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diff --git a/old/3772-8.txt b/old/3772-8.txt new file mode 100644 index 0000000..4e1bd7d --- /dev/null +++ b/old/3772-8.txt @@ -0,0 +1,27674 @@ +The Project Gutenberg Etext of The Student's Elements of Geology +by Sir Charles Lyell + +Copyright laws are changing all over the world, be sure to check +the laws for your country before redistributing these files!!! + +Please take a look at the important information in this header. +We encourage you to keep this file on your own disk, keeping an +electronic path open for the next readers. + +Please do not remove this. + +This should be the first thing seen when anyone opens the book. +Do not change or edit it without written permission. 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Thecosmilia annularis.) + + +WITH MORE THAN 600 ILLUSTRATIONS ON WOOD. + + + +PREFACE. + +The LAST or sixth EDITION of my "Elements of Geology" was already out of print +before the end of 1868, in which year I brought out the tenth edition of my +"Principles of Geology." + +In writing the last-mentioned work I had been called upon to pass in review +almost all the leading points of speculation and controversy to which the rapid +advance of the science had given rise, and when I proposed to bring out a new +edition of the "Elements" I was strongly urged by my friends not to repeat these +theoretical discussions, but to confine myself in the new treatise to those +parts of the "Elements" which were most indispensable to a beginner. This was to +revert, to a certain extent, to the original plan of the first edition; but I +found, after omitting a great number of subjects, that the necessity of bringing +up to the day those which remained, and adverting, however briefly, to new +discoveries, made it most difficult to confine the proposed abridgment within +moderate limits. Some chapters had to be entirely recast, some additional +illustrations to be introduced, and figures of some organic remains to be +replaced by new ones from specimens more perfect than those which had been at my +command on former occasions. By these changes the work assumed a form so +different from the sixth edition of the "Elements," that I resolved to give it a +new title and call it the "Student's Elements of Geology." + +In executing this task I have found it very difficult to meet the requirements +of those who are entirely ignorant of the science. It is only the adept who has +already overcome the first steps as an observer, and is familiar with many of +the technical terms, who can profit by a brief and concise manual. Beginners +wish for a short and cheap book in which they may find a full explanation of the +leading facts and principles of Geology. Their wants, I fear, somewhat resemble +those of the old woman in New England, who asked a bookseller to supply her with +"the cheapest Bible in the largest possible print." + +But notwithstanding the difficulty of reconciling brevity with the copiousness +of illustration demanded by those who have not yet mastered the rudiments of the +science, I have endeavoured to abridge the work in the manner above hinted at, +so as to place it within the reach of many to whom it was before inaccessible. + +CHARLES LYELL. + +73 Harley Street, London, +December, 1870. + + +CONTENTS. + + +CHAPTER I. + +ON THE DIFFERENT CLASSES OF ROCKS. + +Geology defined. +Successive Formation of the Earth's Crust. +Classification of Rocks according to their Origin and Age. +Aqueous Rocks. +Their Stratification and imbedded Fossils. +Volcanic Rocks, with and without Cones and Craters. +Plutonic Rocks, and their Relation to the Volcanic. +Metamorphic Rocks, and their probable Origin. +The term Primitive, why erroneously applied to the Crystalline Formations. +Leading Division of the Work. + + +CHAPTER II. + +AQUEOUS ROCKS-- THEIR COMPOSITION AND FORMS OF STRATIFICATION. + +Mineral Composition of Strata. +Siliceous Rocks. +Argillaceous. +Calcareous. +Gypsum. +Forms of Stratification. +Original Horizontality. +Thinning out. +Diagonal Arrangement. +Ripple-mark. + + +CHAPTER III. + +ARRANGEMENT OF FOSSILS IN STRATA-- FRESH-WATER AND MARINE. + +Successive Deposition indicated by Fossils. +Limestones formed of Corals and Shells. +Proofs of gradual Increase of Strata derived from Fossils. +Serpula attached to Spatangus. +Wood bored by Teredina. +Tripoli formed of Infusoria. +Chalk derived principally from Organic Bodies. +Distinction of Fresh-water from Marine Formations. +Genera of Fresh-water and Land Shells. +Rules for recognising Marine Testacea. +Gyrogonite and Chara. +Fresh-water Fishes. +Alternation of Marine and Fresh-water Deposits. +Lym-Fiord. + + +CHAPTER IV. + +CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS. + +Chemical and Mechanical Deposits. +Cementing together of Particles. +Hardening by Exposure to Air. +Concretionary Nodules. +Consolidating Effects of Pressure. +Mineralization of Organic Remains. +Impressions and Casts: how formed. +Fossil Wood. +Goppert's Experiments. +Precipitation of Stony Matter most rapid where Putrefaction is going on. +Sources of Lime and Silex in Solution. + + +CHAPTER V. + +ELEVATION OF STRATA ABOVE THE SEA.-- HORIZONTAL AND INCLINED STRATIFICATION. + +Why the Position of Marine Strata, above the Level of the Sea, should be +referred to the rising up of the Land, not to the going down of the Sea. +Strata of Deep-sea and Shallow-water Origin alternate. +Also Marine and Fresh-water Beds and old Land Surfaces. +Vertical, inclined, and folded Strata. +Anticlinal and Synclinal Curves. +Theories to explain Lateral Movements. +Creeps in Coal-mines. +Dip and Strike. +Structure of the Jura. +Various Forms of Outcrop. +Synclinal Strata forming Ridges. +Connection of Fracture and Flexure of Rocks. +Inverted Strata. +Faults described. +Superficial Signs of the same obliterated by Denudation. +Great Faults the Result of repeated Movements. +Arrangement and Direction of parallel Folds of Strata. +Unconformability. +Overlapping Strata. + + +CHAPTER VI. + +DENUDATION. + +Denudation defined. +Its Amount more than equal to the entire Mass of Stratified Deposits in the +Earth's Crust. +Subaerial Denudation. +Action of the Wind. +Action of Running Water. +Alluvium defined. +Different Ages of Alluvium. +Denuding Power of Rivers affected by Rise or Fall of Land. +Littoral Denudation. +Inland Sea-Cliffs. +Escarpments. +Submarine Denudation. +Dogger-bank. +Newfoundland Bank. +Denuding Power of the Ocean during Emergence of Land. + + +CHAPTER VII. + +JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH'S +CRUST. + +How we obtain an Insight at the Surface, of the Arrangement of Rocks at great +Depths. +Why the Height of the successive Strata in a given Region is so disproportionate +to their Thickness. +Computation of the average annual Amount of subaerial Denudation. +Antagonism of Volcanic Force to the Levelling Power of running Water. +How far the Transfer of Sediment from the Land to a neighbouring Sea-bottom may +affect Subterranean Movements. +Permanence of Continental and Oceanic Areas. + + +CHAPTER VIII. + +CHRONOLOGICAL CLASSIFICATION OF ROCKS. + +Aqueous, Plutonic, volcanic, and metamorphic Rocks considered chronologically. +Terms Primary, Secondary, and Tertiary; Palaeozoic, Mesozoic, and Cainozoic +explained. +On the different Ages of the aqueous Rocks. +Three principal Tests of relative Age: Superposition, Mineral Character, and +Fossils. +Change of Mineral Character and Fossils in the same continuous Formation. +Proofs that distinct Species of Animals and Plants have lived at successive +Periods. +Distinct Provinces of indigenous Species. +Great Extent of single Provinces. +Similar Laws prevailed at successive Geological Periods. +Relative Importance of mineral and palaeontological Characters. +Test of Age by included Fragments. +Frequent Absence of Strata of intervening Periods. +Tabular Views of fossiliferous Strata. + + +CHAPTER IX. + +CLASSIFICATION OF TERTIARY FORMATIONS. + +Order of Succession of Sedimentary Formations. +Frequent Unconformability of Strata. +Imperfection of the Record. +Defectiveness of the Monuments greater in Proportion to their Antiquity. +Reasons for studying the newer Groups first. +Nomenclature of Formations. +Detached Tertiary Formations scattered over Europe. +Value of the Shell-bearing Mollusca in Classification. +Classification of Tertiary Strata. +Eocene, Miocene, and Pliocene Terms explained. + + +CHAPTER X. + +RECENT AND POST-PLIOCENE PERIODS. + +Recent and Post-pliocene Periods. +Terms defined. +Formations of the Recent Period. +Modern littoral Deposits containing Works of Art near Naples. +Danish Peat and Shell-mounds. +Swiss Lake-dwellings. +Periods of Stone, Bronze, and Iron. +Post-pliocene Formations. +Coexistence of Man with extinct Mammalia. +Reindeer Period of South of France. +Alluvial Deposits of Paleolithic Age. +Higher and Lower-level Valley-gravels. +Loess or Inundation-mud of the Nile, Rhine, etc. +Origin of Caverns. +Remains of Man and extinct Quadrupeds in Cavern Deposits. +Cave of Kirkdale. +Australian Cave-breccias. +Geographical Relationship of the Provinces of living Vertebrata and those of +extinct Post-pliocene Species. +Extinct struthious Birds of New Zealand. +Climate of the Post-pliocene Period. +Comparative Longevity of Species in the Mammalia and Testacea. +Teeth of Recent and Post-pliocene Mammalia. + + +CHAPTER XI. + +POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS. + +Geographical Distribution, Form, and Characters of Glacial Drift. +Fundamental Rocks, polished, grooved, and scratched. +Abrading and striating Action of Glaciers. +Moraines, Erratic Blocks, and "Roches Moutonnees." +Alpine Blocks on the Jura. +Continental Ice of Greenland. +Ancient Centres of the Dispersion of Erratics. +Transportation of Drift by floating Icebergs. +Bed of the Sea furrowed and polished by the running aground of floating Ice- +islands. + + +CHAPTER XII. + +POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS, CONCLUDED. + +Glaciation of Scandinavia and Russia. +Glaciation of Scotland. +Mammoth in Scotch Till. +Marine Shells in Scotch Glacial Drift. +Their Arctic Character. +Rarity of Organic Remains in Glacial Deposits. +Contorted Strata in Drift. +Glaciation of Wales, England, and Ireland. +Marine Shells of Moel Tryfaen. +Erratics near Chichester. +Glacial Formations of North America. +Many Species of Testacea and Quadrupeds survived the Glacial Cold. +Connection of the Predominance of Lakes with Glacial Action. +Action of Ice in preventing the silting up of Lake-basins. +Absence of Lakes in the Caucasus. +Equatorial Lakes of Africa. + + +CHAPTER XIII. + +PLIOCENE PERIOD. + +Glacial Formations of Pliocene Age. +Bridlington Beds. +Glacial Drifts of Ireland. +Drift of Norfolk Cliffs. +Cromer Forest-bed. +Aldeby and Chillesford Beds. +Norwich Crag. +Older Pliocene Strata. +Red Crag of Suffolk. +Coprolitic Bed of Red Crag. +White or Coralline Crag. +Relative Age, Origin, and Climate of the Crag Deposits. +Antwerp Crag. +Newer Pliocene Strata of Sicily. +Newer Pliocene Strata of the Upper Val d'Arno. +Older Pliocene of Italy. +Subapennine Strata. +Older Pliocene Flora of Italy. + + +CHAPTER XIV. + +MIOCENE PERIOD.-- UPPER MIOCENE. + +Upper Miocene Strata of France. +faluns of Touraine. +Tropical Climate implied by Testacea. +Proportion of recent Species of Shells. +faluns more ancient than the Suffolk Crag. +Upper Miocene of Bordeaux and the South of France. +Upper Miocene of Oeningen, in Switzerland. +Plants of the Upper Fresh-water Molasse. +Fossil Fruit and Flowers as well as Leaves. +Insects of the Upper Molasse. +Middle or Marine Molasse of Switzerland. +Upper Miocene Beds of the Bolderberg, in Belgium. +Vienna Basin. +Upper Miocene of Italy and Greece. +Upper Miocene of India; Siwalik Hills. +Older Pliocene and Miocene of the United States. + + +CHAPTER XV. + +LOWER MIOCENE. + +Lower Miocene Strata of France. +Line between Miocene and Eocene. +Lacustrine Strata of Auvergne. +Fossil Mammalia of the Limagne d'Auvergne. +Lower Molasse of Switzerland. +Dense Conglomerates and Proofs of Subsidence. +Flora of the Lower Molasse. +American Character of the Flora. +Theory of a Miocene Atlantis. +Lower Miocene of Belgium. +Rupelian Clay of Hermsdorf near Berlin. +Mayence Basin. +Lower Miocene of Croatia. +Oligocene Strata of Beyrich. +Lower Miocene of Italy. +Lower Miocene of England. +Hempstead Beds. +Bovey Tracey Lignites in Devonshire. +Isle of Mull Leaf-Beds. +Arctic Miocene Flora. +Disco Island. +Lower Miocene of United States. +Fossils of Nebraska. + + +CHAPTER XVI. + +EOCENE FORMATIONS. + +Eocene Areas of North of Europe. +Table of English and French Eocene Strata. +Upper Eocene of England. +Bembridge Beds. +Osborne or St. Helen's Beds. +Headon Series. +Fossils of the Barton Sands and Clays. +Middle Eocene of England. +Shells, Nummulites, Fish and Reptiles of the Bracklesham Beds and Bagshot Sands. +Plants of Alum Bay and Bournemouth. +Lower Eocene of England. +London Clay Fossils. +Woolwich and Reading Beds formerly called "Plastic Clay." +Fluviatile Beds underlying Deep-sea Strata. +Thanet Sands. +Upper Eocene Strata of France. +Gypseous Series of Montmartre and Extinct Quadrupeds. +Fossil Footprints in Paris Gypsum. +Imperfection of the Record. +Calcaire Silicieux. +Gres de Beauchamp. +Calcaire Grossier. +Miliolite Limestone. +Soissonnais Sands. +Lower Eocene of France. +Nummulitic Formations of Europe, Africa, and Asia. +Eocene Strata in the United States. +Gigantic Cetacean. + + +CHAPTER XVII. + +UPPER CRETACEOUS GROUP. + +Lapse of Time between Cretaceous and Eocene Periods. +Table of successive Cretaceous Formations. +Maestricht Beds. +Pisolitic Limestone of France. +Chalk of Faxoe. +Geographical Extent and Origin of the White Chalk. +Chalky Matter now forming in the Bed of the Atlantic. +Marked Difference between the Cretaceous and existing Fauna. +Chalk-flints. +Pot-stones of Horstead. +Vitreous Sponges in the Chalk. +Isolated Blocks of Foreign Rocks in the White Chalk supposed to be ice-borne. +Distinctness of Mineral Character in contemporaneous Rocks of the Cretaceous +Epoch. +Fossils of the White Chalk. +Lower White Chalk without Flints. +Chalk Marl and its Fossils. +Chloritic Series or Upper Greensand. +Coprolite Bed near Cambridge. +Fossils of the Chloritic Series. +Gault. +Connection between Upper and Lower Cretaceous Strata. +Blackdown Beds. +Flora of the Upper Cretaceous Period. +Hippurite Limestone. +Cretaceous Rocks in the United States. + + +CHAPTER XVIII. + +LOWER CRETACEOUS OR NEOCOMIAN FORMATION. + +Classification of marine and fresh-water Strata. +Upper Neocomian. +Folkestone and Hythe Beds. +Atherfield Clay. +Similarity of Conditions causing Reappearance of Species after short Intervals. +Upper Speeton Clay. +Middle Neocomian. +Tealby Series. +Middle Speeton Clay. +Lower Neocomian. +Lower Speeton Clay. +Wealden Formation. +Fresh-water Character of the Wealden. +Weald Clay. +Hastings Sands. +Punfield Beds of Purbeck, Dorsetshire. +Fossil Shells and Fish of the Wealden. +Area of the Wealden. +Flora of the Wealden. + + +CHAPTER XIX. + +JURASSIC GROUP.-- PURBECK BEDS AND OOLITE. + +The Purbeck Beds a Member of the Jurassic Group. +Subdivisions of that Group. +Physical Geography of the Oolite in England and France. +Upper Oolite. +Purbeck Beds. +New Genera of fossil Mammalia in the Middle Purbeck of Dorsetshire. +Dirt-bed or ancient Soil. +Fossils of the Purbeck Beds. +Portland Stone and Fossils. +Kimmeridge Clay. +Lithographic Stone of Solenhofen. +Archaeopteryx. +Middle Oolite. +Coral Rag. +Nerinaea Limestone. +Oxford Clay, Ammonites and Belemnites. +Kelloway Rock. +Lower, or Bath, Oolite. +Great Plants of the Oolite. +Oolite and Bradford Clay. +Stonesfield Slate. +Fossil Mammalia. +Fuller's Earth. +Inferior Oolite and Fossils. +Northamptonshire Slates. +Yorkshire Oolitic Coal-field. +Brora Coal. +Palaeontological Relations of the several Subdivisions of the Oolitic group. + + +CHAPTER XX. + +JURASSIC GROUP-- CONTINUED.-- LIAS. + +Mineral Character of Lias. +Numerous successive Zones in the Lias, marked by distinct Fossils, without +Unconformity in the Stratification, or Change in the Mineral Character of the +Deposits. +Gryphite Limestone. +Shells of the Lias. +Fish of the Lias. +Reptiles of the Lias. +Ichthyosaur and Plesiosaur. +Marine Reptile of the Galapagos Islands. +Sudden Destruction and Burial of Fossil Animals in Lias. +Fluvio-marine Beds in Gloucestershire, and Insect Limestone. +Fossil Plants. +The origin of the Oolite and Lias, and of alternating Calcareous and +Argillaceous Formations. + + +CHAPTER XXI. + +TRIAS, OR NEW RED SANDSTONE GROUP. + +Beds of Passage between the Lias and Trias, Rhaetic Beds. +Triassic Mammifer. +Triple Division of the Trias. +Keuper, or Upper Trias of England. +Reptiles of the Upper Trias. +Foot-prints in the Bunter formation in England. +Dolomitic Conglomerate of Bristol. +Origin of Red Sandstone and Rock-salt. +Precipitation of Salt from inland Lakes and Lagoons. +Trias of Germany. +Keuper. +St. Cassian and Hallstadt Beds. +Peculiarity of their Fauna. +Muschelkalk and its Fossils. +Trias of the United States. +Fossil Foot-prints of Birds and Reptiles in the Valley of the Connecticut. +Triassic Mammifer of North Carolina. +Triassic Coal-field of Richmond, Virginia. +Low Grade of early Mammals favourable to the Theory of Progressive Development. + + +CHAPTER XXII. + +PERMIAN OR MAGNESIAN LIMESTONE GROUP. + +Line of Separation between Mesozoic and Palaeozoic Rocks. +Distinctness of Triassic and Permian Fossils. +Term Permian. +Thickness of calcareous and sedimentary Rocks in North of England. +Upper, Middle, and Lower Permian. +Marine Shells and Corals of the English Magnesian Limestone. +Reptiles and Fish of Permian Marl-slate. +Foot-prints of Reptiles. +Angular Breccias in Lower Permian. +Permian Rocks of the Continent. +Zechstein and Rothliegendes of Thuringia. +Permian Flora. +Its generic Affinity to the Carboniferous. + + +CHAPTER XXIII. + +THE COAL OR CARBONIFEROUS GROUP. + +Principal Subdivisions of the Carboniferous Group. +Different Thickness of the sedimentary and calcareous Members in Scotland and +the South of England. +Coal-measures. +Terrestrial Nature of the Growth of Coal. +Erect fossil Trees. +Uniting of many Coal-seams into one thick Bed. +Purity of the Coal explained. +Conversion of Coal into Anthracite. +Origin of Clay-ironstone. +Marine and brackish-water Strata in Coal. +Fossil Insects. +Batrachian Reptiles. +Labyrinthodont Foot-prints in Coal-measures. +Nova Scotia Coal-measures with successive Growths of erect fossil Trees. +Similarity of American and European Coal. +Air-breathers of the American Coal. +Changes of Condition of Land and Sea indicated by the Carboniferous Strata of +Nova Scotia. + + +CHAPTER XXIV. + +FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD. + +Vegetation of the Coal Period. +Ferns, Lycopodiaceae, Equisetaceae, Sigillariae, Stigmariae, Coniferae. +Angiosperms. +Climate of the Coal Period. +Mountain Limestone. +Marine Fauna of the Carboniferous Period. +Corals. +Bryozoa, Crinoidea. +Mollusca. +Great Number of fossil Fish. +Foraminifera. + + +CHAPTER XXV. + +DEVONIAN OR OLD RED SANDSTONE GROUP. + +Classification of the Old Red Sandstone in Scotland and in Devonshire. +Upper Old Red Sandstone in Scotland, with Fish and Plants. +Middle Old Red Sandstone. +Classification of the Ichthyolites of the Old Red, and their Relation to Living +Types. +Lower Old Red Sandstone, with Cephalaspis and Pterygotus. +Marine or Devonian Type of Old Red Sandstone. +Table of Devonian Series. +Upper Devonian Rocks and Fossils. +Middle. +Lower. +Eifel Limestone of Germany. +Devonian of Russia. +Devonian Strata of the United States and Canada. +Devonian Plants and Insects of Canada. + + +CHAPTER XXVI. + +SILURIAN GROUP. + +Classification of the Silurian Rocks. +Ludlow Formation and Fossils. +Bone-bed of the Upper Ludlow. +Lower Ludlow Shales with Pentamerus. +Oldest known Remains of fossil Fish. +Table of the progressive Discovery of Vertebrata in older Rocks. +Wenlock Formation, Corals, Cystideans and Trilobites. +Llandovery Group or Beds of Passage. +Lower Silurian Rocks. +Caradoc and Bala Beds. +Brachiopoda. +Trilobites. +Cystideae. +Graptolites. +Llandeilo Flags. +Arenig or Stiper-stones Group. +Foreign Silurian Equivalents in Europe. +Silurian Strata of the United States. +Canadian Equivalents. +Amount of specific Agreement of Fossils with those of Europe. + + +CHAPTER XXVII. + +CAMBRIAN AND LAURENTIAN GROUPS. + +Classification of the Cambrian Group, and its Equivalent in Bohemia. +Upper Cambrian Rocks. +Tremadoc Slates and their Fossils. +Lingula Flags. +Lower Cambrian Rocks. +Menevian Beds. +Longmynd Group. +Harlech Grits with large Trilobites. +Llanberis Slates. +Cambrian Rocks of Bohemia. +Primordial Zone of Barrande. +Metamorphosis of Trilobites. +Cambrian Rocks of Sweden and Norway. +Cambrian Rocks of the United States and Canada. +Potsdam Sandstone. +Huronian Series. +Laurentian Group, upper and lower. +Eozoon Canadense, oldest known Fossil. +Fundamental Gneiss of Scotland. + + +CHAPTER XXVIII. + +VOLCANIC ROCKS. + +External Form, Structure, and Origin of Volcanic Mountains. +Cones and Craters. +Hypothesis of "Elevation Craters" considered. +Trap Rocks. +Name whence derived. +Minerals most abundant in Volcanic Rocks. +Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks. +Similar Minerals in Meteorites. +Theory of Isomorphism. +Basaltic Rocks. +Trachytic Rocks. +Special Forms of Structure. +The columnar and globular Forms. +Trap Dikes and Veins. +Alteration of Rocks by volcanic Dikes. +Conversion of Chalk into Marble. +Intrusion of Trap between Strata. +Relation of trappean Rocks to the Products of active Volcanoes. + + +CHAPTER XXIX. + +ON THE AGES OF VOLCANIC ROCKS. + +Tests of relative Age of Volcanic Rocks. +Why ancient and modern Rocks can not be identical. +Tests by Superposition and intrusion. +Test by Alteration of Rocks in Contact. +Test by Organic Remains. +Test of Age by Mineral Character. +Test by Included Fragments. +Recent and Post-pliocene volcanic Rocks. +Vesuvius, Auvergne, Puy de Come, and Puy de Pariou. +Newer Pliocene volcanic Rocks. +Cyclopean Isles, Etna, Dikes of Palagonia, Madeira. +Older Pliocene volcanic Rocks. +Italy. +Pliocene Volcanoes of the Eifel. +Trass. + + +CHAPTER XXX. + +AGE OF VOLCANIC ROCKS-- CONTINUED. + +Volcanic Rocks of the Upper Miocene Period. +Madeira. +Grand Canary. +Azores. +Lower Miocene Volcanic Rocks. +Isle of Mull. +Staffa and Antrim. +The Eifel. +Upper and Lower Miocene Volcanic Rocks of Auvergne. +Hill of Gergovia. +Eocene Volcanic Rocks of Monte Bolca. +Trap of Cretaceous Period. +Oolitic Period. +Triassic Period. +Permian Period. +Carboniferous Period. +Erect Trees buried in Volcanic Ash in the Island of Arran. +Old Red Sandstone Period. +Silurian Period. +Cambrian Period. +Laurentian Volcanic Rocks. + + +CHAPTER XXXI. + +PLUTONIC ROCKS. + +General Aspect of Plutonic Rocks. +Granite and its Varieties. +Decomposing into Spherical Masses. +Rude columnar Structure. +Graphic Granite. +Mutual Penetration of Crystals of Quartz and Feldspar. +Glass Cavities in Quartz of Granite. +Porphyritic, talcose, and syenitic Granite. +Schorlrock and Eurite. +Syenite. +Connection of the Granites and Syenites with the Volcanic Rocks. +Analogy in Composition of Trachyte and Granite. +Granite Veins in Glen Tilt, Cape of Good Hope, and Cornwall. +Metalliferous Veins in Strata near their Junction with Granite. +Quartz Veins. +Exposure of Plutonic Rocks at the surface due to Denudation. + + +CHAPTER XXXII. + +ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS. + +Difficulty in ascertaining the precise Age of a Plutonic Rock. +Test of Age by Relative Position. +Test by Intrusion and Alteration. +Test by Mineral Composition. +Test by included Fragments. +Recent and Pliocene Plutonic Rocks, why invisible. +Miocene Syenite of the Isle of Skye. +Eocene Plutonic Rocks in the Andes. +Granite altering Cretaceous Rocks. +Granite altering Lias in the Alps and in Skye. +Granite of Dartmoor altering Carboniferous Strata. +Granite of the Old Red Sandstone Period. +Syenite altering Silurian Strata in Norway. +Blending of the same with Gneiss. +Most ancient Plutonic Rocks. +Granite protruded in a solid Form. + + +CHAPTER XXXIII. + +METAMORPHIC ROCKS. + +General Character of Metamorphic Rocks. +Gneiss. +Hornblende-schist. +Serpentine. +Mica-schist. +Clay-slate. +Quartzite. +Chlorite-schist. +Metamorphic Limestone. +Origin of the metamorphic Strata. +Their Stratification. +Fossiliferous Strata near intrusive Masses of Granite converted into Rocks +identical with different Members of the metamorphic Series. +Arguments hence derived as to the Nature of Plutonic Action. +Hydrothermal Action, or the Influence of Steam and Gases in producing +Metamorphism. +Objections to the metamorphic Theory considered. + + +CHAPTER XXXIV. + +METAMORPHIC ROCKS-- CONTINUED. + +Definition of slaty Cleavage and Joints. +Supposed Causes of these Structures. +Crystalline Theory of Cleavage. +Mechanical Theory of Cleavage. +Condensation and Elongation of slate Rocks by lateral Pressure. +Lamination of some volcanic Rocks due to Motion. +Whether the Foliation of the crystalline Schists be usually parallel with the +original Planes of Stratification. +Examples in Norway and Scotland. +Causes of Irregularity in the Planes of Foliation. + + +CHAPTER XXXV. + +ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS. + +Difficulty of ascertaining the Age of metamorphic Strata. +Metamorphic Strata of Eocene date in the Alps of Switzerland and Savoy. +Limestone and Shale of Carrara. +Metamorphic Strata of older date than the Silurian and Cambrian Rocks. +Order of Succession in metamorphic Rocks. +Uniformity of mineral Character. +Supposed Azoic Period. +Connection between the Absence of Organic Remains and the Scarcity of calcareous +Matter in metamorphic Rocks. + + +CHAPTER XXXVI. + +MINERAL VEINS. + +Different Kinds of mineral Veins. +Ordinary metalliferous Veins or Lodes. +Their frequent Coincidence with Faults. +Proofs that they originated in Fissures in solid Rock. +Veins shifting other Veins. +Polishing of their Walls or "Slicken sides." +Shells and Pebbles in Lodes. +Evidence of the successive Enlargement and Reopening of veins. +Examples in Cornwall and in Auvergne. +Dimensions of Veins. +Why some alternately swell out and contract. +Filling of Lodes by Sublimation from below. +Supposed relative Age of the precious Metals. +Copper and lead Veins in Ireland older than Cornish Tin. +Lead Vein in Lias, Glamorganshire. +Gold in Russia, California, and Australia. +Connection of hot Springs and mineral Veins. + + +INDEX. + +... + + +STUDENT'S ELEMENTS OF GEOLOGY. + + +CHAPTER I. + +ON THE DIFFERENT CLASSES OF ROCKS. + +Geology defined. +Successive Formation of the Earth's Crust. +Classification of Rocks according to their Origin and Age. +Aqueous Rocks. +Their Stratification and imbedded Fossils. +Volcanic Rocks, with and without Cones and Craters. +Plutonic Rocks, and their Relation to the Volcanic. +Metamorphic Rocks, and their probable Origin. +The term Primitive, why erroneously applied to the Crystalline Formations. +Leading Division of the Work. + +Of what materials is the earth composed, and in what manner are these materials +arranged? These are the first inquiries with which Geology is occupied, a +science which derives its name from the Greek ge, the earth, and logos, a +discourse. Previously to experience we might have imagined that investigations +of this kind would relate exclusively to the mineral kingdom, and to the various +rocks, soils, and metals, which occur upon the surface of the earth, or at +various depths beneath it. But, in pursuing such researches, we soon find +ourselves led on to consider the successive changes which have taken place in +the former state of the earth's surface and interior, and the causes which have +given rise to these changes; and, what is still more singular and unexpected, we +soon become engaged in researches into the history of the animate creation, or +of the various tribes of animals and plants which have, at different periods of +the past, inhabited the globe. + +All are aware that the solid parts of the earth consist of distinct substances, +such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but +previously to observation it is commonly imagined that all these had remained +from the first in the state in which we now see them-- that they were created in +their present form, and in their present position. The geologist soon comes to a +different conclusion, discovering proofs that the external parts of the earth +were not all produced in the beginning of things in the state in which we now +behold them, nor in an instant of time. On the contrary, he can show that they +have acquired their actual configuration and condition gradually, under a great +variety of circumstances, and at successive periods, during each of which +distinct races of living beings have flourished on the land and in the waters, +the remains of these creatures still lying buried in the crust of the earth. + +By the "earth's crust," is meant that small portion of the exterior of our +planet which is accessible to human observation. It comprises not merely all of +which the structure is laid open in mountain precipices, or in cliffs +overhanging a river or the sea, or whatever the miner may reveal in artificial +excavations; but the whole of that outer covering of the planet on which we are +enabled to reason by observations made at or near the surface. These reasonings +may extend to a depth of several miles, perhaps ten miles; and even then it may +be said, that such a thickness is no more than 1/400 part of the distance from +the surface to the centre. The remark is just: but although the dimensions of +such a crust are, in truth, insignificant when compared to the entire globe, yet +they are vast, and of magnificent extent in relation to man, and to the organic +beings which people our globe. Referring to this standard of magnitude, the +geologist may admire the ample limits of his domain, and admit, at the same +time, that not only the exterior of the planet, but the entire earth, is but an +atom in the midst of the countless worlds surveyed by the astronomer. + +The materials of this crust are not thrown together confusedly; but distinct +mineral masses, called rocks, are found to occupy definite spaces, and to +exhibit a certain order of arrangement. The term ROCK is applied indifferently +by geologists to all these substances, whether they be soft or stony, for clay +and sand are included in the term, and some have even brought peat under this +denomination. Our old writers endeavoured to avoid offering such violence to our +language, by speaking of the component materials of the earth as consisting of +rocks and SOILS. But there is often so insensible a passage from a soft and +incoherent state to that of stone, that geologists of all countries have found +it indispensable to have one technical term to include both, and in this sense +we find ROCHE applied in French, ROCCA in Italian, and FELSART in German. The +beginner, however, must constantly bear in mind that the term rock by no means +implies that a mineral mass is in an indurated or stony condition. + +The most natural and convenient mode of classifying the various rocks which +compose the earth's crust, is to refer, in the first place, to their origin, and +in the second to their relative age. I shall therefore begin by endeavouring +briefly to explain to the student how all rocks may be divided into four great +classes by reference to their different origin, or, in other words, by reference +to the different circumstances and causes by which they have been produced. + +The first two divisions, which will at once be understood as natural, are the +aqueous and volcanic, or the products of watery and those of igneous action at +or near the surface. + +AQUEOUS ROCKS. + +The aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a +larger part of the earth's surface than any others. They consist chiefly of +mechanical deposits (pebbles, sand, and mud), but are partly of chemical and +some of them of organic origin, especially the limestones. These rocks are +STRATIFIED, or divided into distinct layers, or strata. The term STRATUM means +simply a bed, or any thing spread out or STREWED over a given surface; and we +infer that these strata have been generally spread out by the action of water, +from what we daily see taking place near the mouths of rivers, or on the land +during temporary inundations. For, whenever a running stream charged with mud or +sand, has its velocity checked, as when it enters a lake or sea, or overflows a +plain, the sediment, previously held in suspension by the motion of the water, +sinks, by its own gravity to the bottom. In this manner layers of mud and sand +are thrown down one upon another. + +If we drain a lake which has been fed by a small stream, we frequently find at +the bottom a series of deposits, disposed with considerable regularity, one +above the other; the uppermost, perhaps, may be a stratum of peat, next below a +more dense and solid variety of the same material; still lower a bed of shell- +marl, alternating with peat or sand, and then other beds of marl, divided by +layers of clay. Now, if a second pit be sunk through the same continuous +lacustrine FORMATION at some distance from the first, nearly the same series of +beds is commonly met with, yet with slight variations; some, for example, of the +layers of sand, clay, or marl, may be wanting, one or more of them having +thinned out and given place to others, or sometimes one of the masses first +examined is observed to increase in thickness to the exclusion of other beds. + +The term "FORMATION," which I have used in the above explanation, expresses in +geology any assemblage of rocks which have some character in common, whether of +origin, age, or composition. Thus we speak of stratified and unstratified, +fresh-water and marine, aqueous and volcanic, ancient and modern, metalliferous +and non-metalliferous formations. + +In the estuaries of large rivers, such as the Ganges and the Mississippi, we may +observe, at low water, phenomena analogous to those of the drained lakes above +mentioned, but on a grander scale, and extending over areas several hundred +miles in length and breadth. When the periodical inundations subside, the river +hollows out a channel to the depth of many yards through horizontal beds of clay +and sand, the ends of which are seen exposed in perpendicular cliffs. These beds +vary in their mineral composition, or colour, or in the fineness or coarseness +of their particles, and some of them are occasionally characterised by +containing drift-wood. At the junction of the river and the sea, especially in +lagoons nearly separated by sand-bars from the ocean, deposits are often formed +in which brackish and salt-water shells are included. + +In Egypt, where the Nile is always adding to its delta by filling up part of the +Mediterranean with mud, the newly deposited sediment is STRATIFIED, the thin +layer thrown down in one season differing slightly in colour from that of a +previous year, and being separable from it, as has been observed in excavations +at Cairo and other places. (See "Principles of Geology" by the Author Index +"Nile" "Rivers" etc.) + +When beds of sand, clay, and marl, containing shells and vegetable matter, are +found arranged in a similar manner in the interior of the earth, we ascribe to +them a similar origin; and the more we examine their characters in minute +detail, the more exact do we find the resemblance. Thus, for example, at various +heights and depths in the earth, and often far from seas, lakes, and rivers, we +meet with layers of rounded pebbles composed of flint, limestone, granite, or +other rocks, resembling the shingles of a sea-beach or the gravel in a torrent's +bed. Such layers of pebbles frequently alternate with others formed of sand or +fine sediment, just as we may see in the channel of a river descending from +hills bordering a coast, where the current sweeps down at one season coarse sand +and gravel, while at another, when the waters are low and less rapid, fine mud +and sand alone are carried seaward. (See Figure 7 Chapter 2.) + +If a stratified arrangement, and the rounded form of pebbles, are alone +sufficient to lead us to the conclusion that certain rocks originated under +water, this opinion is farther confirmed by the distinct and independent +evidence of FOSSILS, so abundantly included in the earth's crust. By a FOSSIL is +meant any body, or the traces of the existence of any body, whether animal or +vegetable, which has been buried in the earth by natural causes. Now the remains +of animals, especially of aquatic species, are found almost everywhere imbedded +in stratified rocks, and sometimes, in the case of limestone, they are in such +abundance as to constitute the entire mass of the rock itself. Shells and corals +are the most frequent, and with them are often associated the bones and teeth of +fishes, fragments of wood, impressions of leaves, and other organic substances. +Fossil shells, of forms such as now abound in the sea, are met with far inland, +both near the surface, and at great depths below it. They occur at all heights +above the level of the ocean, having been observed at elevations of more than +8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above +18,000 feet in the Himalaya. (Colonel R.J. Strachey found oolitic fossils 18,400 +feet high in the Himalaya.) + +These shells belong mostly to marine testacea, but in some places exclusively to +forms characteristic of lakes and rivers. Hence it is concluded that some +ancient strata were deposited at the bottom of the sea, and others in lakes and +estuaries. + +We have now pointed out one great class of rocks, which, however they may vary +in mineral composition, colour, grain, or other characters, external and +internal, may nevertheless be grouped together as having a common origin. They +have all been formed under water, in the same manner as modern accumulations of +sand, mud, shingle, banks of shells, reefs of coral, and the like, and are all +characterised by stratification or fossils, or by both. + +VOLCANIC ROCKS. + +The division of rocks which we may next consider are the volcanic, or those +which have been produced at or near the surface whether in ancient or modern +times, not by water, but by the action of fire or subterranean heat. These rocks +are for the most part unstratified, and are devoid of fossils. They