The Student's Elements of Geology ... With ... illustrations

Front matter

ELEMENTS OF GEOLOGY By the same Author. Twelfth Edition, with Illustrations, 2 vols. 8vo. 32s. The PRINCIPLES of GEOLOGY; or, the Modem Changes of the Earth and its Inhabitants, considered as Illustrative of Geology. Fourth Edition, with Illustrations, 8vo. 14s. The GEOLOGICAL EVIDENCES of the ANTIQUITY of MAN. AVith an Outline of Glacial Post- Tertiary Geology, and Remarks on the Origin of Species, with Special Reference to Man's First Appearance on the Earth. .TOHN MURRAY, Albemarle Street FRONTISPIECE or CAINCZOIC TERTIARY Nummulites lævigata SECONDARY or MESOZOIC Ammonites Rhotomagcnsis PRIMARY Bronteus flahellifer THE STUDENT'S ELEMENTS OF GEOLOGY By Sir CHAELES LYELL, Bart., F.R.S. AUTHOR OK 'THE PRINCIPLES OF GEOLOGY1 'THE ANTIQUITY OP MAN ETC. Thccosm ilia an n ularis THIRD EDITION, REVISED .1 N D^pWRECT E D WITH A TABLE OF BRITISH FOSSILS AND MORE THAN 600 ILLUSTRATIONS LONDON JOHN MURKAY, ALBEMARLE STREET 1878 The right of translation is reserved

Preface

. Between the years 1838 and 18G5 I published six editions of the ' Elements of Geology,' beginning with a small duodecimo volume, which increased with each successive edition, as new facts accumulated, until in 1865 it had become a large and somewhat expensive work. When a seventh edition was called for, I was strongly urged by my friends to attempt to bring the book back again to a size more approaching the original, so that it might te within the reach of the ordinary student. In order to do this I resolved, in the first place, to omit some theoretical discussions which belonged more properly to my ' Principles of Geology,' and further to confine myself to examples of British rocks, wherever this could te done ; only seeking foreign illustrations when, as in the case of the Upper Miocene or Falunian Tertiaries, no good representatives were to be found in this country.

I therefore published in 1871 what was substantially a new work under the title of 'The Student's Elements of Geology,' and the success of the attempt has teen proved by the steady demand which has exhausted an unusually large edition in less than three years. The present work has been carefully revised and corrected with the addition of such new matter as the plan of the volume permitted. "X-o •*■

PREFACE.

. I have also added a new and very important table (see p. 624) illustrative of the successive appearance and development in time of the different forms of animal and vegetable life throughout the British fossiliferous rocks. This table has been compiled for me by Mr. Etheridge, of the London School of Mines, from materials which he has teen collecting for many years. Among the numerous scientific friends who have rendered me valuable assistance in different parts of this new edition, I should wish especially to mention Mr. Searles Wood, Mr. David Forbes, Mr. Judd, and the Rev. T. G. Bonney, of St. John's, Cambridge. Ohari.es Lyell. 73 Hae ley Street February 1874.

PREFACE.

in THE THIRD EDITION The present edition has been revised and corrected throughout, receiving such additions and corrections as "the results of more recent investigations seemed to require. At the same time care has been taken not to alter the original character of the book. With this object in view, the passages in the text which have been added or essentially modified now appear enclosed in square brackets. In preparing this new edition, I wish especially to acknowledge the assistance given me by Professor Judd. particularly in the chapters on Volcanic Rocks. Mr. Etheridge also has been so good as to revise the tables of British fossils which he originally prepared for the previous edition. Leonard Lyell. October 1878. a 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 PAGE 1

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 . . . .11

Chapter III. Arrangement Of Fossils In Strata

— FRESHWATER 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 freshwater from marine formations — Genera of freshwater an d land shells — Rules for recognising marine testacea — Gyrogonite and Chara — Freshwater fishes — Alternation of marine and freshwater deposits — Lym-Fiord . 23

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 — Mineralisation of organic remains — Impressions and casts, how CONTENTS. formed — Fossil wood — Gbppert's experiments — Precipitation of stony matter most rapid where putrefaction is going on — Sources of lime and silex in solution ...... page 37

