Elements of Geology ... Sixth edition, greatly enlarged and illustrated with ... woodcuts

Preface

. The last, or fifth edition of this work, was published in February 1855, so that nearly ten years have since elapsed. I have allowed it to remain several years out of print, having been much occupied in travelling through various parts of Europe, and latterly in writing my " Geological Evidences of the Antiquity of Man," as well as the appendices to the second and third editions of that treatise. In the interval since 1855 I have published several supplements to the " Ele ments," the contents of which are now incorporated in this work. This and other new matter, illustrated by more than 50 new woodcuts, has added 130 pages to the volume, which has thus outgrown the dimensions usually assigned to a Manual. I have, therefore, restored to the book its original title of " The Elements of Geology," under which name it first appeared in 1838, when it consisted of an expansion of the fourth book of my " Principles of Geology," which had at that time reached a fifth edition.

The " Elements " were successively re-edited, and in each case to a great extent recast, in the years 1842, 1851, 1852, and 1855. On former occasions I have given a list of the principal corrections and additions in which each new edition VI

PREFACE.

. differed from its predecessor, but I shall not attempt to offer the reader such a summary in the present case, fearing that that would prove tediously long.* A full index is given in this as in former editions, and the student will observe that all the organic remains of which there are woodcut* figures in the text are printed in italics in the index. CHARLES LYELL. 53 Haeley Street, London : Dec. 20, 1864. * As it is impossible to enable the reader to recognize rocks and minerals at sight by aid of verbal descriptions or figures, he will do well to obtain a well arranged collection of specimens, such as may be procured from Mr. Tennant (149 Strand), teacher of Mineralogy at King's College, London. CONTENTS.

Chapter I. On Ihe Different Classes Of Rocks

Geology defined— Successive formation ofthe earth's crust — Classification of rocks according to their origin and age — Aqueous rocks — Their stratification and embedded 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 — Arenaceous rocks — Argillaceous — Calcareous — Gypsum — Forms of stratification — Original horizontality — Thinning out — Diagonal arrangement — Ripple mark 10

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 and semi-opal formed of infusoria — Chalk derived principally from organic bodies — Distinction of freshwater from marine formations — Genera of freshwater and land shells — Rules for recognizing marine testacea — Gyrogonite and chara — Freshwater fishes — Alternation of marine and freshwater deposits — Lym-Fiord 21

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— Source of lime in solution — Silex derived from decomposition of felspar — Proofs of the lapidification of some fossils soon after burial, of others when much decayed CONTENTS. VIII

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 — Upheaval of extensive masses of horizontal strata — Inclined and vertical stratification — Anticlinal and synclinal lines— Bent strata in east of Scotland— Theory of folding by lateral movement — Creeps— Dip and strike — Structure of the Jura — Various forms of outcrop — Rocks broken by flexure— Inverted position of disturbed strata — Unconformable stratification — Hutton and Playfair on the same — Fractures of strata — Polished surfaces — Faults — Appearance of repeated alternations produced by them — Origin of great faults .... page 44

Chapter VI. Denudation

Denudation defined — Its amount equal to the entire mass of stratified deposits in the earth's crust — Horizontal sandstone denuded in Ross-shire — Levelled surface of countries in which great faults occur — Coalbrook Dale — Denuding power of the ocean during the emergence of land — Origin of Valleys — Obliteration of sea-cliffs — Inland sea-cliffs and terraces in the Morea and Sicily — Limestone pillars at St. Mihiel, in France — in Canada — in the Bermudas . . . .66

Chapter VII. Alluvium

Alluvium described— Due to complicated causes — Of various ages, as shown in Auvergne — How distinguished from rocks in situ — Sandpipes in Chalk — Alluvial terraces caused by oscillation in the level of land . . .79