are more +partially distributed than aqueous formations, at least in respect to horizontal +extension. Among those parts of Europe where they exhibit characters not to be +mistaken, I may mention not only Sicily and the country round Naples, but +Auvergne, Velay, and Vivarais, now the departments of Puy de Dome, Haute Loire, +and Ardeche, towards the centre and south of France, in which are several +hundred conical hills having the forms of modern volcanoes, with craters more or +less perfect on many of their summits. These cones are composed moreover of +lava, sand, and ashes, similar to those of active volcanoes. Streams of lava may +sometimes be traced from the cones into the adjoining valleys, where they have +choked up the ancient channels of rivers with solid rock, in the same manner as +some modern flows of lava in Iceland have been known to do, the rivers either +flowing beneath or cutting out a narrow passage on one side of the lava. +Although none of these French volcanoes have been in activity within the period +of history or tradition, their forms are often very perfect. Some, however, have +been compared to the mere skeletons of volcanoes, the rains and torrents having +washed their sides, and removed all the loose sand and scoriae, leaving only the +harder and more solid materials. By this erosion, and by earthquakes, their +internal structure has occasionally been laid open to view, in fissures and +ravines; and we then behold not only many successive beds and masses of porous +lava, sand, and scoriae, but also perpendicular walls, or DIKES, as they are +called, of volcanic rock, which have burst through the other materials. Such +dikes are also observed in the structure of Vesuvius, Etna, and other active +volcanoes. They have been formed by the pouring of melted matter, whether from +above or below, into open fissures, and they commonly traverse deposits of +VOLCANIC TUFF, a substance produced by the showering down from the air, or +incumbent waters, of sand and cinders, first shot up from the interior of the +earth by the explosions of volcanic gases. + +Besides the parts of France above alluded to, there are other countries, as the +north of Spain, the south of Sicily, the Tuscan territory of Italy, the lower +Rhenish provinces, and Hungary, where spent volcanoes may be seen, still +preserving in many cases a conical form, and having craters and often lava- +streams connected with them. + +There are also other rocks in England, Scotland, Ireland, and almost every +country in Europe, which we infer to be of igneous origin, although they do not +form hills with cones and craters. Thus, for example, we feel assured that the +rock of Staffa, and that of the Giant's Causeway, called basalt, is volcanic, +because it agrees in its columnar structure and mineral composition with streams +of lava which we know to have flowed from the craters of volcanoes. We find also +similar basaltic and other igneous rocks associated with beds of TUFF in various +parts of the British Isles, and forming DIKES, such as have been spoken of; and +some of the strata through which these dikes cut are occasionally altered at the +point of contact, as if they had been exposed to the intense heat of melted +matter. + +The absence of cones and craters, and long narrow streams of superficial lava, +in England and many other countries, is principally to be attributed to the +eruptions having been submarine, just as a considerable proportion of volcanoes +in our own times burst out beneath the sea. But this question must be enlarged +upon more fully in the chapters on Igneous Rocks, in which it will also be +shown, that as different sedimentary formations, containing each their +characteristic fossils, have been deposited at successive periods, so also +volcanic sand and scoriae have been thrown out, and lavas have flowed over the +land or bed of the sea, at many different epochs, or have been injected into +fissures; so that the igneous as well as the aqueous rocks may be classed as a +chronological series of monuments, throwing light on a succession of events in +the history of the earth. + +PLUTONIC ROCKS (GRANITE ETC). + +We have now pointed out the existence of two distinct orders of mineral masses, +the aqueous and the volcanic: but if we examine a large portion of a continent, +especially if it contain within it a lofty mountain range, we rarely fail to +discover two other classes of rocks, very distinct from either of those above +alluded to, and which we can neither assimilate to deposits such as are now +accumulated in lakes or seas, nor to those generated by ordinary volcanic +action. The members of both these divisions of rocks agree in being highly +crystalline and destitute of organic remains. The rocks of one division have +been called Plutonic, comprehending all the granites and certain porphyries, +which are nearly allied in some of their characters to volcanic formations. The +members of the other class are stratified and often slaty, and have been called +by some the CRYSTALLINE SCHISTS, in which group are included gneiss, micaceous- +schist (or mica-slate), hornblende-schist, statuary marble, the finer kinds of +roofing slate, and other rocks afterwards to be described. + +As it is admitted that nothing strictly analogous to these crystalline +productions can now be seen in the progress of formation on the earth's surface, +it will naturally be asked, on what data we can find a place for them in a +system of classification founded on the origin of rocks. I can not, in reply to +this question, pretend to give the student, in a few words, an intelligible +account of the long chain of facts and reasonings from which geologists have +been led to infer the nature of the rocks in question. The result, however, may +be briefly stated. All the various kinds of granites which constitute the +Plutonic family are supposed to be of igneous or aqueo-igneous origin, and to +have been formed under great pressure, at a considerable depth in the earth, or +sometimes, perhaps, under a certain weight of incumbent ocean. Like the lava of +volcanoes, they have been melted, and afterwards cooled and crystallised, but +with extreme slowness, and under conditions very different from those of bodies +cooling in the open air. Hence they differ from the volcanic rocks, not only by +their more crystalline texture, but also by the absence of tuffs and breccias, +which are the products of eruptions at the earth's surface, or beneath seas of +inconsiderable depth. They differ also by the absence of pores or cellular +cavities, to which the expansion of the entangled gases gives rise in ordinary +lava. + +METAMORPHIC, OR STRATIFIED CRYSTALLINE ROCKS. + +The fourth and last great division of rocks are the crystalline strata and +slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist, +marble, and the like, the origin of which is more doubtful than that of the +other three classes. They contain no pebbles, or sand, or scoriae, or angular +pieces of imbedded stone, and no traces of organic bodies, and they are often as +crystalline as granite, yet are divided into beds, corresponding in form and +arrangement to those of sedimentary formations, and are therefore said to be +stratified. The beds sometimes consist of an alternation of substances varying +in colour, composition, and thickness, precisely as we see in stratified +fossiliferous deposits. According to the Huttonian theory, which I adopt as the +most probable, and which will be afterwards more fully explained, the materials +of these strata were originally deposited from water in the usual form of +sediment, but they were subsequently so altered by subterranean heat, as to +assume a new texture. It is demonstrable, in some cases at least, that such a +complete conversion has actually taken place, fossiliferous strata having +exchanged an earthy for a highly crystalline texture for a distance of a quarter +of a mile from their contact with granite. In some cases, dark limestones, +replete with shells and corals, have been turned into white statuary marble; and +hard clays, containing vegetable or other remains, into slates called mica- +schist or hornblende-schist, every vestige of the organic bodies having been +obliterated. + +Although we are in a great degree ignorant of the precise nature of the +influence exerted in these cases, yet it evidently bears some analogy to that +which volcanic heat and gases are known to produce; and the action may be +conveniently called Plutonic, because it appears to have been developed in those +regions where Plutonic rocks are generated, and under similar circumstances of +pressure and depth in the earth. Intensely heated water or steam permeating +stratified masses under great pressure have no doubt played their part in +producing the crystalline texture and other changes, and it is clear that the +transforming influence has often pervaded entire mountain masses of strata. + +In accordance with the hypothesis above alluded to, I proposed in the first +edition of the Principles of Geology (1833) the term "Metamorphic" for the +altered strata, a term derived from meta, trans, and morphe, forma. + +Hence there are four great classes of rocks considered in reference to their +origin-- the aqueous, the volcanic, the Plutonic, and the metamorphic. In the +course of this work it will be shown that portions of each of these four +distinct classes have originated at many successive periods. They have all been +produced contemporaneously, and may even now be in the progress of formation on +a large scale. It is not true, as was formerly supposed, that all granites, +together with the crystalline or metamorphic strata, were first formed, and +therefore entitled to be called "primitive," and that the aqueous and volcanic +rocks were afterwards superimposed, and should, therefore, rank as secondary in +the order of time. This idea was adopted in the infancy of the science, when all +formations, whether stratified or unstratified, earthy or crystalline, with or +without fossils, were alike regarded as of aqueous origin. At that period it was +naturally argued that the foundation must be older than the superstructure; but +it was afterwards discovered that this opinion was by no means in every instance +a legitimate deduction from facts; for the inferior parts of the earth's crust +have often been modified, and even entirely changed, by the influence of +volcanic and other subterranean causes, while superimposed formations have not +been in the slightest degree altered. In other words, the destroying and +renovating processes have given birth to new rocks below, while those above, +whether crystalline or fossiliferous, have remained in their ancient condition. +Even in cities, such as Venice and Amsterdam, it cannot be laid down as +universally true that the upper parts of each edifice, whether of brick or +marble, are more modern than the foundations on which they rest, for these often +consist of wooden piles, which may have rotted and been replaced one after the +other, without the least injury to the buildings above; meanwhile, these may +have required scarcely any repair, and may have been constantly inhabited. So it +is with the habitable surface of our globe, in its relation to large masses of +rock immediately below; it may continue the same for ages, while subjacent +materials, at a great depth, are passing from a solid to a fluid state, and then +reconsolidating, so as to acquire a new texture. + +As all the crystalline rocks may, in some respects, be viewed as belonging to +one great family, whether they be stratified or unstratified, metamorphic or +Plutonic, it will often be convenient to speak of them by one common name. It +being now ascertained, as above stated, that they are of very different ages, +sometimes newer than the strata called secondary, the terms primitive and +primary which were formerly used for the whole must be abandoned, as they would +imply a manifest contradiction. It is indispensable, therefore, to find a new +name, one which must not be of chronological import, and must express, on the +one hand, some peculiarity equally attributable to granite and gneiss (to the +Plutonic as well as the ALTERED rocks), and, on the other, must have reference +to characters in which those rocks differ, both from the volcanic and from the +UNALTERED sedimentary strata. I proposed in the Principles of Geology (first +edition volume 3) the term "hypogene" for this purpose, derived from upo, under, +and ginomai, to be, or to be born; a word implying the theory that granite, +gneiss, and the other crystalline formations are alike NETHERFORMED rocks, or +rocks which have not assumed their present form and structure at the surface. +They occupy the lowest place in the order of superposition. Even in regions such +as the Alps, where some masses of granite and gneiss can be shown to be of +comparatively modern date, belonging, for example, to the period hereafter to be +described as tertiary, they are still UNDERLYING rocks. They never repose on the +volcanic or trappean formations, nor on strata containing organic remains. They +are HYPOGENE, as "being under" all the rest. + +From what has now been said, the reader will understand that each of the four +great classes of rocks may be studied under two distinct points of view; first, +they may be studied simply as mineral masses deriving their origin from +particular causes, and having a certain composition, form, and position in the +earth's crust, or other characters both positive and negative, such as the +presence or absence of organic remains. In the second place, the rocks of each +class may be viewed as a grand chronological series of monuments, attesting a +succession of events in the former history of the globe and its living +inhabitants. + +I shall accordingly proceed to treat of each family of rocks; first, in +reference to those characters which are not chronological, and then in +particular relation to the several periods when they were formed. + + +CHAPTER II. + +AQUEOUS ROCKS.-- THEIR COMPOSITION AND FORMS OF STRATIFICATION. + +Mineral Composition of Strata. +Siliceous Rocks. +Argillaceous. +Calcareous. +Gypsum. +Forms of Stratification. +Original Horizontality. +Thinning out. +Diagonal Arrangement. +Ripple-mark. + +In pursuance of the arrangement explained in the last chapter, we shall begin by +examining the aqueous or sedimentary rocks, which are for the most part +distinctly stratified, and contain fossils. We may first study them with +reference to their mineral composition, external appearance, position, mode of +origin, organic contents, and other characters which belong to them as aqueous +formations, independently of their age, and we may afterwards consider them +chronologically or with reference to the successive geological periods when they +originated. + +I have already given an outline of the data which led to the belief that the +stratified and fossiliferous rocks were originally deposited under water; but, +before entering into a more detailed investigation, it will be desirable to say +something of the ordinary materials of which such strata are composed. These may +be said to belong principally to three divisions, the siliceous, the +argillaceous, and the calcareous, which are formed respectively of flint, clay, +and carbonate of lime. Of these, the siliceous are chiefly made up of sand or +flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter +with a certain proportion, about a fourth in weight, of aluminous earth; and, +lastly, the calcareous rocks, or limestones, of carbonic acid and lime. + +SILICEOUS AND ARENACEOUS ROCKS. + +To speak first of the sandy division: beds of loose sand are frequently met +with, of which the grains consist entirely of silex, which term comprehends all +purely siliceous minerals, as quartz and common flint. Quartz is silex in its +purest form. Flint usually contains some admixture of alumina and oxide of iron. +The siliceous grains in sand are usually rounded, as if by the action of running +water. Sandstone is an aggregate of such grains, which often cohere together +without any visible cement, but more commonly are bound together by a slight +quantity of siliceous or calcareous matter, or by oxide of iron or clay. + +Pure siliceous rocks may be known by not effervescing when a drop of nitric, +sulphuric or other acid is applied to them, or by the grains not being readily +scratched or broken by ordinary pressure. In nature there is every intermediate +gradation, from perfectly loose sand to the hardest sandstone. In MICACEOUS +SANDSTONES mica is very abundant; and the thin silvery plates into which that +mineral divides are often arranged in layers parallel to the planes of +stratification, giving a slaty or laminated texture to the rock. + +When sandstone is coarse-grained, it is usually called GRIT. If the grains are +rounded, and large enough to be called pebbles, it becomes a CONGLOMERATE or +PUDDING-STONE, which may consist of pieces of one or of many different kinds of +rock. A conglomerate, therefore, is simply gravel bound together by cement. + +ARGILLACEOUS ROCKS. + +Clay, strictly speaking, is a mixture of silex or flint with a large proportion, +usually about one fourth, of alumina, or argil; but in common language, any +earth which possesses sufficient ductility, when kneaded up with water, to be +fashioned like paste by the hand, or by the potter's lathe, is called a CLAY; +and such clays vary greatly in their composition, and are, in general, nothing +more than mud derived from the decomposition or wearing down of rocks. The +purest clay found in nature is porcelain clay, or kaolin, which results from the +decomposition of a rock composed of feldspar and quartz, and it is almost always +mixed with quartz. The kaolin of China consists of 71.15 parts of silex, 15.86 +of alumine, 1.92 of lime, and 6.73 of water (W. Phillips Mineralogy page 33.); +but other porcelain clays differ materially, that of Cornwall being composed, +according to Boase, of nearly equal parts of silica and alumine, with 1 per cent +of magnesia. (Phil. Mag. volume 10 1837.) SHALE has also the property, like +clay, of becoming plastic in water: it is a more solid form of clay, or +argillaceous matter, condensed by pressure. It always divides into laminae more +or less regular. + +One general character of all argillaceous rocks is to give out a peculiar, +earthy odour when breathed upon, which is a test of the presence of alumine, +although it does not belong to pure alumine, but, apparently, to the combination +of that substance with oxide of iron. (See W. Phillips Mineralogy "Alumine.") + +CALCAREOUS ROCKS. + +This division comprehends those rocks which, like chalk, are composed chiefly of +lime and carbonic acid. Shells and corals are also formed of the same elements, +with the addition of animal matter. To obtain pure lime it is necessary to +calcine these calcareous substances, that is to say, to expose them to heat of +sufficient intensity to drive off the carbonic acid, and other volatile matter. +White chalk is sometimes pure carbonate of lime; and this rock, although usually +in a soft and earthy state, is occasionally sufficiently solid to be used for +building, and even passes into a COMPACT stone, or a stone of which the separate +parts are so minute as not to be distinguishable from each other by the naked +eye. + +Many limestones are made up entirely of minute fragments of shells and coral, or +of calcareous sand cemented together. These last might be called "calcareous +sandstones;" but that term is more properly applied to a rock in which the +grains are partly calcareous and partly siliceous, or to quartzose sandstones, +having a cement of carbonate of lime. + +The variety of limestone called OOLITE is composed of numerous small egg-like +grains, resembling the roe of a fish, each of which has usually a small fragment +of sand as a nucleus, around which concentric layers of calcareous matter have +accumulated. + +Any limestone which is sufficiently hard to take a fine polish is called MARBLE. +Many of these are fossiliferous; but statuary marble, which is also called +saccharoid limestone, as having a texture resembling that of loaf-sugar, is +devoid of fossils, and is in many cases a member of the metamorphic series. + +SILICEOUS LIMESTONE is an intimate mixture of carbonate of lime and flint, and +is harder in proportion as the flinty matter predominates. + +The presence of carbonate of lime in a rock may be ascertained by applying to +the surface a small drop of diluted sulphuric, nitric, or muriatic acid, or +strong vinegar; for the lime, having a greater chemical affinity for any one of +these acids than for the carbonic, unites immediately with them to form new +compounds, thereby becoming a sulphate, nitrate or muriate of lime. The carbonic +acid, when thus liberated from its union with the lime, escapes in a gaseous +form, and froths up or effervesces as it makes its way in small bubbles through +the drop of liquid. This effervescence is brisk or feeble in proportion as the +limestone is pure or impure, or, in other words, according to the quantity of +foreign matter mixed with the carbonate of lime. Without the aid of this test, +the most experienced eye can not always detect the presence of carbonate of lime +in rocks. + +The above-mentioned three classes of rocks, the siliceous, argillaceous, and +calcareous, pass continually into each other, and rarely occur in a perfectly +separate and pure form. Thus it is an exception to the general rule to meet with +a limestone as pure as ordinary white chalk, or with clay as aluminous as that +used in Cornwall for porcelain, or with sand so entirely composed of siliceous +grains as the white sand of Alum Bay, in the Isle of Wight, employed in the +manufacture of glass, or sandstone so pure as the grit of Fontainebleau, used +for pavement in France. More commonly we find sand and clay, or clay and marl, +intermixed in the same mass. When the sand and clay are each in considerable +quantity, the mixture is called LOAM. If there is much calcareous matter in clay +it is called MARL; but this term has unfortunately been used so vaguely, as +often to be very ambiguous. It has been applied to substances in which there is +no lime; as, to that red loam usually called red marl in certain parts of +England. Agriculturists were in the habit of calling any soil a marl which, like +true marl, fell to pieces readily on exposure to the air. Hence arose the +confusion of using this name for soils which, consisting of loam, were easily +worked by the plough, though devoid of lime. + +MARL SLATE bears the same relation to marl which shale bears to clay, being a +calcareous shale. It is very abundant in some countries, as in the Swiss Alps. +Argillaceous or marly limestone is also of common occurrence. + +There are few other kinds of rock which enter so largely into the composition of +sedimentary strata as to make it necessary to dwell here on their characters. I +may, however, mention two others-- magnesian limestone or dolomite, and gypsum. +MAGNESIAN LIMESTONE is composed of carbonate of lime and carbonate of magnesia; +the proportion of the latter amounting in some cases to nearly one half. It +effervesces much more slowly and feebly with acids than common limestone. In +England this rock is generally of a yellowish colour; but it varies greatly in +mineralogical character, passing from an earthy state to a white compact stone +of great hardness. DOLOMITE, so common in many parts of Germany and France, is +also a variety of magnesian limestone, usually of a granular texture. + +Gypsum is a rock composed of sulphuric acid, lime, and water. It is usually a +soft whitish-yellow rock, with a texture resembling that of loaf-sugar, but +sometimes it is entirely composed of lenticular crystals. It is insoluble in +acids, and does not effervesce like chalk and dolomite, because it does not +contain carbonic acid gas, or fixed air, the lime being already combined with +sulphuric acid, for which it has a stronger affinity than for any other. +Anhydrous gypsum is a rare variety, into which water does not enter as a +component part. GYPSEOUS MARL is a mixture of gypsum and marl. ALABASTER is a +granular and compact variety of gypsum found in masses large enough to be used +in sculpture and architecture. It is sometimes a pure snow-white substance, as +that of Volterra in Tuscany, well known as being carved for works of art in +Florence and Leghorn. It is a softer stone than marble, and more easily wrought. + +FORMS OF STRATIFICATION. + +A series of strata sometimes consists of one of the above rocks, sometimes of +two or more in alternating beds. + +Thus, in the coal districts of England, for example, we often pass through +several beds of sandstone, some of finer, others of coarser grain, some white, +others of a dark colour, and below these, layers of shale and sandstone or beds +of shale, divisible into leaf-like laminae, and containing beautiful impressions +of plants. Then again we meet with beds of pure and impure coal, alternating +with shales and sandstones, and underneath the whole, perhaps, are calcareous +strata, or beds of limestone, filled with corals and marine shells, each bed +distinguishable from another by certain fossils, or by the abundance of +particular species of shells or zoophytes. + +This alternation of different kinds of rock produces the most distinct +stratification; and we often find beds of limestone and marl, conglomerate and +sandstone, sand and clay, recurring again and again, in nearly regular order, +throughout a series of many hundred strata. The causes which may produce these +phenomena are various, and have been fully discussed in my treatise on the +modern changes of the earth's surface. (Consult Index to Principles of Geology, +"Stratification" "Currents" "Deltas" "Water" etc.) It is there seen that rivers +flowing into lakes and seas are charged with sediment, varying in quantity, +composition, colour, and grain according to the seasons; the waters are +sometimes flooded and rapid, at other periods low and feeble; different +tributaries, also, draining peculiar countries and soils, and therefore charged +with peculiar sediment, are swollen at distinct periods. It was also shown that +the waves of the sea and currents undermine the cliffs during wintry storms, and +sweep away the materials into the deep, after which a season of tranquillity +succeeds, when nothing but the finest mud is spread by the movements of the +ocean over the same submarine area. + +It is not the object of the present work to give a description of these +operations, repeated as they are, year after year, and century after century; +but I may suggest an explanation of the manner in which some micaceous +sandstones have originated, namely, those in which we see innumerable thin +layers of mica dividing layers of fine quartzose sand. I observed the same +arrangement of materials in recent mud deposited in the estuary of Laroche St. +Bernard in Brittany, at the mouth of the Loire. The surrounding rocks are of +gneiss, which, by its waste, supplies the mud: when this dries at low water, it +is found to consist of brown laminated clay, divided by thin seams of mica. The +separation of the mica in this case, or in that of micaceous sandstones, may be +thus understood. If we take a handful of quartzose sand, mixed with mica, and +throw it into a clear running stream, we see the materials immediately sorted by +the water, the grains of quartz falling almost directly to the bottom, while the +plates of mica take a much longer time to reach the bottom, and are carried +farther down the stream. At the first instant the water is turbid, but +immediately after the flat surfaces of the plates of mica are seen all alone, +reflecting a silvery light, as they descend slowly, to form a distinct micaceous +lamina. The mica is the heavier mineral of the two; but it remains a longer time +suspended in the fluid, owing to its greater extent of surface. It is easy, +therefore, to perceive that where such mud is acted upon by a river or tidal +current, the thin plates of mica will be carried farther, and not deposited in +the same places as the grains of quartz; and since the force and velocity of the +stream varies from time to time, layers of mica or of sand will be thrown down +successively on the same area. + +ORIGINAL HORIZONTALITY. + +It is said generally that the upper and under surfaces of strata, or the "planes +of stratification," are parallel. Although this is not strictly true, they make +an approach to parallelism, for the same reason that sediment is usually +deposited at first in nearly horizontal layers. Such an arrangement can by no +means be attributed to an original evenness or horizontality in the bed of the +sea: for it is ascertained that in those places where no matter has been +recently deposited, the bottom of the ocean is often as uneven as that of the +dry land, having in like manner its hills, valleys, and ravines. Yet if the sea +should go down, or be removed from near the mouth of a large river where a delta +has been forming, we should see extensive plains of mud and sand laid dry, +which, to the eye, would appear perfectly level, although, in reality, they +would slope gently from the land towards the sea. + +This tendency in newly-formed strata to assume a horizontal position arises +principally from the motion of the water, which forces along particles of sand +or mud at the bottom, and causes them to settle in hollows or depressions where +they are less exposed to the force of a current than when they are resting on +elevated points. The velocity of the current and the motion of the superficial +waves diminish from the surface downward, and are least in those depressions +where the water is deepest. + +(FIGURE 1. Layers of sand and ashes on uneven ground.) + +A good illustration of the principle here alluded to may be sometimes seen in +the neighbourhood of a volcano, when a section, whether natural or artificial, +has laid open to view a succession of various-coloured layers of sand and ashes, +which have fallen in showers upon uneven ground. Thus let A B (Figure 1) be two +ridges, with an intervening valley. These original inequalities of the surface +have been gradually effaced by beds of sand and ashes c, d, e, the surface at e +being quite level. It will be seen that, although the materials of the first +layers have accommodated themselves in a great degree to the shape of the ground +A B, yet each bed is thickest at the bottom. At first a great many particles +would be carried by their own gravity down the steep sides of A and B, and +others would afterwards be blown by the wind as they fell off the ridges, and +would settle in the hollow, which would thus become more and more effaced as the +strata accumulated from c to e. Now, water in motion can exert this levelling +power on similar materials more easily than air, for almost all stones lose in +water more than a third of the weight which they have in air, the specific +gravity of rocks being in general as 2 1/2 when compared to that of water, which +is estimated at 1. But the buoyancy of sand or mud would be still greater in the +sea, as the density of salt-water exceeds that of fresh. + +(FIGURE 2. Section of strata of sandstone, grit, and conglomerate.) + +Yet, however uniform and horizontal may be the surface of new deposits in +general, there are still many disturbing causes, such as eddies in the water, +and currents moving first in one and then in another direction, which frequently +cause irregularities. We may sometimes follow a bed of limestone, shale, or +sandstone, for a distance of many hundred yards continuously; but we generally +find at length that each individual stratum thins out, and allows the beds which +were previously above and below it to meet. If the materials are coarse, as in +grits and conglomerates, the same beds can rarely be traced many yards without +varying in size, and often coming to an end abruptly. (See Figure 2.) + +DIAGONAL OR CROSS STRATIFICATION. + +(FIGURE 3. Section of sand at Sandy Hill, near Biggleswade, Bedfordshire. Height +20 feet. (Green-sand formation.)) + +(FIGURE 4. Layers of sediment on a bank.) + +(FIGURE 5. Nearly horizontal layers of sediment over sloping strata.) + +(FIGURE 6. Cliff between mismer and Dunwich.) + +There is also another phenomenon of frequent occurrence. We find a series of +larger strata, each of which is composed of a number of minor layers placed +obliquely to the general planes of stratification. To this diagonal arrangement +the name of "false or cross bedding" has been given. Thus in the section (Figure +3) we see seven or eight large beds of loose sand, yellow and brown, and the +lines a, b, c mark some of the principal planes of stratification, which are +nearly horizontal. But the greater part of the subordinate laminae do not +conform to these planes, but have often a steep slope, the inclination being +sometimes towards opposite points of the compass. When the sand is loose and +incoherent, as in the case here represented, the deviation from parallelism of +the slanting laminae can not possibly be accounted for by any rearrangement of +the particles acquired during the consolidation of the rock. In what manner, +then, can such irregularities be due to original deposition? We must suppose +that at the bottom of the sea, as well as in the beds of rivers, the motions of +waves, currents, and eddies often cause mud, sand, and gravel to be thrown down +in heaps on particular spots, instead of being spread out uniformly over a wide +area. Sometimes, when banks are thus formed, currents may cut passages through +them, just as a river forms its bed. Suppose the bank A (Figure 4) to be thus +formed with a steep sloping side, and, the water being in a tranquil state, the +layer of sediment No. 1 is thrown down upon it, conforming nearly to its +surface. Afterwards the other layers, 2, 3, 4, may be deposited in succession, +so that the bank B C D is formed. If the current then increases in velocity, it +may cut away the upper portion of this mass down to the dotted line e, and +deposit the materials thus removed farther on, so as to form the layers 5, 6, 7, +8. We have now the bank B, C, D, E (Figure 5), of which the surface is almost +level, and on which the nearly horizontal layers, 9, 10, 11, may then +accumulate. It was shown in Figure 3 that the diagonal layers of successive +strata may sometimes have an opposite slope. This is well seen in some cliffs of +loose sand on the Suffolk coast. A portion of one of these is represented in +Figure 6, where the layers, of which there are about six in the thickness of an +inch, are composed of quartzose grains. This arrangement may have been due to +the altered direction of the tides and currents in the same place. + +(FIGURE 7. Section from Monte Calvo to the sea by the valley of the Magnan, near +Nice. +A. Dolomite and sandstone. (Green-sand formation?) +a, b, d. Beds of gravel and sand. +c. Fine marl and sand of Ste. Madeleine, with marine (Pliocene) shells.) + +The description above given of the slanting position of the minor layers +constituting a single stratum is in certain cases applicable on a much grander +scale to masses several hundred feet thick, and many miles in extent. A fine +example may be seen at the base of the Maritime Alps near Nice. The mountains +here terminate abruptly in the sea, so that a depth of one hundred fathoms is +often found within a stone's throw of the beach, and sometimes a depth of 3000 +feet within half a mile. But at certain points, strata of sand, marl, or +conglomerate intervene between the shore and the mountains, as in the section +(Figure 7), where a vast succession of slanting beds of gravel and sand may be +traced from the sea to Monte Calvo, a distance of no less than nine miles in a +straight line. The dip of these beds is remarkably uniform, being always +southward or towards the Mediterranean, at an angle of about 25 degrees. They +are exposed to view in nearly vertical precipices, varying from 200 to 600 feet +in height, which bound the valley through which the river Magnan flows. +Although, in a general view, the strata appear to be parallel and uniform, they +are nevertheless found, when examined closely, to be wedge-shaped, and to thin +out when followed for a few hundred feet or yards, so that we may suppose them +to have been thrown down originally upon the side of a steep bank where a river +or Alpine torrent discharged itself into a deep and tranquil sea, and formed a +delta, which advanced gradually from the base of Monte Calvo to a distance of +nine miles from the original shore. If subsequently this part of the Alps and +bed of the sea were raised 700 feet, the delta may have emerged, a deep channel +may then have been cut through it by the river, and the coast may at the same +time have acquired its present configuration. + +(FIGURE 8. Slab of ripple-marked (New Red) sandstone from Cheshire.) + +It is well known that the torrents and streams which now descend from the Alpine +declivities to the shore, bring down annually, when the snow melts, vast +quantities of shingle and sand, and then, as they subside, fine mud, while in +summer they are nearly or entirely dry; so that it may be safely assumed that +deposits like those of the valley of the Magnan, consisting of coarse gravel +alternating with fine sediment, are still in progress at many points, as, for +instance, at the mouth of the Var. They must advance upon the Mediterranean in +the form of great shoals terminating in a steep talus; such being the original +mode of accumulation of all coarse materials conveyed into deep water, +especially where they are composed in great part of pebbles, which can not be +transported to indefinite distances by currents of moderate velocity. By +inattention to facts and inferences of this kind, a very exaggerated estimate +has sometimes been made of the supposed depth of the ancient ocean. There can be +no doubt, for example, that the strata a, Figure 7, or those nearest to Monte +Calvo, are older than those indicated by b, and these again were formed before +c; but the vertical depth of gravel and sand in any one place can not be proved +to amount even to 1000 feet, although it may perhaps be much greater, yet +probably never exceeding at any point 3000 or 4000 feet. But were we to assume +that all the strata were once horizontal, and that their present dip or +inclination was due to subsequent movements, we should then be forced to +conclude that a sea several miles deep had been filled up with alternate layers +of mud and pebbles thrown down one upon another. + +In the locality now under consideration, situated a few miles to the west of +Nice, there are many geological data, the details of which can not be given in +this place, all leading to the opinion that, when the deposit of the Magnan was +formed, the shape and outline of the Alpine declivities and the shore greatly +resembled what we now behold at many points in the neighbourhood. That the beds +a, b, c, d are of comparatively modern date is proved by this fact, that in +seams of loamy marl intervening between the pebbly beds are fossil shells, half +of which belong to species now living in the Mediterranean. + +RIPPLE-MARK. + +The ripple-mark, so common on the surface of sandstones of all ages (see Figure +8), and which is so often seen on the sea-shore at low tide, seems to originate +in the drifting of materials along the bottom of the water, in a manner very +similar to that which may explain the inclined layers above described. This +ripple is not entirely confined to the beach between high and low water mark, +but is also produced on sands which are constantly covered by water. Similar +undulating ridges and furrows may also be sometimes seen on the surface of drift +snow and blown sand. + +The ripple-mark is usually an indication of a sea-beach, or of water from six to +ten feet deep, for the agitation caused by waves even during storms extends to a +very slight depth. To this rule, however, there are some exceptions, and recent +ripple-marks have been observed at the depth of 60 or 70 feet. It has also been +ascertained that currents or large bodies of water in motion may disturb mud and +sand at the depth of 300 or even 450 feet. (Darwin Volcanic Islands page 134.) +Beach ripple, however, may usually be distinguished from current ripple by +frequent changes in its direction. In a slab of sandstone, not more than an inch +thick, the furrows or ridges of an ancient ripple may often be seen in several +successive laminae to run towards different points of the compass. + + +CHAPTER III. + +ARRANGEMENT OF FOSSILS IN STRATA.