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 freshwater 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 . . . . .47

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 . . . . .74

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 amouut 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 . . 87

Chapter VIII. Chronological Classification Of Rocks

Aqueous, plutonic, volcanic, and metamorphic rocks considered chronologically — Terms Primary, Secondary, and Tertiary ; Palæozoic, Mesozoic, and Cainozoic explained — On tlie different ages of the aqueous roeks — CONTENTS. 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 palæontological characters — Test of age by included fragments — Frequent absence of strata of intervening periods — Tabular views of fossiliferous strata page 100

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 . 116

Chapter X. Recent And Pleistocene Periods

Recent and Pleistocene periods — Terms defined — Formations ofthe 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 — Pleistocene 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, &c. — 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 Pleistocene species — Extinct struthious birds of New Zealand — Climate of the Pleistocene period — Comparative longevity of species in the mammalia and testacea — Teeth or Recent and Pleistocene mammalia . . 124

Chapter XI

pleistocene 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 . ..... CONTENTS.

Chapter XII. Pleistocene Period

COnt. — 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 .... page 154

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 168

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 Œningen in Switzerland— Plants of the Upper freshwater 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 . . 192

Chapter XV. Lower Miocene

Lower Miocene strata of France— Line between Miocene and Eocene- Lacustrine strata of Auvergue— 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— 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 CONTENTS. XV —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 . . . page 212

Chapter XVI. Eocene Formations

Eocene areas of North of Europe — Table of English and French Eocenestrata — 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 Ba}- and Bourne mouth—Lower Eocene of England — London Clay fossils — Woolwich and Reading beds formerly called ' Plastic Clay ' — Fluviatile beds underlyingdeep-sea strata — Thanet sands — Upper Eocene strata of France — Gypseous series of Montmartre and extinct quadrupeds— Fossil footprints iu Paris gypsum — Imperfection of the Record — Calcaire siliceux — Gre's de Beauchamp — Calcaire grossier — Miliolite limestone — Soissonnais sands — Lower Eocene of France — Nummulitic formations of Europe, Africa, and Asia — Eocene strata in United States — Gigantic cetacean . . . 232

Chapter XVII. Upper Cretaceous Group

[.apse of time between Cretaceous aud 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 — Potstones of Horstead — Vitreous sponges in the Chalk — Isolated blocks of foreign rocks in the white chalk, supposed to be iceborne — Distinctness of miueral 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 Cam bridge — 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 264

Chapter XVIII. Lower Cretaceous Or Neocomian Formation

lassificatiou of marine and freshwater 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 — Freshwater character of the Wealden — Weald clay — Hastings sand — Punfield beds of Purbeck, Dorset shire Fossil shells and fish ofthe Wealden — Area of the Wealden — Flora of the Wealden CONTENTS.

Chapter XIX. Jurassic Croup

— PURBECK BEDS AND OOLITE. I'he 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 ofthe Purbeck beds— Portland stone and fossils— Kimmeridge clay— Lithographic stone of Solenhofen— Archæopteryx— Middle Oolite— Coral rag— Nerinæa limestone—Oxford Clay, Ammonites and Belemnites— Kelloway rock- Lower, or Bath Oolite— Plants of the Great Oolite— Oolite and Bradford elay — Stonesfield slate— Fossil mammalia— Fuller's earth— Inferior Oolite and fossils— Northamptonshire slates— Yorkshire Oolitic coal-field— Brora coal— Palæontological relations of the several subdivisions of the Oolitic group .....•• page 307

Chapter XX

Jurassic group — continued, lias. Mineral character of Lias — Numerous successive Zones iu the Lias, marked by distinct fossils, without unconformity in the stratification, or change in the mineral character of the deposits — Gryplute 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 — Origin of the Oolite and Lias, and of alternating calcareous and argillaceous formations .... 340

