be mentioned, this percentage must be only regarded as provisional. It must also
be borne in mind that the proportion of recent shells would be augmented if the
uppermost beds at Bramerton, near Norwich, which belong to the most modern or
Chillesford division of the Crag, had been included, as they were formerly, by
Mr. Woodward and myself, in the Norwich series. Arctic shells, which formed so
large a proportion in the Chillesford and Aldeby beds, are more rare in the
Norwich Crag, though many northern species-- such as Rhynchonella psittacea,
Scalaria Groenlandica, Astarte borealis, Panopaea Norvegia, and others-- still
occur. The Nucula Cobboldiae and Tellina obliqua, Figures 119 and 120, before
mentioned, are frequent in these beds, as are also Littorina littorea, Cardium
edule, and Turritella communis, of our seas, proving the littoral origin of the
beds.
OLDER PLIOCENE STRATA.
RED CRAG.
(FIGURE 121. Section through (left) sea, Red Crag, London Clay and Chalk
(right).)
Among the English Pliocene beds the next in antiquity is the Red Crag, which
often rests immediately on the London Clay, as in the county of Essex,
illustrated in Figure 121.
It is chiefly in the county of Suffolk that it is found, rarely exceeding twenty
feet in thickness, and sometimes overlying another Pliocene deposit, the
Coralline Crag, to be mentioned in the sequel. It has yielded-- exclusive of 25
species regarded by Mr. Wood as derivative-- 256 species of mollusca, of which
65, or 25 per cent, are extinct. Thus, apart from its order of superposition,
its greater antiquity than the Norwich and glacial beds, already described, is
proved by the greater departure from the fauna of our seas. It may also be
observed that in most of the deposits of this Red Crag, the northern forms of
the Norwich Crag, and of such glacial formations as Bridlington, are less
numerous, while those having a more southern aspect begin to make their
appearance. Both the quartzose sand, of which it chiefly consists, and the
included shells, are most commonly distinguished by a deep ferruginous or
ochreous colour, whence its name. The shells are often rolled, sometimes
comminuted, and the beds have much the appearance of having been shifting sand-
banks, like those now forming on the Dogger-bank, in the sea, sixty miles east
of the coast of Northumberland. Cross stratification is almost always present,
the planes of the strata being sometimes directed towards one point of the
compass, sometimes to the opposite, in beds immediately overlying. That such a
structure is not deceptive or due to any subsequent concretionary rearrangement
of particles, or to mere bands of colour produced by the iron, is proved by each
bed being made up of flat pieces of shell which lie parallel to the planes of
the smaller strata.
(FIGURE 122. Purpura tetragona, Sowerby; natural size.)
(FIGURE 123. Voluta Lamberti, Sowerby. Variety characteristic of Suffolk Crag.
Pliocene.)
(FIGURE 124. Voluta Lamberti, young individual, Cor. and Red Crag.)
It has long been suspected that the different patches of Red Crag are not all of
the same age, although their chronological relation can not be decided by
superposition. Separate masses are characterised by shells specifically distinct
or greatly varying in relative abundance, in a manner implying that the deposits
containing them were separated by intervals of time. At Butley, Tunstall,
Sudbourn, and in the Red Crag of Chillesford, the mollusca appear to assume
their most modern aspect when the climate was colder than when the earliest
deposits of the same period were formed. At Butley, Nucula Cobboldiae, so common
in the Norwich and certain glacial beds, is found, and Purpura tetragona (Figure
122) is very abundant. On the other hand, at Walton-on-the-Naze, in Essex, we
seem to have an exhibition of the oldest phase of the Red Crag; and a warmer
climate seems indicated, not only by the absence of many northern forms, but
also by the abundance of some now living in the British seas and the
Mediterranean. Voluta Lamberti (see Figures 123 and 124), an extinct form, which
seems to have flourished chiefly in the antecedent Coralline Crag period, is
still represented here by individuals of every age.
(FIGURE 125. Trophon antiquum, Muller. (Fusus contrarius) half natural size.)
The reversed whelk (Figure 125) is common at Walton, where the dextral form of
that shell is unknown. Here also we find most frequently specimens of
lamellibranchiate molluscs, with both the valves united, showing that they
belonged to this sea of the Upper Crag, and were not washed in from an older
bed, such as the Coralline, in which case the ligament would not have held
together the valves in strata so often showing signs of the boisterous action of
the waves. No less than forty species of lamellibranchiate molluscs, with double
valves, have been collected by Mr. Bell from the various localities of the Red
Crag.
At and near the base of the Red Crag is a loose bed of brown nodules, first
noticed by Professor Henslow as containing a large percentage of earthy
phosphates. This bed of coprolites (as it is called, because they were
originally supposed to be the faeces of animals) does not always occur at one
level, but is generally in largest quantity at the junction of the Crag and the
underlying formation. In thickness it usually varies from six to eighteen
inches, and in some rare cases amounts to many feet. It has been much used in
agriculture for manure, as not only the nodules, but many of the separate bones
associated with them, are largely impregnated with phosphate of lime, of which
there is sometimes as much as sixty per cent. They are not unfrequently covered
with barnacles, showing that they were not formed as concretions in the stratum
where they now lie buried, but had been previously consolidated. The phosphatic
nodules often collect fossil crabs and fishes from the London Clay, together
with the teeth of gigantic sharks. In the same bed have been found many ear-
bones of whales, and the teeth of Mastodon arvernensis, Rhinoceros
Schleiermacheri, Tapirus priscus, and Hipparion (a quadruped of the horse
family), and antlers of a stag, Cervus anoceros. Organic remains also of the
older chalk and Lias are met with, showing how great was the denudation of
previous formations during the Pliocene period. As the older White Crag,
presently to be mentioned, contains similar phosphatic nodules near its base,
those of the Red Crag may be partly derived from this source.
WHITE OR CORALLINE CRAG.
The lower or Coralline Crag is of very limited extent, ranging over an area
about twenty miles in length, and three or four in breadth, between the rivers
Stour and Alde, in Suffolk. It is generally calcareous and marly-- often a mass
of comminuted shells, and the remains of bryozoa (or polyzoa), passing
occasionally into a soft building-stone. (Ehrenberg proposed in 1831 the term
Bryozoum, or "Moss-animal," for the molluscous or ascidian form of polyp,
characterised by having two openings to the digestive sack, as in Eschara,
Flustra, Retepora, and other zoophytes popularly included in the corals, but now
classed by naturalists as mollusca. The term Polyzoum, synonymous with Bryozoum,
was, it seems, proposed in 1830, or the year before, by Mr. J.O. Thompson.) At
Sudbourn and Gedgrave, near Orford, this building-stone has been largely
quarried. At some places in the neighbourhood the softer mass is divided by thin
flags of hard limestone, and bryozoa placed in the upright position in which
they grew. From the abundance of these coralloid mollusca the lowest or White
Crag obtained its popular name, but true corals, as now defined, or zoantharia,
are very rare in this formation.
The Coralline Crag rarely, if ever, attains a thickness of thirty feet in any
one section. Mr. Prestwich imagines that if the beds found at different
localities were united in the probable order of their succession, they might
exceed eighty feet in thickness, but Mr. Searles Wood does not believe in the
possibility of establishing such a chronological succession by aid of the
organic remains, and questions whether proof could be obtained of more than
forty feet. I was unable to come to any satisfactory opinion on the subject,
although at Orford, especially at Gedgrave, in the neighbourhood of that place,
I saw many sections in pits, where this crag is cut through. These pits are so
unconnected, and of such limited extent, that no continuous section of any
length can be obtained, so that speculations as to the thickness of the whole
deposit must be very vague. At the base of the formation at Sutton a bed of
phosphatic nodules, very similar to that before alluded to in the Red Crag, with
remains of mammalia, has been met with.
(FIGURE 126. Section near Woodbridge, in Suffolk.
Through Sutton (left), Shottisham Creek, Ramsholt (right) and R. Deben.
a. Red Crag.
b. Coralline Crag.
c. London Clay.)