-- FRESH-WATER AND MARINE FOSSILS. + +Successive Deposition indicated by Fossils. +Limestones formed of Corals and Shells. +Proofs of gradual Increase of Strata derived from Fossils. +Serpula attached to Spatangus. +Wood bored by Teredina. +Tripoli formed of Infusoria. +Chalk derived principally from Organic Bodies. +Distinction of Fresh-water from Marine Formations. +Genera of Fresh-water and Land Shells. +Rules for recognising Marine Testacea. +Gyrogonite and Chara. +Fresh-water Fishes. +Alternation of Marine and Fresh-water Deposits. +Lym-Fiord. + +Having in the last chapter considered the forms of stratification so far as they +are determined by the arrangement of inorganic matter, we may now turn our +attention to the manner in which organic remains are distributed through +stratified deposits. We should often be unable to detect any signs of +stratification or of successive deposition, if particular kinds of fossils did +not occur here and there at certain depths in the mass. At one level, for +example, univalve shells of some one or more species predominate; at another, +bivalve shells; and at a third, corals; while in some formations we find layers +of vegetable matter, commonly derived from land plants, separating strata. + +It may appear inconceivable to a beginner how mountains, several thousand feet +thick, can have become full of fossils from top to bottom; but the difficulty is +removed, when he reflects on the origin of stratification, as explained in the +last chapter, and allows sufficient time for the accumulation of sediment. He +must never lose sight of the fact that, during the process of deposition, each +separate layer was once the uppermost, and immediately in contact with the water +in which aquatic animals lived. Each stratum, in fact, however far it may now +lie beneath the surface, was once in the state of shingle, or loose sand or soft +mud at the bottom of the sea, in which shells and other bodies easily became +enveloped. + +RATE OF DEPOSITION INDICATED BY FOSSILS. + +By attending to the nature of these remains, we are often enabled to determine +whether the deposition was slow or rapid, whether it took place in a deep or +shallow sea, near the shore or far from land, and whether the water was salt, +brackish, or fresh. Some limestones consist almost exclusively of corals, and in +many cases it is evident that the present position of each fossil zoophyte has +been determined by the manner in which it grew originally. The axis of the +coral, for example, if its natural growth is erect, still remains at right +angles to the plane of stratification. If the stratum be now horizontal, the +round spherical heads of certain species continue uppermost, and their points of +attachment are directed downward. This arrangement is sometimes repeated +throughout a great succession of strata. From what we know of the growth of +similar zoophytes in modern reefs, we infer that the rate of increase was +extremely slow, and some of the fossils must have flourished for ages like +forest-trees, before they attained so large a size. During these ages, the water +must have been clear and transparent, for such corals can not live in turbid +water. + +(FIGURE 9. Fossil Gryphaea, covered both on the outside and inside with fossil +Serpulae.) + +In like manner, when we see thousands of full-grown shells dispersed everywhere +throughout a long series of strata, we can not doubt that time was required for +the multiplication of successive generations; and the evidence of slow +accumulation is rendered more striking from the proofs, so often discovered, of +fossil bodies having lain for a time on the floor of the ocean after death +before they were imbedded in sediment. Nothing, for example, is more common than +to see fossil oysters in clay, with Serpulae, or barnacles (acorn-shells), or +corals, and other creatures, attached to the inside of the valves, so that the +mollusk was certainly not buried in argillaceous mud the moment it died. There +must have been an interval during which it was still surrounded with clear +water, when the creatures whose remains now adhere to it grew from an embryonic +to a mature state. Attached shells which are merely external, like some of the +Serpulae (a) in Figure 9, may often have grown upon an oyster or other shell +while the animal within was still living; but if they are found on the inside, +it could only happen after the death of the inhabitant of the shell which +affords the support. Thus, in Figure 9, it will be seen that two Serpulae have +grown on the interior, one of them exactly on the place where the adductor +muscle of the Gryphaea (a kind of oyster) was fixed. + +(FIGURE 10. Serpula attached to a fossil Micraster from the Chalk.) + +(FIGURE 11. Recent Spatangus with the spines removed from one side. +b. Spine and tubercles, natural size. +a. The same magnified.) + +Some fossil shells, even if simply attached to the OUTSIDE of others, bear full +testimony to the conclusion above alluded to, namely, that an interval elapsed +between the death of the creature to whose shell they adhere, and the burial of +the same in mud or sand. The sea-urchins, or Echini, so abundant in white chalk, +afford a good illustration. It is well known that these animals, when living, +are invariably covered with spines supported by rows of tubercles. These last +are only seen after the death of the sea-urchin, when the spines have dropped +off. In Figure 11 a living species of Spatangus, common on our coast, is +represented with one half of its shell stripped of the spines. In Figure 10 a +fossil of a similar and allied genus from the white chalk of England shows the +naked surface which the individuals of this family exhibit when denuded of their +bristles. The full-grown Serpula, therefore, which now adheres externally, could +not have begun to grow till the Micraster had died, and the spines became +detached. + +(FIGURE 12. +a. Ananchytes from the chalk with lower valve of Crania attached. +b. Upper valve of Crania detached.) + +Now the series of events here attested by a single fossil may be carried a step +farther. Thus, for example, we often meet with a sea-urchin (Ananchytes) in the +chalk (see Figure 12) which has fixed to it the lower valve of a Crania, a genus +of bivalve mollusca. The upper valve (b, Figure 12) is almost invariably +wanting, though occasionally found in a perfect state of preservation in white +chalk at some distance. In this case, we see clearly that the sea-urchin first +lived from youth to age, then died and lost its spines, which were carried away. +Then the young Crania adhered to the bared shell, grew and perished in its turn; +after which the upper valve was separated from the lower before the Ananchytes +became enveloped in chalky mud. + +(FIGURES 13 AND 14. Fossil and recent wood drilled by perforating Mollusca. + +(FIGURE 13. +a. Fossil wood from London Clay, bored by Teredina. +b. Shell and tube of Teredina personata, the right-hand figure the ventral, the +left the dorsal view.) + +(FIGURE 14. +e. Recent wood bored by Toredo. +d. Shell and tube of Teredo navalis, from the same. +c. Anterior and posterior view of the valves of same detached from the tube.)) + +It may be well to mention one more illustration of the manner in which single +fossils may sometimes throw light on a former state of things, both in the bed +of the ocean and on some adjoining land. We meet with many fragments of wood +bored by ship-worms at various depths in the clay on which London is built. +Entire branches and stems of trees, several feet in length, are sometimes found +drilled all over by the holes of these borers, the tubes and shells of the +mollusk still remaining in the cylindrical hollows. In Figure 14, e, a +representation is given of a piece of recent wood pierced by the Teredo navalis, +or common ship-worm, which destroys wooden piles and ships. When the cylindrical +tube d has been extracted from the wood, the valves are seen at the larger or +anterior extremity, as shown at c. In like manner, a piece of fossil wood (a, +Figure 13) has been perforated by a kindred but extinct genus, the Teredina of +Lamarck. The calcareous tube of this mollusk was united and, as it were, +soldered on to the valves of the shell (b), which therefore can not be detached +from the tube, like the valves of the recent Teredo. The wood in this fossil +specimen is now converted into a stony mass, a mixture of clay and lime; but it +must once have been buoyant and floating in the sea, when the Teredinae lived +upon, and perforated it. Again, before the infant colony settled upon the drift +wood, part of a tree must have been floated down to the sea by a river, +uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: +and thus our thoughts are carried back to a prior period, when the tree grew for +years on dry land, enjoying a fit soil and climate. + +STRATA OF ORGANIC ORIGIN. + +(FIGURE 15. Gaillonella ferruginea, Ehb.) + +(FIGURE 16. Gaillonella distans, Ehb.) + +(FIGURE 17. Bacillaria paradoxa. +a. Front view. +b. Side view.) + +It has been already remarked that there are rocks in the interior of continents, +at various depths in the earth, and at great heights above the sea, almost +entirely made up of the remains of zoophytes and testacea. Such masses may be +compared to modern oyster-beds and coral-reefs; and, like them, the rate of +increase must have been extremely gradual. But there are a variety of stone +deposits in the earth's crust, now proved to have been derived from plants and +animals of which the organic origin was not suspected until of late years, even +by naturalists. Great surprise was therefore created some years since by the +discovery of Professor Ehrenberg, of Berlin, that a certain kind of siliceous +stone, called tripoli, was entirely composed of millions of the remains of +organic beings, which were formerly referred to microscopic Infusoria, but which +are now admitted to be plants. They abound in rivulets, lakes, and ponds in +England and other countries, and are termed Diatomaceae by those naturalists who +believe in their vegetable origin. The subject alluded to has long been well- +known in the arts, under the name of infusorial earth or mountain meal, and is +used in the form of powder for polishing stones and metals. It has been +procured, among other places, from the mud of a lake at Dolgelly, in North +Wales, and from Bilin, in Bohemia, in which latter place a single stratum, +extending over a wide area, is no less than fourteen feet thick. This stone, +when examined with a powerful microscope, is found to consist of the siliceous +plates or frustules of the above-figured Diatomaceae, united together without +any visible cement. It is difficult to convey an idea of their extreme +minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 +millions of individuals of the Gaillonella distans (see Figure 16) in every +cubic inch (which weighs about 220 grains), or about 187 millions in a single +grain. At every stroke, therefore, that we make with this polishing powder, +several millions, perhaps tens of millions, of perfect fossils are crushed to +atoms. + +A well-known substance, called bog-iron ore, often met with in peat-mosses, has +often been shown by Ehrenberg to consist of innumerable articulated threads, of +a yellow ochre colour, composed of silica, argillaceous matter, and peroxide of +iron. These threads are the cases of a minute microscopic body, called +Gaillonella ferruginea (Figure 15), associated with the siliceous frustules of +other fresh-water algae. Layers of this iron ore occurring in Scotch peat bogs +are often called "the pan," and are sometimes of economical value. + +It is clear much time must have been required for the accumulation of strata to +which countless generations of Diatomaceae have contributed their remains; and +these discoveries lead us naturally to suspect that other deposits, of which the +materials have been supposed to be inorganic, may in reality be composed chiefly +of microscopic organic bodies. That this is the case with the white chalk, has +often been imagined, and is now proved to be the fact. It has, moreover, been +lately discovered that the chambers into which these Foraminifera are divided +are actually often filled with thousands of well-preserved organic bodies, which +abound in every minute grain of chalk, and are especially apparent in the white +coating of flints, often accompanied by innumerable needle-shaped spiculae of +sponges (see Chapter 17.). + +"The dust we tread upon was once alive!"-- Byron. + +How faint an idea does this exclamation of the poet convey of the real wonders +of nature! for here we discover proofs that the calcareous and siliceous dust of +which hills are composed has not only been once alive, but almost every +particle, albeit invisible to the naked eye, still retains the organic structure +which, at periods of time incalculably remote, was impressed upon it by the +powers of life. + +FRESH-WATER AND MARINE FOSSILS. + +Strata, whether deposited in salt or fresh water, have the same forms; but the +imbedded fossils are very different in the two cases, because the aquatic +animals which frequent lakes and rivers are distinct from those inhabiting the +sea. In the northern part of the Isle of Wight formations of marl and limestone, +more than 50 feet thick occur, in which the shells are of extinct species. Yet +we recognise their fresh-water origin, because they are of the same genera as +those now abounding in ponds, lakes, and rivers, either in our own country or in +warmer latitudes. + +In many parts of France-- in Auvergne, for example-- strata occur of limestone, +marl, and sandstone hundreds of feet thick, which contain exclusively fresh- +water and land shells, together with the remains of terrestrial quadrupeds. The +number of land-shells scattered through some of these fresh-water deposits is +exceedingly great; and there are districts in Germany where the rocks scarcely +contain any other fossils except snail-shells (helices); as, for instance, the +limestone on the left bank of the Rhine, between Mayence and Worms, at +Oppenheim, Findheim, Budenheim, and other places. In order to account for this +phenomenon, the geologist has only to examine the small deltas of torrents which +enter the Swiss lakes when the waters are low, such as the newly-formed plain +where the Kander enters the Lake of Thun. He there sees sand and mud strewn over +with innumerable dead land-shells, which have been brought down from the valleys +in the Alps in the preceding spring, during the melting of the snows. Again, if +we search the sands on the borders of the Rhine, in the lower part of its +course, we find countless land-shells mixed with others of species belonging to +lakes, stagnant pools, and marshes. These individuals have been washed away from +the alluvial plains of the great river and its tributaries, some from +mountainous regions, others from the low country. + +Although fresh-water formations are often of great thickness, yet they are +usually very limited in area when compared to marine deposits, just as lakes and +estuaries are of small dimensions in comparison with seas. + +The absence of many fossil forms usually met with in marine strata, affords a +useful negative indication of the fresh-water origin of a formation. For +example, there are no sea-urchins, no corals, no chambered shells, such as the +nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In +distinguishing the latter from formations accumulated in the sea, we are chiefly +guided by the forms of the mollusca. In a fresh-water deposit, the number of +individual shells is often as great as in a marine stratum, if not greater; but +there is a smaller variety of species and genera. This might be anticipated from +the fact that the genera and species of recent fresh-water and land shells are +few when contrasted with the marine. Thus, the genera of true mollusca according +to Woodward's system, excluding those altogether extinct and those without +shells, amount to 446 in number, of which the terrestrial and fresh-water genera +scarcely form more than a fifth. (See Woodward's Manual of Mollusca 1856.) + +(FIGURE 18. Cyrena obovata, Sowerby; fossil. Hants.) + +(FIGURE 19. Cyrena (Corbicella) fluminalis, Moll.; fossil. Grays, Essex.) + +(FIGURE 20. Anodonta Cordierii; D'Orbigny; fossil. Paris.) + +(FIGURE 21. Anodonta latimarginata; recent. Bahia.) + +(FIGURE 22. Unio littoralis. Lamarck; recent. Auvergne.) + +(FIGURE 23. Gryphaea incurva, Sowerby; (G. arcuata, Lamarck) upper valve. Lias.) + +Almost all bivalve shells, or those of acephalous mollusca, are marine, about +sixteen only out of 140 genera being fresh-water. Among these last, the four +most common forms, both recent and fossil, are Cyclas, Cyrena, Unio, and +Anodonta (see Figures 18-22); the two first and two last of which are so nearly +allied as to pass into each other. + +Lamarck divided the bivalve mollusca into the Dimyary, or those having two large +muscular impressions in each valve, as a, b in the Cyclas, Figure 18, and Unio, +Figure 22, and the Monomyary, such as the oyster and scallop, in which there is +only one of these impressions, as is seen in Figure 23. Now, as none of these +last, or the unimuscular bivalves, are fresh-water, we may at once presume a +deposit containing any of them to be marine. (The fresh-water Mulleria, when +young, forms a single exception to the rule, as it then has two muscular +impressions, but it has only one in the adult state.) + +(FIGURE 24. Planorbis euomphalus, Sowerby; fossil. Isle of Wight.) + +(FIGURE 25. Limnaea longiscala, Brongniart; fossil. Isle of Wight.) + +(FIGURE 26. Paludina lenta, Brand.; fossil. Isle of Wight.) + +(FIGURE 27. Succinea amphibia, Drap. (S. putris, L.); fossil. Loess, Rhine.) + +(FIGURE 28. Ancylus velletia (A. elegans), Sowerby; fossil. Isle of Wight.) + +(FIGURE 29. Valvata piscinalis, Mull.; fossil. Grays, Essex.) + +(FIGURE 30. Physa hypnorum, Linne; recent. Isle of Wight.) + +(FIGURE 31. Auricula; recent. Ava.) + +(FIGURE 32. Melania inquinata, Def. Paris basin.) + +(FIGURE 33. Physa columnaris, Desh. Paris basin.) + +(FIGURE 34. Melanopsis buccinoidea, Ferr.; recent. Asia.) + +The univalve shells most characteristic of fresh-water deposits are, Planorbis, +Limnaea, and Paludina. (See Figures 24-26.) But to these are occasionally added +Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides, and Neritina +(see Figures 27-34), the four last being usually found in estuaries. + +(FIGURE 35. Neritina globulus, Def. Paris basin.) + +(FIGURE 36. Nerita granulosa, Desh. Paris basin.) + +Some naturalists include Neritina (Figure 35) and the marine Nerita (Figure 36) +in the same genus, it being scarcely possible to distinguish the two by good +generic characters. But, as a general rule, the fluviatile species are smaller, +smoother, and more globular than the marine; and they have never, like the +Neritae, the inner margin of the outer lip toothed or crenulated. (See Figure +36.) + +(FIGURE 37. Potamides cinctus, Sowerby. Paris basin.) + +The Potamides inhabit the mouths of rivers in warm latitudes, and are +distinguishable from the marine Cerithia by their orbicular and multispiral +opercula. The genus Auricula (Figure 31) is amphibious, frequenting swamps and +marshes within the influence of the tide. + +(FIGURE 38. Helix Turonensis, Desh.; faluns, Touraine.) + +(FIGURE 39. Cyclostoma elegans, Mull.; Loess.) + +(FIGURE 40. Pupa tridens, Drap.; Loess.) + +(FIGURE 41. Clausilia bidens, Drap.; Loess.) + +(FIGURE 42. Bulimus lubricus, Mull.; Loess, Rhine.) + +The terrestrial shells are all univalves. The most important genera among these, +both in a recent and fossil state, are Helix (Figure 38), Cyclostoma (Figure +39), Pupa (Figure 40), Clausilia (Figure 41) Bulimus (Figure 42), Glandina and +Achatina. + +(FIGURE 43. Ampullaria glauca, from the Jumna.) + +Ampullaria (Figure 43) is another genus of shells inhabiting rivers and ponds in +hot countries. Many fossil species formerly referred to this genus, and which +have been met with chiefly in marine formations, are now considered by +conchologists to belong to Natica and other marine genera. + +(FIGURE 44. Pleurotoma exorta, Brand. Upper and Middle Eocene. Barton and +Bracklesham.) + +(FIGURE 45. Ancillaria subulata, Sowerby. Barton clay. Eocene.) + +All univalve shells of land and fresh-water species, with the exception of +Melanopsis (Figure 34), and Achatina, which has a slight indentation, have +entire mouths; and this circumstance may often serve as a convenient rule for +distinguishing fresh-water from marine strata; since, if any univalves occur of +which the mouths are not entire, we may presume that the formation is marine. +The aperture is said to be entire in such shells as the fresh-water Ampullaria +and the land-shells (Figures 38-42), when its outline is not interrupted by an +indentation or notch, such as that seen at b in Ancillaria (Figure 45); or is +not prolonged into a canal, as that seen at a in Pleurotoma (Figure 44). + +The mouths of a large proportion of the marine univalves have these notches or +canals, and almost all species are carnivorous; whereas nearly all testacea +having entire mouths are plant-eaters, whether the species be marine, fresh- +water, or terrestrial. + +There is, however, one genus which affords an occasional exception to one of the +above rules. The Potamides (Figure 37), a subgenus of Cerithium, although +provided with a short canal, comprises some species which inhabit salt, others +brackish, and others fresh-water, and they are said to be all plant-eaters. + +Among the fossils very common in fresh-water deposits are the shells of Cypris, +a minute bivalve crustaceous animal. (For figures of fossil species of Purbeck +see below, Chapter 19.) Many minute living species of this genus swarm in lakes +and stagnant pools in Great Britain; but their shells are not, if considered +separately, conclusive as to the fresh-water origin of a deposit, because the +majority of species in another kindred genus of the same order, the Cytherina of +Lamarck, inhabit salt-water; and, although the animal differs slightly, the +shell is scarcely distinguishable from that of the Cypris. + +FRESH-WATER FOSSIL PLANTS. + +(FIGURE 46. Chara medicaginula; fossil. Upper Eocene, Isle of Wight.) + +The seed-vessels and stems of Chara, a genus of aquatic plants, are very +frequent in fresh-water strata. These seed-vessels were called, before their +true nature was known, gyrogonites, and were supposed to be foraminiferous +shells. (See Figure 46, a.) + +(FIGURE 47. Chara elastica; recent, Italy. +a. Sessile seed-vessel between the divisions of the leaves of the female plant. +b. Magnified transverse section of a branch, with five seed-vessels, seen from +below upward.) + +The Charae inhabit the bottom of lakes and ponds, and flourish mostly where the +water is charged with carbonate of lime. Their seed-vessels are covered with a +very tough integument, capable of resisting decomposition; to which circumstance +we may attribute their abundance in a fossil state. Figure 47 represents a +branch of one of many new species found by Professor Amici in the lakes of +Northern Italy. The seed-vessel in this plant is more globular than in the +British Charae, and therefore more nearly resembles in form the extinct fossil +species found in England, France, and other countries. The stems, as well as the +seed-vessels, of these plants occur both in modern shell-marl and in ancient +fresh-water formations. They are generally composed of a large central tube +surrounded by smaller ones; the whole stem being divided at certain intervals by +transverse partitions or joints. (See b, Figure 46.) + +It is not uncommon to meet with layers of vegetable matter, impressions of +leaves, and branches of trees, in strata containing fresh-water shells; and we +also find occasionally the teeth and bones of land quadrupeds, of species now +unknown. The manner in which such remains are occasionally carried by rivers +into lakes, especially during floods, has been fully treated of in the +"Principles of Geology." + +FRESH-WATER AND MARINE FISH. + +The remains of fish are occasionally useful in determining the fresh-water +origin of strata. Certain genera, such as carp, perch, pike, and loach +(Cyprinus, Perca, Esox, and Cobitis), as also Lebias, being peculiar to fresh- +water. Other genera contain some fresh-water and some marine species, as Cottus, +Mugil, and Anguilla, or eel. The rest are either common to rivers and the sea, +as the salmon; or are exclusively characteristic of salt-water. The above +observations respecting fossil fishes are applicable only to the more modern or +tertiary deposits; for in the more ancient rocks the forms depart so widely from +those of existing fishes, that it is very difficult, at least in the present +state of science, to derive any positive information from ichthyolites +respecting the element in which strata were deposited. + +The alternation of marine and fresh-water formations, both on a small and large +scale, are facts well ascertained in geology. When it occurs on a small scale, +it may have arisen from the alternate occupation of certain spaces by river- +water and the sea; for in the flood season the river forces back the ocean and +freshens it over a large area, depositing at the same time its sediment; after +which the salt-water again returns, and, on resuming its former place, brings +with it sand, mud, and marine shells. + +There are also lagoons at the mouth of many rivers, as the Nile and Mississippi, +which are divided off by bars of sand from the sea, and which are filled with +salt and fresh water by turns. They often communicate exclusively with the river +for months, years, or even centuries; and then a breach being made in the bar of +sand, they are for long periods filled with salt-water. + +LYM-FIORD. + +The Lym-Fiord in Jutland offers an excellent illustration of analogous changes; +for, in the course of the last thousand years, the western extremity of this +long frith, which is 120 miles in length, including its windings, has been four +times fresh and four times salt, a bar of sand between it and the ocean having +been often formed and removed. The last irruption of salt water happened in +1824, when the North Sea entered, killing all the fresh-water shells, fish, and +plants; and from that time to the present, the sea-weed Fucus vesiculosus, +together with oysters and other marine mollusca, have succeeded the Cyclas, +Lymnaea, Paludina, and Charae. (See Principles Index "Lym-Fiord.") + +But changes like these in the Lym-Fiord, and those before mentioned as occurring +at the mouths of great rivers, will only account for some cases of marine +deposits of partial extent resting on fresh-water strata. When we find, as in +the south-east of England (Chapter 18), a great series of fresh-water beds, 1000 +feet in thickness, resting upon marine formations and again covered by other +rocks, such as the Cretaceous, more than 1000 feet thick, and of deep-sea +origin, we shall find it necessary to seek for a different explanation of the +phenomena. + + +CHAPTER IV. + +CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS. + +Chemical and Mechanical Deposits. +Cementing together of Particles. +Hardening by Exposure to Air. +Concretionary Nodules. +Consolidating Effects of Pressure. +Mineralization of Organic Remains. +Impressions and Casts: how formed. +Fossil Wood. +Goppert's Experiments. +Precipitation of Stony Matter most rapid where Putrefaction is going on. +Sources of Lime and Silex in Solution. + +Having spoken in the preceding chapters of the characters of sedimentary +formations, both as dependent on the deposition of inorganic matter and the +distribution of fossils, I may next treat of the consolidation of stratified +rocks, and the petrifaction of imbedded organic remains. + +CHEMICAL AND MECHANICAL DEPOSITS. + +A distinction has been made by geologists between deposits of a mechanical, and +those of a chemical, origin. By the name mechanical are designated beds of mud, +sand, or pebbles produced by the action of running water, also accumulations of +stones and scoriae thrown out by a volcano, which have fallen into their present +place by the force of gravitation. But the matter which forms a chemical deposit +has not been mechanically suspended in water, but in a state of solution until +separated by chemical action. In this manner carbonate of lime is occasionally +precipitated upon the bottom of lakes in a solid form, as may be well seen in +many parts of Italy, where mineral springs abound, and where the calcareous +stone, called travertin, is deposited. In these springs the lime is usually held +in solution by an excess of carbonic acid, or by heat if it be a hot spring, +until the water, on issuing from the earth, cools or loses part of its acid. The +calcareous matter then falls down in a solid state, incrusting shells, fragments +of wood and leaves, and binding them together. + +That similar travertin is formed at some points in the bed of the sea where +calcareous springs issue can not be doubted, but as a general rule the quantity +of lime, according to Bischoff, spread through the waters of the ocean is very +small, the free carbonic acid gas in the same waters being five times as much as +is necessary to keep the lime in a fluid state. Carbonate of lime, therefore, +can rarely be precipitated at the bottom of the sea by chemical action alone, +but must be produced by vital agency as in the case of coral reefs. + +In such reefs, large masses of limestone are formed by the stony skeletons of +zoophytes; and these, together with shells, become cemented together by +carbonate of lime, part of which is probably furnished to the sea-water by the +decomposition of dead corals. Even shells, of which the animals are still living +on these reefs, are very commonly found to be incrusted over with a hard coating +of limestone. + +If sand and pebbles are carried by a river into the sea, and these are bound +together immediately by carbonate of lime, the deposit may be described as of a +mixed origin, partly chemical, and partly mechanical. + +Now, the remarks already made in Chapter 2 on the original horizontality of +strata are strictly applicable to mechanical deposits, and only partially to +those of a mixed nature. Such as are purely chemical may be formed on a very +steep slope, or may even incrust the vertical walls of a fissure, and be of +equal thickness throughout; but such deposits are of small extent, and for the +most part confined to vein-stones. + +CONSOLIDATION OF STRATA. + +It is chiefly in the case of calcareous rocks that solidification takes place at +the time of deposition. But there are many deposits in which a cementing process +comes into operation long afterwards. We may sometimes observe, where the water +of ferruginous or calcareous springs has flowed through a bed of sand or gravel, +that iron or carbonate of lime has been deposited in the interstices between the +grains or pebbles, so that in certain places the whole has been bound together +into a stone, the same set of strata remaining in other parts loose and +incoherent. + +Proofs of a similar cementing action are seen in a rock at Kelloway, in +Wiltshire. A peculiar band of sandy strata belonging to the group called Oolite +by geologists may be traced through several counties, the sand being for the +most part loose and unconsolidated, but becoming stony near Kelloway. In this +district there are numerous fossil shells which have decomposed, having for the +most part left only their casts. The calcareous matter hence derived has +evidently served, at some former period, as a cement to the siliceous grains of +sand, and thus a solid sandstone has been produced. If we take fragments of many +other argillaceous grits, retaining the casts of shells, and plunge them into +dilute muriatic or other acid, we see them immediately changed into common sand +and mud; the cement of lime, derived from the shells, having been dissolved by +the acid. + +Traces of impressions and casts are often extremely faint. In some loose sands +of recent date we meet with shells in so advanced a stage of decomposition as to +crumble into powder when touched. It is clear that water percolating such strata +may soon remove the calcareous matter of the shell; and unless circumstances +cause the carbonate of lime to be again deposited, the grains of sand will not +be cemented together; in which case no memorial of the fossil will remain. + +In what manner silex and carbonate of lime may become widely diffused in small +quantities through the waters which permeate the earth's crust will be spoken of +presently, when the petrifaction of fossil bodies is considered; but I may +remark here that such waters are always passing in the case of thermal springs +from hotter to colder parts of the interior of the earth; and, as often as the +temperature of the solvent is lowered, mineral matter has a tendency to separate +from it and solidify. Thus a stony cement is often supplied to sand, pebbles, or +any fragmentary mixture. In some conglomerates, like the pudding-stone of +Hertfordshire (a Lower Eocene deposit), pebbles of flint and grains of sand are +united by a siliceous cement so firmly, that if a block be fractured, the rent +passes as readily through the pebbles as through the cement. + +It is probable that many strata became solid at the time when they emerged from +the waters in which they were deposited, and when they first formed a part of +the dry land. A well-known fact seems to confirm this idea: by far the greater +number of the stones used for building and road-making are much softer when +first taken from the quarry than after they have been long exposed to the air; +and these, when once dried, may afterwards be immersed for any length of time in +water without becoming soft again. Hence it is found desirable to shape the +stones which are to be used in architecture while they are yet soft and wet, and +while they contain their "quarry-water," as it is called; also to break up stone +intended for roads when soft, and then leave it to dry in the air for months +that it may harden. Such induration may perhaps be accounted for by supposing +the water, which penetrates the minutest pores of rocks, to deposit, on +evaporation, carbonate of lime, iron, silex, and other minerals previously held +in solution, and thereby to fill up the pores partially. These particles, on +crystallising, would not only be themselves deprived of freedom of motion, but +would also bind together other portions of the rock which before were loosely +aggregated. On the same principle wet sand and mud become as hard as stone when +frozen; because one ingredient of the mass, namely, the water, has crystallised, +so as to hold firmly together all the separate particles of which the loose mud +and sand were composed. + +Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like dough +when first found; and some simple minerals, which are rigid and as hard as glass +in our cabinets, are often flexible and soft in their native beds: this is the +case with asbestos, sahlite, tremolite, and chalcedony, and it is reported also +to happen in the case of the beryl. (Dr. MacCulloch System of Geology volume 1 +page 123.) + +The marl recently deposited at the bottom of Lake Superior, in North America, is +soft, and often filled with fresh-water shells; but if a piece be taken up and +dried, it becomes so hard that it can only be broken by a smart blow of the +hammer. If the lake, therefore, was drained, such a deposit would be found to +consist of strata of marlstone, like that observed in many ancient European +formations, and, like them, containing fresh-water shells. + +CONCRETIONARY STRUCTURE. + +(FIGURE 48. Calcareous nodules in Lias.) + +It is probable that some of the heterogeneous materials which rivers transport +to the sea may at once set under water, like the artificial mixture called +pozzolana, which consists of fine volcanic sand charged with about twenty per +cent of oxide of iron, and the addition of a small quantity of lime. This +substance hardens, and becomes a solid stone in water, and was used by the +Romans in constructing the foundations of buildings in the sea. Consolidation in +such cases is brought about by the action of chemical affinity on finely +comminuted matter previously