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The Student's Elements of Geology
by Sir Charles Lyell
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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.)

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