Whenever the Red and Coralline Crag occur in the same district, the Red Crag
lies uppermost; and in some cases, as in the section represented in Figure 126,
which I had an opportunity of seeing exposed to view in 1839, it is clear that
the older deposit, or Coralline Crag, b, had suffered denudation, before the
newer formation, a, was thrown down upon it. At D there was not only seen a
distinct cliff, eight or ten feet high, of Coralline Crag, running in a
direction N.E. and S.W., against which the Red Crag abuts with its horizontal
layers, but this cliff occasionally overhangs. The rock composing it is drilled
everywhere by Pholades, the holes which they perforated having been afterwards
filled with sand, and covered over when the newer beds were thrown down. The
older formation is shown by its fossils to have accumulated in a deeper sea, and
contains none of those littoral forms such as the limpet, Patella, found in the
Red Crag. So great an amount of denudation could scarcely take place, in such
incoherent materials, without some of the fossils of the inferior beds becoming
mixed up with the overlying crag, so that considerable difficulty must be
occasionally experienced by the palaeontologist in deciding which species belong
severally to each group.
(FIGURE 127. Fascicularia aurantium, Milne Edwards. Family, Tubuliporidae, of
same author. Bryozoan of extinct genus, from the inferior or Coralline Crag,
Suffolk.
a. Exterior.
b. Vertical section of interior.
c. Portion of exterior magnified.
d. Portion of interior magnified, showing that it is made up of long, thin,
straight tubes, united in conical bundles.)
(FIGURE 128. Astarte omalii, laj.; species common to Upper and lower crag.)
Mr. Searles Wood estimates the total number of marine testaceous mollusca of the
Coralline Crag at 350, of which 110 are not known as living, being in the
proportion of thirty-one per cent extinct. No less than 130 species of bryozoa
have been found in the Coralline Crag, and some belong to genera unknown in the
living creation, and of a very peculiar structure; as, for example, that
represented in Figure 127, which is one of several species having a globular
form. Among the testacea the genus Astarte (see Figure 128) is largely
represented, no less than fourteen species being known, and many of these being
rich in individuals. There is an absence of genera peculiar to hot climates,
such as Conus, Oliva, Fasciolaria, Crassatella, and others. The absence also of
large cowries (Cyprea), those found belonging exclusively to the section Trivia,
is remarkable. The large volute, called Voluta Lamberti (Figure 123), may seem
an exception; but it differs in form from the volutes of the torrid zone, and,
like the living Voluta Magellanica, must have been fitted for an extra-tropical
climate.
(FIGURE 129. Lingula Dumortieri, Nyst; Suffolk and Antwerp Crag.)
(FIGURE 130. Pyrula reticulata, Lam.; Coralline Crag, Ramsholt.)
(FIGURE 131. Temnechinus excavatus, Forbes; Temnopleurus excavatus, Wood;
Coralline Crag, Ramsholt.)
The occurrence of a species of Lingula at Sutton (see Figure 129) is worthy of
remark, as these Brachiopoda seem now confined to more equatorial latitudes; and
the same may be said still more decidedly of a species of Pyrula, supposed by
Mr. Wood to be identical with P. reticulata (Figure 130), now living in the
Indian Ocean. A genus also of echinoderms, called by Professor Forbes
Temnechinus (Figure 131), occurs in the Red and Coralline Crag of Suffolk, and
until lately was unknown in a living state, but it has been brought to light as
an existing form by the deep-sea dredgings, both of the United States survey,
off Florida, at a depth of from 180 to 480 feet, and more recently (1869), in
the British seas, during the explorations of the "Porcupine."
CLIMATE OF THE CRAG DEPOSITS.
One of the most interesting conclusions deduced from a careful comparison of the
shells of the British Pliocene strata and the fauna of our present seas has been
pointed out by Professor E. Forbes. It appears that, during the Glacial period,
a period intermediate, as we have seen, between that of the Crag and our own
time, many shells, previously established in the temperate zone, retreated
southward to avoid an uncongenial climate, and they have been found fossil in
the Newer Pliocene strata of Sicily, Southern Italy, and the Grecian
Archipelago, where they may have enjoyed, during the era of floating icebergs, a
climate resembling that now prevailing in higher European latitudes. (E. Forbes
Mem. Geological Survey of Great Britain volume 1 page 386.) The Professor gave a
list of fifty shells which inhabited the British seas while the Coralline and
Red Crag were forming, and which, though now living in our seas, were wanting,
as far as was then known, in the glacial deposits. Some few of these species
have subsequently been found in the glacial drift, but the general conclusion of
Forbes remains unshaken.
The transport of blocks by ice, when the Red Crag was being deposited, appears
to me evident from the large size of some huge, irregular, quite unrounded chalk
flints, retaining their white coating, and 2 feet long by 18 inches broad, in
beds worked for phosphatic nodules at Foxhall, four miles south-east of Ipswich.
These must have been tranquilly drifted to the spot by floating ice. Mr.
Prestwich also mentions the occurrence of a large block of porphyry in the base
of the Coralline Crag at Sutton, which would imply that the ice-action had begun
in our seas even in this older period. The cold seems to have gone on increasing
from the time of the Coralline to that of the Norwich Crag, and became more and
more severe, not perhaps without some oscillations of temperature, until it
reached its maximum in what has been called the Glacial period, or at the close
of the Newer Pliocene, and in the Post-pliocene periods.
RELATION OF THE FAUNA OF THE CRAG TO THAT OF THE RECENT SEAS.
By far the greater number of the recent marine species occurring in the several
Crag formations are still inhabitants of the British seas; but even these differ
considerably in their relative abundance, some of the commonest of the Crag
shells being now extremely scarce-- as, for example, Buccinum Dalei-- while
others, rarely met with in a fossil state, are now very common, as Murex
erinaceus and Cardium echinatum. Some of the species also, the identity of which
with the living would not be disputed by any conchologist, are nevertheless
distinguishable as varieties, whether by slight deviations in form or a
difference in average dimensions. Since Mr. Searles Wood first described the
marine testacea of the Crags, the additions made to that fossil fauna have not
been considerable, whereas we have made in the same period immense progress in
our knowledge of the living testacea of the British and arctic seas, and of the
Mediterranean. By this means the naturalist has been enabled to identify with
existing species many forms previously supposed to be extinct.
In the forthcoming supplement to the invaluable monograph communicated by Mr.
Wood to the Palaeontographical Society, in which he has completed his figures
and descriptions of the British crag shells of every age, list will be found of
all the fossil shells, of which a summary is given in the table below.
TABLE OF NUMBER OF KNOWN SPECIES OF MARINE TESTACEA IN THE CRAG.
COLUMN 1: KNOWN SPECIES.
COLUMN 2: TOTAL NUMBER OF KNOWN SPECIES.
COLUMN 3: NUMBER OF SPECIES NOT KNOWN AS LIVING.
CHILLESFORD AND ALDEBY BEDS:
Bivalves: 61 : 4.
Univalves: 33 : 5.
Brachiopods: 0 : 0.
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 9.5.
NORWICH OR FLUVIO-MARINE CRAG:
Bivalves: 61 : 10.
Univalves: 64 : 12.
Brachiopods: 1 : 0.
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 17.5.
RED CRAG (Exclusive of many derivative shells):
Bivalves: 128 : 31.
Univalves: 127 : 33.
Brachiopods: 1 : 1.
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 25.0.
CORALLINE CRAG:
Bivalves: 161 : 47.
Univalves: 184 : 60
Brachiopods: 5 : 3
PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 31.5
To begin with the uppermost or Chillesford beds, it will be seen that about 9
per cent only are extinct, or not known as living, whereas in the Norwich, which
succeeds in the descending order, seventeen in a hundred are extinct. Formerly,
when the Norwich or Fluvio-marine Crag was spoken of, both these formations were
included under the same head, for both at Bramerton and Thorpe, the chief
localities where the Norwich Crag was studied, an overlying deposit occurs
referable to the Chillesford age. If now the two were fused together as of old,
their shells would, according to Mr. Wood, yield a percentage of fifteen in a
hundred of species extinct or not known as living.