suspended in water. After deposition similar +particles seem often to exert a mutual attraction on each other, and congregate +together in particular spots, forming lumps, nodules, and concretions. Thus in +many argillaceous deposits there are calcareous balls, or spherical concretions, +ranged in layers parallel to the general stratification; an arrangement which +took place after the shale or marl had been thrown down in successive laminae; +for these laminae are often traceable through the concretions, remaining +parallel to those of the surrounding unconsolidated rock. (See Figure 48.) Such +nodules of limestone have often a shell or other foreign body in the centre. + +(FIGURE 49. Spheroidal concretions in magnesian limestone.) + +Among the most remarkable examples of concretionary structure are those +described by Professor Sedgwick as abounding in the magnesian limestone of the +north of England. The spherical balls are of various sizes, from that of a pea +to a diameter of several feet, and they have both a concentric and radiated +structure, while at the same time the laminae of original deposition pass +uninterruptedly through them. In some cliffs this limestone resembles a great +irregular pile of cannon-balls. Some of the globular masses have their centre in +one stratum, while a portion of their exterior passes through to the stratum +above or below. Thus the larger spheroid in the section (Figure 49) passes from +the stratum b upward into a. In this instance we must suppose the deposition of +a series of minor layers, first forming the stratum b, and afterwards the +incumbent stratum a; then a movement of the particles took place, and the +carbonates of lime and magnesia separated from the more impure and mixed matter +forming the still unconsolidated parts of the stratum. Crystallisation, +beginning at the centre, must have gone on forming concentric coats around the +original nucleus without interfering with the laminated structure of the rock. + +(FIGURE 50. Section through strata of grit.) + +When the particles of rocks have been thus rearranged by chemical forces, it is +sometimes difficult or impossible to ascertain whether certain lines of division +are due to original deposition or to the subsequent aggregation of several +particles. Thus suppose three strata of grit, A, B, C, are charged unequally +with calcareous matter, and that B is the most calcareous. If consolidation +takes place in B, the concretionary action may spread upward into a part of A, +where the carbonate of lime is more abundant than in the rest; so that a mass, d +e f, forming a portion of the superior stratum, becomes united with B into one +solid mass of stone. The original line of division, d e, being thus effaced, the +line d f would generally be considered as the surface of the bed B, though not +strictly a true plane of stratification. (Figure 50.) + +PRESSURE AND HEAT. + +When sand and mud sink to the bottom of a deep sea, the particles are not +pressed down by the enormous weight of the incumbent ocean; for the water, which +becomes mingled with the sand and mud, resists pressure with a force equal to +that of the column of fluid above. The same happens in regard to organic remains +which are filled with water under great pressure as they sink, otherwise they +would be immediately crushed to pieces and flattened. Nevertheless, if the +materials of a stratum remain in a yielding state, and do not set or solidify, +they will be gradually squeezed down by the weight of other materials +successively heaped upon them, just as soft clay or loose sand on which a house +is built may give way. By such downward pressure particles of clay, sand, and +marl may become packed into a smaller space, and be made to cohere together +permanently. + +Analogous effects of condensation may arise when the solid parts of the earth's +crust are forced in various directions by those mechanical movements hereafter +to be described, by which strata have been bent, broken, and raised above the +level of the sea. Rocks of more yielding materials must often have been forced +against others previously consolidated, and may thus by compression have +acquired a new structure. A recent discovery may help us to comprehend how fine +sediment derived from the detritus of rocks may be solidified by mere pressure. +The graphite or "black lead" of commerce having become very scarce, Mr. +Brockedon contrived a method by which the dust of the purer portions of the +mineral found in Borrowdale might be recomposed into a mass as dense and compact +as native graphite. The powder of graphite is first carefully prepared and freed +from air, and placed under a powerful press on a strong steel die, with air- +tight fittings. It is then struck several blows, each of a power of 1000 tons; +after which operation the powder is so perfectly solidified that it can be cut +for pencils, and exhibits when broken the same texture as native graphite. + +But the action of heat at various depths in the earth is probably the most +powerful of all causes in hardening sedimentary strata. To this subject I shall +refer again when treating of the metamorphic rocks, and of the slaty and jointed +structure. + +MINERALISATION OF ORGANIC REMAINS. + +(FIGURE 51. Phasianella Heddingtonensis, and cast of the same. Coral Rag.) + +(FIGURE 52. Pleurotomaria Anglica, and cast. Lias.) + +The changes which fossil organic bodies have undergone since they were first +imbedded in rocks, throw much light on the consolidation of strata. Fossil +shells in some modern deposits have been scarcely altered in the course of +centuries, having simply lost a part of their animal matter. But in other cases +the shell has disappeared, and left an impression only of its exterior, or, +secondly, a cast of its interior form, or, thirdly, a cast of the shell itself, +the original matter of which has been removed. These different forms of +fossilisation may easily be understood if we examine the mud recently thrown out +from a pond or canal in which there are shells. If the mud be argillaceous, it +acquires consistency on drying, and on breaking open a portion of it we find +that each shell has left impressions of its external form. If we then remove the +shell itself, we find within a solid nucleus of clay, having the form of the +interior of the shell. This form is often very different from that of the outer +shell. Thus a cast such as a, Figure 51, commonly called a fossil screw, would +never be suspected by an inexperienced conchologist to be the internal shape of +the fossil univalve, b, Figure 51. Nor should we have imagined at first sight +that the shell a and the cast b, Figure 52, belong to one and the same fossil. +The reader will observe, in the last-mentioned figure (b, Figure 52), that an +empty space shaded dark, which the SHELL ITSELF once occupied, now intervenes +between the enveloping stone and the cast of the smooth interior of the whorls. +In such cases the shell has been dissolved and the component particles removed +by water percolating the rock. If the nucleus were taken out, a hollow mould +would remain, on which the external form of the shell with its tubercles and +striae, as seen in a, Figure 52, would be seen embossed. Now if the space +alluded to between the nucleus and the impression, instead of being left empty, +has been filled up with calcareous spar, flint, pyrites, or other mineral, we +then obtain from the mould an exact cast both of the external and internal form +of the original shell. In this manner silicified casts of shells have been +formed; and if the mud or sand of the nucleus happen to be incoherent, or +soluble in acid, we can then procure in flint an empty shell, which in shape is +the exact counterpart of the original. This cast may be compared to a bronze +statue, representing merely the superficial form, and not the internal +organisation; but there is another description of petrifaction by no means +uncommon, and of a much more wonderful kind, which may be compared to certain +anatomical models in wax, where not only the outward forms and features, but the +nerves, blood-vessels, and other internal organs are also shown. Thus we find +corals, originally calcareous, in which not only the general shape, but also the +minute and complicated internal organisation is retained in flint. + +(FIGURE 53. Section of a tree from the coal-measures, magnified (Witham), +showing texture of wood.) + +Such a process of petrifaction is still more remarkably exhibited in fossil +wood, in which we often perceive not only the rings of annual growth, but all +the minute vessels and medullary rays. Many of the minute cells and fibres of +plants, and even those spiral vessels which in the living vegetable can only be +discovered by the microscope, are preserved. Among many instances, I may mention +a fossil tree, seventy-two feet in length, found at Gosforth, near Newcastle, in +sandstone strata associated with coal. By cutting a transverse slice so thin as +to transmit light, and magnifying it about fifty-five times, the texture, as +seen in Figure 53, is exhibited. A texture equally minute and complicated has +been observed in the wood of large trunks of fossil trees found in the +Craigleith quarry near Edinburgh, where the stone was not in the slightest +degree siliceous, but consisted chiefly of carbonate of lime, with oxide of +iron, alumina, and carbon. The parallel rows of vessels here seen are the rings +of annual growth, but in one part they are imperfectly preserved, the wood +having probably decayed before the mineralising matter had penetrated to that +portion of the tree. + +In attempting to explain the process of petrifaction in such cases, we may first +assume that strata are very generally permeated by water charged with minute +portions of calcareous, siliceous, and other earths in solution. In what manner +they become so impregnated will be afterwards considered. If an organic +substance is exposed in the open air to the action of the sun and rain, it will +in time putrefy, or be dissolved into its component elements, consisting usually +of oxygen, hydrogen, nitrogen, and carbon. These will readily be absorbed by the +atmosphere or be washed away by rain, so that all vestiges of the dead animal or +plant disappear. But if the same substances be submerged in water, they +decompose more gradually; and if buried in earth, still more slowly; as in the +familiar example of wooden piles or other buried timber. Now, if as fast as each +particle is set free by putrefaction in a fluid or gaseous state, a particle +equally minute of carbonate of lime, flint, or other mineral, is at hand ready +to be precipitated, we may imagine this inorganic matter to take the place just +before left unoccupied by the organic molecule. In this manner a cast of the +interior of certain vessels may first be taken, and afterwards the more solid +walls of the same may decay and suffer a like transmutation. Yet when the whole +is lapidified, it may not form one homogeneous mass of stone or metal. Some of +the original ligneous, osseous, or other organic elements may remain mingled in +certain parts, or the lapidifying substance itself may be differently coloured +at different times, or so crystallised as to reflect light differently, and thus +the texture of the original body may be faithfully exhibited. + +The student may perhaps ask whether, on chemical principles, we have any ground +to expect that mineral matter will be thrown down precisely in those spots where +organic decomposition is in progress? The following curious experiments may +serve to illustrate this point: Professor Goppert of Breslau, with a view of +imitating the natural process of petrifaction, steeped a variety of animal and +vegetable substances in waters, some holding siliceous, others calcareous, +others metallic matter in solution. He found that in the period of a few weeks, +or sometimes even days, the organic bodies thus immersed were mineralised to a +certain extent. Thus, for example, thin vertical slices of deal, taken from the +Scotch fir (Pinus sylvestris), were immersed in a moderately strong solution of +sulphate of iron. When they had been thoroughly soaked in the liquid for several +days they were dried and exposed to a red-heat until the vegetable matter was +burnt up and nothing remained but an oxide of iron, which was found to have +taken the form of the deal so exactly that casts even of the dotted vessels +peculiar to this family of plants were distinctly visible under the microscope. + +The late Dr. Turner observes, that when mineral matter is in a "nascent state," +that is to say, just liberated from a previous state of chemical combination, it +is most ready to unite with other matter, and form a new chemical compound. +Probably the particles or atoms just set free are of extreme minuteness, and +therefore move more freely, and are more ready to obey any impulse of chemical +affinity. Whatever be the cause, it clearly follows, as before stated, that +where organic matter newly imbedded in sediment is decomposing, there will +chemical changes take place most actively. + +An analysis was lately made of the water which was flowing off from the rich mud +deposited by the Hooghly River in the Delta of the Ganges after the annual +inundation. This water was found to be highly charged with carbonic acid holding +lime in solution. (Piddington Asiatic Researches volume 18 page 226.) Now if +newly-deposited mud is thus proved to be permeated by mineral matter in a state +of solution, it is not difficult to perceive that decomposing organic bodies, +naturally imbedded in sediment, may as readily become petrified as the +substances artificially immersed by Professor Goppert in various fluid mixtures. + +It is well known that the waters of all springs are more or less charged with +earthy, alkaline, or metallic ingredients derived from the rocks and mineral +veins through which they percolate. Silex is especially abundant in hot springs, +and carbonate of lime is almost always present in greater or less quantity. The +materials for the petrifaction of organic remains are, therefore, usually at +hand in a state of chemical solution wherever organic remains are imbedded in +new strata. + + +CHAPTER V. + +ELEVATION OF STRATA ABOVE THE SEA.-- HORIZONTAL AND INCLINED STRATIFICATION. + +Why the Position of Marine Strata, above the Level of the Sea, should be +referred to the rising up of the Land, not to the going down of the Sea. +Strata of Deep-sea and Shallow-water Origin alternate. +Also Marine and Fresh-water Beds and old Land Surfaces. +Vertical, inclined, and folded Strata. +Anticlinal and Synclinal Curves. +Theories to explain Lateral Movements. +Creeps in Coal-mines. +Dip and Strike. +Structure of the Jura. +Various Forms of Outcrop. +Synclinal Strata forming Ridges. +Connection of Fracture and Flexure of Rocks. +Inverted Strata. +Faults described. +Superficial Signs of the same obliterated by Denudation. +Great Faults the Result of repeated Movements. +Arrangement and Direction of parallel Folds of Strata. +Unconformability. +Overlapping Strata. + +LAND HAS BEEN RAISED, NOT THE SEA LOWERED. + +It has been already stated that the aqueous rocks containing marine fossils +extend over wide continental tracts, and are seen in mountain chains rising to +great heights above the level of the sea (Chapter 1). Hence it follows, that +what is now dry land was once under water. But if we admit this conclusion, we +must imagine, either that there has been a general lowering of the waters of the +ocean, or that the solid rocks, once covered by water, have been raised up +bodily out of the sea, and have thus become dry land. The earlier geologists, +finding themselves reduced to this alternative, embraced the former opinion, +assuming that the ocean was originally universal, and had gradually sunk down to +its actual level, so that the present islands and continents were left dry. It +seemed to them far easier to conceive that the water had gone down, than that +solid land had risen upward into its present position. It was, however, +impossible to invent any satisfactory hypothesis to explain the disappearance of +so enormous a body of water throughout the globe, it being necessary to infer +that the ocean had once stood at whatever height marine shells might be +detected. It moreover appeared clear, as the science of geology advanced, that +certain spaces on the globe had been alternately sea, then land, then estuary, +then sea again, and, lastly, once more habitable land, having remained in each +of these states for considerable periods. In order to account for such phenomena +without admitting any movement of the land itself, we are required to imagine +several retreats and returns of the ocean; and even then our theory applies +merely to cases where the marine strata composing the dry land are horizontal, +leaving unexplained those more common instances where strata are inclined, +curved, or placed on their edges, and evidently not in the position in which +they were first deposited. + +Geologists, therefore, were at last compelled to have recourse to the doctrine +that the solid land has been repeatedly moved upward or downward, so as +permanently to change its position relatively to the sea. There are several +distinct grounds for preferring this conclusion. First, it will account equally +for the position of those elevated masses of marine origin in which the +stratification remains horizontal, and for those in which the strata are +disturbed, broken, inclined, or vertical. Secondly, it is consistent with human +experience that land should rise gradually in some places and be depressed in +others. Such changes have actually occurred in our own days, and are now in +progress, having been accompanied in some cases by violent convulsions, while in +others they have proceeded so insensibly as to have been ascertainable only by +the most careful scientific observations, made at considerable intervals of +time. On the other hand, there is no evidence from human experience of a rising +or lowering of the sea's level in any region, and the ocean can not be raised or +depressed in one place without its level being changed all over the globe. + +These preliminary remarks will prepare the reader to understand the great +theoretical interest attached to all facts connected with the position of +strata, whether horizontal or inclined, curved or vertical. + +Now the first and most simple appearance is where strata of marine origin occur +above the level of the sea in horizontal position. Such are the strata which we +meet with in the south of Sicily, filled with shells for the most part of the +same species as those now living in the Mediterranean. Some of these rocks rise +to the height of more than 2000 feet above the sea. Other mountain masses might +be mentioned, composed of horizontal strata of high antiquity, which contain +fossil remains of animals wholly dissimilar from any now known to exist. In the +south of Sweden, for example, near Lake Wener, the beds of some of the oldest +fossiliferous deposits, called Silurian and Cambrian by geologists, occur in as +level a position as if they had recently formed part of the delta of a great +river, and been left dry on the retiring of the annual floods. Aqueous rocks of +equal antiquity extend for hundreds of miles over the lake-district of North +America, and exhibit in like manner a stratification nearly undisturbed. The +Table Mountain at the Cape of Good Hope is another example of highly elevated +yet perfectly horizontal strata, no less than 3500 feet in thickness, and +consisting of sandstone of very ancient date. + +Instead of imagining that such fossiliferous rocks were always at their present +level, and that the sea was once high enough to cover them, we suppose them to +have constituted the ancient bed of the ocean, and to have been afterwards +uplifted to their present height. This idea, however startling it may at first +appear, is quite in accordance, as before stated, with the analogy of changes +now going on in certain regions of the globe. Thus, in parts of Sweden, and the +shores and islands of the Gulf of Bothnia, proofs have been obtained that the +land is experiencing, and has experienced for centuries, a slow upheaving +movement. (See "Principles of Geology" 1867 page 314.) + +It appears from the observations of Mr. Darwin and others, that very extensive +regions of the continent of South America have been undergoing slow and gradual +upheaval, by which the level plains of Patagonia, covered with recent marine +shells, and the Pampas of Buenos Ayres, have been raised above the level of the +sea. On the other hand, the gradual sinking of the west coast of Greenland, for +the space of more than 600 miles from north to south, during the last four +centuries, has been established by the observations of a Danish naturalist, Dr. +Pingel. And while these proofs of continental elevation and subsidence, by slow +and insensible movements, have been recently brought to light, the evidence has +been daily strengthened of continued changes of level effected by violent +convulsions in countries where earthquakes are frequent. There the rocks are +rent from time to time, and heaved up or thrown down several feet at once, and +disturbed in such a manner as to show how entirely the original position of +strata may be modified in the course of centuries. + +Mr. Darwin has also inferred that, in those seas where circular coral islands +and barrier reefs abound, there is a slow and continued sinking of the submarine +mountains on which the masses of coral are based; while there are other areas of +the South Sea where the land is on the rise, and where coral has been upheaved +far above the sea-level. + +ALTERNATIONS OF MARINE AND FRESH-WATER STRATA. + +It has been shown in the third chapter that there is such a difference between +land, fresh-water, and marine fossils as to enable the geologist to determine +whether particular groups of strata were formed at the bottom of the ocean or in +estuaries, rivers, or lakes. If surprise was at first created by the discovery +of marine corals and shells at the height of several miles above the sea-level, +the imagination was afterwards not less startled by observing that in the +successive strata composing the earth's crust, especially if their total +thickness amounted to thousands of feet, they comprised in some parts formations +of shallow-sea as well as of deep-sea origin; also beds of brackish or even of +purely fresh-water formation, as well as vegetable matter or coal accumulated on +ancient land. In these cases we as frequently find fresh-water beds below a +marine set or shallow-water under those of deep-sea origin as the reverse. Thus, +if we bore an artesian well below London, we pass through a marine clay, and +there reach, at the depth of several hundred feet, a shallow-water and +fluviatile sand, beneath which comes the white chalk originally formed in a deep +sea. Or if we bore vertically through the chalk of the North Downs, we come, +after traversing marine chalky strata, upon a fresh-water formation many +hundreds of feet thick, called the Wealden, such as is seen in Kent and Surrey, +which is known in its turn to rest on purely marine beds. In like manner, in +various parts of Great Britain we sink vertical shafts through marine deposits +of great thickness, and come upon coal which was formed by the growth of plants +on an ancient land-surface sometimes hundreds of square miles in extent. + +VERTICAL, INCLINED, AND CURVED STRATA. + +(FIGURE 54. Vertical conglomerate and sandstone.) + +It has been stated that marine strata of different ages are sometimes found at a +considerable height above the sea, yet retaining their original horizontality; +but this state of things is quite exceptional. As a general rule, strata are +inclined or bent in such a manner as to imply that their original position has +been altered. + +(FIGURE 55. Section of Forfarshire, from N.W. to S.E., from the foot of the +Grampians to the sea at Arbroath (volcanic or trap rocks omitted). Length of +section twenty miles. +From S.E. (left) Sea: Whiteness, Arbroath: Strata a, 2, 3: Leys Mill: Strata 4: +Sidlaw Hills. Viney R.: Strata B: Pitmuies: Strata 4: Position and nature of the +rocks below No. 4 unknown: Turin: Findhaven: Strata 3, 2, A: Valley of +Strathmore: Strata 1, 2, 3: W. Ogle: Strata 4 and Clay-Slate: to N.W. (right).) + +The most unequivocal evidence of such a change is afforded by their standing up +vertically, showing their edges, which is by no means a rare phenomenon, +especially in mountainous countries. Thus we find in Scotland, on the southern +skirts of the Grampians, beds of pudding-stone alternating with thin layers of +fine sand, all placed vertically to the horizon. When Saussure first observed +certain conglomerates in a similar position in the Swiss Alps, he remarked that +the pebbles, being for the most part of an oval shape, had their longer axes +parallel to the planes of stratification (see Figure 54). From this he inferred +that such strata must, at first, have been horizontal, each oval pebble having +settled at the bottom of the water, with its flatter side parallel to the +horizon, for the same reason that an egg will not stand on either end if +unsupported. Some few, indeed, of the rounded stones in a conglomerate +occasionally afford an exception to the above rule, for the same reason that in +a river's bed, or on a shingle beach, some pebbles rest on their ends or edges; +these having been shoved against or between other stones by a wave or current, +so as to assume this position. + +ANTICLINAL AND SYNCLINAL CURVES. + +Vertical strata, when they can be traced continuously upward or downward for +some depth, are almost invariably seen to be parts of great curves, which may +have a diameter of a few yards, or of several miles. I shall first describe two +curves of considerable regularity, which occur in Forfarshire, extending over a +country twenty miles in breadth, from the foot of the Grampians to the sea near +Arbroath. + +The mass of strata here shown may be 2000 feet in thickness, consisting of red +and white sandstone, and various coloured shales, the beds being distinguishable +into four principal groups, namely, No. 1, red marl or shale; No. 2, red +sandstone, used for building; No. 3, conglomerate; and No. 4, grey paving-stone, +and tile-stone, with green and reddish shale, containing peculiar organic +remains. A glance at the section (Figure 55.) will show that each of the +formations 2, 3, 4 are repeated thrice at the surface, twice with a southerly, +and once with a northerly inclination or DIP, and the beds in No. 1, which are +nearly horizontal, are still brought up twice by a slight curvature to the +surface, once on each side of A. Beginning at the north-west extremity, the +tile-stones and conglomerates, No. 4 and No. 3, are vertical, and they generally +form a ridge parallel to the southern skirts of the Grampians. The superior +strata, Nos. 2 and 1, become less and less inclined on descending to the valley +of Strathmore, where the strata, having a concave bend, are said by geologists +to lie in a "trough" or "basin." Through the centre of this valley runs an +imaginary line A, called technically a "synclinal line," where the beds, which +are tilted in opposite directions, may be supposed to meet. It is most important +for the observer to mark such lines, for he will perceive by the diagram that, +in travelling from the north to the centre of the basin, he is always passing +from older to newer beds; whereas, after crossing the line A, and pursuing his +course in the same southerly direction, he is continually leaving the newer, and +advancing upon older strata. All the deposits which he had before examined begin +then to recur in reversed order, until he arrives at the central axis of the +Sidlaw hills, where the strata are seen to form an arch, or SADDLE, having an +ANTICLINAL line, B, in the centre. On passing this line, and continuing towards +the S.E., the formations 4, 3, and 2, are again repeated, in the same relative +order of superposition, but with a southerly dip. At Whiteness (see Figure 55) +it will be seen that the inclined strata are covered by a newer deposit, a, in +horizontal beds. These are composed of red conglomerate and sand, and are newer +than any of the groups, 1, 2, 3, 4, before described, and rest UNCONFORMABLY +upon strata of the sandstone group, No. 2. + +An example of curved strata, in which the bends or convolutions of the rock are +sharper and far more numerous within an equal space, has been well described by +Sir James Hall. (Edinburgh Transactions volume 7 plate 3.) It occurs near St. +Abb's Head, on the east coast of Scotland, where the rocks consist principally +of a bluish slate, having frequently a ripple-marked surface. The undulations of +the beds reach from the top to the bottom of cliffs from 200 to 300 feet in +height, and there are sixteen distinct bendings in the course of about six +miles, the curvatures being alternately concave and convex upward. + +FOLDING BY LATERAL MOVEMENT. + +(FIGURE 56. Curved strata of slate near St. Abb's Head, Berwickshire. (Sir J. +Hall.) + +(FIGURE 57. Curved strata in line of cliff.) + +(FIGURE 58. Folded cloths imitating bent strata.) + +An experiment was made by Sir James Hall, with a view of illustrating the manner +in which such strata, assuming them to have been originally horizontal, may have +been forced into their present position. A set of layers of clay were placed +under a weight, and their opposite ends pressed towards each other with such +force as to cause them to approach more nearly together. On the removal of the +weight, the layers of clay were found to be curved and folded, so as to bear a +miniature resemblance to the strata in the cliffs. We must, however, bear in +mind that in the natural section or sea-cliff we only see the foldings +imperfectly, one part being invisible beneath the sea, and the other, or upper +portion, being supposed to have been carried away by DENUDATION, or that action +of water which will be explained in the next chapter. The dark lines in the plan +(Figure 57) represent what is actually seen of the strata in the line of cliff +alluded to; the fainter lines, that portion which is concealed beneath the sea- +level, as also that which is supposed to have once existed above the present +surface. + +We may still more easily illustrate the effects which a lateral thrust might +produce on flexible strata, by placing several pieces of differently coloured +cloths upon a table, and when they are spread out horizontally, cover them with +a book. Then apply other books to each end, and force them towards each other. +The folding of the cloths (see Figure 58) will imitate those of the bent strata; +the incumbent book being slightly lifted up, and no longer touching the two +volumes on which it rested before, because it is supported by the tops of the +anticlinal ridges formed by the curved cloths. In like manner there can be no +doubt that the squeezed strata, although laterally condensed and more closely +packed, are yet elongated and made to rise upward, in a direction perpendicular +to the pressure. + +Whether the analogous flexures in stratified rocks have really been due to +similar sideway movements is a question which we can not decide by reference to +our own observation. Our inability to explain the nature of the process is, +perhaps, not simply owing to the inaccessibility of the subterranean regions +where the mechanical force is exerted, but to the extreme slowness of the +movement. The changes may sometimes be due to variation in the temperature of +mountain masses of rock causing them, while still solid, to expand or contract; +or melting them, and then again cooling them and allowing them to crystallise. +If such be the case, we have scarcely more reason to expect to witness the +operation of the process within the limited periods of our scientific +observation than to see the swelling of the roots of a tree, by which, in the +course of years, a wall of solid masonry may be lifted up, rent or thrown down. +In both instances the force may be irresistible, but though adequate, it need +not be visible by us, provided the time required for its development be very +great. The lateral pressure arising from the unequal expansion of rocks by heat +may cause one mass lying in the same horizontal plane gradually to occupy a +larger space, so as to press upon another rock, which, if flexible, may be +squeezed into a bent and folded form. It will also appear, when the volcanic and +granitic rocks are described, that some of them have, when melted in the +interior of the earth's crust, been injected forcibly into fissures, and after +the solidification of such intruded matter, other sets of rents, crossing the +first, have been formed and in their turn filled by melted rock. Such repeated +injections imply a stretching, and often upheaval, of the whole mass. + +We also know, especially by the study of regions liable to earthquakes, that +there are causes at work in the interior of the earth capable of producing a +sinking in of the ground, sometimes very local, but often extending over a wide +area. The continuance of such a downward movement, especially if partial and +confined to linear areas, may produce regular folds in the strata. + +CREEPS IN COAL-MINES. + +The "creeps," as they are called in coal-mines, afford an excellent illustration +of this fact.