Chapter VIII. Chronological Classification Of Rocks

Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically — Lehman's division into primitive and secondary — Werner's addition of a transition class — Neptunian theory — Hutron on igneous origin of granite — How the name of primary was still retained for granite — The term " transition," why faulty — The adherence to the old chronological nomenclature retarded the progress of geology — New hypothesis invented to reconcile the igneous origin of granite to the notion of its high antiquity — Explanation of the chronological nomenclature adopted in this work, so far as regards primary, secondarv, and tertiary periods .86

Chapter IX. On The Different Ages Of The Aqueous Rocks

On the 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 a°*e bv included fragments — Frequent absence of strata of intervening periods— Principal groups of strata in western Europe — Tabular views of fossiliferous strata . . CONTENTS. IX

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 — Form of human skulls of the Recent period — Post-pliocene formations — Coexistence of man with extinct mammalia — Higher and Lower-level Valleygravels — Loess or inundation mud of the Nile, Rhine, &c. — Antiquity of Post-pliocene Lake-terraces in Switzerland — Upraised marine strata in Sardinia — Origin of caverns — Remains of man and extinct quadrupeds in cavern deposits — Cave of Kirkdale — Reindeer period of south of France — Australian cave-breccias — Geographical relationship of the provinces of living vertebrata and those of extinct Post-pliocene species — Extinct struthious birds of New Zealand — Fluctuations of climate in Post-glacial period — Comparative longevity of species in the mammalia and testacea — Teeth in Recent and Post-pliocene mammalia page 107

Chapter XI. Post-pliocene Period

Continued. — GLACIAL ErOCH. Geographical distribution, form, and characters of glacial drift — Fundamental rocks, poli.hed, grooved, and scratched — Abrading and striating action of glaciers — Moraines, erratic blocks, and "Roches Moutonnees " — Alpine blocks on the Jura — Colossal size of ancient Swiss glaciers — Continental ice of Greenland — Ancient centres ofthe dispersion of erratics — Transportation of drift by floating icebergs — Bed of the sea furrowed and polished by the running aground of floating ice-islands — How to distinguish glacial drift of submarine from that of terrestrial origin . 136

Chapter XII

post-pliocene teriod continued. — glacial epoch concluded. Glaciation of Scandinavia and Russia — Glaciation of Scotland — 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 — Norfolk drift — Glacial formations of North America — How far of submarine origin — Many species of testacea and quadrupeds survived the glacial cold — Connection ofthe predominance of lakes with glacial action — Morainic lakes — Objections to the hypothesis ofthe erosion of large lake-basins by ice — Conversion of valleys of denudation into lakes by upward and downward movements — Action of ice in preventing the silting-up of lake-basins— How the bed ofa sea where icebergs have abounded may, on emerging, afford lake-basins — General causes of change of climate — Submergence of the Sahara in the Post-pliocene period a cause of Alpine cold — Meteorites in drift 149

Chapter XIII. Classification Of Tertiary Formations

— PLIOCENE PERIOD. Order of succession of sedimentary formations — Imperfection of the record — Defectiveness and obscurity of the monuments greater in proportion to their antiquity — Reasons for studying the newer groups first — General principles of classification of tertiary strata — Detached formations scattered over Europe — Strata of Paris and London — More modern groups. — Peculiar difficulties in determining the chronology of tertiary formations — Increasing proportion of X CONTENTS. living species of shells in strata of newer origin —Eocene, Miocene, and Pliocene terms explained — Formations of the Newer Pliocene period — Island of Ischia — Eastern base of Mount Etna — Newer Pliocene strata of great height and extent in Sicily — Formations of same age in the Upper Val d'Arno— Norwich Crag — Chillesford beds— Bridlington beds — Older Pliocene strata — Red Crag of Suffolk — White or Coralline Crag — Successive refrigeration of climate proved by the Pliocene shells of Suffolk and Norfolk— Antwerp Crag— Subapennine strata — Aralo-Caspian formations page 177