Chapter XXI. Trias, Or New Red Sandstone Group

Beds of passage between the Lias anctTrias, Rhætic beds — Triassic mammifer — Triple division of the Trias — Keuper, or Upper Trias of England — Reptiles of the Upper Trias — Footprints 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 tlieir fauna — Muschelkalk and its fossils — Trias of the United States — Fossil footprints 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 ' . . . . . . . 354

Chapter XXII. Permian Or Magnesian Limestone Group

Line of separation between Mesozoic and Palæozoic rocks — Distinctness of Triassic and Permian fossils — Term Permian — Thickness of calcareous and CONTENTS. XVII 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 — Footprints of reptiles — Angular breccias in Lower Permian — Extent of land in the Permian Period — Per mian rocks of the Continent — Zechstein and Rothliegendes of Thuringia — Permian flora — Its generic affinity with the Carboniferous page 375

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 — Batra- , chian reptiles — Carboniferous Amphibia — Labyrinthodont footprints in coal-measures — Nova Scotia coal-measures with successive growths of erect fossil trees — Structure 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 . . . 385

Chapter XXIV. Flora And Fauna Of The Carboniferous Period

Vegetation of the Coal period — Ferns, Lycopodiaceæ, Equisetaceæ, Sigillariæ, Stigmariæ, Coniferæ — Monocotyledon of the coal-measures — Climate of the Coal period — Mountain Limestone — Marine Fauna of the Carboniferous period — Corals — Polyzoa, Crinoidea — Mollusca — Great number of fossil fish — Foraminifera ...... 413

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 Cephalapsis 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 ...... . 434 a CONTENTS.

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— Cystideæ— Graptolites— Llandeilo Flags— Arenig or Stiper-stones group — Foreign Silurian equivalents in Europe— Barrande's Silurian fauna— Silurian strata of the United States— Canadian equivalents— Amount of specific agreement of fossils with those of Europe ...... page 455

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 ofScotland . . . . . . . .480

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 roeks to the products of active volcanos . . 493

Chapter XXIX. On The Ages Of Volcanic Rocks

Test of relative age of volcanic rocks — Why ancient and modem rocks cannot 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 — Post-Tertianvolcanic rocks CONTENTS. XIX Vesuvius, Auvergne, Puy de Come, and Puy de Pariou— Newer Pliocene volcanic rocks — Cyclopean Isles, Etna, Dikes of Patagonia, Madeira — Older Pliocene volcanic rocks — Italy — Pliocene volcanos of the Eifel —Trass ........ page 520

Chapter XXX

age of volcanic rocks — continued. Volcanic rocks of the Upper Miocene Period — Madeira — Grand Canary — Azores — Lower Miocene volcanic rocks — Isle of Midi — Staffa and Antrim — The Eifel — Upper and Lower Miocene volcanic rocks of Auvergne — Hill of Gergovia — Eocene volcanic rocks of the Hebrides — 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 . . . 536

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 felspar — Glass cavities in quartz of granite — Porphyritic, talcose, and hornblendic granite — Eurite — Syenite — Diorite — Connection of the plutonic 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 ..... 551

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 — 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 .... 565

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 — CONTENTS. XX 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—Objec tions to the metamorphictheory considered . . • page 5.6

Chapter XXXIV

metamorphic rooks — 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 ........ 589

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 ....... 599

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 rocks — 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 — Origin of mineral veins . . . 607 TABLE OF BRITISH FOSSILS 622 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 1 These are the first enquiries with which Geology is occupied, a science whicli derives its name from the Greek yi), ge, the earth, and Xoyos, 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 v. [ch. i. 2 CLASSIFICATION OF ROCKS. commonly imagined that all these have 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 oxtr planet which is accessible to human observation. It comprises not merely all of wiiich 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 sur face.

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 j^th part of the distance from the sur face to the centre. The remark is just ; but although the di mensions of such a crust are, in truth, insignificant when com pared 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 con fusedly ; but distinct mineral masses, called rocks, are found to occupy definite spaces, and to exhibit a certain order of arrange ment.

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 in clude both, and in this sense we find roclie applied in French CH. I.] AQUEOUS EOCKS. 3 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.