-- First, it may be stated generally, that the excavation of coal +at a considerable depth causes the mass of overlying strata to sink down bodily, +even when props are left to support the roof of the mine. "In Yorkshire," says +Mr. Buddle, "three distinct subsidences were perceptible at the surface, after +the clearing out of three seams of coal below, and innumerable vertical cracks +were caused in the incumbent mass of sandstone and shale which thus settled +down." (Proceedings of Geological Society volume 3 page 148.) The exact amount +of depression in these cases can only be accurately measured where water +accumulates on the surface, or a railway traverses a coal-field. + +(FIGURE 59. Section of carboniferous strata at Wallsend, Newcastle, showing +"creeps." (J. Buddle, Esq.) +Horizontal length of section 174 feet. The upper seam, or main coal, here worked +out, was 630 feet below the surface. +Section through, from top to bottom: +Siliceous sandstone. +Shale. +1. Main coal, 6 feet 6 inches, with creeps a, b, c, d. +Shale eighteen yards thick. +2. Metal coal, 3 feet, with fractures e, f, g, h.) + +When a bed of coal is worked out, pillars or rectangular masses of coal are left +at intervals as props to support the roof, and protect the colliers. Thus in +Figure 59, representing a section at Wallsend, Newcastle, the galleries which +have been excavated are represented by the white spaces a, b, while the +adjoining dark portions are parts of the original coal seam left as props, beds +of sandy clay or shale constituting the floor of the mine. When the props have +been reduced in size, they are pressed down by the weight of overlying rocks (no +less than 630 feet thick) upon the shale below, which is thereby squeezed and +forced up into the open spaces. + +Now it might have been expected that, instead of the floor rising up, the +ceiling would sink down, and this effect, called a "thrust," does, in fact, take +place where the pavement is more solid than the roof. But it usually happens, in +coalmines, that the roof is composed of hard shale, or occasionally of +sandstone, more unyielding than the foundation, which often consists of clay. +Even where the argillaceous substrata are hard at first, they soon become +softened and reduced to a plastic state when exposed to the contact of air and +water in the floor of a mine. + +The first symptom of a "creep," says Mr. Buddle, is a slight curvature at the +bottom of each gallery, as at a, Figure 59: then the pavement, continuing to +rise, begins to open with a longitudinal crack, as at b; then the points of the +fractured ridge reach the roof, as at c; and, lastly, the upraised beds close up +the whole gallery, and the broken portions of the ridge are reunited and +flattened at the top, exhibiting the flexure seen at d. Meanwhile the coal in +the props has become crushed and cracked by pressure. It is also found that +below the creeps a, b, c, d, an inferior stratum, called the "metal coal," which +is 3 feet thick, has been fractured at the points e, f, g, h, and has risen, so +as to prove that the upward movement, caused by the working out of the "main +coal," has been propagated through a thickness of 54 feet of argillaceous beds, +which intervene between the two coal-seams. This same displacement has also been +traced downward more than 150 feet below the metal coal, but it grows +continually less and less until it becomes imperceptible. + +No part of the process above described is more deserving of our notice than the +slowness with which the change in the arrangement of the beds is brought about. +Days, months, or even years, will sometimes elapse between the first bending of +the pavement and the time of its reaching the roof. Where the movement has been +most rapid, the curvature of the beds is most regular, and the reunion of the +fractured ends most complete; whereas the signs of displacement or violence are +greatest in those creeps which have required months or years for their entire +accomplishment. Hence we may conclude that similar changes may have been wrought +on a larger scale in the earth's crust by partial and gradual subsidences, +especially where the ground has been undermined throughout long periods of time; +and we must be on our guard against inferring sudden violence, simply because +the distortion of the beds is excessive. + +Engineers are familiar with the fact that when they raise the level of a railway +by heaping stone or gravel on a foundation of marsh, quicksand, or other +yielding formation, the new mound often sinks for a time as fast as they attempt +to elevate it; when they have persevered so as to overcome this difficulty, they +frequently find that some of the adjoining flexible ground has risen up in one +or more parallel arches or folds, showing that the vertical pressure of the +sinking materials has given rise to a lateral folding movement. + +In like manner, in the interior of the earth, the solid parts of the earth's +crust may sometimes, as before mentioned, be made to expand by heat, or may be +pressed by the force of steam against flexible strata loaded with a great weight +of incumbent rocks. In this case the yielding mass, squeezed, but unable to +overcome the resistance which it meets with in a vertical direction, may be +gradually relieved by lateral folding. + +DIP AND STRIKE. + +(FIGURE 60. Series of inclined strata dipping to the north at an angle of 45 +degrees.) + +In describing the manner in which strata depart from their original +horizontality, some technical terms, such as "dip" and "strike," "anticlinal" +and "synclinal" line or axis, are used by geologists. I shall now proceed to +explain some of these to the student. If a stratum or bed of rock, instead of +being quite level, be inclined to one side, it is said to DIP; the point of the +compass to which it is inclined is called the POINT OF DIP, and the degree of +deviation from a level or horizontal line is called THE AMOUNT OF DIP, or THE +ANGLE OF DIP. Thus, in the diagram (Figure 60), a series of strata are inclined, +and they dip to the north at an angle of forty-five degrees. The STRIKE, or LINE +OF BEARING, is the prolongation or extension of the strata in a direction AT +RIGHT ANGLES to the dip; and hence it is sometimes called the DIRECTION of the +strata. Thus, in the above instance of strata dipping to the north, their strike +must necessarily be east and west. We have borrowed the word from the German +geologists, streichen signifying to extend, to have a certain direction. Dip and +strike may be aptly illustrated by a row of houses running east and west, the +long ridge of the roof representing the strike of the stratum of slates, which +dip on one side to the north, and on the other to the south. + +A stratum which is horizontal, or quite level in all directions, has neither dip +nor strike. + +It is always important for the geologist, who is endeavouring to comprehend the +structure of a country, to learn how the beds dip in every part of the district; +but it requires some practice to avoid being occasionally deceived, both as to +the point of dip and the amount of it. + +(FIGURE 61. Apparent horizontality of inclined strata.) + +If the upper surface of a hard stony stratum be uncovered, whether artificially +in a quarry, or by waves at the foot of a cliff, it is easy to determine towards +what point of the compass the slope is steepest, or in what direction water +would flow if poured upon it. This is the true dip. But the edges of highly +inclined strata may give rise to perfectly horizontal lines in the face of a +vertical cliff, if the observer see the strata in the line of the strike, the +dip being inward from the face of the cliff. If, however, we come to a break in +the cliff, which exhibits a section exactly at right angles to the line of the +strike, we are then able to ascertain the true dip. In the drawing (Figure 61), +we may suppose a headland, one side of which faces to the north, where the beds +would appear perfectly horizontal to a person in the boat; while in the other +side facing the west, the true dip would be seen by the person on shore to be at +an angle of 40 degrees. If, therefore, our observations are confined to a +vertical precipice facing in one direction, we must endeavour to find a ledge or +portion of the plane of one of the beds projecting beyond the others, in order +to ascertain the true dip. + +(FIGURE 62. Two hands used to determine the inclination of strata.) + +If not provided with a clinometer, a most useful instrument, when it is of +consequence to determine with precision the inclination of the strata, the +observer may measure the angle within a few degrees by standing exactly opposite +to a cliff where the true dip is exhibited, holding the hands immediately before +the eyes, and placing the fingers of one in a perpendicular, and of the other in +a horizontal position, as in Figure 62. It is thus easy to discover whether the +lines of the inclined beds bisect the angle of 90 degrees, formed by the meeting +of the hands, so as to give an angle of 45 degrees, or whether it would divide +the space into two equal or unequal portions. You have only to change hands to +get the line of dip on the upper side of the horizontal hand. + +(FIGURE 63. Section illustrating the structure of the Swiss Jura.) + +It has been already seen, in describing the curved strata on the east coast of +Scotland, in Forfarshire and Berwickshire, that a series of concave and convex +bendings are occasionally repeated several times. These usually form part of a +series of parallel waves of strata, which are prolonged in the same direction, +throughout a considerable extent of country. Thus, for example, in the Swiss +Jura, that lofty chain of mountains has been proved to consist of many parallel +ridges, with intervening longitudinal valleys, as in Figure 63, the ridges being +formed by curved fossiliferous strata, of which the nature and dip are +occasionally displayed in deep transverse gorges, called "cluses," caused by +fractures at right angles to the direction of the chain. (Thurmann "Essai sur +les Soulevemens Jurassiques de Porrentruy" Paris 1832.) Now let us suppose these +ridges and parallel valleys to run north and south, we should then say that the +STRIKE of the beds is north and south, and the DIP east and west. Lines drawn +along the summits of the ridges, A, B, would be anticlinal lines, and one +following the bottom of the adjoining valleys a synclinal line. + +OUTCROP OF STRATA. + +(FIGURE 64. Ground-plan of the denuded ridge C, Figure 63.) + +(FIGURE 65. Transverse section of the denuded ridge C, Figure 63..) + +It will be observed that some of these ridges, A, B, are unbroken on the summit, +whereas one of them, C, has been fractured along the line of strike, and a +portion of it carried away by denudation, so that the ridges of the beds in the +formations a, b, c come out to the day, or, as the miners say, CROP OUT, on the +sides of a valley. The ground-plan of such a denuded ridge as C, as given in a +geological map, may be expressed by the diagram, Figure 64, and the cross- +section of the same by Figure 65. The line D E, Figure 64, is the anticlinal +line, on each side of which the dip is in opposite directions, as expressed by +the arrows. The emergence of strata at the surface is called by miners their +OUTCROP, or BASSET. + +If, instead of being folded into parallel ridges, the beds form a boss or dome- +shaped protuberance, and if we suppose the summit of the dome carried off, the +ground-plan would exhibit the edges of the strata forming a succession of +circles, or ellipses, round a common centre. These circles are the lines of +strike, and the dip being always at right angles is inclined in the course of +the circuit to every point of the compass, constituting what is termed a qua- +quaversal dip-- that is, turning every way. + +There are endless variations in the figures described by the basset-edges of the +strata, according to the different inclination of the beds, and the mode in +which they happen to have been denuded. One of the simplest rules, with which +every geologist should be acquainted, relates to the V-like form of the beds as +they crop out in an ordinary valley. First, if the strata be horizontal, the V- +like form will be also on a level, and the newest strata will appear at the +greatest heights. + +(FIGURE 66. Slope of valley 40 degrees, dip of strata 20 degrees.) + +Secondly, if the beds be inclined and intersected by a valley sloping in the +same direction, and the dip of the beds be less steep than the slope of the +valley, then the V's, as they are often termed by miners, will point upward (see +Figure 66), those formed by the newer beds appearing in a superior position, and +extending highest up the valley, as A is seen above B. + +(FIGURE 67. Slope of valley 20 degrees, dip of strata 50 degrees.) + +Thirdly, if the dip of the beds be steeper than the slope of the valley, then +the V's will point downward (see Figure 67), and those formed of the older beds +will now appear uppermost, as B appears above A. + +(FIGURE 68. Slope of valley 20 degrees, dip of strata 20 degrees, in opposite +directions.) + +Fourthly, in every case where the strata dip in a contrary direction to the +slope of the valley, whatever be the angle of inclination, the newer beds will +appear the highest, as in the first and second cases. This is shown by the +drawing (Figure 68), which exhibits strata rising at an angle of 20 degrees, and +crossed by a valley, which declines in an opposite direction at 20 degrees. + +These rules may often be of great practical utility; for the different degrees +of dip occurring in the two cases represented in Figures 66 and 67 may +occasionally be encountered in following the same line of flexure at points a +few miles distant from each other. A miner unacquainted with the rule, who had +first explored the valley Figure 66, may have sunk a vertical shaft below the +coal-seam A, until he reached the inferior bed, B. He might then pass to the +valley, Figure 67, and discovering there also the outcrop of two coal-seams, +might begin his workings in the uppermost in the expectation of coming down to +the other bed A, which would be observed cropping out lower down the valley. But +a glance at the section will demonstrate the futility of such hopes. (I am +indebted to the kindness of T. Sopwith, Esq., for three models which I have +copied in the above diagrams; but the beginner may find it by no means easy to +understand such copies, although, if he were to examine and handle the +originals, turning them about in different ways, he would at once comprehend +their meaning, as well as the import of others far more complicated, which the +same engineer has constructed to illustrate FAULTS.) + +SYNCLINAL STRATA FORMING RIDGES. + +(FIGURE 69. Section of carboniferous rocks of Lancashire. (E. Hull. (Edward +Hull, Quarterly Geological Journal volume 24 page 324. 1868.)) +a. Synclinal. Grits and shales. +c. Anticlinal. Mountain limestone. +b. Synclinal. Grits and shales.) + +Although in many cases an anticlinal axis forms a ridge, and a synclinal axis a +valley, as in A B, Figure 63, yet this can by no means be laid down as a general +rule, as the beds very often slope inward from either side of a mountain, as at +a, b, Figure 69, while in the intervening valley, c, they slope upward, forming +an arch. + +It would be natural to expect the fracture of solid rocks to take place chiefly +where the bending of the strata has been sharpest, and such rending may produce +ravines giving access to running water and exposing the surface to atmospheric +waste. The entire absence, however, of such cracks at points where the strain +must have been greatest, as at a, Figure 63, is often very remarkable, and not +always easy of explanation. We must imagine that many strata of limestone, +chert, and other rocks which are now brittle, were pliant when bent into their +present position. They may have owed their flexibility in part to the fluid +matter which they contained in their minute pores, as before described, and in +part to the permeation of sea-water while they were yet submerged. + +(FIGURE 70. Strata of chert, grit, and marl, near St. Jean de Luz.) + +At the western extremity of the Pyrenees, great curvatures of the strata are +seen in the sea-cliffs, where the rocks consist of marl, grit, and chert. At +certain points, as at a, Figure 70, some of the bendings of the flinty chert are +so sharp that specimens might be broken off well fitted to serve as ridge-tiles +on the roof of a house. Although this chert could not have been brittle as now, +when first folded into this shape, it presents, nevertheless, here and there, at +the points of greatest flexure, small cracks, which show that it was solid, and +not wholly incapable of breaking at the period of its displacement. The numerous +rents alluded to are not empty, but filled with chalcedony and quartz. + +(FIGURE 71. Bent and undulating gypseous marl. +g. Gypsum. m. Marl.) + +Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous +marls occur, with here and there thin beds of solid gypsum interstratified. +Sometimes these solid layers have been broken into detached fragments, still +preserving their sharp edges (g, g, Figure 71), while the continuity of the more +pliable and ductile marls, m, m, has not been interrupted. + +(FIGURE 72. Folded strata.) + +(FIGURE 73. Folded strata.) + +We have already explained, Figure 69, that stratified rocks have usually their +strata bent into parallel folds forming anticlinal and synclinal axes, a group +of several of these folds having often been subjected to a common movement, and +having acquired a uniform strike or direction. In some disturbed regions these +folds have been doubled back upon themselves in such a manner that it is often +difficult for an experienced geologist to determine correctly the relative age +of the beds by superposition. Thus, if we meet with the strata seen in the +section, Figure 72, we should naturally suppose that there were twelve distinct +beds, or sets of beds, No. 1 being the newest, and No. 12 the oldest of the +series. But this section may perhaps exhibit merely six beds, which have been +folded in the manner seen in Figure 73, so that each of them is twice repeated, +the position of one half being reversed, and part of No. 1, originally the +uppermost, having now become the lowest of the series. + +These phenomena are observable on a magnificent scale in certain regions in +Switzerland, in precipices often more than 2000 feet in perpendicular height, +and there are flexures not inferior in dimensions in the Pyrenees. The upper +part of the curves seen in this diagram, Figure 73, and expressed in fainter +lines, has been removed by what is called denudation, to be afterwards +explained. + +FRACTURES OF THE STRATA AND FAULTS. + +Numerous rents may often be seen in rocks which appear to have been simply +broken, the fractured parts still remaining in contact; but we often find a +fissure, several inches or yards wide, intervening between the disunited +portions. These fissures are usually filled with fine earth and sand, or with +angular fragments of stone, evidently derived from the fracture of the +contiguous rocks. + +The face of each wall of the fissure is often beautifully polished, as if +glazed, striated, or scored with parallel furrows and ridges, such as would be +produced by the continued rubbing together of surfaces of unequal hardness. +These polished surfaces are called by miners "slickensides." It is supposed that +the lines of the striae indicate the direction in which the rocks were moved. +During one of the minor earthquakes in Chili, in 1840, the brick walls of a +building were rent vertically in several places, and made to vibrate for several +minutes during each shock, after which they remained uninjured, and without any +opening, although the line of each crack was still visible. When all movement +had ceased, there were seen on the floor of the house, at the bottom of each +rent, small heaps of fine brick-dust, evidently produced by trituration. + +(FIGURE 74. Faults. A B perpendicular, C D oblique to the horizon.) + +(FIGURE 75. E F, fault or fissure filled with rubbish, on each side of which the +shifted strata are not parallel.) + +It is not uncommon to find the mass of rock on one side of a fissure thrown up +above or down below the mass with which it was once in contact on the other +side. "This mode of displacement is called a fault, shift, slip, or throw." "The +miner," says Playfair, describing a fault, "is often perplexed, in his +subterranean journey, by a derangement in the strata, which changes at once all +those lines and bearings which had hitherto directed his course. When his mine +reaches a certain plane, which is sometimes perpendicular, as in A B, Figure 74, +sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock +broken asunder, those on the one side of the plane having changed their place, +by sliding in a particular direction along the face of the others. In this +motion they have sometimes preserved their parallelism, as in Figure 74, so that +the strata on each side of faults A B, C D, continue parallel to one another; in +other cases, the strata on each side are inclined, as in a, b, c, d (Figure 75), +though their identity is still to be recognised by their possessing the same +thickness and the same internal characters." (Playfair, Illustration of Hutt. +Theory paragraph 42.) + +In Coalbrook Dale, says Mr. Prestwich (Geological Transactions second series +volume 5 page 452.), deposits of sandstone, shale, and coal, several thousand +feet thick, and occupying an area of many miles, have been shivered into +fragments, and the broken remnants have been placed in very discordant +positions, often at levels differing several hundred feet from each other. The +sides of the faults, when perpendicular, are commonly several yards apart, and +are sometimes as much as 50 yards asunder, the interval being filled with broken +debris of the strata. In following the course of the same fault it is sometimes +found to produce in different places very unequal changes of level, the amount +of shift being in one place 300, and in another 700 feet, which arises from the +union of two or more faults. In other words, the disjointed strata have in +certain districts been subjected to renewed movements, which they have not +suffered elsewhere. + +We may occasionally see exact counterparts of these slips, on a small scale, in +pits of loose sand and gravel, many of which have doubtless been caused by the +drying and shrinking of argillaceous and other beds, slight subsidences having +taken place from failure of support. Sometimes, however, even these small slips +may have been produced during earthquakes; for land has been moved, and its +level, relatively to the sea, considerably altered, within the period when much +of the alluvial sand and gravel now covering the surface of continents was +deposited. + +I have already stated that a geologist must be on his guard, in a region of +disturbed strata, against inferring repeated alternations of rocks, when, in +fact, the same strata, once continuous, have been bent round so as to recur in +the same section, and with the same dip. A similar mistake has often been +occasioned by a series of faults. + +(FIGURE 76. Apparent alternations of strata caused by vertical faults.) + +If, for example, the dark line A H (Figure 76) represent the surface of a +country on which the strata a, b, c frequently crop out, an observer who is +proceeding from H to A might at first imagine that at every step he was +approaching new strata, whereas the repetition of the same beds has been caused +by vertical faults, or downthrows. Thus, suppose the original mass, A, B, C, D, +to have been a set of uniformly inclined strata, and that the different masses +under E F, F G, and G D sank down successively, so as to leave vacant the spaces +marked in the diagram by dotted lines, and to occupy those marked by the +continuous lines, then let denudation take place along the line A H, so that the +protruding masses indicated by the fainter lines are swept away-- a miner, who +has not discovered the faults, finding the mass a, which we will suppose to be a +bed of coal four times repeated, might hope to find four beds, workable to an +indefinite depth, but first, on arriving at the fault G, he is stopped suddenly +in his workings, for he comes partly upon the shale b, and partly on the +sandstone c; the same result awaits him at the fault F, and on reaching E he is +again stopped by a wall composed of the rock d. + +The very different levels at which the separated parts of the same strata are +found on the different sides of the fissure, in some faults, is truly +astonishing. One of the most celebrated in England is that called the "ninety- +fathom dike," in the coal-field of Newcastle. This name has been given to it, +because the same beds are ninety fathoms (540 feet) lower on the northern than +they are on the southern side. The fissure has been filled by a body of sand, +which is now in the state of sandstone, and is called the dike, which is +sometimes very narrow, but in other places more than twenty yards wide. +(Conybeare and Phillips Outlines, etc. page 376.) The walls of the fissure are +scored by grooves, such as would have been produced if the broken ends of the +rock had been rubbed along the plane of the fault. (Phillips Geology Lardner's +Cyclop. page 41.) In the Tynedale and Craven faults, in the north of England, +the vertical displacement is still greater, and the fracture has extended in a +horizontal direction for a distance of thirty miles or more. + +GREAT FAULTS THE RESULT OF REPEATED MOVEMENTS. + +It must not, however, be supposed that faults generally consist of single linear +rents; there are usually a number of faults springing off from the main one, and +sometimes a long strip of country seems broken up into fragments by sets of +parallel and connecting transverse faults. Oftentimes a great line of fault has +been repeated, or the movements have been continued through successive periods, +so that, newer deposits having covered the old line of displacement, the strata +both newer and older have given way along the old line of fracture. Some +geologists have considered it necessary to imagine that the upward or downward +movement in these cases was accomplished at a single stroke, and not by a series +of sudden but interrupted movements. They appear to have derived this idea from +a notion that the grooved walls have merely been rubbed in one direction, which +is far from being a constant phenomenon. Not only are some sets of striae not +parallel to others, but the clay and rubbish between the walls, when squeezed or +rubbed, have been streaked in different directions, the grooves which the harder +minerals have impressed on the softer being frequently curved and irregular. + +(FIGURE 77. Faults and denuded coal-strata, Ashby de la Zouch. (Mammatt.)) + +The usual absence of protruding masses of rock forming precipices or ridges +along the lines of great faults has already been alluded to in explaining Figure +76, and the same remarkable fact is well exemplified in every coal-field which +has been extensively worked. It is in such districts that the former relation of +the beds which have been shifted is determinable with great accuracy. Thus in +the coal-field of Ashby de la Zouch, in Leicestershire (see Figure 77), a fault +occurs, on one side of which the coal-beds a, b, c, d must once have risen to +the height of 500 feet above the corresponding beds on the other side. But the +uplifted strata do not stand up 500 feet above the general surface; on the +contrary, the outline of the country, as expressed by the line z z, is uniformly +undulating, without any break, and the mass indicated by the dotted outline must +have been washed away. (See Mammatt's Geological Facts etc. page 90 and plate.) + +The student may refer to Mr. Hull's measurement of faults, observed in the +Lancashire coal-field, where the vertical displacement has amounted to thousands +of feet, and yet where all the superficial inequalities which must have resulted +from such movements have been obliterated by subsequent denudation. In the same +memoir proofs are afforded of there having been two periods of vertical movement +in the same fault-- one, for example, before, and another after, the Triassic +epoch. (Hull Quarterly Geological Journal volume 24 page 318. 1868.) + +The shifting of the beds by faults is often intimately connected with those same +foldings which constitute the anticlinal and synclinal axes before alluded to, +and there is no doubt that the subterranean causes of both forms of disturbance +are to a great extent the same. A fault in Virginia, believed to imply a +displacement of several thousand feet, has been traced for more than eighty +miles in the same direction as the foldings of the Appalachian chain. (H.D. +Rogers Geology of Pennsylvania page 897.) An hypothesis which attributes such a +change of position to a succession of movements, is far preferable to any theory +which assumes each fault to have been accomplished by a single upcast or +downthrow of several thousand feet. For we know that there are operations now in +progress, at great depths in the interior of the earth, by which both large and +small tracts of ground are made to rise above and sink below their former level, +some slowly and insensibly, others suddenly and by starts, a few feet or yards +at a time; whereas there are no grounds for believing that, during the last 3000 +years at least, any regions have been either upheaved or depressed, at a single +stroke, to the amount of several hundred, much less several thousand feet. + +It is certainly not easy to understand how in the subterranean regions one mass +of solid rock should have been folded up by a continued series of movements, +while another mass in contact, or only separated by a line of fissure, has +remained stationary or has perhaps subsided. But every volcano, by the +intermittent action of the steam, gases, and lava evolved during an eruption, +helps us to form some idea of the manner in which such operations take place. +For eruptions are repeated at uncertain intervals throughout the whole or a +large part of a geological period, some of the surrounding and contiguous +districts remaining quite undisturbed. And in most of the instances with which +we are best acquainted the emission of lava, scoria, and steam is accompanied by +the uplifting of the solid crust. Thus in Vesuvius, Etna, the Madeiras, the +Canary Islands, and the Azores there is evidence of marine deposits of recent +and tertiary date having been elevated to the height of a thousand feet, and +sometimes more, since the commencement of the volcanic explosions. There is, +moreover, a general tendency in contemporaneous volcanic vents to affect a +linear arrangement, extending in some instances, as in the Andes or the Indian +Archipelago, to distances equalling half the circumference of the globe. Where +volcanic heat, therefore, operates at such a depth as not to obtain vent at the +surface, in the form of an eruption, it may nevertheless be conceived to give +rise to upheavals, foldings, and faults in certain linear tracts. And marine +denudation, to be treated of in the next chapter, will help us to understand why +that which should be the protruding portion of the faulted rocks is missing at +the surface. + +ARRANGEMENT AND DIRECTION OF PARALLEL FOLDS OF STRATA. + +The possible causes of the folding of strata by lateral movements have been +considered in a former part of this chapter. No European chain of mountains +affords so remarkable an illustration of the persistency of such flexures for a +great distance as the Appalachians before alluded to, and none has been studied +and described by many good observers with more accuracy. The chain extends from +north to south, or rather N.N.E. to S.S.W., for nearly 1500 miles, with a +breadth of 50 miles, throughout which the Palaeozoic strata have been so bent as +to form a series of parallel anticlinal and synclinal ridges and troughs, +comprising usually three or four principal and many smaller plications, some of +them forming broad and gentle arches, others narrower and steeper ones, while +some, where the bending has been greatest, have the position of their beds +inverted, as before shown in Figure 73. + +The strike of the parallel ridges, after continuing in a straight line for many +hundred miles, is then found to vary for a more limited distance as much as 30 +degrees, the folds wheeling round together in the new direction and continuing +to be parallel, as if they had all obeyed the same movement. The date of the +movements by which the great flexures were brought about must, of course, be +subsequent to the formation of the uppermost part of the coal or the newest of +the bent rocks, but the disturbance must have ceased before the Triassic strata +were deposited on the denuded edges of the folded beds. + +The manner in which the numerous parallel folds, all simultaneously formed, +assume a new direction common to the whole of them, and sometimes varying at an +angle of 30 degrees from the normal strike of the chain, shows what deviation +from an otherwise uniform strike of the beds may be experienced when the +geographical area through which they are traced is on so vast a scale. + +The disturbances in the case here adverted to occurred between the Carboniferous +period and that of the Trias, and this interval is so vast that they may have +occupied a great lapse of time, during which their parallelism was always +preserved. But, as a rule, wherever after a long geological interval the +recurrence of lateral movements gives rise to a new set of folds, the strike of +these last is different. Thus, for example, Mr. Hull has pointed out that three +principal lines of disturbance, all later than the Carboniferous period, have +affected the stratified rocks of Lancashire. The first of these, having an +E.N.E. direction, took place at the close of the Carboniferous period. The next, +running north and south, at the close of the Permian, and the third, having a +N.N.W. direction, at the close of the Jurassic period. (Edward Hull Quarterly +Geological Journal volume 24 page 323.) + +UNCONFORMABILITY OF STRATA. + +(FIGURE 78. Unconformable junction of old red sandstone and Silurian schist at +the Siccar Point, near St. Abb's Head, Berwickshire.) + +Strata are said to be unconformable when one series is so placed over another +that the planes of the superior repose on the edges of the inferior (see Figure +78.) In this case it is evident that a period had elapsed between the production +of the two sets of strata, and that, during this interval, the older series had +been tilted and disturbed. Afterwards the upper series was thrown down in +horizontal strata upon it. If these superior beds, d d Figure 78, are also +inclined, it is plain that the lower strata a a, have been twice displaced; +first, before the deposition of the newer beds, d d, and a second time when +these same strata were upraised out of the sea, and thrown slightly out of the +horizontal position. + +(FIGURE 79. Junction of unconformable strata near Mons, in Belgium.) + +It often happens that in the interval between the deposition of two sets of +unconformable strata, the inferior rock has not only been denuded, but drilled +by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons, +beds of an ancient (primary or palaeozoic) limestone, highly inclined, and often +bent, are covered with horizontal strata of greenish and whitish marls of the +Cretaceous formation. The lowest, and therefore the oldest, bed of the +horizontal series is usually the sand and conglomerate, a, in which are rounded +fragments of stone, from an inch to two feet in diameter. These fragments have +often adhering shells attached to them, and have been bored by perforating +mollusca. The solid surface of the inferior limestone has also been bored, so as +to exhibit cylindrical and pear-shaped cavities, as at c, the work of saxicavous +mollusca; and many rents, as at b, which descend several feet or yards into the +limestone, have been filled with sand and shells, similar to those in the +stratum a. + +OVERLAPPING STRATA. + +Strata are said to overlap when an upper bed extends beyond the limits of a +lower one. This may be produced in various ways; as, for example, when +alterations of physical geography cause the arms of a river or channels of +discharge to vary, so that sediment brought down is deposited over a wider area +than before, or when the sea-bottom has been raised up and again depressed +without disturbing the horizontal position of the strata. In this case the newer +strata may rest for the most part conformably on the older, but, extending +farther, pass over their edges. Every intermediate state between unconformable +and over-lapping beds may occur, because there may be every gradation between a +slight derangement of position, and a considerable disturbance and denudation of +the older formation before the newer beds come on. + + +CHAPTER VI. + +DENUDATION. + +Denudation defined. +Its Amount more than equal to the entire Mass of Stratified Deposits in the +Earth's Crust. +Subaerial Denudation. +Action of the Wind. +Action of Running Water. +Alluvium defined. +Different Ages of Alluvium. +Denuding Power of Rivers affected by Rise or Fall of Land. +Littoral Denudation. +Inland Sea-Cliffs. +Escarpments. +Submarine Denudation. +Dogger-bank. +Newfoundland Bank. +Denuding Power of the Ocean during Emergence of Land. + +Denudation, which has been occasionally spoken of in the preceding chapters, is +the removal of solid matter by water in motion, whether of rivers or of the +waves and currents of the sea, and the consequent laying bare of some inferior +rock. This operation has exerted an influence on the structure of the earth's +crust as universal and important as sedimentary deposition itself; for +denudation is the necessary antecedent of the production of all new strata of +mechanical origin. The formation of every new deposit by the transport of +sediment and pebbles necessarily implies that there has been, somewhere else, a +grinding down of rock into rounded fragments, sand, or mud, equal in quantity to +the new strata. All deposition, therefore, except in the case of a shower of +volcanic ashes, and the outflow of lava, and the growth of certain organic +formations, is the sign of superficial waste going on contemporaneously, and to +an equal amount, elsewhere. The gain at one point is no more than sufficient to +balance the loss at some other. Here a lake has grown shallower, there a ravine +has been deepened. Here the depth of the sea has been augmented by the removal +of a sandbank during a storm, there its bottom has been raised and shallowed by +the accumulation in its bed of the same sand transported from the bank. + +When we see a stone building, we know that somewhere, far or near, a quarry has +been opened. The courses of stone in the building may be compared to successive +strata, the quarry to a ravine or valley which has suffered denudation. As the +strata, like the courses of hewn stone, have been laid one upon another +gradually, so the excavation both of the valley and quarry have been gradual. To +pursue the comparison still farther, the superficial heaps of mud, sand, and +gravel, usually called alluvium, may be likened to the rubbish of a quarry which +has been rejected as useless by the workmen, or has fallen upon the road between +the quarry and the building, so as to lie scattered at random over the ground. + +But we occasionally find in a conglomerate large rounded pebbles of an older +conglomerate, which had previously been derived from a variety of different +rocks. In such cases we are reminded that, the same materials having been used +over and over again, it is not enough to affirm that the entire mass of +stratified deposits in the earth's crust affords a monument and measure of the +denudation which has taken place, for in truth the quantity of matter now extant +in the form of stratified rock represents but a fraction of the material removed +by water and redeposited in past ages. + +SUBAERIAL DENUDATION. + +Denudation may be divided into subaerial, or the action of wind, rain, and +rivers; and submarine, or that effected by the waves of the sea, and its tides +and currents. With the operation of the first of these we are best acquainted, +and it may be well to give it our first attention. + +ACTION OF THE WIND. + +In desert regions where no rain falls, or where, as in parts of the Sahara, the +soil is so salt as to be without any covering of vegetation, clouds of dust and +sand attest the power of the wind to cause the shifting of the unconsolidated or +disintegrated rock. + +In examining volcanic countries I have been much struck with the great +superficial changes brought about by this power in the course of centuries. The +highest peak of Madeira is about 6050 feet above the sea, and consists of the +skeleton of a volcanic cone now 250 feet high, the beds of which once dipped +from a centre in all directions at an angle of more than 30 degrees. The summit +is formed of a dike of basalt with much olivine, fifteen feet wide, apparently +the remains of a column of lava which once rose to the crater. Nearly all the +scoriae of the upper part of the cone have been swept away, those portions only +remaining which were hardened by the contact or proximity of the dike. While I +was myself on this peak on January 25, 1854, I saw the wind, though it was not +stormy weather, removing sand and dust derived from the decomposing scoriae. +There had been frost in the night, and some ice was still seen in the crevices +of the rock. + +On the highest platform of the Grand Canary, at an elevation of 6000 feet, there +is a cylindrical column of hard lava, from which the softer matter has been +carried away; and other similar remnants of the dikes of cones of eruption +attest the denuding power of the wind at points where running water could never +have exerted any influence. The waste effected by wind aided by frost and snow, +may not be trifling, even in a single winter, and when multiplied by centuries +may become indefinitely great. + +ACTION OF RUNNING WATER. + +(FIGURE 80. Section through several eroded formations. +a. Older alluvium or drift. +b. Modern alluvium.) + +There are different classes of phenomena which attest in a most striking manner +the vast spaces left vacant by the erosive power of water. I may allude, first, +to those valleys on both sides of which the same strata are seen following each +other in the same order, and having the same mineral composition and fossil +contents. We may observe, for example, several formations, as Nos. 1, 2, 3, 4, +in the diagram (Figure 80): No. 1, conglomerate, No. 2, clay, No. 3, grit, and +No. 4, limestone, each repeated in a series of hills separated by valleys +varying in depth. When we examine the subordinate parts of these four +formations, we find, in like manner, distinct beds in each, corresponding, on +the opposite sides of the valleys, both in composition and order of position. No +one can doubt that the strata were originally continuous, and that some cause +has swept away the portions which once connected the whole series. A torrent on +the side of a mountain produces similar interruptions; and when we make +artificial cuts in lowering roads, we expose, in like manner, corresponding beds +on either side. But in nature, these appearances