Chapter XIV. Miocen'e Period

Upper Miocene strata of France — Faluns of Touraine — Depth of sea and littoral character of fauna — Tropical climate implied by the testacea — Proportion of recent species of shells — Faluns more ancient than the Suffolk Crag — Varieties of Voluta Lamberti peculiar to Faluns and to Suffolk Crag — The same Species are common to more than one geological Period — Lower Miocene strata of France — Remarks on classification, and where to draw the line of separation between Miocene and Eocene strata — Relations of the Gres de Fontainebleau to the Faluns and to the Cal-aire Grassier — Lower Miocene strata of Central France — Lacustrine strata of Auvergne — Indusia! limestone — Fossil mammalia of the Limagne d'Auvergne — Freshwater strata ofthe Cantal — Its resemblance in some places to white chalk with flints — Proofs of gradual deposition — Miocene strata of Bordeaux and south of France — Upper Miocene strata of Gers — Drjopitliecus — Belgian and British Miocene formations — Edegham beds near Antwerp — Diest sands of Belgium and contemporaneous iron-sands of North Downs — Upper Miocene beds of Belgium — Bolderberg — Lower Miocene strata of Kleyn Spawen — Hempstead beds, Isle of Wight — Bovey Tracey Lignites in Devonshire — Lie of Mull Leaf-beds — Miocene formations of Germany — Mayence basin — Upper Miocene beds of Vienna basin — Lower Miocene of Croatia — Fossil Lepidoptera — Oligocene strata of Professor Beyrich — Miocene strata of Italy 210

Chapter XV

miocene formations — continued. Miocene strata of Switzerland — Upper Miocene beds of Œningen — Importance of Fossil Plants — Heer's work on the Swiss Miocene flora — Plants and insects of Œningen embedded in different seasons — Fossil fruits and flowers, as well as leaves — Middle or Marine Molasse of Switzerland — Lower Molasse, or Lower Miocene — Dense conglomerates and proofs of subsidence — Fossil plants of Lower Miocene period more tropical — Preponderance of arborescent species — Supposed discordance in relative numbers of living species of plants and shells in Upper Miocene formations — Theory of a Miocene Atlantis — Whether the American plants abounding in the Miocene of Europe migrated by a westerly or an easterly route — Objections derived from depth and width of the Atlantic — Arguments in favour of a Trans-Asiatic migration — Miocene fossils of Oregon — Agreement of Miocene corals ofthe West Indies and Europe opposed to the theory of an Atlantic Continent— Upper Miocene formations of India Sub-Himalayan or Siwalik Hills — Older Pliocene and Miocene formations in the United States of America 246

Chapter XVI. Eocene Formations

Upper Eocene strata of England — Fluvio-marine series in the Isle of Wight and Hampshire — Successive groups of Eocene Mammalia — Boundary-line between CONTENTS. XI Lower Miocene and Eocene — Fossils of Barton Clay — British Middle Eocene — Shells, nummulites, fishes, and reptiles of the Bagshot and Bracklesham beds — Vegetation of Middle Eocene period — Lower Eocene strata of England— Fossil plants and shells of the London Clay proper — Strata of Kyson in Suffolk — Plastic clays and sands — Thanet sands— Eocene formations of France — Gyp seous series of Montmartre and extinct quadrupeds — Fossil footprints — Calcaire Grossier — Miliolites — Lower Eocene in France — Nummulitic formations of Europe, Africa, and Asia — Their wide extent — referable to the Middle Eocene period — Eocene strata in the United States — Section at Claiborne, Alabama — ■ Colossal cetacean — Oibitoidal limestone — Burr stone . . . page 278

Chapter XVII. Cretaceous Group

Lapse of time between the Cretaceous and Eocene periods — Whether certain lormilions in Belgium and France are of intermediate age — Pisolitic limestone — Divisions of the Cretaceous series in North-Western Europe — Maestricht beds — Chalk of Faxoe — White chalk — Its geographical extent and origin — Formed in an open and deep sea — How far derived from shells and coralsv— A similar rock now in progress in the depths of the Atlantic made up of Globigerina. — Origin of Flint in Chalk — Siliceous Diatomaceæ of the Atlantic — By what intermittent action the alternate layers of white chalk and flint may have been caused — Pot-stones of Horstead — Isolated pebbles of quartz and foreign rocks in chalk — Fossils of the Upper Cretaceous rocks — Echinoderms, Mollusca, Bryozoa, Sponges — Upper Greensand and Gault— Blackdown beds — Flora of the Upper Cretaceous period — Fossil plants of Aix-la-Chapelle — Large proportion of Dicotyledonous Angiosperms — Their co-existence with large extinct genera of reptiles — Chalk of South of Europe — Hippurite limestone — Cretaceous rocks of the United States 310