To come next to the Red Crag, the reader will observe that a percentage of 25 is
given of shells unknown as living, and this increases to 31 in the antecedent
Coralline Crag. But the gap between these two stages of our Pliocene deposits is
really wider than these numbers would indicate, for several reasons. In the
first place, the Coralline Crag is more strictly the product of a single period,
the Red Crag, as we have seen, consisting of separate and independent patches,
slightly varying in age, of which the newest is probably not much anterior to
the Norwich Crag. Secondly, there was a great change of conditions, both as to
the depth of the sea and climate, between the periods of the Coralline and Red
Crag, causing the fauna in each to differ far more widely than would appear from
the above numerical results.
The value of the analysis given in the above table of the shells of the Red and
Coralline Crags is in no small degree enhanced by the fact that they were all
either collected by Mr. Wood himself, or obtained by him direct from their
discoverers, so that he was enabled in each case to test their authenticity, and
as far as possible to avoid those errors which arise from confounding together
shells belonging to the sea of a newer deposit, and those washed into it from a
formation of older date. The danger of this confusion may be conceived when we
remember that the number of species rejected from the Red Crag as derivative by
Mr. Wood is no less than 25. Some geologists have held that on the same grounds
it is necessary to exclude as spurious some of the species found in the Norwich
Crag proper; but Mr. Wood does not entertain this view, believing that the
spurious shells which have sometimes found their way into the lists of this crag
have been introduced by want of care from strata of Red Crag.
There can be no doubt, on the other hand, that conchologists have occasionally
rejected from the Red and Norwich Crags, as derivative, shells which really
belonged to the seas of those periods, because they were extinct or unknown as
living, which in their eyes afforded sufficient ground for suspecting them to be
intruders. The derivative origin of a species may sometimes be indicated by the
extreme scarcity of the individuals, their colour, and worn condition; whereas
an opposite conclusion may be arrived at by the integrity of the shells,
especially when they are of delicate and tender structure, or their abundance,
and, in the case of the lamellibranchiata, by their being held together by the
ligament, which often happens when the shells have been so broken that little
more than the hinges of the two valves are preserved. As to the univalves, I
have seen from a pit of Red Crag, near Woodbridge, a large individual of the
extinct Voluta Lamberti, seven inches in length, of which the lip, then perfect,
had in former stages of its growth been frequently broken, and as often
repaired. It had evidently lived in the sea of the Red Crag, where it had been
exposed to rough usage, and sustained injuries like those which the reversed
whelk, Trophon antiquum, so characteristic of the same formation, often
exhibits. Additional proofs, however, have lately been obtained by Mr. Searles
Wood that this shell had not died out in the era of the Red Crag by the
discovery of the same fossil near Southwold, in beds of the later Norwich Crag.
ANTWERP CRAG.
Strata of the same age as the Red and Coralline Crag of Suffolk have been long
known in the country round Antwerp, and on the banks of the Scheldt, below that
city; and the lowest division, or Black Crag, there found, is shown by the
shells to be somewhat more ancient than any of our British series, and probably
forms the first links of a downward passage from the strata of the Pliocene to
those of the Upper Miocene period.
NEWER PLIOCENE STRATA OF SICILY.
(FIGURE 132. Murex vaginatus, Phil.)
At several points north of Catania, on the eastern sea-coast of Sicily-- as at
Aci-Castello, for example, Trezza, and Nizzeti-- marine strata, associated with
volcanic tuffs and basaltic lavas, are seen, which belong to a period when the
first igneous eruptions of Mount Etna were taking place in a shallow bay of the
Mediterranean. They contain numerous fossil shells, and out of 142 species that
have been collected all but eleven are identical with species now living. Some
few of these eleven shells may possibly still linger in the depths of the
Mediterranean, like Murex vaginatus, see Figure 132. The last-mentioned shell
had already become rare when the associated marine and volcanic strata above
alluded to were formed. On the whole, the modern character of the testaceous
fauna under consideration is expressed not only by the small proportion of
extinct species, but by the relative number of individuals by which most of the
other species are represented, for the proportion agrees with that observed in
the present fauna of the Mediterranean. The rarity of individuals in the extinct
species is such as to imply that they were already on the point of dying out,
having flourished chiefly in the earlier Pliocene times, when the Subapennine
strata were in progress.
Yet since the accumulation of these Newer Pliocene sands and clays, the whole
cone of Etna, 11,000 feet in height and about 90 miles in circumference at its
base, has been slowly built up; an operation requiring many tens of thousands of
years for its accomplishment, and to estimate the magnitude of which it is
necessary to study in detail the internal structure of the mountain, and to see
the proofs of its double axis, or the evidence of the lavas of the present great
centre of eruption having gradually overwhelmed and enveloped a more ancient
cone, situated 3 1/2 miles to the east of the present one. (See a Memoir on the
Lavas and Mode of Origin of Mount Etna by the Author in Philosophical
Transactions 1858.)
It appears that while Etna was increasing in bulk by a series of eruptions, its
whole mass, comprising the foundations of subaqueous origin above alluded to,
was undergoing a slow upheaval, by which those marine strata were raised to the
height of 1200 feet above the sea, as seen at Catera, and perhaps to greater
heights, for we can not trace their extension westward, owing to the dense and
continuous covering of modern lava under which they are buried. During the
gradual rise of these Newer Pliocene formations (consisting of clays, sands, and
basalts) other strata of Post-pliocene date, marine as well as fluviatile,
accumulated round the base of the mountain, and these, in their turn, partook of
the upward movement, so that several inland cliffs and terraces at low levels,
due partly to the action of the sea and partly to the river Simeto, originated
in succession. Fossil remains of the elephant, and other extinct quadrupeds,
have been found in these Post-Pliocene strata, associated with recent shells.
There is probably no part of Europe where the Newer Pliocene formations enter so
largely into the structure of the earth's crust, or rise to such heights above
the level of the sea, as Sicily. They cover nearly half the island, and near its
centre, at Castrogiovanni, reach an elevation of 3000 feet. They consist
principally of two divisions, the upper calcareous and the lower argillaceous,
both of which may be seen at Syracuse, Girgenti, and Castrogiovanni. According
to Philippi, to whom we are indebted for the best account of the tertiary shells
of this island, thirty-five species out of one hundred and twenty-four obtained
from the beds in central Sicily are extinct.
A geologist, accustomed to see nearly all the Newer Pliocene formations in the
north of Europe occupying low grounds and very incoherent in texture, is
naturally surprised to behold formations of the same age so solid and stony, of
such thickness, and attaining so great an elevation above the level of the sea.
The upper or calcareous member of this group in Sicily consists in some places
of a yellowish-white stone, like the Calcaire Grossier of Paris; in others, of a
rock nearly as compact as marble. Its aggregate thickness amounts sometimes to
700 or 800 feet. It usually occurs in regular horizontal beds, and is
occasionally intersected by deep valleys, such as those of Sortino and
Pentalica, in which are numerous caverns. The fossils are in every stage of
preservation, from shells retaining portions of their animal matter and colour
to others which are mere casts. The limestone passes downward into a sandstone
and conglomerate, below which is clay and blue marl, from which perfect shells
and corals may be disengaged. The clay sometimes alternates with yellow sand.
South of the plain of Catania is a region in which the tertiary beds are
intermixed with volcanic matter, which has been for the most part the product of
submarine eruptions. It appears that, while the clay, sand, and yellow limestone
before mentioned were in course of deposition at the bottom of the sea,
volcanoes burst out beneath the waters, like that of Graham Island, in 1831, and
these explosions recurred again and again at distant intervals of time. Volcanic
ashes and sand were showered down and spread by the waves and currents so as to
form strata of tuff, which are found intercalated between beds of limestone and
clay containing marine shells, the thickness of the whole mass exceeding 2000
feet. The fissures through which the lava rose may be seen in many places,
forming what are called DIKES.