occur in mountains several +thousand feet high, and separated by intervals of many miles or leagues in +extent. + +In the "Memoirs of the Geological Survey of Great Britain" (volume 1), Professor +Ramsay has shown that the missing beds, removed from the summit of the Mendips, +must have been nearly a mile in thickness; and he has pointed out considerable +areas in South Wales and some of the adjacent counties of England, where a +series of primary (or palaeozoic) strata, no less than 11,000 feet in thickness, +have been stripped off. All these materials have of course been transported to +new regions, and have entered into the composition of more modern formations. On +the other hand, it is shown by observations in the same "Survey," that the +Palaeozoic strata are from 20,000 to 30,000 feet thick. It is clear that such +rocks, formed of mud and sand, now for the most part consolidated, are the +monuments of denuding operations, which took place on a grand scale at a very +remote period in the earth's history. For, whatever has been given to one area +must always have been borrowed from another; a truth which, obvious as it may +seem when thus stated, must be repeatedly impressed on the student's mind, +because in many geological speculations it is taken for granted that the +external crust of the earth has been always growing thicker in consequence of +the accumulation, period after period, of sedimentary matter, as if the new +strata were not always produced at the expense of pre-existing rocks, stratified +or unstratified. By duly reflecting on the fact that all deposits of mechanical +origin imply the transportation from some other region, whether contiguous or +remote, of an equal amount of solid matter, we perceive that the stony exterior +of the planet must always have grown thinner in one place, whenever, by +accessions of new strata, it was acquiring thickness in another. + +It is well known that generally at the mouths of large rivers, deltas are +forming and the land is encroaching upon the sea; these deltas are monuments of +recent denudation and deposition; and it is obvious that if the mud, sand, and +gravel were taken from them and restored to the continents they would fill up a +large part of the gullies and valleys which are due to the excavating and +transporting power of torrents and rivers. + +ALLUVIUM. + +Between the superficial covering of vegetable mould and the subjacent rock there +usually intervenes in every district a deposit of loose gravel, sand, and mud, +to which when it occurs in valleys the name of alluvium has been popularly +applied. The term is derived from alluvio, an inundation, or alluo, to wash, +because the pebbles and sand commonly resemble those of a river's bed or the mud +and gravel washed over low lands by a flood. + +In the course of those changes in physical geography which may take place during +the gradual emergence of the bottom of the sea and its conversion into dry land, +any spot may either have been a sunken reef, or a bay, or estuary, or sea-shore, +or the bed of a river. The drainage, moreover, may have been deranged again and +again by earthquakes, during which temporary lakes are caused by landslips, and +partial deluges occasioned by the bursting of the barriers of such lakes. For +this reason it would be unreasonable to hope that we should ever be able to +account for all the alluvial phenomena of each particular country, seeing that +the causes of their origin are so various. Besides, the last operations of water +have a tendency to disturb and confound together all pre-existing alluviums. +Hence we are always in danger of regarding as the work of a single era, and the +effect of one cause, what has in reality been the result of a variety of +distinct agents, during a long succession of geological epochs. Much useful +instruction may therefore be gained from the exploration of a country like +Auvergne, where the superficial gravel of very different eras happens to have +been preserved and kept separate by sheets of lava, which were poured out one +after the other at periods when the denudation, and probably the upheaval, of +rocks were in progress. That region had already acquired in some degree its +present configuration before any volcanoes were in activity, and before any +igneous matter was superimposed upon the granitic and fossiliferous formations. +The pebbles therefore in the older gravels are exclusively constituted of +granite and other aboriginal rocks; and afterwards, when volcanic vents burst +forth into eruption, those earlier alluviums were covered by streams of lava, +which protected them from intermixture with gravel of subsequent date. In the +course of ages, a new system of valleys was excavated, so that the rivers ran at +lower levels than those at which the first alluviums and sheets of lava were +formed. When, therefore, fresh eruptions gave rise to new lava, the melted +matter was poured out over lower grounds; and the gravel of these plains +differed from the first or upland alluvium, by containing in it rounded +fragments of various volcanic rocks, and often fossil bones belonging to species +of land animals different from those which had previously flourished in the same +country and been buried in older gravels. + +(FIGURE 81. Lavas of Auvergne resting on alluviums of different ages.) + +Figure 81 will explain the different heights at which beds of lava and gravel, +each distinct from the other in composition and age, are observed, some on the +flat tops of hills, 700 or 800 feet high, others on the slope of the same hills, +and the newest of all in the channel of the existing river where there is +usually gravel alone, although in some cases a narrow strip of solid lava shares +the bottom of the valley with the river. + +The proportion of extinct species of quadrupeds is more numerous in the fossil +remains of the gravel No. 1 than in that indicated as No. 2; and in No. 3 they +agree more closely, sometimes entirely, with those of the existing fauna. The +usual absence or rarity of organic remains in beds of loose gravel and sand is +partly owing to the friction which originally ground down the rocks into small +fragments, and partly to the porous nature of alluvium, which allows the free +percolation through it of rain-water, and promotes the decomposition and removal +of fossil remains. + +The loose transported matter on the surface of a large part of the land now +existing in the temperate and arctic regions of the northern hemisphere, must be +regarded as being in a somewhat exceptional state, in consequence of the +important part which ice has played in comparatively modern geological times. +This subject will be more specially alluded to when we describe, in the eleventh +chapter, the deposits called "glacial." + +DENUDING POWER OF RIVERS AFFECTED BY RISE OR FALL OF LAND. + +It has long been a matter of common observation that most rivers are now cutting +their channels through alluvial deposits of greater depth and extent than could +ever have been formed by the present streams. From this fact it has been +inferred that rivers in general have grown smaller, or become less liable to be +flooded than formerly. It may be true that in the history of almost every +country the rivers have been both larger and smaller than they are at the +present moment. For the rainfall in particular regions varies according to +climate and physical geography, and is especially governed by the elevation of +the land above the sea, or its distance from it and other conditions equally +fluctuating in the course of time. But the phenomenon alluded to may sometimes +be accounted for by oscillations in the level of the land, experienced since the +existing valleys originated, even where no marked diminution in the quantity of +rain and in the size of the rivers has occurred. + +We know that many large areas of land are rising and others sinking, and unless +it could be assumed that both the upward and downward movements are everywhere +uniform, many of the existing hydrographical basins ought to have the appearance +of having been temporary lakes first filled with fluviatile strata and then +partially re-excavated. + +Suppose, for example, part of a continent, comprising within it a large +hydrographical basin like that of the Mississippi, to subside several inches or +feet in a century, as the west coast of Greenland, extending 600 miles north and +south, has been sinking for three or four centuries, between the latitudes 60 +and 69 degrees N. (Principles of Geology 7th edition page 506; 10th edition +volume 2 page 196.) It will rarely happen that the rate of subsidence will be +everywhere equal, and in many cases the amount of depression in the interior +will regularly exceed that of the region nearer the sea. Whenever this happens, +the fall of the waters flowing from the upland country will be diminished, and +each tributary stream will have less power to carry its sand and sediment into +the main river, and the main river less power to convey its annual burden of +transported matter to the sea. All the rivers, therefore, will proceed to fill +up partially their ancient channels, and, during frequent inundations, will +raise their alluvial plains by new deposits. If then the same area of land be +again upheaved to its former height, the fall, and consequently the velocity, of +every river will begin to augment. Each of them will be less given to overflow +its alluvial plain; and their power of carrying earthy matter seaward, and of +scouring out and deepening their channels, will be sustained till, after a lapse +of many thousand years, each of them has eroded a new channel or valley through +a fluviatile formation of comparatively modern date. The surface of what was +once the river-plain at the period of greatest depression, will then remain +fringing the valley-sides in the form of a terrace apparently flat, but in +reality sloping down with the general inclination of the river. Everywhere this +terrace will present cliffs of gravel and sand, facing the river. That such a +series of movements has actually taken place in the main valley of the +Mississippi and in its tributary valleys during oscillations of level, I have +endeavoured to show in my description of that country (Second Visit to the +United States volume 1 chapter 34.); and the fresh-water shells of existing +species and bones of land quadrupeds, partly of extinct races, preserved in the +terraces of fluviatile origin, attest the exclusion of the sea during the whole +process of filling up and partial re-excavation. + +LITTORAL DENUDATION. + +Part of the action of the waves between high and low watermark must be included +in subaerial denudation, more especially as the undermining of cliffs by the +waves is facilitated by land-springs, and these often lead to the sliding down +of great masses of land into the sea. Along our coasts we find numerous +submerged forests, only visible at low water, having the trunks of the trees +erect and their roots attached to them and still spreading through the ancient +soil as when they were living. They occur in too many places, and sometimes at +too great a depth, to be explained by a mere change in the level of the tides, +although as the coasts waste away and alter in shape, the height to which the +tides rise and fall is always varying, and the level of high tide at any given +point may, in the course of many ages, differ by several feet or even fathoms. +It is this fluctuation in the height of the tides, and the erosion and +destruction of the sea-coast by the waves, that makes it exceedingly difficult +for us in a few centuries, or even perhaps in a few thousand years, to determine +whether there is a change by subterranean movement in the relative level of sea +and land. + +We often behold, as on the coasts of Devonshire and Pembrokeshire, facts which +appear to lead to opposite conclusions. In one place a raised beach with marine +littoral shells, and in another immediately adjoining a submerged forest. These +phenomena indicate oscillations of level, and as the movements are very gradual, +they must give repeated opportunities to the breakers to denude the land which +is thus again and again exposed to their fury, although it is evident that the +submergence is sometimes effected in such a manner as to allow the trees which +border the coast not to be carried away. + +INLAND SEA-CLIFFS. + +In countries where hard limestone rocks abound, inland cliffs have often +retained faithfully for ages the characters which they acquired when they +constituted the boundary of land and sea. Thus, in the Morea, no less than three +or even four ranges of cliffs are well-preserved, rising one above the other at +different distances from the actual shore, the summit of the highest and oldest +occasionally attaining 1000 feet in elevation. A consolidated beach with marine +shells is usually found at the base of each cliff, and a line of littoral +caverns. These ranges of cliff probably imply pauses in the process of upheaval +when the waves and currents had time to undermine and clear away considerable +masses of rock. + +But the beginner should be warned not to expect to find evidence of the former +sojourn of the sea on all those lands which we are nevertheless sure have been +submerged at periods comparatively modern; for notwithstanding the enduring +nature of the marks left by littoral action on some rocks, especially +limestones, we can by no means detect sea-beaches and inland cliffs everywhere. +On the contrary, they are, upon the whole, extremely partial, and are often +entirely wanting in districts composed of argillaceous and sandy formations, +which must, nevertheless, have been upheaved at the same time, and by the same +intermittent movements, as the adjoining harder rocks. + +ESCARPMENTS. + +Besides the inland cliffs above alluded to which mark the ancient limits of the +sea, there are other abrupt terminations of rocks of various kinds which +resemble sea-cliffs, but which have in reality been due to subaerial denudation. +These have been called "escarpments," a term which it is useful to confine to +the outcrop of particular formations having a scarped outline, as distinct from +cliffs due to marine action. + +I formerly supposed that the steep line of cliff-like slopes seen along the +outcrop of the chalk, when we follow the edge of the North or South Downs, was +due to marine action; but Professor Ramsay has shown (Physical Geography and +Geology of Great Britain page 78 1864.) that the present outline of the physical +geography is more in favour of the idea of the escarpments having been due to +gradual waste since the rocks were exposed in the atmosphere to the action of +rain and rivers. + +Mr. Whittaker has given a good summary of the grounds for ascribing these +apparent sea-cliffs to waste in the open air. 1. There is an absence of all +signs of ancient sea-beaches or littoral deposits at the base of the escarpment. +2. Great inequality is observed in the level of the base line. 3. The +escarpments do not intersect, like sea-cliffs, a series of distinct rocks, but +are always confined to the boundary-line of the same formation. 4. There are +sometimes different contiguous and parallel escarpments-- those, for example, of +the greensand and chalk-- which are so near each other, and occasionally so +similar in altitude, that we can not imagine any existing archipelago if +converted into dry land to present a like outline. + +The above theory is by no means inconsistent with the opinion that the limits of +the outcrop of the chalk and greensand which the escarpments now follow, were +originally determined by marine denudation. When the south-east of England last +emerged from beneath the level of the sea, it was acted upon, no doubt, by the +tide, waves, and currents, and the chalk would form from the first a mass +projecting above the more destructible clay called Gault. Still the present +escarpments so much resembling sea-cliffs have no doubt, for reasons above +stated, derived their most characteristic features subsequently to emergence +from subaerial waste by rain and rivers. + +SUBMARINE DENUDATION. + +When we attempt to estimate the amount of submarine denudation, we become +sensible of the disadvantage under which we labour from our habitual incapacity +of observing the action of marine currents on the bed of the sea. We know that +the agitation of the waves, even during storms, diminishes at a rapid rate, so +as to become very insignificant at the depth of a few fathoms, and is quite +imperceptible at the depth of about sixteen fathoms; but when large bodies of +water are transferred by a current from one part of the ocean to another, they +are known to maintain at great depths such a velocity as must enable them to +remove the finer, and sometimes even the coarser, materials of the rocks over +which they flow. As the Mississippi when more than 150 feet deep can keep open +its channel and even carry down gravel and sand to its delta, the surface +velocity being not more than two or three miles an hour, so a gigantic current, +like the Gulf Stream, equal in volume to many hundred Mississippis, and having +in parts a surface velocity of more than three miles, may act as a propelling +and abrading power at still greater depths. But the efficacy of the sea as a +denuding agent, geologically considered, is not dependent on the power of +currents to preserve at great depths a velocity sufficient to remove sand and +mud, because, even where the deposition or removal of sediment is not in +progress, the depth of water does not remain constant throughout geological +time. Every page of the geological record proves to us that the relative levels +of land and sea, and the position of the ocean and of continents and islands, +has been always varying, and we may feel sure that some portions of the +submarine area are now rising and others sinking. The force of tidal and other +currents and of the waves during storms is sufficient to prevent the emergence +of many lands, even though they may be undergoing continual upheaval. It is not +an uncommon error to imagine that the waste of sea-cliffs affords the measure of +the amount of marine denudation of which it probably constitutes an +insignificant portion. + +DOGGER-BANK. + +That great shoal called the Dogger-bank, about sixty miles east of the coast of +Northumberland, and occupying an area about as large as Wales, has nowhere a +depth of more than ninety feet, and in its shallower parts is less than forty +feet under water. It might contribute towards the safety of the navigation of +our seas to form an artificial island, and to erect a light-house on this bank; +but no engineer would be rash enough to attempt it, as he would feel sure that +the ocean in the first heavy gale would sweep it away as readily as it does +every temporary shoal that accumulates from time to time around a sunk vessel on +the same bank. (Principles 10th edition volume 1 page 569.) + +No observed geographical changes in historical times entitle us to assume that +where upheaval may be in progress it proceeds at a rapid rate. Three or four +feet rather than as many yards in a century may probably be as much as we can +reckon upon in our speculations; and if such be the case, the continuance of the +upward movement might easily be counteracted by the denuding force of such +currents aided by such waves as, during a gale, are known to prevail in the +German Ocean. What parts of the bed of the ocean are stationary at present, and +what areas may be rising or sinking, is a matter of which we are very ignorant, +as the taking of accurate soundings is but of recent date. + +NEWFOUNDLAND BANK. + +The great bank of Newfoundland may be compared in size to the whole of England. +This part of the bottom of the Atlantic is surrounded on three sides by a +rapidly deepening ocean, the bank itself being from twenty to fifty fathoms (or +from 120 to 300 feet) under water. We are unable to determine by the comparison +of different charts made at distant periods, whether it is undergoing any change +of level, but if it be gradually rising we can not anticipate on that account +that it will become land, because the breakers in an open sea would exercise a +prodigious force even on solid rock brought up to within a few yards of the +surface. We know, for example, that when a new volcanic island rose in the +Mediterranean in 1831, the waves were capable in a few years of reducing it to a +sunken rock. + +In the same way currents which flow over the Newfoundland bank a great part of +the year at the rate of two miles an hour, and are known to retain a +considerable velocity to near the bottom, may carry away all loose sand and mud, +and make the emergence of the shoal impossible, in spite of the accessions of +mud, sand, and boulders derived occasionally from melting icebergs which, coming +from the northern glaciers, are frequently stranded on various parts of the +bank. They must often leave at the bottom large erratic blocks which the marine +currents may be incapable of moving, but the same rocky fragments may be made to +sink by the undermining of beds consisting of finer matter on which the blocks +and gravel repose. In this way gravel and boulders may continue to overspread a +submarine bottom after the latter has been lowered for hundreds of feet, the +surface never having been able to emerge and become land. It is by no means +improbable that the annual removal of an average thickness of half an inch of +rock might counteract the ordinary upheaval which large submarine areas are +undergoing; and the real enigma which the geologist has to solve is not the +extensive denudation of the white chalk or of our tertiary sands and clays, but +the fact that such incoherent materials have ever succeeded in lifting up their +heads above water in an open sea. Why were they not swept away during storms +into some adjoining abysses, the highest parts of each shoal being always planed +off down to the depth of a few fathoms? The hardness and toughness of some rocks +already exposed to windward and acting as breakwaters may perhaps have assisted; +nor must we forget the protection afforded by a dense and unbroken covering of +barnacles, limpets, and other creatures which flourish most between high and low +water and shelter some newly risen coasts from the waves. + + +CHAPTER VII. + +JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH'S +CRUST. + +How we obtain an Insight at the Surface, of the Arrangement of Rocks at great +Depths. +Why the Height of the successive Strata in a given Region is so disproportionate +to their Thickness. +Computation of the average annual Amount of subaerial Denudation. +Antagonism of Volcanic Force to the Levelling Power of running Water. +How far the Transfer of Sediment from the Land to a neighbouring Sea-bottom may +affect Subterranean Movements. +Permanence of Continental and Oceanic Areas. + +HOW WE OBTAIN AN INSIGHT AT THE SURFACE, OF THE ARRANGEMENT OF ROCKS AT GREAT +DEPTHS. + +The reader has been already informed that, in the structure of the earth's +crust, we often find proofs of the direct superposition of marine to fresh-water +strata, and also evidence of the alternation of deep-sea and shallow-water +formations. In order to explain how such a series of rocks could be made to form +our present continents and islands, we have not only to assume that there have +been alternate upward and downward movements of great vertical extent, but that +the upheaval in the areas which we at present inhabit has, in later geological +times, sufficiently predominated over subsidence to cause these portions of the +earth's crust to be land instead of sea. The sinking down of a delta beneath the +sea-level may cause strata of fluviatile or even terrestrial origin, such as +peat with trees proper to marshes, to be covered by deposits of deep-sea origin. +There is also no end to the thickness of mud and sand which may accumulate in +shallow water, provided that fresh sediment is brought down from the wasting +land at a rate corresponding to that of the sinking of the bed of the sea. The +latter, again, may sometimes sink so fast that the earthy matter, being +intercepted in some new landward depression, may never reach its former resting- +place, where, the water becoming clear may favour the growth of shells and +corals, and calcareous rocks of organic origin may thus be superimposed on +mechanical deposits. + +The succession of strata here alluded to would be consistent with the occurrence +of gradual downward and upward movements of the land and bed of the sea without +any disturbance of the horizontality of the several formations. But the +arrangement of rocks composing the earth's crust differs materially from that +which would result from a mere series of vertical movements. Had the volcanic +forces been confined to such movements, and had the stratified rocks been first +formed beneath the sea and then raised above it, without any lateral +compression, the geologist would never have obtained an insight into the +monuments of various ages, some of extremely remote antiquity. + +What we have said in Chapter 5 of dip and strike, of the folding and inversion +of strata, of anticlinal and synclinal flexures, and in Chapter 6 of denudation +at different periods, whether subaerial or submarine, must be understood before +the student can comprehend what may at first seem to him an anomaly, but which +it is his business particularly to understand. I allude to the small height +above the level of the sea attained by strata often many miles in thickness, and +about the chronological succession of which, in one and the same region, there +is no doubt whatever. Had stratified rocks in general remained horizontal, the +waves of the sea would have been enabled during oscillations of level to plane +off entirely the uppermost beds as they rose or sank during the emergence or +submergence of the land. But the occurrence of a series of formations of widely +different ages, all remaining horizontal and in conformable stratification, is +exceptional, and for this reason the total annihilation of the uppermost strata +has rarely taken place. We owe, indeed, to the side way movements of LATERAL +COMPRESSION those anticlinal and synclinal curves of the beds already described +(Figure 55 Chapter 4), which, together with denudation, subaerial and submarine, +enable us to investigate the structure of the earth's crust many miles below +those points which the miner can reach. I have already shown in Figure 56 +Chapter 4, how, at St. Abb's Head, a series of strata of indefinite thickness +may become vertical, and then denuded, so that the edges of the beds alone shall +be exposed to view, the altitude of the upheaved ridges being reduced to a +moderate height above the sea-level; and it may be observed that although the +incumbent strata of Old Red Sandstone are in that place nearly horizontal, yet +these same newer beds will in other places be found so folded as to present +vertical strata, the edges of which are abruptly cut off, as in 2, 3, 4 on the +right-hand side of the diagram, Figure 55 Chapter 4. + +WHY THE HEIGHT OF THE SUCCESSIVE STRATA IN A GIVEN REGION IS SO DISPROPORTIONATE +TO THEIR THICKNESS. + +We can not too distinctly bear in mind how dependent we are on the joint action +of the volcanic and aqueous forces, the one in disturbing the original position +of rocks, and the other in destroying large portions of them, for our power of +consulting the different pages and volumes of those stony records of which the +crust of the globe is composed. Why, it may be asked, if the ancient bed of the +sea has been in many regions uplifted to the height of two or three miles, and +sometimes twice that altitude, and if it can be proved that some single +formations are of themselves two or three miles thick, do we so often find +several important groups resting one upon the other, yet attaining only the +height of a few hundred feet above the level of the sea? + +The American geologists, after carefully studying the Allegheny or Appalachian +mountains, have ascertained that the older fossiliferous rocks of that chain +(from the Silurian to the Carboniferous inclusive) are not less than 42,000 feet +thick, and if they were now superimposed on each other in the order in which +they were thrown down, they ought to equal in height the Himalayas with the Alps +piled upon them. Yet they rarely reach an altitude of 5000 feet, and their +loftiest peaks are no more than 7000 feet high. The Carboniferous strata forming +the highest member of the series, and containing beds of coal, can be shown to +be of shallow-water origin, or even sometimes to have originated in swamps in +the open air. But what is more surprising, the lowest part of this great +Palaeozoic series, instead of having been thrown down at the bottom of an abyss +more than 40,000 feet deep, consists of sediment (the Potsdam sandstone), +evidently spread out on the bottom of a shallow sea, on which ripple-marked +sands were occasionally formed. This vast thickness of 40,000 feet is not +obtained by adding together the maximum density attained by each formation in +distant parts of the chain, but by measuring the successive groups as they are +exposed in a very limited area, and where the denuded edges of the vertical +strata forming the parallel folds alluded to in Chapter 5 "crop out" at the +surface. Our attention has been called by Mr. James Hall, Palaeontologist of New +York, to the fact that these Palaeozoic rocks of the Appalachian chain, which +are of such enormous density, where they are almost entirely of mechanical +origin, thin out gradually as they are traced to the westward, where evidently +the contemporaneous seas allowed organic rocks to be formed by corals, +echinoderms, and encrinites in clearer water, and where, although the same +successive periods are represented, the total mass of strata from the Silurian +to the Carboniferous, instead of being 40,000 is only 4000 feet thick. + +A like phenomenon is exhibited in every mountainous country, as, for example, in +the European Alps; but we need not go farther than the north of England for its +illustration. Thus in Lancashire and central England the thickness of the +Carboniferous formation, including the Millstone Grit and Yoredale beds, is +computed to be more than 18,000 feet; to this we may add the Mountain Limestone, +at least 2000 feet in thickness, and the overlying Permian and Triassic +formations, 3000 or 4000 feet thick. How then does it happen that the loftiest +hills of Yorkshire and Lancashire, instead of being 24,000 feet high, never rise +above 3000 feet? For here, as before pointed out in the Alleghenies, all the +great thicknesses are sometimes found in close approximation and in a region +only a few miles in diameter. It is true that these same sets of strata do not +preserve their full force when followed for indefinite distances. Thus the +18,000 feet of Carboniferous grits and shales in Lancashire, before alluded to, +gradually thin out, as Mr. Hull has shown, as they extend southward, by +attenuation or original deficiency of sediment, and not in consequence of +subsequent denudation, so that when we have followed them for about 100 miles +into Leicestershire, they have dwindled away to a thickness of only 3000 feet. +In the same region the Carboniferous limestone attains so unusual a thickness-- +namely, more than 4000 feet-- as to appear to compensate in some measure for the +deficiency of contemporaneous sedimentary rock. (Hull Quarterly Geological +Journal volume 24 page 322 1868.) + +(FIGURE 82. Unconformable Palaeozoic strata, Sutherlandshire (Murchison). +Queenaig (2673 feet). +1. Laurentian gneiss. +2. Cambrian conglomerate and sandstone. +3, 3'. Quartzose Lower Silurian, with annelid burrows.) + +It is admitted that when two formations are unconformable their fossil remains +almost always differ considerably. The break in the continuity of the organic +forms seems connected with a great lapse of time, and the same interval has +allowed extensive disturbance of the strata, and removal of parts of them by +denudation, to take place. The more we extend our investigations the more +numerous do the proofs of these breaks become, and they extend to the most +ancient rocks yet discovered. The oldest examples yet brought to light in the +British Isles are on the borders of Rosshire and Sutherlandshire, and have been +well described by Sir Roderick Murchison, by whom their chronological relations +were admirably worked out, and proved to be very different from those which +previous observers had imagined them to be. I had an opportunity in the autumn +of 1869 of verifying the splendid section given in Figure 82 by climbing in a +few hours from the banks of Loch Assynt to the summit of the mountain called +Queenaig, 2673 feet high. + +The formations 1, 2, 3, the Laurentian, Cambrian, and Silurian, to be explained +in Chapters 25 and 26, not only occur in succession in this one mountain, but +their unconformable junctions are distinctly exposed to view. + +To begin with the oldest set of rocks, No. 1; they consist chiefly of +hornblendic gneiss, and in the neighbouring Hebrides form whole islands, +attaining a thickness of thousands of feet, although they have suffered such +contortions and denudation that they seldom rise more than a few hundred feet +above the sea-level. In discordant stratification upon the edges of this gneiss +reposes No. 2, a group of conglomerate and purple sandstone referable to the +Cambrian (or Longmynd) formation, which can elsewhere be shown to be +characterised by its peculiar organic remains. On this again rests No. 3, a +lower member of the important group called Silurian, an outlier of which, 3', +caps the summit of Queenaig, attesting the removal by denudation of rocks of the +same age, which once extended from the great mass 3 to 3'. Although this rock +now consists of solid quartz, it is clear that in its original state it was +formed of fine sand, perforated by numerous lob-worms or annelids, which left +their burrows in the shape of tubular hollows (Chapter 26, Figure 563 of +Arenicolites), hundreds, nay thousands, of which I saw as I ascended the +mountain. + +(FIGURE 83. Diagrammatic section of the same groups near Queenaig (Murchison) +through west (left), Suilvein, Assynt and Ben More, east (right). +1. Laurentian gneiss. +2. Cambrian conglomerate and sandstone. +3, 3'. Quartzose Lower Silurian, with annelid burrows. +3a. Fossiliferous Silurian limestone. +3b. Quartzose, micaceous and gneissose rocks (altered Silurian).) + +In Queenaig we only behold this single quartzose member of the Silurian series, +but in the neighbouring country (see Figure 83) it is seen to the eastward to be +followed by limestones, 3a, and schists, 3b, presenting numerous folds, and +becoming more and more metamorphic and crystalline, until at length, although +very different in age and strike, they much resemble in appearance the group No. +1. It is very seldom that in the same country one continuous formation, such as +the Silurian, is, as in this case, more fossiliferous and less altered by +volcanic heat in its older than in its newer strata, and still more rare to find +an underlying and unconformable group like the Cambrian retaining its original +condition of a conglomerate and sandstone more perfectly than the overlying +formation. Here also we may remark in regard to the origin of these Cambrian +rocks that they were evidently produced at the expense of the underlying +Laurentian, for the rounded pebbles occurring in them are identical in +composition and texture with that crystalline gneiss which constitutes the +contorted beds of the inferior formation No. 1. When the reader has studied the +chapter on metamorphism, and has become aware how much modification by heat, +pressure, and chemical action is required before the conversion of sedimentary +into crystalline strata can be brought about, he will appreciate the insight +which we thus gain into the date of the changes which had already been effected +in the Laurentian rocks long before the Cambrian pebbles of quartz and gneiss +were derived from them. The Laurentian is estimated by Sir William Logan to +amount in Canada to 30,000 feet in thickness. As to the Cambrian, it is supposed +by Sir Roderick Murchison that the fragment left in Sutherlandshire is about +3500 feet thick, and in Wales and the borders of Shropshire this formation may +equal 10,000 feet, while the Silurian strata No. 3, difficult as it may be to +measure them in their various foldings to the eastward, where they have been +invaded by intrusive masses of granite, are supposed many times to surpass the +Cambrian in volume and density. + +But although we are dealing here with stratified rocks, each of which would be +several miles in thickness, if they were fully represented, the whole of them do +not attain the elevation of a single mile above the level of the sea. + +COMPUTATION OF THE AVERAGE ANNUAL AMOUNT OF SUBAERIAL DENUDATION. + +The geology of the district above alluded to may assist our imagination in +conceiving the extent to which groups of ancient rocks, each of which may in +their turn have formed continents and oceanic basins, have been disturbed, +folded, and denuded even in the course of a few out of many of those geological +periods to which our imperfect records relate. It is not easy for us to +overestimate the effects which causes in every day action must produce when the +multiplying power of time is taken into account. + +Attempts were made by Manfredi in 1736, and afterwards by Playfair in 1802, to +calculate the time which it would require to enable the rivers to deliver over +the whole of the land into the basin of the ocean. The data were at first too +imperfect and vague to allow them even to approximate to safe conclusions. But +in our own time similar investigations have been renewed with more prospect of +success, the amount brought down by many large rivers to the sea having been +more accurately ascertained. Mr. Alfred Tylor, in 1850, inferred that the +quantity of detritus now being distributed over the sea-bottom would, at the end +of 10,000 years, cause an elevation of the sea-level to the extent of at least +three inches. (Tylor Philosophical Magazine 4th series page 268 1850.) +Subsequently Mr. Croll, in 1867, and again, with more exactness, in 1868, +deduced from the latest measurement of the sediment transported by European and +American rivers the rate of subaerial denudation to which