Chapter XVIII. Lower Cretaceous And Wealden Formations

Lower Greensand — Term " Neocomian " — Atherfield section, Isle of Wight — Fossils of Lower Greensand — Palæontological relations ofthe Upper and Lower Cretaceous strata — Wealden Formation — Freshwater strata intercalated between two marine groups — Weald Clay and Hastings Sand — Tunbridge rocks — Fossil shells, fish, and plants of Wealden — Their relation to the Cretaceous type — Geographical extent of Wealden — Movements in the earth's crust to which the Wealden owed its origin and submergence 339

Chapter XIX. Denudation Of Ihe Chalk And Tvealden

Physical geography of certain districts composed of Cretaceous and Wealden strata — Lines of inland chalk-cliffs on the Seine in Normandy — Outstanding pillars and needles of chalk — Denudation of the chalk and Wealden in Surrey, Kent, and Sussex — Chalk once continuous from the North to the South Downs — Anticlinal axis and parallel ridges — Longitudinal and transverse valleys — Chalk escarpments — Rise and denudation ofthe strata gradual — Ridges formed by harder, valleys by softer beds — At what periods the Weald Valley was denuded — Why no alluvium, or wreck of the chalk, in the central district of the Weald — Successive periods of marine denudation — The latest of these posterior to the Upper Miocene era — Elephant bed, Brighton — Sangatte Cliff* — The great escarpments and transverse valleys of the chalk mainly due to the waves and tides of the sea —Paroxysmal causes unnecessary for explaining the external configuration of the Wealden . . . . . . .351 CONTENTS.

Chapter XX. 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 sgil— Fossils of the Purbeck beds— Portland stone and fossils— Lithographic stone of Solenhofen— Archæopteryx— Middle Oolite — Coral rag — Zoophytes — Nerinæan limestone — Diceras limestone— - Oxford Clay, Ammonites and Belemnites — Kelloway Rock — Lower Oolite, Crinoideans— Great Oolite and Bradford clay— Stonesfield slate— Fossil mammalia — Resemblance to an Australian fauna — Northamptonshire slates — Yorkshire Oolitic coal-field — Brora coal — Fuller's earth — Inferior Oolite and fossils — Palæontological relations ofthe several subdivisions ofthe Oolitic group, page 375

Chapter XXI. 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 — Name of Gryphite limestone — Fossil shells and fish — Radiata — Ichthyodorulites — 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 ..... 413

Chapter XXII. Trias Or New Red Sandstone Group

Distinction between New and Old Red Sandstone — Between Upper and Lower New Red — The Trias and its three divisions — Most largely developed in Germany — Recognition of a Marine equivalent of the Upper Trias in the Austrian Alps — True position of the St. Cassian and Hallstadt Beds — 800 new species of triassic Mollusca and Radiata — Links thus supplied for connecting the Palæozoic and Neozoic faunas — Keuper and its fossils — Muschelkalk and fossils — Fossil plants of the Bunter — Triassic group in England — Bone-bed of Axmouth and Aust — Red Sandstone of Warwickshire and Cheshire — Footsteps of Cheirotherium in England and Germany — Osteology of the Labyrinthodon — Whether this Batrachian was identical with Cheirotherium — Dolomitic Conglomerate of Bristol — Origin of Red Sandstone and Rock-salt — Hypothesis of saline volcanic exhalations — Theory of the precipitation of salt from inland lakes or lagoons — Saltness ofthe Red Sea — Triassic coal-field of Eastern Virginia, near Richmond— New Red Sandstone intheUnited States— Fossil footprints of birds and reptiles in the valley of the Connecticut — Antiquity of the Red Sandstone containing them — Triassic mammifer of North Carolina .... 429