(FIGURE 133. Pecten jacobaeus; half natural size.)
No shell is more conspicuous in these Sicilian strata than the great scallop,
Pecten jacobaeus (Figure 133), now so common in the neighbouring seas. The more
we reflect on the preponderating number of this and other recent shells, the
more we are surprised at the great thickness, solidity, and height above the sea
of the rocky masses in which they are entombed, and the vast amount of
geographical change which has taken place since their origin. It must be
remembered that, before they began to emerge, the uppermost strata of the whole
must have been deposited under water. In order, therefore, to form a just
conception of their antiquity, we must first examine singly the innumerable
minute parts of which the whole is made up, the successive beds of shells,
corals, volcanic ashes, conglomerates, and sheets of lava; and we must
afterwards contemplate the time required for the gradual upheaval of the rocks,
and the excavation of the valleys. The historical period seems scarcely to form
an appreciable unit in this computation, for we find ancient Greek temples, like
those of Girgenti (Agrigentum), built of the modern limestone of which we are
speaking, and resting on a hill composed of the same; the site having remained
to all appearances unaltered since the Greeks first colonised the island.
It follows, from the modern geological date of these rocks, that the fauna and
flora of a large part of Sicily are of higher antiquity than the country itself.
The greater part of the island has been raised above the sea since the epoch of
existing species, and the animals and plants now inhabiting it must have
migrated from adjacent countries, with whose productions the species are now
identical. The average duration of species would seem to be so great that they
are destined to outlive many important changes in the configuration of the
earth's surface, and hence the necessity for those innumerable contrivances by
which they are enabled to extend their range to new lands as they are formed,
and to escape from those which sink beneath the sea.
NEWER PLIOCENE STRATA OF THE UPPER VAL D'ARNO.
When we ascend the Arno for about ten miles above Florence, we arrive at a deep
narrow valley called the Upper Val d'Arno, which appears once to have been a
lake, at a time when the valley below Florence was an arm of the sea. The
horizontal lacustrine strata of this upper basin are twelve miles long and two
broad. The depression which they fill has been excavated out of Eocene and
Cretaceous rocks, which form everywhere the sides of the valley in highly
inclined stratification. The thickness of the more modern and unconformable beds
is about 750 feet, of which the upper 200 feet consist of Newer Pliocene strata,
while the lower are Older Pliocene. The newer series are made up of sands and a
conglomerate called "sansino." Among the imbedded fossil mammalia are Mastodon
arvernensis, Elephas meridionalis, Rhinoceros etruscus, Hippopotamus major, and
remains of the genera bear, hyaena, and felis, nearly all of which occur in the
Cromer forest-bed (see Chapter 13).
In the same upper strata are found, according to M. Gaudin, the leaves and cones
of Glyptostrobus europaeus, a plant closely allied to G. heterophyllus, now
inhabiting the north of China and Japan. This conifer had a wide range in time,
having been traced back to the Lower Miocene strata of Switzerland, and being
common at Oeningen in the Upper Miocene, as we shall see in the sequel (Chapter
14.)
OLDER PLIOCENE OF ITALY.-- SUBAPENNINE STRATA.
The Apennines, it is well-known, are composed chiefly of Secondary or Mesozoic
rocks, forming a chain which branches off from the Ligurian Alps and passes down
the middle of the Italian peninsula. At the foot of these mountains, on the side
both of the Adriatic and the Mediterranean, are found a series of tertiary
strata, which form, for the most part, a line of low hills occupying the space
between the older chain and the sea. Brocchi was the first Italian geologist who
described this newer group in detail, giving it the name of the Subapennine.
Though chiefly composed of Older Pliocene strata, it belongs, nevertheless, in
part, both to older and newer members of the tertiary series. The strata, for
example, of the Superga, near Turin, are Miocene; those of Asti and Parma Older
Pliocene, as is the blue marl of Sienna; while the shells of the incumbent
yellow sand of the same territory approach more nearly to the recent fauna of
the Mediterranean, and may be Newer Pliocene.
We have seen that most of the fossil shells of the Older Pliocene strata of
Suffolk which are of recent species are identical with testacea now living in
British seas, yet some of them belong to Mediterranean species, and a few even
of the genera are those of warmer climates. We might therefore expect, in
studying the fossils of corresponding age in countries bordering the
Mediterranean, to find among them some species and genera of warmer latitudes.
Accordingly, in the marls belonging to this period at Asti, Parma, Sienna, and
parts of the Tuscan and Roman territories, we observe the genera Conus, Cypraea,
Strombus, Pyrula, Mitra, Fasciolaria, Sigaretus, Delphinula, Ancillaria, Oliva,
Terebellum, Terebra, Perna, Plicatula, and Corbis, some characteristic of
tropical seas, others represented by species more numerous or of larger size
than those now proper to the Mediterranean.
OLDER PLIOCENE FLORA OF ITALY.
(FIGURE 134. Oreodaphne Heerii.
Leaf half natural size. (Feuilles fossiles de la Toscane.))
I have already alluded to the Newer Pliocene deposits of the Upper Val d'Arno
above Florence, and stated that below those sands and conglomerates, containing
the remains of the Elephas meridionalis and other associated quadrupeds, lie an
older horizontal and conformable series of beds, which may be classed as Older
Pliocene. They consist of blue clays with some subordinate layers of lignite,
and exhibit a richer flora than the overlying Newer Pliocene beds, and one
receding farther from the existing vegetation of Europe. They also comprise more
species common to the antecedent Miocene period. Among the genera of flowering
plants, M. Gaudin enumerates pine, oak, evergreen oak, plum, plane, alder, elm,
fig, laurel, maple, walnut, birch, buckthorn, hickory, sumach, sarsaparilla,
sassafras, cinnamon, Glyptostrobus, Taxodium, Sequoia, Persea, Oreodaphne
(Figure 134), Cassia, and Psoralea, and some others. This assemblage of plants
indicates a warm climate, but not so subtropical an one as that of the Upper
Miocene period, which will presently be considered.
(FIGURE 135. Liquidambar europaeum, var. trilobatum, A. Br. (sometimes four-
lobed, and more commonly five-lobed).
a. Leaf, half natural size.
b. Part of same, natural size.
c. Fruit, natural size.
d. Seed, natural size. Oeningen.)
M. Gaudin, jointly with the Marquis Strozzi, has thrown much light on the botany
of beds of the same age in another part of Tuscany, at a place called Montajone,
between the rivers Elsa and Evola, where, among other plants, is found the
Oreodaphne Heerii, Gaud. (See Figure 134), which is probably only a variety of
Oreodaphne foetens, or the laurel called the Til in Madeira, where, as in the
Canaries, it constitutes a large portion of the native woods, but can not now
endure the climate of Europe. In the fossil specimens the same glands or
protuberances are preserved (see Figure 134) as those which are seen in the
axils of the primary veins of the leaves in the recent Til. (Contributions a la
Flore fossile Italienne. Gaudin and Strozzi. Plate 11 Figure 3. Gaudin page 22.)
Another plant also indicating a warmer climate is the Liquidambar europaeum,
Brong. (see Figure 135), a species nearly allied to L. styracifluum, L., which
flourishes in most places in the Southern States of North America, on the
borders of the Gulf of Mexico.
CHAPTER XIV.
MIOCENE PERIOD.-- UPPER MIOCENE.
Upper Miocene Strata of France.-- faluns of Touraine.
Tropical Climate implied by Testacea.
Proportion of recent Species of Shells.
faluns more ancient than the Suffolk Crag.
Upper Miocene of Bordeaux and the South of France.
Upper Miocene of Oeningen, in Switzerland.
Plants of the Upper Fresh-water Molasse.
Fossil Fruit and Flowers as well as Leaves.
Insects of the Upper Molasse.
Middle or Marine Molasse of Switzerland.
Upper Miocene Beds of the Bolderberg, in Belgium.