the surface of large +continents is exposed, taking especially the hydrographical basin of the +Mississippi as affording the best available measure of the average waste of the +land. The conclusion arrived at in his able memoir was that the whole +terrestrial surface is denuded at the rate of one foot in 6000 years (Croll +Philosophical Magazine 1868 page 381.), and this opinion was simultaneously +enforced by his fellow-labourer, Mr. Geikie, who, being jointly engaged in the +same line of inquiry, published a luminous essay on the subject in 1868. + +The student, by referring to my "Principles of Geology" (Volume 1 page 442 +1867.) may see that Messrs. Humphrey and Abbot, during their survey of the +Mississippi, attempted to make accurate measurements of the proportion of +sediment carried down annually to the sea by that river, including not only the +mud held in suspension, but also the sand and gravel forced along the bottom. + +It is evident that when we know the dimensions of the area which is drained, and +the annual quantity of earthy matter taken from it and borne into the sea, we +can affirm how much on an average has been removed from the general surface in +one year, and there seems no danger of our overrating the mean rate of waste by +selecting the Mississippi as our example, for that river drains a country equal +to more than half the continent of Europe, extends through twenty degrees of +latitude, and therefore through regions enjoying a great variety of climate, and +some of its tributaries descend from mountains of great height. The Mississippi +is also more likely to afford us a fair test of ordinary denudation, because, +unlike the St. Lawrence and its tributaries, there are no great lakes in which +the fluviatile sediment is thrown down and arrested in its way to the sea. In +striking a general average we have to remember that there are large deserts in +which there is scarcely any rainfall, and tracts which are as rainless as parts +of Peru, and these must not be neglected as counterbalancing others, in the +tropics, where the quantity of rain is in excess. If then, argues Mr. Geikie, we +assume that the Mississippi is lowering the surface of the great basin which it +drains at the rate of one foot in 6000 years, 10 feet in 60,000 years, 100 feet +in 600,000 years, and 1000 feet in 6,000,000 years, it would not require more +than about 4,500,000 years to wear away the whole of the North American +continent if its mean height is correctly estimated by Humboldt at 748 feet. And +if the mean height of all the land now above the sea throughout the globe is +1000 feet, as some geographers believe, it would only require six million years +to subject a mass of rock equal in volume to the whole of the land to the action +of subaerial denudation. It may be objected that the annual waste is partial, +and not equally derived from the general surface of the country, inasmuch as +plains, water-sheds, and level ground at all heights remain comparatively +unaltered; but this, as Mr. Geikie has well pointed out, does not affect our +estimate of the sum total of denudation. The amount remains the same, and if we +allow too little for the loss from the surface of table-lands we only increase +the proportion of the loss sustained by the sides and bottoms of the valleys, +and vice versa. (Transactions of the Geological Society Glasgow volume 3 page +169.) + +ANTAGONISM OF VOLCANIC FORCE TO THE LEVELLING POWER OF RUNNING WATER. + +In all these estimates it is assumed that the entire quantity of land above the +sea-level remains on an average undiminished in spite of annual waste. Were it +otherwise the subaerial denudation would be continually lessened by the +diminution of the height and dimensions of the land exposed to waste. +Unfortunately we have as yet no accurate data enabling us to measure the action +of that force by which the inequalities of the surface of the earth's crust may +be restored, and the height of the continents and depth of the seas made to +continue unimpaired. I stated in 1830 in the "Principles of Geology" (1st +edition chapter 10 page 167 1830; see also 10th edition volume 1 chapter 15 page +327 1867.), that running water and volcanic action are two antagonistic forces; +the one labouring continually to reduce the whole of the land to the level of +the sea, the other to restore and maintain the inequalities of the crust on +which the very existence of islands and continents depends. I stated, however, +that when we endeavour to form some idea of the relation of these destroying and +renovating forces, we must always bear in mind that it is not simply by upheaval +that subterranean movements can counteract the levelling force of running water. +For whereas the transportation of sediment from the land to the ocean would +raise the general sea-level, the subsidence of the sea-bottom, by increasing its +capacity, would check this rise and prevent the submergence of the land. I have, +indeed, endeavoured to show that unless we assume that there is, on the whole, +more subsidence than upheaval, we must suppose the diameter of the planet to be +always increasing, by that quantity of volcanic matter which is annually poured +out in the shape of lava or ashes, whether on the land or in the bed of the sea, +and which is derived from the interior of the earth. The abstraction of this +matter causes, no doubt, subterranean vacuities and a corresponding giving way +of the surface; if it were not so, the average density of parts of the interior +would be always lessening and the size of the planet increasing. (Principles +volume 2 page 237; also 1st edition page 447 1830.) + +Our inability to estimate the amount or direction of the movements due to +volcanic power by no means renders its efficacy as a land-preserving force in +past times a mere matter of conjecture. The student will see in Chapter 24 that +we have proofs of Carboniferous forests hundreds of miles in extent which grew +on the lowlands or deltas near the sea, and which subsided and gave place to +other forests, until in some regions fluviatile and shallow-water strata with +occasional seams of coal were piled one over the other, till they attained a +thickness of many thousand feet. Such accumulations, observed in Great Britain +and America on opposite sides of the Atlantic, imply the long-continued +existence of land vegetation, and of rivers draining a former continent placed +where there is now deep sea. + +It will be also seen in Chapter 25 that we have evidence of a rich terrestrial +flora, the Devonian, even more ancient than the Carboniferous; while on the +other hand, the later Triassic, Oolitic, Cretaceous, and successive Tertiary +periods have all supplied us with fossil plants, insects, or terrestrial +mammalia; showing that, in spite of great oscillations of level and continued +changes in the position of land and sea, the volcanic forces have maintained a +due proportion of dry land. We may appeal also to fresh-water formations, such +as the Purbeck and Wealden, to prove that in the Oolitic and Neocomian eras +there were rivers draining ancient lands in Europe in times when we know that +other spaces, now above water, were submerged. + +HOW FAR THE TRANSFER OF SEDIMENT FROM THE LAND TO A NEIGHBOURING SEA-BOTTOM MAY +AFFECT SUBTERRANEAN MOVEMENTS. + +Little as we understand at present the laws which govern the distribution of +volcanic heat in the interior and crust of the globe, by which mountain chains, +high table-lands, and the abysses of the ocean are formed, it seems clear that +this heat is the prime mover on which all the grander features in the external +configuration of the planet depend. + +It has been suggested that the stripping off by denudation of dense masses from +one part of a continent and the delivery of the same into the bed of the ocean +must have a decided effect in causing changes of temperature in the earth's +crust below, or, in other words, in causing the subterranean isothermals to +shift their position. If this be so, one part of the crust may be made to rise, +and another to sink, by the expansion and contraction of the rocks, of which the +temperature is altered. + +I can not, at present, discuss this subject, of which I have treated more fully +elsewhere (Principles volume 2 page 229 1868.), but may state here that I +believe this transfer of sediment to play a very subordinate part in modifying +those movements on which the configuration of the earth's crust depends. In +order that strata of shallow-water origin should be able to attain a thickness +of several thousand feet, and so come to exert a considerable downward pressure, +there must have been first some independent and antecedent causes at work which +have given rise to the incipient shallow receptacle in which the sediment began +to accumulate. The same causes there continuing to depress the sea-bottom, room +would be made for fresh accessions of sediment, and it would only be by a long +repetition of the depositing process that the new matter could acquire weight +enough to affect the temperature of the rocks far below, so as to increase or +diminish their volume. + +PERMANENCE OF CONTINENTAL AND OCEANIC AREAS. + +If the thickness of more than 40,000 feet of sedimentary strata before alluded +to in the Appalachians proves a preponderance of downward movements in +Palaeozoic times in a district now forming the eastern border of North America, +it also proves, as before hinted, the continued existence and waste of some +neighbouring continent, probably formed of Laurentian rocks, and situated where +the Atlantic now prevails. Such an hypothesis would be in perfect harmony with +the conclusions forced upon us by the study of the present configuration of our +continents, and the relation of their height to the depth of the oceanic basins; +also to the considerable elevation and extent sometimes reached by drift +containing shells of recent species, and still more by the fact of sedimentary +strata, several thousand feet thick, as those of central Sicily, or such as +flank the Alps and Apennines, containing fossil Mollusca sometimes almost wholly +identical with species still living. + +I have remarked elsewhere (Principles volume 1 page 265 1867.) that upward and +downward movements of 1000 feet or more would turn much land into sea and sea +into land in the continental areas and their borders, whereas oscillations of +equal magnitude would have no corresponding effect in the bed of the ocean +generally, believed as it is to have a mean depth of 15,000 feet, and which, +whether this estimate be correct or not, is certainly of great profundity. +Subaerial denudation would not of itself lessen the area of the land, but would +tend to fill up with sediment seas of moderate depth adjoining the coast. The +coarser matter falls to the bottom near the shore in the first still water which +it reaches, and whenever the sea-bottom on which this matter has been thrown is +slightly elevated, it becomes land, and an upheaval of a thousand feet causes it +to attain the mean elevation of continents in general. + +Suppose, therefore, we had ascertained that the triturating power of subaerial +denudation might in a given time-- in three, or six, or a greater number of +millions of years-- pulverise a volume of rock equal in dimensions to all the +present land, we might yet find, could we revisit the earth at the end of such a +period, that the continents occupied very much the same position which they held +before; we should find the rivers employed in carrying down to the sea the very +same mud, sand, and pebbles with which they had been charged in our own time, +the superficial alluvial matter as well as a great thickness of sedimentary +strata would inclose shells, all or a great part of which we should recognise as +specifically identical with those already known to us as living. Every geologist +is aware that great as have been the geographical changes in the northern +hemisphere since the commencement of the Glacial Period, there having been +submergence and re-emergence of land to the extent of 1000 feet vertically, and +in the temperate latitudes great vicissitudes of climate, the marine mollusca +have not changed, and the same drift which had been carried down to the sea at +the beginning of the period is now undergoing a second transportation in the +same direction. + +As when we have measured a fraction of time in an hour-glass we have only to +reverse the position of our chronometer and we make the same sand measure over +again the duration of a second equal period, so when the volcanic force has +remoulded the form of a continent and the adjoining sea-bottom, the same +materials are made to do duty a second time. It is true that at each oscillation +of level the solid rocks composing the original continent suffer some fresh +denudation, and do not remain unimpaired like the wooden and glass framework of +the hour-glass, still the wear and tear suffered by the larger area exposed to +subaerial denudation consists either of loose drift or of sedimentary strata, +which were thrown down in seas near the land, and subsequently upraised, the +same continents and oceanic basins remaining in existence all the while. + +From all that we know of the extreme slowness of the upward and downward +movements which bring about even slight geographical changes, we may infer that +it would require a long succession of geological periods to cause the submarine +and supramarine areas to change places, even if the ascending movements in the +one region and the descending in the other were continuously in one direction. +But we have only to appeal to the structure of the Alps, where there are so many +shallow and deep water formations of various ages crowded into a limited area, +to convince ourselves that mountain chains are the result of great oscillations +of level. High land is not produced simply by uniform upheaval, but by a +predominance of elevatory over subsiding movements. Where the ocean is extremely +deep it is because the sinking of the bottom has been in excess, in spite of +interruptions by upheaval. + +Yet persistent as may be the leading features of land and sea on the globe, they +are not immutable. Some of the finest mud is doubtless carried to indefinite +distances from the coast by marine currents, and we are taught by deep-sea +dredgings that in clear water at depths equalling the height of the Alps organic +beings may flourish, and their spoils slowly accumulate on the bottom. We also +occasionally obtain evidence that submarine volcanoes are pouring out ashes and +streams of lava in mid-ocean as well as on land (see Principles volume 2 page +64), and that wherever mountains like Etna, Vesuvius, and the Canary Islands are +now the site of eruptions, there are signs of accompanying upheaval, by which +beds of ashes full of recent marine shells have been uplifted many hundred feet. +We need not be surprised, therefore, if we learn from geology that the +continents and oceans were not always placed where they now are, although the +imagination may well be overpowered when it endeavours to contemplate the +quantity of time required for such revolutions. + +We shall have gained a great step if we can approximate to the number of +millions of years in which the average aqueous denudation going on upon the land +would convey seaward a quantity of matter equal to the average volume of our +continents, and this might give us a gauge of the minimum of volcanic force +necessary to counteract such levelling power of running water; but to discover a +relation between these great agencies and the rate at which species of organic +beings vary, is at present wholly beyond the reach of our computation, though +perhaps it may not prove eventually to transcend the powers of man. + + +CHAPTER VIII. + +CHRONOLOGICAL CLASSIFICATION OF ROCKS. + +Aqueous, Plutonic, volcanic, and metamorphic Rocks considered chronologically. +Terms Primary, Secondary, and Tertiary; Palaeozoic, Mesozoic, and Cainozoic +explained. +On the different Ages of the aqueous Rocks. +Three principal Tests of relative Age: Superposition, Mineral Character, and +Fossils. +Change of Mineral Character and Fossils in the same continuous Formation. +Proofs that distinct Species of Animals and Plants have lived at successive +Periods. +Distinct Provinces of indigenous Species. +Great Extent of single Provinces. +Similar Laws prevailed at successive Geological Periods. +Relative Importance of mineral and palaeontological Characters. +Test of Age by included Fragments. +Frequent Absence of Strata of intervening Periods. +Tabular Views of fossiliferous Strata. + +CHRONOLOGY OF ROCKS. + +In the first chapter it was stated that the four great classes of rocks, the +aqueous, the volcanic, the Plutonic, and the metamorphic, would each be +considered not only in reference to their mineral characters, and mode of +origin, but also to their relative age. In regard to the aqueous rocks, we have +already seen that they are stratified, that some are calcareous, others +argillaceous or siliceous, some made up of sand, others of pebbles; that some +contain fresh-water, others marine fossils, and so forth; but the student has +still to learn which rocks, exhibiting some or all of these characters, have +originated at one period of the earth's history, and which at another. + +To determine this point in reference to the fossiliferous formations is more +easy than in any other class, and it is therefore the most convenient and +natural method to begin by establishing a chronology for these strata, and then +to refer as far as possible to the same divisions, the several groups of +Plutonic, volcanic, and metamorphic rocks. Such a system of classification is +not only recommended by its greater clearness and facility of application, but +is also best fitted to strike the imagination by bringing into one view the +contemporaneous revolutions of the inorganic and organic creations of former +times. For the sedimentary formations are most readily distinguished by the +different species of fossil animals and plants which they inclose, and of which +one assemblage after another has flourished and then disappeared from the earth +in succession. + +In the present work, therefore, the four great classes of rocks, the aqueous, +Plutonic, volcanic, and metamorphic, will form four parallel, or nearly +parallel, columns in one chronological table. They will be considered as four +sets of monuments relating to four contemporaneous, or nearly contemporaneous, +series of events. I shall endeavour, in a subsequent chapter on the Plutonic +rocks, to explain the manner in which certain masses belonging to each of the +four classes of rocks may have originated simultaneously at every geological +period, and how the earth's crust may have been continually remodelled, above +and below, by aqueous and igneous causes, from times indefinitely remote. In the +same manner as aqueous and fossiliferous strata are now formed in certain seas +or lakes, while in other places volcanic rocks break out at the surface, and are +connected with reservoirs of melted matter at vast depths in the bowels of the +earth, so, at every era of the past, fossiliferous deposits and superficial +igneous rocks were in progress contemporaneously with others of subterranean and +Plutonic origin, and some sedimentary strata were exposed to heat, and made to +assume a crystalline or metamorphic structure. + +It can by no means be taken for granted, that during all these changes the solid +crust of the earth has been increasing in thickness. It has been shown, that so +far as aqueous action is concerned, the gain by fresh deposits, and the loss by +denudation, must at each period have been equal (see Chapter 6); and in like +manner, in the inferior portion of the earth's crust, the acquisition of new +crystalline rocks, at each successive era, may merely have counterbalanced the +loss sustained by the melting of materials previously consolidated. As to the +relative antiquity of the crystalline foundations of the earth's crust, when +compared to the fossiliferous and volcanic rocks which they support, I have +already stated, in the first chapter, that to pronounce an opinion on this +matter is as difficult as at once to decide which of the two, whether the +foundations or superstructure of an ancient city built on wooden piles may be +the oldest. We have seen that, to answer this question, we must first be +prepared to say whether the work of decay and restoration had gone on most +rapidly above or below; whether the average duration of the piles has exceeded +that of the buildings, or the contrary. So also in regard to the relative age of +the superior and inferior portions of the earth's crust; we can not hazard even +a conjecture on this point, until we know whether, upon an average, the power of +water above, or that of heat below, is most efficacious in giving new forms to +solid matter. + +The early geologists gave to all the crystalline and non-fossiliferous rocks the +name of Primitive or Primary, under the idea that they were formed anterior to +the appearance of life upon the earth, while the aqueous or fossiliferous strata +were termed Secondary, and alluviums or other superficial deposits, Tertiary. +The meaning of these terms, has, however, been gradually modified with advancing +knowledge, and they are now used to designate three great chronological +divisions under which all geological formations can be classed, each of them +being characterised by the presence of distinctive groups of organic remains +rather than by any mechanical peculiarities of the strata themselves. If, +therefore, we retain the term "primary," it must not be held to designate a set +of crystalline rocks some of which have been proved to be even of Tertiary age, +but must be applied to all rocks older than the secondary formations. Some +geologists, to avoid misapprehension, have introduced the term Palaeozoic for +primary, from palaion, "ancient," and zoon, "an organic being," still retaining +the terms secondary and tertiary; Mr. Phillips, for the sake of uniformity, has +proposed Mesozoic, for secondary, from mesos, "middle," etc.; and Cainozoic, for +tertiary, from kainos, "recent," etc.; but the terms primary, secondary, and +tertiary have the claim of priority in their favour, and are of corresponding +value. + +It may perhaps be suggested that some metamorphic strata, and some granites, may +be anterior in date to the oldest of the primary fossiliferous rocks. This +opinion is doubtless true, and will be discussed in future chapters; but I may +here observe, that when we arrange the four classes of rocks in four parallel +columns in one table of chronology, it is by no means assumed that these columns +are all of equal length; one may begin at an earlier period than the rest, and +another may come down to a later point of time, and we may not be yet acquainted +with the most ancient of the primary fossiliferous beds, or with the newest of +the hypogene. + +For reasons already stated, I proceed first to treat of the aqueous or +fossiliferous formations considered in chronological order or in relation to the +different periods at which they have been deposited. + +There are three principal tests by which we determine the age of a given set of +strata; first, superposition; secondly, mineral character; and, thirdly, organic +remains. Some aid can occasionally be derived from a fourth kind of proof, +namely, the fact of one deposit including in it fragments of a pre-existing +rock, by which the relative ages of the two may, even in the absence of all +other evidence, be determined. + +SUPERPOSITION. + +The first and principal test of the age of one aqueous deposit, as compared to +another, is relative position. It has been already stated, that, where strata +are horizontal, the bed which lies uppermost is the newest of the whole, and +that which lies at the bottom the most ancient. So, of a series of sedimentary +formations, they are like volumes of history, in which each writer has recorded +the annals of his own times, and then laid down the book, with the last written +page uppermost, upon the volume in which the events of the era immediately +preceding were commemorated. In this manner a lofty pile of chronicles is at +length accumulated; and they are so arranged as to indicate, by their position +alone, the order in which the events recorded in them have occurred. + +In regard to the crust of the earth, however, there are some regions where, as +the student has already been informed, the beds have been disturbed, and +sometimes extensively thrown over and turned upside down. (See Chapter 5.) But +an experienced geologist can rarely be deceived by these exceptional cases. When +he finds that the strata are fractured, curved, inclined, or vertical, he knows +that the original order of superposition must be doubtful, and he then +endeavours to find sections in some neighbouring district where the strata are +horizontal, or only slightly inclined. Here, the true order of sequence of the +entire series of deposits being ascertained, a key is furnished for settling the +chronology of those strata where the displacement is extreme. + +MINERAL CHARACTER. + +The same rocks may often be observed to retain for miles, or even hundreds of +miles, the same mineral peculiarities, if we follow the planes of +stratification, or trace the beds, if they be undisturbed, in a horizontal +direction. But if we pursue them vertically, or in any direction transverse to +the planes of stratification, this uniformity ceases almost immediately. In that +case we can scarcely ever penetrate a stratified mass for a few hundred yards +without beholding a succession of extremely dissimilar rocks, some of fine, +others of coarse grain, some of mechanical, others of chemical origin; some +calcareous, others argillaceous, and others siliceous. These phenomena lead to +the conclusion that rivers and currents have dispersed the same sediment over +wide areas at one period, but at successive periods have been charged, in the +same region, with very different kinds of matter. The first observers were so +astonished at the vast spaces over which they were able to follow the same +homogeneous rocks in a horizontal direction, that they came hastily to the +opinion, that the whole globe had been environed by a succession of distinct +aqueous formations, disposed round the nucleus of the planet, like the +concentric coats of an onion. But, although, in fact, some formations may be +continuous over districts as large as half of Europe, or even more, yet most of +them either terminate wholly within narrower limits, or soon change their +lithological character. Sometimes they thin out gradually, as if the supply of +sediment had failed in that direction, or they come abruptly to an end, as if we +had arrived at the borders of the ancient sea or lake which served as their +receptacle. It no less frequently happens that they vary in mineral aspect and +composition, as we pursue them horizontally. For example, we trace a limestone +for a hundred miles, until it becomes more arenaceous, and finally passes into +sand, or sandstone. We may then follow this sandstone, already proved by its +continuity to be of the same age, throughout another district a hundred miles or +more in length. + +ORGANIC REMAINS. + +This character must be used as a criterion of the age of a formation, or of the +contemporaneous origin of two deposits in distant places, under very much the +same restrictions as the test of mineral composition. + +First, the same fossils may be traced over wide regions, if we examine strata in +the direction of their planes, although by no means for indefinite distances. +Secondly, while the same fossils prevail in a particular set of strata for +hundreds of miles in a horizontal direction, we seldom meet with the same +remains for many fathoms, and very rarely for several hundred yards, in a +vertical line, or a line transverse to the strata. This fact has now been +verified in almost all parts of the globe, and has led to a conviction that at +successive periods of the past, the same area of land and water has been +inhabited by species of animals and plants even more distinct than those which +now people the antipodes, or which now co-exist in the arctic, temperate, and +tropical zones. It appears that from the remotest periods there has been ever a +coming in of new organic forms, and an extinction of those which pre-existed on +the earth; some species having endured for a longer, others for a shorter, time; +while none have ever reappeared after once dying out. The law which has governed +the succession of species, whether we adopt or reject the theory of +transmutation, seems to be expressed in the verse of the poet:-- + +Natura il fece, e poi ruppe la stampa. Ariosto. + +Nature made him, and then broke the die. + +And this circumstance it is, which confers on fossils their highest value as +chronological tests, giving to each of them, in the eyes of the geologist, that +authority which belongs to contemporary medals in history. + +The same can not be said of each peculiar variety of rock; for some of these, as +red marl and red sandstone, for example, may occur at once at the top, bottom, +and middle of the entire sedimentary series; exhibiting in each position so +perfect an identity of mineral aspect as to be undistinguishable. Such exact +repetitions, however, of the same mixtures of sediment have not often been +produced, at distant periods, in precisely the same parts of the globe; and even +where this has happened, we are seldom in any danger of confounding together the +monuments of remote eras, when we have studied their imbedded fossils and their +relative position. + +ZOOLOGICAL PROVINCES. + +It was remarked that the same species of organic remains can not be traced +horizontally, or in the direction of the planes of stratifications for +indefinite distances. This might have been expected from analogy; for when we +inquire into the present distribution of living beings, we find that the +habitable surface of the sea and land may be divided into a considerable number +of distinct provinces, each peopled by a peculiar assemblage of animals and +plants. In the "Principles of Geology," I have endeavoured to point out the +extent and probable origin of these separate divisions; and it was shown that +climate is only one of many causes on which they depend, and that difference of +longitude as well as latitude is generally accompanied by a dissimilarity of +indigenous species. + +As different seas, therefore, and lakes are inhabited, at the same period, by +different aquatic animals and plants, and as the lands adjoining these may be +peopled by distinct terrestrial species, it follows that distinct fossils will +be imbedded in contemporaneous deposits. If it were otherwise-- if the same +species abounded in every climate, or in every part of the globe where, so far +as we can discover, a corresponding temperature and other conditions favourable +to their existence are found-- the identification of mineral masses of the same +age, by means of their included organic contents, would be a matter of still +greater certainty. + +Nevertheless, the extent of some single zoological provinces, especially those +of marine animals, is very great; and our geological researches have proved that +the same laws prevailed at remote periods; for the fossils are often identical +throughout wide spaces, and in detached deposits, consisting of rocks varying +entirely in their mineral nature. + +The doctrine here laid down will be more readily understood, if we reflect on +what is now going on in the Mediterranean. That entire sea may be considered as +one zoological province; for although certain species of testacea and zoophytes +may be very local, and each region has probably some species peculiar to it, +still a considerable number are common to the whole Mediterranean. If, +therefore, at some future period, the bed of this inland sea should be converted +into land, the geologist might be enabled, by reference to organic remains, to +prove the contemporaneous origin of various mineral masses scattered over a +space equal in area to half of Europe. + +Deposits, for example, are well known to be now in progress in this sea in the +deltas of the Po, Rhone, Nile, and other rivers, which differ as greatly from +each other in the nature of their sediment as does the composition of the +mountains which their drain. There are also other quarters of the Mediterranean, +as off the coast of Campania, or near the base of Etna, in Sicily, or in the +Grecian Archipelago, where another class of rocks is now forming; where showers +of volcanic ashes occasionally fall into the sea, and streams of lava overflow +its bottom; and where, in the intervals between volcanic eruptions, beds of sand +and clay are frequently derived from the waste of cliffs, or the turbid waters +of rivers. Limestones, moreover, such as the Italian travertins, are here and +there precipitated from the waters of mineral springs, some of which rise up +from the bottom of the sea. In all these detached formations, so diversified in +their lithological characters, the remains of the same shells, corals, +crustacea, and fish are becoming inclosed; or, at least, a sufficient number +must be common to the different localities to enable the zoologist to refer them +all to one contemporaneous assemblage of species. + +There are, however, certain combinations of geographical circumstances which +cause distinct provinces of animals and plants to be separated from each other +by very narrow limits; and hence it must happen that strata will be sometimes +formed in contiguous regions, differing widely both in mineral contents and +organic remains. Thus, for example, the testacea, zoophytes, and fish of the Red +Sea are, as a group, extremely distinct from those inhabiting the adjoining +parts of the Mediterranean, although the two seas are separated only by the +narrow isthmus of Suez. Calcareous formations have accumulated on a great scale +in the Red Sea in modern times, and fossil shells of existing species are well +preserved therein; and we know that at the mouth of the Nile large deposits of +mud are amassed, including the remains of Mediterranean species. It follows, +therefore, that if at some future period the bed of the Red Sea should be laid +dry, the geologist might experience great difficulties in endeavouring to +ascertain the relative age of these formations, which, although dissimilar both +in organic and mineral characters, were of synchronous origin. + +But, on the other hand, we must not forget that the north-western shores of the +Arabian Gulf, the plains of Egypt, and the Isthmus of Suez, are all parts of one +province of TERRESTRIAL species. Small streams, therefore, occasional land- +floods, and those winds which drift clouds of sand along the deserts, might +carry down into the Red Sea the same shells of fluviatile and land testacea +which the Nile is sweeping into its delta, together with some remains of +terrestrial plants and the bones of quadrupeds, whereby the groups of strata +before alluded to might, notwithstanding the discrepancy of their mineral +composition and MARINE organic fossils, be shown to have belonged to the same +epoch. + +Yet, while rivers may thus carry down the same fluviatile and terrestrial spoils +into two or more seas inhabited by different marine species, it will much more +frequently happen that the coexistence of terrestrial species of distinct +zoological and botanical provinces will be proved by the identity of the marine +beings which inhabited the intervening space. Thus, for example, the land +quadrupeds and shells of the valley of the Mississippi, of central America, and +of the West India islands differ very considerably, yet their remains are all +washed down by rivers flowing from these three zoological provinces into the +Gulf of Mexico. + +In some parts of the globe, at the present period, the line of demarkation +between distinct provinces of animals and plants is not very strongly marked, +especially where the change is determined by temperature, as it is in seas +extending from the temperate to the tropical zone, or from the temperate to the +arctic regions. Here a gradual passage takes place from one set of species to +another. In like manner the geologist, in studying particular formations of +remote periods, has sometimes been able to trace the gradation from one ancient +province to another, by observing carefully the fossils of all the intermediate +places. His success in thus acquiring a knowledge of the zoological or botanical +geography of very distant eras has been mainly owing to this circumstance, that +the mineral character has no tendency to be affected by climate. A large river +may convey yellow or red mud into some part of the ocean, where it may be +dispersed by a current over an area several hundred leagues in length, so as to +pass from the tropics into the temperate zone. If the bottom of the sea be +afterwards upraised, the organic remains imbedded in such yellow or red strata +may indicate the different animals or plants which once inhabited at the same +time the temperate and equatorial regions. + +It may be true, as a general rule, that groups of the same species of animals +and plants may extend over wider areas than deposits of homogeneous composition; +and if so, palaeontological characters will be of more importance in geological +classification than the test of mineral composition; but it is idle to discuss +the relative value of these tests, as the aid of both is indispensable, and it +fortunately happens, that where the one criterion fails, we can often avail +ourselves of the other. + +TEST BY INCLUDED FRAGMENTS OF OLDER ROCKS. + +It was stated, that proof may sometimes be obtained of the relative date of two +formations by fragments of an older rock being included in a newer one. This +evidence may sometimes be of great use, where a geologist is at a loss to +determine the relative age of two formations from want of clear sections +exhibiting their true order of position, or because the strata of each group are +vertical. In such cases we sometimes discover that the more modern rock has been +in part derived from the degradation of the older. Thus, for example, we may +find chalk in one part of a country, and in another strata of clay, sand, and +pebbles. If some of these pebbles consist of that peculiar flint, of which +layers more or less continuous are characteristic of the chalk, and which +include fossil shells, sponges, and foraminifera of cretaceous species, we may +confidently infer that the chalk was the oldest of the two formations. + +CHRONOLOGICAL GROUPS. + +The number of groups into which the fossiliferous strata may be separated are +more or less numerous, according to the views of classification which different +geologists entertain; but when we have adopted a certain system of arrangement, +we immediately find that a few only of the entire series of groups occur one +upon the other in any single section or district. + +(FIGURE 84. Seven fossiliferous groups.) + +The thinning out of individual strata was before described (Chapter 2). But let +the diagram (Figure 84) represent seven fossiliferous groups, instead of as many +strata. It will then be seen that in the middle all the superimposed formations +are present; but in consequence of some of them thinning out, No. 2 and No. 5 +are absent at one extremity of the section, and No. 4 at the other. + +(FIGURE 85. Section South of Bristol (A.C. Ramsay.) +Dundry Hill. +Length of section 4 miles. +a-b. Level of the sea. +1. Inferior Oolite. +2. Lias. +3. New Red Sandstone. +4. Dolomitic or magnesian conglomerate. +5. Upper coal-measures (shales, etc.) +6. Pennant rock (sandstone.) +7. Lower coal-measures (shales, etc.) +8. Carboniferous or mountain limestone. +9. Old Red Sandstone.) + +In another diagram (Figure 85), a real section of the geological formations in +the neighbourhood of Bristol and the Mendip Hills is presented to the reader, as +laid down on a true scale by Professor Ramsay, where the newer groups 1, 2, 3, 4 +rest unconformably on the formations 5, 6, 7 and 8. At the southern end of the +line of section we meet with the beds No. 3 (the New Red Sandstone) resting +immediately on Nos. 7 and 8, while farther north as at Dundry Hill in +Somersetshire, we behold eight groups superimposed one upon the other, +comprising all the strata from the inferior Oolite, No. 1, to the coal and +carboniferous limestone. The limited horizontal extension of the groups 1 and 2 +is owing to denudation, as these formations end abruptly, and have left outlying +patches to attest the fact of their having originally covered a much wider area. + +In order, therefore, to establish a chronological succession of fossiliferous +groups, a geologist must begin with a single section in which several sets of +strata lie one upon the other. He must then trace these formations, by attention +to their mineral character and fossils, continuously, as far as possible, from +the starting-point. As often as he meets with new groups, he must ascertain by +superposition their age relatively to those first examined, and thus learn how +to intercalate them in a tabular arrangement of the whole. + +By this means the German, French, and English geologists have determined the +succession of strata throughout a great part of Europe, and have adopted pretty +generally the following groups, almost all of which have their representatives +in the British Islands. + +ABRIDGED GENERAL TABLE OF FOSSILIFEROUS STRATA. + +1. RECENT.