Chapter XXIII. Permian Ok Magnesian Limestone Group

Fossil of Magnesian Limestone and Lower New Red distinct from the Triassic — Term Permian — English and German equivalents — Marine shells and corals of English Magnesian Limestone — Palæoniscus and other fish of the marl-slate — Zechstein and Rothliegendes of Thuringia — Permian Flora — Its generic affinity to the Carboniferous — Psaronites or tree-ferns CONTENTS. XIII

Chapter XXIV. The Coal, Or Carboniferous Group

Carboniferous strata in the south-west of England — Superposition of Coal-measures to Mountain Limestone — Departure from this type in North of England and Scotland — Carboniferous series in Ireland — Section in South Wales — Underclays with Stigmaria — Carboniferous Flora — Ferns, Lepidodendra, Equisetaceæ, Catamites, Asterophyllites, Sigillariæ, Stigmariæ — Coniferæ — Sternbergia — Trigonocarpon — Grade of Coniferæ in the Vegetable Kingdom — Absence of Angiosperms — Coal, how formed — Erect fossil trees — Parkfield Colliery — St. Etienne Coal-field — Oblique trees or snags — Fossil forests in Nova Scotia — Rain-prints — Purity of the Coal explained — Time required for the accumulation of the Coal-measures— Brackish-water and marine strata — Crustaceans of the Coal — Origin of Clay-iron-stone .... . . page 463

Chapter XXV

carboniferous group — continued. Coal-fields ofthe United States — Section ofthe country between the Atlantic and Mississippi — Position of land in the carboniferous period eastward of the Alleghanies — Mechanically formed rocks thinning out westward, and limestones thickening — Uniting of many coal-seams into one thick bed — Horizontal coal at Brownsville, Pennsylvania — Vast extent and continuity of single seams of coal — Ancient river-channel in Forest of Dean coal-field — Climate of Carboniferous period — Insects in coal — Rarity of air-breathing animals — Great number of fossil fish — First discovery of the skeletons of fossil reptiles — Footprints of reptilians — First land-shell found — Rarity of air-breathers, whether vertebrate or invertebrate, in Coal-measures — Mountain limestone — Its corals and marine shells 493

Chapter XXVI. Old Red Sandstone, Or Devonian Group

Old Red Sandstone of the Borders of Wales — Of Scotland and the South of Ireland—Fossil Devonian plants at Kilkenny — Holoptychius of the Middle and Cephalaspis oi the Lower Old Red of Forfarshire— Pterygotus and supposed eggs of Crustaceans — Northern type of Old Red in Scotland — Classification of the Ichthyolites of the Old Red, and their relation to living types — Distinct lithological type of Old Red in Devon and Cornwall — Term Devonian — Organic remains of intermediate character between those of the Carboniferous and Silurian systems— Devonian series of England and the Continent — Upper Devonian rocks and fossils — Middle — Lower— Old Red Sandstone of Russia- Preponderance of Brachiopoda — Devonian Strata of the United States and Canada — Coral reefs at the falls of the Ohio — Gaspe Sandstone — Vegetation of the Devonian period 520 CHAPTER XXVH.