Vienna Basin.
Upper Miocene of Italy and Greece.
Upper Miocene of India; Siwalik Hills.
Older Pliocene and Miocene of the United States.
UPPER MIOCENE STRATA OF FRANCE.-- FALUNS OF TOURAINE.
The strata which we meet with next in the descending order are those called by
many geologists "Middle Tertiary," for which in 1833 I proposed the name of
Miocene, selecting the "faluns" of the valley of the Loire, in France, as my
example or type. I shall now call these falunian deposits Upper Miocene, to
distinguish them from others to which the name of Lower Miocene will be given.
No British strata have a distinct claim to be regarded as Upper Miocene, and as
the Lower Miocene are also but feebly represented in the British Isles, we must
refer to foreign examples in illustration of this important period in the
earth's history. The term "faluns" is given provincially by French
agriculturists to shelly sand and marl spread over the land in Touraine, just as
similar shelly deposits were formerly much used in Suffolk to fertilise the
soil, before the coprolitic or phosphatic nodules came into use. Isolated masses
of such faluns occur from near the mouth of the Loire, in the neighbourhood of
Nantes, to as far inland as a district south of Tours. They are also found at
Pontlevoy, on the Cher, about seventy miles above the junction of that river
with the Loire, and thirty miles south-east of Tours. Deposits of the same age
also appear under new mineral conditions near the towns of Dinan and Rennes, in
Brittany. I have visited all the localities above enumerated, and found the beds
on the Loire to consist principally of sand and marl, in which are shells and
corals, some entire, some rolled, and others in minute fragments. In certain
districts, as at Doue, in the Department of Maine and Loire, ten miles south-
west of Saumur, they form a soft building-stone, chiefly composed of an
aggregate of broken shells, bryozoa, corals, and echinoderms, united by a
calcareous cement; the whole mass being very like the Coralline Crag near
Aldborough, and Sudbourn in Suffolk. The scattered patches of faluns are of
slight thickness, rarely exceeding fifty feet; and between the district called
Sologne and the sea they repose on a great variety of older rocks; being seen to
rest successively upon gneiss, clay-slate, various secondary formations,
including the chalk; and, lastly, upon the upper fresh-water limestone of the
Parisian tertiary series, which, as before mentioned (Chapter 9), stretches
continuously from the basin of the Seine to that of the Loire.
(FIGURE 136. Dinotherium giganteum, Kaup.)
At some points, as at Louans, south of Tours, the shells are stained of a
ferruginous colour, not unlike that of the Red Crag of Suffolk. The species are,
for the most part, marine, but a few of them belong to land and fluviatile
genera. Among the former, Helix turonensis (Figure 38, Chapter 3) is the most
abundant. Remains of terrestrial quadrupeds are here and there intermixed,
belonging to the genera Dinotherium (Figure 136), Mastodon, Rhinoceros,
Hippopotamus, Chaeropotamus, Dichobune, Deer, and others, and these are
accompanied by cetacea, such as the Lamantin, Morse, Sea-calf, and Dolphin, all
of extinct species.
The fossil testacea of the faluns of the Loire imply, according to the late
Edward Forbes, that the beds were formed partly on the shore itself at the level
of low water, and partly at very moderate depths, not exceeding ten fathoms
below that level. The molluscan fauna is, on the whole, much more littoral than
that of the Pliocene Red and Coralline Crag of Suffolk, and implies a shallower
sea. It is, moreover, contrasted with the Suffolk Crag by the indications it
affords of an extra-European climate. Thus it contains seven species of Cypraea,
some larger than any existing cowry of the Mediterranean, several species of
Oliva, Ancillaria, Mitra, Terebra, Pyrula, Fasciolaria, and Conus. Of the cones
there are no less than eight species, some very large, whereas the only European
cone now living is of diminutive size. The genus Nerita, and many others, are
also represented by individuals of a type now characteristic of equatorial seas,
and wholly unlike any Mediterranean forms. These proofs of a more elevated
temperature seem to imply the higher antiquity of the faluns as compared with
the Suffolk Crag, and are in perfect accordance with the fact of the smaller
proportion of testacea of recent species found in the faluns.
Out of 290 species of shells, collected by myself in 1840 at Pontlevoy, Louans,
Bossee, and other villages twenty miles south of Tours, and at Savigne, about
fifteen miles north-west of that place, seventy-two only could be identified
with recent species, which is in the proportion of twenty-five per cent. A large
number of the 290 species are common to all the localities, those peculiar to
each not being more numerous than we might expect to find in different bays of
the same sea.
The total number of species of testaceous mollusca from the faluns in my
possession is 302, of which forty-five only, or fourteen per cent, were found by
Mr. Wood to be common to the Suffolk Crag. The number of corals, including
bryozoa and zoantharia, obtained by me at Doue and other localities before
adverted to, amounts to forty-three, as determined by Mr. Lonsdale, of which
seven (one of them a zoantharian) agree specifically with those of the Suffolk
Crag. Some of the genera occurring fossil in Touraine, as the corals Astrea and
Dendrophyllia, and the bryozoan Lunulites, have not been found in European seas
north of the Mediterranean; nevertheless, the zoantharia of the faluns do not
seem to indicate, on the whole, so warm a climate as would be inferred from the
shells.
It was stated that, on comparing about 300 species of Touraine shells with about
450 from the Suffolk Crag, forty-five only were found to be common to both,
which is in the proportion of only fifteen per cent. The same small amount of
agreement is found in the corals also. I formerly endeavoured to reconcile this
marked difference in species with the supposed co-existence of the two faunas,
by imagining them to have severally belonged to distinct zoological provinces or
two seas, the one opening to the north and the other to the south, with a
barrier of land between them, like the Isthmus of Suez, now separating the Red
Sea and the Mediterranean. But I now abandon that idea for several reasons;
among others, because I succeeded in 1841 in tracing the Crag fauna southward in
Normandy to within seventy miles of the Falunian type, near Dinan, yet found
that both assemblages of fossils retained their distinctive characters, showing
no signs of any blending of species or transition of climate.
The principal grounds, however, for referring the English Crag to the older
Pliocene and the French faluns to the Upper Miocene epochs, consist in the
predominance of fossil shells in the British strata identifiable with species
not only still living, but which are now inhabitants of neighbouring seas, while
the accompanying extinct species are of genera such as characterise Europe. In
the faluns, on the contrary, the recent species are in a decided minority; and
most of them are now inhabitants of the Mediterranean, the coast of Africa, and
the Indian Ocean; in a word, less northern in character, and pointing to the
prevalence of a warmer climate. They indicate a state of things receding farther
from the present condition of Central Europe in physical geography and climate,
and doubtless, therefore, receding farther from our era in time.
(FIGURE 137. Voluta Lamberti, Sowerby. Variety characteristic of Faluns of
Touraine. Miocene.)
Among the conspicuous fossils common to the faluns of the Loire and the Suffolk
Crag is a variety of the Voluta Lamberti, a shell already alluded to (Figure
123). The specimens of this shell which I have myself collected in Touraine, or
have seen in museums, are thicker and heavier than British individuals of the
same species, and shorter in proportion to their width, and have the folds on
the columella less oblique, as represented in Figure 137.
UPPER MIOCENE OF BORDEAUX AND THE SOUTH OF FRANCE.
A great extent of country between the Pyrenees and the Gironde is overspread by
tertiary deposits of various ages, and chiefly of Miocene date. Some of these,
near Bordeaux, coincide in age with the faluns of Touraine, already mentioned,
but many of the species of shells are peculiar to the south. The succession of
beds in the basin of the Gironde implies several oscillations of level by which
the same wide area was alternately converted into sea and land and into
brackish-water lagoons, and finally into fresh-water ponds and lakes.
Among the fresh-water strata of this age near the base of the Pyrenees are
marls, limestones and sands, in which the eminent comparative anatomist, M.