SILURIAN AND CAMBRIAN GROUPS. Silurian strata formerly called Transition — Term Grauwacke — Subdivisions of Upper, Middle, and Lower Silurians — Ludlow formation and fossils — Oldest known remains of fossil fish — Wenlock formation, corals, cystideans, trilobites — Middle Silurian or Llandovery beds — Lower Silurian rocks — Caradoc and Bala beds — Upper and Lower Llandeilo formations — Cystideæ — Trilobites — Graptolites — Vast thickness of Lower Silurian strata, sedimentary and volcanic, in CONTENTS. Wales — Foreign Silurian equivalents in Europe — Silurian strata of the United States — Amount of specific agreement of fossils with those of Europe — Canadian equivalents — Whether Silurian strata of deep-sea origin — Cambrian rocks — Classification and nomenclature — Barrande's primordial fauna — Upper Cambrian of Wales — Tremadoc slates — Lingula flags— Lower Cambrian — Long mynd group — Oldest organic remains known in Europe — Foreign equivalents of the Cambrian group— Primordial zone of Bohemia — Characteristic trilobites — Metamorphosis of trilobites — Alum schists of Sweden and Norway — Potsdam sandstone of United States and Canada — Footprints near Montreal — Quebec strata and Huronian rocks — Minnesota trilobites — Rocks older than the Cam brian — Laurentian group — Upper and Lower — Oldest known fossil, Eozoon Cana dense — No remains of vertebrate animals known in strata below the Upper Silurian — Progressive discovery of vertebrata in older rocks — Theoretical inferences from the rarity or absence of vertebrata in the most ancient fossiliferous formations page 546

Chapter XXVIII. Volcanic Rocks

Trap rocks — Name, whence derived — Their igneous origin at first doubted — Their general appearance and character — Volcanic cones and craters, how formed — Mineral composition and texture of volcanic rocks — Varieties of felspar — Hornblende and augite — Isomorphism — Rocks, how to be studied — Basalt, trachyte, greenstone, porphyry, scoria, amygdaloid, lava, tuff* — Agglomerate- Laterite — Alphabetical list, and explanation of names and synonyms, of volcanic rocks — Table of the analyses of minerals most abundant in the volcanic and hypogene rocks ......... 587

Chapter XXIX. Volcanic Rocks

— continued. Trap dikes— sometimes project — sometimes leave fissures vacant by decomposition — Branches and veins of trap — Dikes more crystalline in the centre Strata altered at or near the contact — Obliteration of organic remains Conversion of chalk into marble — Trap interposed between strata— Columnar and globular structure — Relation of trappean rocks to tbe products of active volcanos - Form, external structure, and origin of volcanic mountains — Craters and Calderas — Sandwich Islands — Lava flowing underground — Truncation of cones — Javanese calderas — Canary Islands— Structure and origin of the Caldera of Palma — Older and newer volcanic rocks in, unconformable — Aqueous conglomerate in Palma — Hypothesis of upheaval considered — Slope on which stony lavas may form— Extent and nature of aqueous erosion in Falma — Island of St. Paul in the Indian Ocean — Peak of Teneriffe, and ruins of older cone Madeira — Its volcanic rocks, partly of marine, and partly of subaerial origin Central axis of eruptions— Varying dip of solid lavas near the axis, and further from it— Leaf-bed, and fossil land-plants— Central valleys of Madeira not craters, or calderas ..... . gno

Chapter XXX. On The Different Ages Of The Volcanic Rocks

Tests of relative age of volcanic rocks— 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— Volcanic rocks of the Post-pliocene period— Basalt of the Bay of Trezza in Sicily— Post-pliocene volcanic rocks near Naples— Dikes of Somma CONTENTS. XV

Chapter XXXI. On The Different Ages Of The Volcanic Rocks

— Continued. Volcanic rocks of the Newer Pliocene period — Val di Noto — Sicilian dikes — Region of Olot in Catalonia — Volcanic rocks of the Older Pliocene period — Tuscany — Rome— Volcanic region of Olot in Catalonia — Cones and lava-currents — Ravines and ancient gravel-beds — Jets of air called Bufadors — Age of the Catalonian volcanos — Upper Miocene period — Volcanic archipelagos of Madeira, the Canaries, and the Azores — Lower Miocene period — Brown-coal of the Eisel and contemporaneous trachytic breccias — Age of the brown-coal — Peculiar characters of the volcanos of the upper and lower Eisel — Lake Craters — Trass — Hungarian volcanos ...... page 658

Chapter XXXII. On The Different Ages Of The Volcanic Rocks

— Continued. Volcanic rocks of the Tertiary period, continued — Extinct volcanos of Auvergne — Mont Dor — Breccias and alluviums of Mont Perrier, with bones of quadrupeds — River dammed up by lava-current — Range of minor cones from Auvergne to the Vivarais — Monts Dome — Puy de Come— Puy de Pariou — Cones not denuded by general flood — Lower Miocene volcanic rocks near Clermont — Hill of Gergovia — Eocene volcanic rocks of Monte Bolca — Trap of Cretaceous period — Oolitic period — New Red Sandstone period- Carboniferous period — "Rock and Spindle " near St. Andrew's — Old Red Sandstone period — Silurian period — Cambrian period — Laurentian volcanic rocks 677

Chapter XXXIII. Plutonic Rocks

— GRANITE. General aspect of granite — Decomposing into spherical masses — Rude columnar structure — Analogy and difference of volcanic and plutonic formations — Minerals in granite, and their arrangement — Graphic and porphyritic granite — Mutual penetration of crystals of quartz and felspar — Occasional minerals — Syenite — Syenitic, talcose, and schorly granites— Eurite — Passage of granite into trap — Examples near Christiania and in Aberdeenshire — Analogy in composition of trachyte and granite — Granite veins in Glen Tilt, Cornwall, the Valorsine, and other countries — Different composition of veins from main body of granite — Metalliferous veins in strata near their junction with granite — Apparent isolation of nodules of granite — Quartz veins — Whether plutonic rocks are ever overlying — Their exposure at the surface due to denudation .... 694

Chapter XXXIV. 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 Tertiary 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— On the probable age of the granites of Arran, in Scotland CONTENTS.

Chapter XXXV. Metamorphic Rocks

General character of metamorphic rocks — Gneiss — Hornblende-schist — Mica-schist — Clay-slate — Quartzite — Chlorite-schist — Metamorphic limestone — Alphabetical list and explanation of the more abundant rocks of this family — 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 — Time may enable this action to pervade denser masses — From what kinds of sedimentary rock each variety of the metamorphic class may be derived — Certain objections to the metamorphic theory considered — Partial conversion of Eocene slate into gneiss .... page 724

Chapter XXXVI

metamorphic rocks — continued. Definition of joints, slaty cleavage and foliation — Supposed causes of these structures — Mechanical theory of cleavage — Condensation and elongation of slate rocks by lateial pressure — Supposed combination of crystalline and mechanical forces — 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 — Foliation in homogeneous rocks may coincide with planes of cleavage, and in uncleaved rocks with those of stratification — Causes of irregularity in the planes of foliation 738

Chapter XXXVII

, OF THE DIFFERENT AGES OF THE METAMORPHIC ROCKS, Age of each set of metamorphic strata twofold — Test of age by fossils and mineral character not available — Test by superposition ambiguous — Conversion of dense mass of fossiliferous strata into metamorphic rocks— Limestone and shale of Carrara — Metamorphic' strata of older date than the Cambrian rocks — Others of Lower Silurian origin — Others of the Jurassic and Eocene periods in the Alps of Switzerland and Savoy — Why scarcely any of the visible crystalline strata are very modern — Order of succession in metamorphic rocks Uniformity of mineral character — Why the metamorphic strata are less calcareous than the fossiliferous ••■••..... 749 CHAPTER XXXVTII. MINERAL VEINS. Werner's doctrine that mineral veins were fissures filled from above Veins of segregation — Ordinary metalliferous veins or lodes — Their frequent coincidence with faults — Proofs that they originated in fissures in solid rock Veins shiftm** other veins— Polishing of their walls or " slicken-sides " — Shells and pebbles in lodes— Evidence of the successive enlargement and reopenino* of veins Fournet's observations in Auvergne — Dimensions of veins — Why some alternately swell out and contract— Filling of lodes by sublimation from below- Chemical and electrical action— 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 — Concluding remarks ,758 ELEMENTS OF GEOLOGY.