Lartet, has obtained a great number of fossil mammalia common to the faluns of
the Loire and the Upper Miocene beds of Switzerland, such as Dinotherium
giganteum and Mastodon angustidens; also the bones of quadrumana, or of the ape
and monkey tribe, which were discovered in 1837, the first of that order of
quadrupeds detected in Europe. They were found near Auch, in the Department of
Gers, in latitude 43 degrees 39' N. About forty miles west of Toulouse. They
were referred by MM. Lartet and Blainville to a genus closely allied to the
Gibbon, to which they gave the name of Pliopithecus. Subsequently, in 1856, M.
Lartet described another species of the same family of long-armed apes
(Hylobates), which he obtained from strata of the same age at Saint-Gaudens, in
the Haute Garonne. The fossil remains of this animal consisted of a portion of a
lower jaw with teeth and the shaft of a humerus. It is supposed to have been a
tree-climbing frugivorous ape, equalling man in stature. As the trunks of oaks
are common in the lignite beds in which it lay, it has received the generic name
of Dryopithecus. The angle formed by the ascending ramus of the jaw and the
alveolar border is less open, and therefore more like the human subject, than in
the Chimpanzee, and what is still more remarkable, the fossil, a young but adult
individual, had all its milk teeth replaced by the second set, while its last
true molar (or wisdom-tooth) was still undeveloped, or only existed as a germ in
the jaw-bone. In the mode, therefore, of the succession of its teeth (which, as
in all the old-World apes, exactly agree in number with those in man) it
differed from the Gorilla and Chimpanzee, and corresponded with the human
species.
UPPER MIOCENE BEDS OF OENINGEN, IN SWITZERLAND.
The faluns of the Loire first served, as already stated, as the type of the
Miocene formations in Europe. They yielded a plentiful harvest of marine fossil
shells and corals, but were entirely barren of plants and insects. In
Switzerland, on the other hand, deposits of the same age have been discovered,
remarkable for their botanical and entomological treasures. We are indebted to
Professor Heer, of Zurich, for the description, restoration, and classification
of several hundred species and varieties of these fossil plants, the whole of
which he has illustrated by excellent figures in his "Flora Tertiaria
Helvetiae." This great work, and those of Adolphe Brongniart, Unger, Goppert and
others, show that this class of fossils is beginning to play the same important
part in the classification of the tertiary strata containing lignite or brown
coal as an older flora has long played in enabling us to understand the ancient
coal or carboniferous formation. No small skepticism has always prevailed among
botanists as to whether the leaves alone and the wood of plants could ever
afford sufficient data for determining even genera and families in the vegetable
kingdom. In truth, before such remains could be rendered available a new science
had to be created. It was necessary to study the outlines, nervation, and
microscopic structure of the leaves, with a degree of care which had never been
called for in the classification of living plants, where the flower and fruit
afforded characters so much more definite and satisfactory. As geologists, we
can not be too grateful to those who, instead of despairing when so difficult a
task was presented to them, or being discouraged when men of the highest
scientific attainments treated the fossil leaves as worthless, entered with full
faith and enthusiasm into this new and unexplored field. That they should
frequently have fallen into errors was unavoidable, but it is remarkable,
especially if we inquire into the history of Professor Heer's researches, how
often early conjectures as to the genus and family founded on the leaves alone
were afterwards confirmed when fuller information was obtained. As examples to
be found on comparing Heer's earlier and later works, I may instance the
chestnut, elm, maple, cinnamon, magnolia, buckbean or Menyanthes, vine,
buckthorn (Rhamnus), Andromeda and Myrica, and among the conifers Sequoia and
Taxodium. In all these cases the plants were first recognised by their leaves,
and the accuracy of the determination was afterwards confirmed when the fruit,
and in some instances both fruit and flower, were found attached to the same
stem as the leaves.
But let us suppose that no fruit, seed, or flower had ever been met with in a
fossil state, we should still have been indebted to the persevering labours of
botanical palaeontologists for one of the grandest scientific discoveries for
which the present century is remarkable-- namely, the proofs now established of
the prevalence of a mild climate and a rich arborescent flora in the arctic
regions in that Miocene epoch on the history of which we are now entering. It
may be useful if I endeavour to give the reader in a few words some idea of the
nature of the evidence of these important conclusions, to show how far they may
be safely based on fossil leaves alone. When we begin by studying the fossils of
the Newer Pliocene deposits, such as those of the Upper Val d'Arno, before
alluded to, we perceive that the fossil foliage agrees almost entirely with the
trees and shrubs of a modern European forest. In the plants of the Older
Pliocene strata of the same region we observe a larger proportion of species and
genera which, although they may agree with well-known Asiatic or other foreign
types, are at present wanting in Italy. If we then examine the Miocene
formations of the same country, exotic forms become more abundant, especially
the palms, whether they belong to the European or American fan-palms, Chamaerops
and Sabal, or to the more tropical family of the date-palms or Phoenicites,
which last are conspicuous in the Lower Miocene beds of Central Europe. Although
we have not found the fruit or flower of these palms in a fossil state, the
leaves are so characteristic that no one doubts the family to which they belong,
or hesitates to accept them as indications of a warm and sub-tropical climate.
When the Miocene formations are traced to the northward of the 50th degree of
latitude, the fossil palms fail us, but the greater proportion of the leaves,
whether identical with those of existing European trees or of forms now unknown
in Europe, which had accompanied the Miocene palms, still continue to
characterise rocks of the same age, until we meet with them not only in Iceland,
but in Greenland, in latitude 70 degrees N., and in Spitzbergen, latitude 78
degrees 56', or within about 11 degrees of the pole, and under circumstances
which clearly show them to have been indigenous in those regions, and not to
have been drifted from the south (see Chapter 15). Not only, therefore, has the
botanist afforded the geologist much palaeontological assistance in identifying
distinct tertiary formations in distant places by his power of accurately
discriminating the forms, veining, and microscopic structure of leaves or wood,
but, independently of that exact knowledge derivable from the organs of
fructification, we are indebted to him for one of the most novel, unexpected
results of modern scientific inquiry.
The Miocene formations of Switzerland have been called MOLASSE, a term derived
from the French MOL, and applied to a SOFT, incoherent, greenish sandstone,
occupying the country between the Alps and the Jura. This molasse comprises
three divisions, of which the middle one is marine, and being closely related by
its shells to the faluns of Touraine, may be classed as Upper Miocene. The two
others are fresh-water, the upper of which may be also grouped with the faluns,
while the lower must be referred to the Lower Miocene, as defined in the next
chapter.
UPPER FRESH-WATER MOLASSE.
This formation is best seen at Oeningen, in the valley of the Rhine, between
Constance and Schaffhausen, a locality celebrated for having produced in the
year 1700 the supposed human skeleton called by Scheuchzer "homo diluvii
testis," a fossil afterwards demonstrated by Cuvier to be a reptile, or aquatic
salamander, of larger dimensions than even its great living representative, the
salamander of Japan.
The Oeningen strata consist of a series of marls and limestones, many of them
thinly laminated, and which appear to have slowly accumulated in a lake probably
fed by springs holding carbonate of lime in solution. The elliptical area over
which this fresh-water formation has been traced extends, according to Sir
Roderick Murchison, for a distance of ten miles east and west from Berlingen, on
the right bank of the river to Wangen, and to Oeningen, near Stein, on the left
bank. The organic remains have been chiefly derived from two quarries, the lower
of which is about 550 feet above the level of the Lake of Constance, while the
upper quarry is 150 feet higher. In this last, a section thirty feet deep
displays a great succession of beds, most of them splitting into slabs and some
into very thin laminae. Twenty-one beds are enumerated by Professor Heer, the
uppermost a bluish-grey marl seven feet thick, with organic remains, resting on
a limestone with fossil plants, including leaves of poplar, cinnamon, and pond-
weed (Potamogeton), together with some insects; while in the bed No. 4, below,
is a bituminous rock, in which the Mastodon tapiroides, a characteristic Upper
Miocene quadruped, has been met with. The 5th bed, two or three inches thick,
contains fossil fish, e.g., Leuciscus (roach), and the larvae of dragon-flies,
with plants such as the elm (Ulmus), and the aquatic Chara. Below this are other
plant-beds; and then, in No. 9, the stone in which the great salamander (Andrias
Scheuchzeri) and some fish were found. Below this other strata occur with fish,
tortoises, the great salamander before alluded to, fresh-water mussels, and
plants. In No. 16 the fossil fox of Oeningen, galecynus Oeningensis, Owen, was
obtained by Sir R. Murchison. To this succeed other beds with mammalia
(Lagomys), reptiles, (Emys), fish, and plants, such as walnut, maple, and
poplar. In the 19th bed are numerous fish, insects, and plants, below which are
marls of a blue indigo colour.
In the lower quarry eleven beds are mentioned, in which, as in the upper, both
land and fresh-water plants and many insects occur. In the 6th, reckoning from
the top, many plants have been obtained, such as Liquidambar, Daphnogene,
Podogonium, and Ulmus, together with tortoises, besides the bones and teeth of a
ruminant quadruped, named by H. von Meyer Palaeomeryx eminens. No. 9 is called
the insect-bed, a layer only a few inches thick, which, when exposed to the
frost, splits into leaves as thin as paper. In these thin laminae plants such as
Liquidambar, Daphnogene, and Glyptostrobus, occur, with innumerable insects in a
wonderful state of preservation, usually found singly. Below this is an indigo-
blue marl, like that at the bottom of the higher quarry, resting on yellow marl
ascertained to be at least thirty feet thick.
(FIGURE 138. Cinnamomum polymorphum, Ad. Brong. Upper and Lower Miocene.
a. Leaf.
b. Flower, natural size; Heer Plate 93 Figure 28.
c. Ripe fruit of Cinnamomum polymorphum, from Oeningen; Heer, Plate 94 Figure
14.
d. Fruit of recent Cinnamomum camphorum of Japan; Heer, Plate 152 Figure 18.)
All the above fossil-bearing strata were evidently formed with extreme slowness.
Although the fossiliferous beds are, in the aggregate, no more than a few yards
in thickness, and have only been examined in the small area comprised in the two
quarries just alluded to, they give us an insight into the state of animal and
vegetable life in part of the Upper Miocene period, such as no other region in
the world has elsewhere supplied. In the year 1859, Professor Heer had already
determined no less than 475 species of plants and more than 800 insects from
these Oeningen beds. He supposes that a river entering a lake floated into it
some of the leaves and land insects, together with the carcasses of quadrupeds,
among others a great Mastodon. Occasionally, during tempests, twigs and even
boughs of trees with their leaves were torn off and carried for some distance so
as to reach the lake. Springs, containing carbonate of lime, seem at some points
to have supplied calcareous matter in solution, giving origin locally to a kind
of travertin, in which organic bodies sinking to the bottom became hermetically
sealed up. The laminae, says Heer, which immediately succeed each other were not
all formed at the same season, for it can be shown that, when some of them
originated, certain plants were in flower, whereas, when the next of these
layers was produced, the same plants had ripened their fruit. This inference is
confirmed by independent proofs derived from insects. The principal insect-bed
is rarely two inches thick, and is composed, says Heer, of about 250 leaf-like
laminae, some of which were deposited in the spring, when the Cinnamomum
polymorphum (Figure 138) was in flower, others in summer, when winged ants were
numerous, and when the poplar and willow had matured their seed; others, again,
in autumn, when the same Cinnamomum polymorphum (Figure 138) was in fruit, as
well as the liquidambar, oak, clematis, and many other plants. The ancient lake
seems to have had a belt of poplars and willows round its borders, countless
leaves of which were imbedded in mud, and together with them, at some points, a
species of reed, Arundo, which was very common.
One of the most characteristic shrubs is a papilionaceous and leguminous plant
of an extinct genus, called by Heer Podogonium, of which two species are known.
Entire twigs have been found with flowers, and always without leaves, as the
flowers evidently came out, as in the poplar and willow tribe, before any leaves
made their appearance. Other specimens have been obtained with ripe fruits
accompanied by leaves, which resemble those of the tamarind, to which it was
evidently allied, being of the family Caesalpineae, now proper to warmer
regions.
(FIGURE 139. Acer trilobatum, normal form; Heer, Flora Tert. Helv. Plate 114
Figure 2. Size 1/2 diam. (Part only of the long stalk of the original fossil
specimen is here given ). Upper Miocene, Oeningen; also found in Lower Miocene
of Switzerland.)
(FIGURE 140. Acer trilobatum.
a. Abnormal variety of leaf; Heer, Plate 110 Figure 16.
b. Flower and bracts, normal form; Heer, Plate 111 Figure 21.
c. Half a seed-vessel; Heer, Plate 111 Figure 5.)
(FIGURE 141. Platanus aceroides, Gopp.; Heer, Plate 88 Figures 5-8. Size 2/3
diam. Upper Miocene, Oeningen.
a. Leaf.
b. The core of a bundle of pericarps.
c. Single fruit or pericarp, natural size.)
The Upper Miocene flora of Oeningen is peculiarly important, in consequence of
the number of genera of which not merely the leaves, but, as in the case of the
Podogonium just mentioned, the fruit also and even the flower are known. Thus
there are nineteen species of maple, ten of which have already been found with
fruit. Although in no one region of the globe do so many maples now flourish, we
need not suspect Professor Heer of having made too many species in this genus
when we consider the manner in which he has dealt with one of them, Acer
trilobatum, Figures 139 and 140. Of this plant the number of marked varieties
figured and named is very great, and no less than three of them had been
considered as distinct species by other botanists, while six of the others might
have laid claim, with nearly equal propriety, to a like distinction. The common
form, called Acer trilobatum, Figure 139, may be taken as a normal
representative of the Oeningen fossil, and Figure 140, as one of the most
divergent varieties, having almost four lobes in the leaf instead of three.
(FIGURE 142. Smilax sagittifera; Heer, Plate 30 Figure 7. Size 1/2 diameter.
a. Leaf.
b. Flower magnified, one of the six petals wanting at d. Upper Miocene,
Oeningen.
c. Smilax obtusifolia; Heer, Plate 30 Figure 9; natural size. Upper Miocene,
Oeningen.)
(FIGURE 143. Fruit of the fossil and recent species of Hakea, a genus of
Proteaceae.
a. Leaf of fossil species, Hakea salicina. Upper Miocene, Oeningen; Heer Plate
97 Figure 29. 1/3 diameter.
b. Impression of woody fruit of same, showing thick stalk. 2/3 diameter.
c. Seed of same, natural size.
d. Fruit of living Australian species, Hakea saligna, R. Brown. 1/2 diameter.
e. Seed of same, natural size.)
Among the conspicuous genera which abounded in the Miocene period in Europe is
the plane-tree, Platanus, the fossil species being considered by Heer to come
nearer to the American P. occidentalis than to P. orientalis of Greece and Asia
Minor. In some of the fossil specimens the male flowers are preserved. Among
other points of resemblance with the living plane-trees, as we see them in the
parks and squares of London, fossil fragments of the trunk are met with, having
pieces of their bark peeling off.
The vine of Oeningen, Vitis teutonica, Ad. Brong, is of a North American type.
Both the leaves and seeds have been found at Oeningen, and bunches of compressed
grapes of the same species have been met with in the brown coal of Wetteravia in
Germany. No less than eight species of smilax, a monocotyledonous genus, occur
at Oeningen and in other Upper Miocene localities, the flowers of some of them,
as well as the leaves, being preserved; as in the case of the very common
fossil, S. sagittifera, Figure 142, a.
Leaves of plants supposed to belong to the order Proteaceae have been obtained
partly from Oeningen and partly from the lacustrine formation of the same age at
Locle in the Jura. They have been referred to the genera Banksia, Grevillea,
Hakea, and Persoonia. Of Hakea there is the impression of a supposed seed-
vessel, with its characteristic thick stalk and seeds, but as the fruit is
without structure, and has not yet been found attached to the same stem as the
leaf, the proof is incomplete.
To whatever family the foliage hitherto regarded as proteaceous by many able
palaeontologists may eventually be shown to belong, we must be careful not to
question their affinity to that order of plants on those geographical
considerations which have influenced some botanists. The nearest living
Proteaceae now feel the in Abyssinia in latitude 20 degrees N., but the greatest
number are confined to the Cape and Australia. The ancestors, however, of the
Oeningen fossils ought not to be looked for in such distant regions, but from
that European land which in Lower Miocene times bore trees with similar foliage,
and these had doubtless an Eocene source, for cones admitted by all botanists to
be proteaceous have been met with in one division of that older Tertiary group
(see Figure 206 Chapter 16). The source of these last, again, must not be sought
in the antipodes, for in the white chalk of Aix-la-Chapelle leaves like those of
Grevillea and other proteaceous genera have been found in abundance, and, as we
shall see in Chapter 17, in a most perfect state of preservation. All geologists
agree that the distribution of the Cretaceous land and sea had scarcely any
connection with the present geography of the globe.
(FIGURE 144. Glyptostrobus Europaeus.
Branch with ripe fruit; Heer, Plate 20 Figure 1. Upper Miocene, Oeningen.)
In the same beds with the supposed Proteaceae there occurs at Locle a fan-palm
of the American type Sabal (for genus see Figure 151), a genus which ranges
throughout the low country near the sea from the Carolinas to Florida and
Louisiana. Among the Coniferae of Upper Miocene age is found a deciduous cypress
nearly allied to the Taxodium distichum of North America, and a Glyptostrobus
(Figure 144), very like the Japanese G. heterophyllus, now common in our
shrubberies.
Before the appearance of Heer's work on the Miocene Flora of Switzerland, Unger
and Goppert had already pointed out the large proportion of living North
American genera which distinguished the vegetation of the Miocene period in
Central Europe. Next in number, says Heer, to these American forms at Oeningen
the European genera preponderate, the Asiatic ranking in the third, the African
in the fourth, and the Australian in the fifth degree. The American forms are
more numerous than in the Italian Pliocene flora, and the whole vegetation
indicates a warmer climate than the Pliocene, though not so high a temperature
as that of the older or Lower Miocene period.
The conclusions drawn from the insects are for the most part in perfect harmony
with those derived from the plants, but they have a somewhat less tropical and
less American aspect, the South European types being more numerous. On the
whole, the insect fauna is richer than that now inhabiting any part of Europe.
No less than 844 species are reckoned by Heer from the Oeningen beds alone, the
number of specimens which he has examined being 5080. The entire list of Swiss
species from the Upper and Lower Miocene together amount to 1322. Almost all the
living families of Coleoptera are represented, but, as we might have anticipated
from the preponderance of arborescent and ligneous plants, the wood-eating
beetles play the most conspicuous part, the Buprestidae and other long-horned
beetles being particularly abundant.
(FIGURE 145. Harpactor maculipes, Heer. Upper Miocene, Oeningen.)
The patterns and some remains of the colours both of Coleoptera and Hemiptera
are preserved at Oeningen, as, for example in Harpactor (Figure 145), in which
the antennae, one of the eyes, and the legs and wings are retained. The
characters, indeed, of many of the insects are so well defined as to incline us
to believe that if this class of the invertebrata were not so rare and local,
they might be more useful than even the plants and shells in settling
chronological points in geology.
MIDDLE OR MARINE MOLASSE (UPPER MIOCENE) OF SWITZERLAND.
It was before stated that the Miocene formation of Switzerland consisted of,
first, the upper fresh-water molasse, comprising the lacustrine marls of
Oeningen; secondly, the marine molasse, corresponding in age to the faluns of
Touraine; and thirdly, the lower fresh-water molasse. Some of the beds of the
marine or middle series reach a height of 2470 feet above the sea. A large
number of the shells are common to the faluns of Touraine, the Vienna basin, and
other Upper Miocene localities. The terrestrial plants play a subordinate part
in the fossiliferous beds, yet more than ninety of them are enumerated by Heer
as belonging to this falunian division, and of these more than half are common
to subjacent Lower Miocene beds, while a proportion of about forty-five in one
hundred are common to the overlying Oeningen flora. Twenty-six of the ninety-two
species are peculiar.
UPPER MIOCENE OF THE BOLDERBERG, IN BELGIUM.
(FIGURE 146. Oliva Dufresnii, Bast. Bolderberg, Belgium; natural size.
a. Front view.
b. Back view.)
In a small hill or ridge called the Bolderberg, which I visited in 1851,
situated near Hasselt, about forty miles E.N.E. of Brussels, strata of sand and
gravel occur, to which M. Dumont first called attention as appearing to
constitute a northern representative of the faluns of Touraine. On the whole,
they are very distinct in their fossils from the two upper divisions of the
Antwerp Crag before mentioned (Chapter 13), and contain shells of the genera
Oliva, Conus, Ancillaria, Pleurotoma, and Cancellaria in abundance. The most
common shell is an Olive (Figure 146), called by Nyst Oliva Dufresnii; and
constituting, as M. Bosquet observes, a smaller and shorter variety of the
Bordeaux species.
So far as the shells of the Bolderberg are known, the proportion of recent
species agrees with that in the faluns of Touraine, and the climate must have
been warmer than that of the Coralline Crag of England.
UPPER MIOCENE BEDS OF THE VIENNA BASIN.
In South Germany the general resemblance of the shells of the Vienna tertiary
basin with those of the faluns of Touraine has long been acknowledged. In the
late Dr. Hornes's excellent work on the fossil mollusca of that formation, we
see accurate figures of many shells, clearly of the same species as those found
in the falunian sands of Touraine.
According to Professor Suess, the most ancient and purely marine of the Miocene
strata in this basin consist of sands, conglomerates, limestones, and clays, and
they are inclined inward, or from the borders of the trough towards the centre,
their outcropping edges rising much higher than the newer beds, whether Miocene
or Pliocene, which overlie them, and which occupy a smaller area at an inferior
elevation above the sea. M. Hornes has described no less than 500 species of
gasteropods, of which he identifies one-fifth with living species of the
Mediterranean, Indian, or African seas, but the proportion of existing species
among the lamellibranchiate bivalves exceeds this average. Among many univalves
agreeing with those of Africa on the eastern side of the Atlantic are Cypraea
sanguinolenta, Buccinum lyratum, and Oliva flammulata. In the lowest marine beds
of the Vienna basin the remains of several mammalia have been found, and among
them a species of Dinotherium, a Mastodon of the Trilophodon family, a
Rhinoceros (allied to R. megarhinus, Christol), also an animal of the hog tribe,
Listriodon, von Meyer, and a carnivorous animal of the canine family. The Helix
turonensis (Figure 38 Chapter 3), the most common land shell of the French
faluns, accompanies the above land animals. In a higher member of the Vienna
Miocene series are found Dinotherium giganteum (Figure 136 Chapter 14), Mastodon
longirostris, Rhinoceros Schleiermacheri, Acerotherium incisivum, and
Hippotherium gracile, all of them equally characteristic of an Upper Miocene
deposit occurring at Eppelsheim, in Hesse Darmstadt; a locality also remarkable
as having furnished in latitude 49 degrees 50 north the bone of a large ape of
the Gibbon kind, the most northerly example yet discovered of a quadrumanous
animal.
(FIGURE 147. Amphistegina Hauerina, d'Orbigny. Upper Miocene strata, Vienna.)
M. Alcide d'Orbigny has shown that the foraminifera of the Vienna basin differ
alike from the Eocene and Pliocene species, and agree with those of the faluns,
so far as the latter are known. Among the Vienna foraminifera, the genus
Amphistegina (Figure 147) is very characteristic, and is supposed by d'Archiac
to t 
The Student's Elements of Geology / Lyell, Charles, Sir, 1797-1875