-- POST-TERTIARY.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +2. POST-PLIOCENE.-- POST-TERTIARY.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +3. NEWER-PLIOCENE.-- PLIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +4. OLDER PLIOCENE.-- PLIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +5. UPPER MIOCENE.-- MIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +6. LOWER MIOCENE.-- MIOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +7. UPPER EOCENE.-- EOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +8. MIDDLE EOCENE.-- EOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +9. LOWER EOCENE.-- EOCENE.-- TERTIARY OR CAINOZOIC.-- NEOZOIC. + +10. MAESTRICHT BEDS.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +11. WHITE CHALK.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +12. CHLORITIC SERIES.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +13. GAULT.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +14. NEOCOMIAN.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +15. WEALDEN.-- CRETACEOUS.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +16. PURBECK BEDS.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +17. PORTLAND STONE.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +18. KIMMERIDGE CLAY.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +19. CORAL RAG.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +20. OXFORD CLAY.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +21. GREAT or BATH OOLITE.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +22. INFERIOR OOLITE.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +23. LIAS.-- JURASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +24. UPPER TRIAS.-- TRIASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +25. MIDDLE TRIAS.-- TRIASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +26. LOWER TRIAS.-- TRIASSIC.-- SECONDARY OR MESOZOIC.-- NEOZOIC. + +27. PERMIAN.-- PERMIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +28. COAL-MEASURES.-- CARBONIFEROUS.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +29. CARBONIFEROUS LIMESTONE.-- CARBONIFEROUS.-- -- PRIMARY OR PALAEOZOIC.-- +PALAEOZOIC. + +30. UPPER DEVONIAN.-- DEVONIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +31. MIDDLE DEVONIAN.-- DEVONIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +32. LOWER DEVONIAN.-- DEVONIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +33. UPPER SILURIAN.-- SILURIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +34. LOWER SILURIAN.-- SILURIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +35. UPPER CAMBRIAN.-- CAMBRIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +36. LOWER CAMBRIAN.-- CAMBRIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +37. UPPER LAURENTIAN.-- LAURENTIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +38. LOWER LAURENTIAN.-- LAURENTIAN.-- PRIMARY OR PALAEOZOIC.-- PALAEOZOIC. + +TABULAR VIEW OF THE FOSSILIFEROUS STRATA, + +SHOWING THE ORDER OF SUPERPOSITION OR CHRONOLOGICAL SUCCESSION OF THE PRINCIPAL +GROUPS DESCRIBED IN THIS WORK (CITING EXAMPLES). + +POST-TERTIARY. + +1. RECENT. Shells and mammalia, all of living species. + +BRITISH. +Clyde marine strata, with canoes (Chapter 10.) + +FOREIGN. +Danish kitchen middens (Chapter 10.) +Lacustrine mud, with remains of Swiss lake-dwellings (Chapter 10.) +Marine strata inclosing Temple of Serapis, at Puzzuoli (Chapter 10.) + +2. POST-PLIOCENE. Shells, recent mammalia in part extinct. + +BRITISH. +Loam of Brixham cave, with flint implements and bones of extinct and living +quadrupeds. (Chapter 10.) +Drift near Salisbury, with bones of mammoth, Spermophilus, and stone implements. +(Chapter 10.) +Glacial drift of Scotland, with marine shells and remains of mammoth. (Chapter +11.) +Erratics of Pagham and Selsey Bill. (Chapter 11.) +Glacial drift of Wales, with marine fossil shells, about 1400 feet high, on Moel +Tryfaen. (Chapter 11.) + +FOREIGN. +Dordogne caves of the reindeer period. (Chapter 10.) +Older valley-gravels of Amiens, with flint implements and bones of extinct +mammalia. (Chapter 10.) +Loess of Rhine. (Chapter 10.) +Ancient Nile-mud forming river-terraces. (Chapter 10.) +Loam and breccia of Liege caverns, with human remains. (Chapter 10.) +Australian cave breccias, with bones of extinct marsupials. (Chapter 10.) +Glacial drift of Northern Europe. (Chapters 11 and 12.) + +TERTIARY OR CAINOZOIC. + +PLIOCENE. + +3. NEWER PLIOCENE. The shells almost all of living species. + +BRITISH. +Bridlington beds, marine Arctic fauna. (Chapter 13.) +Glacial boulder formation of Norfolk cliffs. (Chapter 13.) +Forest-bed of Norfolk cliffs, with bones of Elephas meridionalis, etc. (Chapter +13.) +Chillesford and Aldeby beds, with marine shells, chiefly Arctic. (Chapter 13.) +Norwich Crag. (Chapter 13.) + +FOREIGN. +Eastern base of Mount Etna, with marine shells. (Chapter 13.) +Sicilian calcareous and tufaceous strata. (Chapter 13.) +Lacustrine strata of Upper Val d'Arno. (Chapter 13.) +Madeira leaf-bed and land-shells. (Chapter 29.) + +4. OLDER PLIOCENE. Extinct species of shells forming a large minority. + +BRITISH. +Red crag of Suffolk, marine shells, some of northern forms. (Chapter 13.) +White or coralline crag of Suffolk. (Chapter 13.) + +FOREIGN. +Antwerp crag. (Chapter 13.) +Subapennine marls and sands. (Chapter 13.) + +MIOCENE. + +5. UPPER MIOCENE. Majority of the shells extinct. + +BRITISH. +Wanting. + +FOREIGN. +faluns of Touraine (Chapter 14.) +faluns, proper, of Bordeaux. (Chapter 14.) +Fresh-water strata of Gers. (Chapter 14.) +Swiss Oeningen beds, rich in plants and insects. (Chapter 14.) +Marine Molasse, Switzerland. (Chapter 14.) +Bolderberg beds of Belgium. (Chapter 14.) +Vienna basin. (Chapter 14.) +Beds of the Superga, near Turin. (Chapter 14.) +Deposit at Pikerme, near Athens. (Chapter 14.) +Strata of the Siwalik hills, India. (Chapter 14.) +Marine strata of the Atlantic border in the United States. (Chapter 14.) +Volcanic tuff and limestone of Madeira, the Canaries, and the Azores. (Chapter +30.) + +6. LOWER MIOCENE. Nearly all the shells extinct. + +BRITISH. +Hempstead beds, marine and fresh-water strata. (Chapter 15.) +Lignites and clays of Bovey Tracey. (Chapter 15.) +Isle of Mull leaf-bed, volcanic tuff. (Chapter 15.) + +FOREIGN. +Calcaire de la Beauce, etc. (Chapter 15.) +Gres de Fontainebleau. (Chapter 15.) +Lacustrine strata of the Limagne d'Auvergne, and the Cantal. (Chapter 15.) +Mayence basin. (Chapter 15.) +Radaboj beds of Croatia. (Chapter 15.) +Brown coal of Germany. (Chapter 15.) +Lower Molasse of Switzerland, fresh-water and brackish. (Chapter 15.) +Rupelmonde, Kleynspawen, and Tongrian beds of Belgium. (Chapter 15.) +Nebraska beds, United States. (Chapter 15.) +Lower Miocene beds of Italy. (Chapter 15.) +Miocene flora of North Greenland. (Chapter 15.) + +7. UPPER EOCENE. + +BRITISH. +Bembridge fluvio-marine strata. (Chapter 16.) +Osborne or St. Helen's series. (Chapter 16.) +Headon series, with marine and fresh-water shells. (Chapter 16.) +Barton sands and clays (Chapter 16.) + +FOREIGN. +Gypsum of Montmartre, fresh-water with Palaeotherium. (Chapter 16.) +Calcaire silicieux, or Travertin inferieur. (Chapter 16.) +Gres de Beauchamp, or Sables moyens. (Chapter 16.) + +8. MIDDLE EOCENE. + +BRITISH. +Bracklesham beds and Bagshot sands. (Chapter 16.) +White clays of Alum Bay and Bournemouth. (Chapter 16.) + +FOREIGN. +Calcaire grossier, miliolitic limestone. (Chapter 16.) +Soissonnais sands, or Lits coquilliers, with Nummulites planulata. (Chapter 16.) +Claiborne beds of the United States, with Orbitoides and Zeuglodon. (Chapter +16.) + +9. LOWER EOCENE. + +Nummulitic formation of Europe, Asia, etc. (Chapter 16.) + +BRITISH. +London Clay proper. (Chapter 16.) +Woolwich and Reading series, fluvio-marine. (Chapter 16.) +Thanet sands. (Chapter 16.) + +FOREIGN. +Argile de Londres, near Dunkirk. (Chapter 16.) +Argile plastique. (Chapter 16.) +Sables de Bracheux. (Chapter 16.) + +SECONDARY OR MESOZOIC. + +CRETACEOUS. + +10. UPPER CRETACEOUS. + +BRITISH. +Upper white chalk, with flints. (Chapter 17.) +Lower white chalk, without flints. (Chapter 17.) +Chalk marl. (Chapter 17.) +Chloritic series (or Upper Greensand), fire-stone of Surrey. (Chapter 17.) +Gault. (Chapter 17.) +Blackdown beds. (Chapter 17.) + +FOREIGN. +Maestricht beds and Faxoe chalk. (Chapter 17.) +Pisolitic limestone of France. (Chapter 17.) +White chalk of France, Sweden, and Russia. (Chapter 17.) +Planer-kalk of Saxony. (Chapter 17.) +Sands and clays of Aix-la-Chapelle. (Chapter 17.) +Hippurite limestone of South of France. (Chapter 17.) +New Jersey, U.S., sands and marls. (Chapter 17.) + +11. LOWER CRETACEOUS OR NEOCOMIAN. + +BRITISH. +Sands of Folkestone, Sandgate, and Hythe. (Chapter 18.) +Atherfield clay, with Perna mulleti. (Chapter 18.) +Punfield marine beds, with Vicarya lujana. (Chapter 18.) +Speeton clay of Flamborough Head and Tealby. (Chapter 18.) +Weald clay of Surrey, Kent, and Sussex, fresh-water, with Cypris. (Chapter 18.) +Hastings sands. + +FOREIGN. +Neocomian of Neufchatel, and Hils conglomerate of North Germany. (Chapter 18.) +Wealden beds of Hanover. (Chapter 18.) + +OOLITE. + +12. UPPER OOLITE. + +BRITISH. +Upper Purbeck beds, fresh-water. (Chapter 19.) +Middle Purbeck, with numerous marsupial quadrupeds, etc. (Chapter 19.) +Lower Purbeck, fresh-water, with intercalated dirt-bed. (Chapter 19.) +Portland stone and sand. (Chapter 19.) +Kimmeridge clay. (Chapter 19.) + +FOREIGN. +Marnes a gryphees virgules of Argonne. (Chapter 19.) +Lithographic-stone of Solenhofen, with Archaeopteryx. (Chapter 19.) + +13. MIDDLE OOLITE. + +BRITISH. +Coral rag of Berkshire, Wilts, and Yorkshire. (Chapter 19.) +Oxford clay, with belemnites and Ammonite. (Chapter 19.) +Kelloway rock of Wilts and Yorkshire. (Chapter 19.) + +FOREIGN. +Nerinaean limestone of the Jura. + +14. LOWER OOLITE. + +BRITISH. +Cornbrash and forest marble. (Chapter 19.) +Great or Bath oolite of Bradford. (Chapter 19.) +Stonesfield slate, with marsupials and Araucaria. (Chapter 19.) +Fuller's earth of Bath. (Chapter 19.) +Inferior oolite. (Chapter 19.) + +LIAS. + +15. LIAS. + +Upper Lias, argillaceous, with Ammonites striatulus. (Chapter 20.) +Shale and limestone, with Ammonites bifrons. (Chapter 20.) +Middle Lias or Marlstone series, with zones containing characteristic Ammonites. +(Chapter 20.) +Lower Lias, also with zones characterised by peculiar Ammonites. (Chapter 20.) + +TRIAS. + +16. UPPER TRIAS. + +BRITISH. +Rhaetic, Penarth or Avicula contorta beds (beds of passage). (Chapter 21.) +Keuper or Upper New Red sandstone, etc. (Chapter 21.) +Red shales of Cheshire and Lancashire, with rock-salt. (Chapter 21.) +Dolomite conglomerate of Bristol (Chapter 21.) + +FOREIGN. +Keuper beds of Germany. (Chapter 21.) +St. Cassian or Hallstadt beds, with rich marine fauna. (Chapter 21.) +Coal-field of Richmond, Virginia. (Chapter 21.) +Chatham coal-field, North Carolina. (Chapter 21.) + +17. MIDDLE TRIAS. + +BRITISH. +Wanting. + +FOREIGN. +Muschelkalk of Germany. (Chapter 21.) + +18. LOWER TRIAS. + +BRITISH. +Bunter or Lower New Red sandstone of Lancashire and Cheshire. (Chapter 21.) + +FOREIGN. +Bunter-sandstein of Germany. (Chapter 21.) +Red sandstone of Connecticut Valley, with footprints of birds and reptiles. +(Chapter 21.) + +PRIMARY OR PALAEOZOIC. + +PERMIAN. + +19. PERMIAN. + +BRITISH. +Upper Permian of St. Bees' Head, Cumberland. (Chapter 22.) +Middle Permian, magnesian limestone, and marl-slate of Durham and Yorkshire, +with Protosaurus. (Chapter 22.) +Lower Permian sandstones and breccias of Penrith and Dumfriesshire, +intercalated. (Chapter 22.) + +FOREIGN. +Dark-coloured shales of Thuringia. (Chapter 22.) +Zechstein or Dolomitic limestone. (Chapter 22.) +Mergel-schiefer or Kupfer-schiefer. (Chapter 22.) +Rothliegendes of Thuringia, with Psaronius. (Chapter 22.) +Magnesian limestones, etc., of Russia. (Chapter 22.) + +CARBONIFEROUS. + +20. UPPER CARBONIFEROUS. + +BRITISH. +Coal-measures of South Wales, with underclays inclosing Stigmaria. (Chapter 23.) +Coal-measures of north and central England. (Chapter 23.) +Millstone grit. (Chapter 23.) +Yoredale series of Yorkshire. (Chapter 23.) +Coal-field of Kilkenny with Labyrinthodont. (Chapter 23.) + +FOREIGN. +Coal-field of Saarbruck, with Archegosaurus. (Chapter 23.) +Carboniferous strata of South Joggins, Nova Scotia. (Chapter 23.) +Pennsylvania coal-field. (Chapter 23.) + +21. LOWER CARBONIFEROUS. + +BRITISH. +Mountain limestone of Wales and South of England. (Chapter 24.) +Same in Ireland. (Chapter 24.) +Carboniferous limestone of Scotland alternating with coal-bearing sandstones. +(Chapter 23.) +Erect trees in volcanic ash in the Island of Arran. (Chapter 30.) + +FOREIGN. +Mountain limestone of Belgium. (Chapter 24.) + +DEVONIAN OR OLD RED SANDSTONE. + +22. UPPER DEVONIAN. + +BRITISH. +Yellow sandstone of Dura Den, with Holoptychius, etc. (Chapter 25.); and of +Ireland with Anodon Jukesii. (Chapter 25.) +Sandstones of Forfarshire and Perthshire, with Holoptychius, etc. (Chapter 25.) +Pilton group of North Devon. (Chapter 25.) +Petherwyn group of Cornwall, with Clymenia and Cypridina. (Chapter 25.) + +FOREIGN. +Clymenien-kalk and Cypridinen-schiefer of Germany. (Chapter 25.) + +23. MIDDLE DEVONIAN. + +BRITISH. +Bituminous schists of Gamrie, Caithness, etc., with numerous fish. (Chapter 25.) +Ilfracombe beds with peculiar trilobites and corals. (Chapter 25.) +Limestones of Torquay, with broad-winged Spirifers. (Chapter 25.) + +FOREIGN. (Chapter 25.) +Eifel limestone, with underlying schists containing Calceola. (Chapter 25.) +Devonian strata of Russia. (Chapter 25.) + +24. LOWER DEVONIAN. + +BRITISH. +Arbroath paving-stones, with Cephalaspis and Pterygotus. (Chapter 25.) +Lower sandstones of Forfarshire, with Pterygotus. (Chapter 25.) +Sandstones and slates of the Foreland and Linton. (Chapter 25.) + +FOREIGN. +Oriskany sandstone of Western Canada and New York. (Chapter 25.) +Sandstones of Gaspe, with Cephalaspis. (Chapter 25.) + +SILURIAN. + +25. UPPER SILURIAN. + +BRITISH. +Upper Ludlow formation, Downton sandstone, with bone-bed. (Chapter 26.) +Lower Ludlow formation, with oldest known fish remains. (Chapter 26.) +Wenlock limestone and shale. (Chapter 26.) +Woolhope limestone and grit. (Chapter 26.) +Tarannon shales. (Chapter 26.) +Beds of passage between Upper and Lower Silurian: +Upper Llandovery, or May-hill sandstone, with Pentamerus oblongus, etc. (Chapter +26.) +Lower Llandovery slates. (Chapter 26.) + +FOREIGN. +Niagara limestone, with Calymene, Homalonotus, etc. (Chapter 26.) +Clinton group of America, with Pentamerus oblongus, etc. (Chapter 26.) +Silurian strata of Russia, with Pentamerus. (Chapter 26.) + +26. LOWER SILURIAN. + +BRITISH. +Bala and Caradoc beds. (Chapter 26.) +Llandeilo flags. (Chapter 26.) +Arenig or Stiper-stones group (Lower Llandeilo of Murchison.) (Chapter 26.) + +FOREIGN. +Ungulite or Obolus grit of Russia. (Chapter 26.) +Trenton limestone, and other Lower Silurian groups of North America. (Chapter +26.) +Lower Silurian of Sweden. (Chapter 26.) + +CAMBRIAN. + +27. UPPER CAMBRIAN. + +BRITISH. +Tremadoc slates. (Chapter 27.) +Lingula flags, with Lingula Davisii. (Chapter 27.) + +FOREIGN. +"Primordial" zone of Bohemia in part, with trilobites of the genera Paradoxides, +etc. (Chapter 27.) +Alum schists of Sweden and Norway. (Chapter 27.) +Potsdam sandstone, with Dikelocephalus and Obolella. (Chapter 27.) + +28. LOWER CAMBRIAN. + +BRITISH. +Menevian beds of Wales, with Paradoxides Davidis, etc. (Chapter 27.) +Longmynd group, comprising the Harlech grits and Llanberis slates. (Chapter 27.) + +FOREIGN. +Lower portion of Barrande's "Primordial" zone in Bohemia. (Chapter 27.) +Fucoid sandstones of Sweden. (Chapter 27.) +Huronian series of Canada? (Chapter 27.) + +LAURENTIAN. + +29. UPPER LAURENTIAN. + +BRITISH. +Fundamental gneiss of the Hebrides? (Chapter 27.) +Hypersthene rocks of Skye? (Chapter 27.) + +FOREIGN. +Labradorite series north of the river St. Lawrence in Canada. (Chapter 27.) +Adirondack mountains of New York. (Chapter 27.) + +30. LOWER LAURENTIAN. + +BRITISH. +Wanting? + +FOREIGN. +Beds of gneiss and quartzite, with interstratified limestones, in one of which, +1000 feet thick, occurs a foraminifer, Eozoon Canadense, the oldest known +fossil. (Chapter 27.) + + +CHAPTER IX. + +CLASSIFICATION OF TERTIARY FORMATIONS. + +Order of Succession of Sedimentary Formations. +Frequent Unconformability of Strata. +Imperfection of the Record. +Defectiveness of the Monuments greater in Proportion to their Antiquity. +Reasons for studying the newer Groups first. +Nomenclature of Formations. +Detached Tertiary Formations scattered over Europe. +Value of the Shell-bearing Mollusca in Classification. +Classification of Tertiary Strata. +Eocene, Miocene, and Pliocene Terms explained. + +By reference to the tables given at the end of the last chapter the reader will +see that when the fossiliferous rocks are arranged chronologically, we have +first to consider the Post-tertiary and then the Tertiary or Cainozoic +formations, and afterwards to pass on to those of older date. + +ORDER OF SUPERPOSITION. + +(FIGURE 86. Section through Primary (left), Secondary, Tertiary and Post- +tertiary (right) Strata. +1. Laurentian. +2. Cambrian. +3. Silurian. +4. Devonian. +5. Carboniferous. +6. Permian. +7. Triassic. +8. Jurassic. +9. Cretaceous. +10. Eocene. +11. Miocene. +12. Pliocene. +13. Post-pliocene. +14. Recent. +Sea.) + +The diagram (Figure 86.) will show the order of superposition of these deposits, +assuming them all to be visible in one continuous section. In nature, as before +hinted (Chapter 6), we have never an opportunity of seeing the whole of them so +displayed in a single region; first, because sedimentary deposition is confined, +during any one geological period, to limited areas; and secondly, because +strata, after they have been formed, are liable to be utterly annihilated over +wide areas by denudation. But wherever certain members of the series are +present, they overlie one another in the order indicated in the diagram, though +not always in the exact manner there represented, because some of them repose +occasionally in unconformable stratification on others. This mode of +superposition has been already explained (Chapters 5 and 7), where I pointed out +that the discordance which implies a considerable lapse of time between two +formations in juxtaposition is almost invariably accompanied by a great +dissimilarity in the species of organic remains. + +FREQUENT UNCONFORMABILITY OF STRATA. + +Where the widest gaps appear in the sequence of the fossil forms, as between the +Permian and Triassic rocks, or between the Cretaceous and Eocene, examples of +such unconformability are very frequent. But they are also met with in some part +or other of the world at the junction of almost all the other principal +formations, and sometimes the subordinate divisions of any one of the leading +groups may be found lying unconformably on another subordinate member of the +same-- the Upper, for example, on the Lower Silurian, or the superior division +of the Old Red Sandstone on a lower member of the same, and so forth. Instances +of such irregularities in the mode of succession of the strata are the more +intelligible the more we extend our survey of the fossiliferous formations, for +we are continually bringing to light deposits of intermediate date, which have +to be intercalated between those previously known, and which reveal to us a long +series of events, of which antecedently to such discoveries we had no knowledge. + +But while unconformability invariably bears testimony to a lapse of +unrepresented time, the conformability of two sets of strata in contact by no +means implies that the newer formation immediately succeeded the older one. It +simply implies that the ancient rocks were subjected to no movements of such a +nature as to tilt, bend, or break them before the more modern formation was +superimposed. It does not show that the earth's crust was motionless in the +region in question, for there may have been a gradual sinking or rising, +extending uniformly over a large surface, and yet, during such movement, the +stratified rocks may have retained their original horizontality of position. +There may have been a conversion of a wide area from sea into land and from land +into sea, and during these changes of level some strata may have been slowly +removed by aqueous action, and after this new strata may be superimposed, +differing perhaps in date by thousands of years or centuries, and yet resting +conformably on the older set. There may even be a blending of the materials +constituting the older deposit with those of the newer, so as to give rise to a +passage in the mineral character of the one rock into the other as if there had +been no break or interruption in the depositing process. + +IMPERFECTION OF THE RECORD. + +Although by the frequent discovery of new sets of intermediate strata the +transition from one type of organic remains to another is becoming less and less +abrupt, yet the entire series of records appears to the geologists now living +far more fragmentary and defective than it seemed to their predecessors half a +century ago. The earlier inquirers, as often as they encountered a break in the +regular sequence of formations, connected it theoretically with a sudden and +violent catastrophe, which had put an end to the regular course of events that +had been going on uninterruptedly for ages, annihilating at the same time all or +nearly all the organic beings which had previously flourished, after which, +order being re-established, a new series of events was initiated. In proportion +as our faith in these views grows weaker, and the phenomena of the organic or +inorganic world presented to us by geology seem explicable on the hypothesis of +gradual and insensible changes, varied only by occasional convulsions, on a +scale comparable to that witnessed in historical times; and in proportion as it +is thought possible that former fluctuations in the organic world may be due to +the indefinite modifiability of species without the necessity of assuming new +and independent acts of creation, the number and magnitude of the gaps which +still remain, or the extreme imperfection of the record, become more and more +striking, and what we possess of the ancient annals of the earth's history +appears as nothing when contrasted with that which has been lost. + +When we examine a large area such as Europe, the average as well as the extreme +height above the sea attained by the older formations is usually found to exceed +that reached by the more modern ones, the primary or palaeozoic rising higher +than the secondary, and these in their turn than the tertiary; while in +reference to the three divisions of the tertiary, the lowest or Eocene group +attains a higher summit-level than the Miocene, and these again a greater height +than the Pliocene formations. Lastly, the post-tertiary deposits, such, at +least, as are of marine origin, are most commonly restricted to much more +moderate elevations above the sea-level than the tertiary strata. + +It is also observed that strata, in proportion as they are of newer date, bear +the nearest resemblance in mineral character to those which are now in the +progress of formation in seas or lakes, the newest of all consisting principally +of soft mud or loose sand, in some places full of shells, corals, or other +organic bodies, animal or vegetable, in others wholly devoid of such remains. +The farther we recede from the present time, and the higher the antiquity of the +formations which we examine, the greater are the changes which the sedimentary +deposits have undergone. Time, as I have explained in Chapters 5, 6, and 7, has +multiplied the effects of condensation by pressure and cementation, and the +modification produced by heat, fracture, contortion, upheaval, and denudation. +The organic remains also have sometimes been obliterated entirely, or the +mineral matter of which they were composed has been removed and replaced by +other substances. + +WHY NEWER GROUPS SHOULD BE STUDIED FIRST. + +We likewise observe that the older the rocks the more widely do their organic +remains depart from the types of the living creation. First, we find in the +newer tertiary rocks a few species which no longer exist, mixed with many living +ones, and then, as we go farther back, many genera and families at present +unknown make their appearance, until we come to strata in which the fossil +relics of existing species are nowhere to be detected, except a few of the +lowest forms of invertebrate, while some orders of animals and plants wholly +unrepresented in the living world begin to be conspicuous. + +When we study, therefore, the geological records of the earth and its +inhabitants, we find, as in human history, the defectiveness and obscurity of +the monuments always increasing the remoter the era to which we refer, and the +difficulty of determining the true chronological relations of rocks is more and +more enhanced, especially when we are comparing those which were formed +simultaneously in very distant regions of the globe. Hence we advance with +securer steps when we begin with the study of the geological records of later +times, proceeding from the newer to the older, or from the more to the less +known. + +In thus inverting what might at first seem to be the more natural order of +historical research, we must bear in mind that each of the periods above +enumerated, even the shortest, such as the Post-tertiary, or the Pliocene, +Miocene, or Eocene, embrace a succession of events of vast extent, so that to +give a satisfactory account of what we already know of any one of them would +require many volumes. When, therefore, we approach one of the newer groups +before endeavouring to decipher the monuments of an older one, it is like +endeavouring to master the history of our own country and that of some +contemporary nations, before we enter upon Roman History, or like investigating +the annals of Ancient Italy and Greece before we approach those of Egypt and +Assyria. + +NOMENCLATURE. + +The origin of the terms Primary and Secondary, and the synonymous terms +Palaeozoic, and Mesozoic, were explained in Chapter 8. + +The Tertiary or Cainozoic strata (see Chapter 8) were so called because they +were all posterior in date to the Secondary series, of which last the Chalk of +Cretaceous, No. 9, Figure 86, constitutes the newest group. The whole of them +were at first confounded with the superficial alluviums of Europe; and it was +long before their real extent and thickness, and the various ages to which they +belong, were fully recognised. They were observed to occur in patches, some of +fresh-water, others of marine origin, their geographical area being usually +small as compared to the secondary formations, and their position often +suggesting the idea of their having been deposited in different bays, lakes, +estuaries, or inland seas, after a large portion of the space now occupied by +Europe had already been converted into dry land. + +The first deposits of this class, of which the characters were accurately +determined, were those occurring in the neighbourhood of Paris, described in +1810 by MM. Cuvier and Brongniart. They were ascertained to consist of +successive sets of strata, some of marine, others of fresh-water origin, lying +one upon the other. The fossil shells and corals were perceived to be almost all +of unknown species, and to have in general a near affinity to those now +inhabiting warmer seas. The bones and skeletons of land animals, some of them of +large size, and belonging to more than forty distinct species, were examined by +Cuvier, and declared by him not to agree specifically, nor most of them even +generically, with any hitherto observed in the living creation. + +Strata were soon afterwards brought to light in the vicinity of London, and in +Hampshire, which, although dissimilar in mineral composition, were justly +inferred by Mr. T. Webster to be of the same age as those of Paris, because the +greater number of the fossil shells were specifically identical. For the same +reason, rocks found on the Gironde, in the South of France, and at certain +points in the North of Italy, were suspected to be of contemporaneous origin. + +Another important discovery was soon afterwards made by Brocchi in Italy, who +investigated the argillaceous and sandy deposits, replete with shells, which +form a low range of hills, flanking the Apennines on both sides, from the plains +of the Po to Calabria. These lower hills were called by him the Subapennines, +and were formed of strata chiefly marine, and newer than those of Paris and +London. + +Another tertiary group occurring in the neighbourhood of Bordeaux and Dax, in +the South of France, was examined by M. de Basterot in 1825, who described and +figured several hundred species of shells, which differed for the most part both +from the Parisian series and those of the Subapennine hills. It was soon, +therefore, suspected that this fauna might belong to a period intermediate +between that of the Parisian and Subapennine strata, and it was not long before +the evidence of superposition was brought to bear in support of this opinion; +for other strata, contemporaneous with those of Bordeaux, were observed in one +district (the Valley of the Loire), to overlie the Parisian formation, and in +another (in Piedmont) to underlie the Subapennine beds. The first example of +these was pointed out in 1829 by M. Desnoyers, who ascertained that the sand and +marl of marine origin called faluns, near Tours, in the basin of the Loire, full +of sea-shells and corals, rested upon a lacustrine formation, which constitutes +the uppermost subdivision of the Parisian group, extending continuously +throughout a great table-land intervening between the basin of the Seine and +that of the Loire. The other example occurs in Italy, where strata containing +many fossils similar to those of Bordeaux were observed by Bonelli and others in +the environs of Turin, subjacent to strata belonging to the Subapennine group of +Brocchi. + +VALUE OF TESTACEAN FOSSILS IN CLASSIFICATION. + +It will be observed that in the foregoing allusions to organic remains, the +testacea or the shell-bearing mollusca are selected as the most useful and +convenient class for the purposes of general classification. In the first place, +they are more universally distributed through strata of every age than any other +organic bodies. Those families of fossils which are of rare and casual +occurrence are absolutely of no avail in establishing a chronological +arrangement. If we have plants alone in one group of strata and the bones of +mammalia in another, we can draw no conclusion respecting the affinity or +discordance of the organic beings of the two epochs compared; and the same may +be said if we have plants and vertebrated animals in one series and only shells +in another. Although corals are more abundant, in a fossil state, than plants, +reptiles, or fish, they are still rare when contrasted with shells, because they +are more dependent for their well-being on the constant clearness of the water, +and are, therefore, less likely to be included in rocks which endure in +consequence of their thickness and the copiousness of sediment which prevailed +when they originated. The utility of the testacea is, moreover, enhanced by the +circumstance that some forms are proper to the sea, others to the land, and +others to fresh water. Rivers scarcely ever fail to carry down into their deltas +some land-shells, together with species which are at once fluviatile and +lacustrine. By this means we learn what terrestrial, fresh-water, and marine +species coexisted at particular eras of the past: and having thus identified +strata formed in seas with others which originated contemporaneously in inland +lakes, we are then enabled to advance a step farther, and show that certain +quadrupeds or aquatic plants, found fossil in lacustrine formations, inhabited +the globe at the same period when certain fish, reptiles, and zoophytes lived in +the ocean. + +Among other characters of the molluscous animals, which render them extremely +valuable in settling chronological questions in geology, may be mentioned, +first, the wide geographical range of many species; and, secondly, what is +probably a consequence of the former, the great duration of species in this +class, for they appear to have surpassed in longevity the greater number of the +mammalia and fish. Had each species inhabited a very limited space, it could +never, when imbedded in strata, have enabled the geologist to identify deposits +at distant points; or had they each lasted but for a brief period, they could +have thrown no light on the connection of rocks placed far from each other in +the chronological, or, as it is often termed, vertical series. + +CLASSIFICATION OF TERTIARY STRATA. + +Many authors have divided the European Tertiary strata into three groups-- +lower, middle, and upper; the lower comprising the oldest formations of Paris +and London before mentioned; the middle those of Bordeaux and Touraine; and the +upper all those newer than the middle group. + +In the first edition of the Principles of Geology, I divided the whole of the +Tertiary formations into four groups, characterised by the percentage of recent +shells which they contained. The lower tertiary strata of London and Paris were +thought by M. Deshayes to contain only 3 1/2 per cent of recent species, and +were termed Eocene. The middle tertiary of the Loire and Gironde had, according +to the specific determinations of the same conchologist, 17 per cent, and formed +the Miocene division. The Subapennine beds contained 35 to 50 per cent, and were +termed Older Pliocene, while still more recent beds in Sicily, which had from 90 +to 95 per cent of species identical with those now living, were called Newer +Pliocene. The first of the above terms, Eocene, is derived from eos, dawn, and +cainos, recent, because the fossil shells of this period contain an extremely +small proportion of living species, which may be looked upon as indicating the +dawn of the existing state of the testaceous fauna, no recent species having +been detected in the older or secondary rocks. + +The term Miocene (from meion, less, and cainos, recent) is intended to express a +minor proportion of recent species (of testacea), the term Pliocene (from +pleion, more, and cainos, recent) a comparative plurality of the same. It may +assist the memory of students to remind them, that the MI-ocene contain a MI-nor +proportion, and PL-iocene a comparative PL-urality of recent species; and that +the greater number of recent species always implies the more modern origin of +the strata. + +It has sometimes been objected to this nomenclature that certain species of +infusoria found in the chalk are still existing, and, on the other hand, the +Miocene and Older Pliocene deposits often contain the remains of mammalia, +reptiles, and fish, exclusively of extinct species. But the reader must bear in +mind that the terms Eocene, Miocene, and Pliocene were originally invented with +reference purely to conchological data, and in that sense have always been and +are still used by me. + +Since the year 1830 the number of known shells, both recent and fossil, has +largely increased, and their identification has been more accurately determined. +Hence some modifications have been required in the classifications founded on +less perfect materials. The Eocene, Miocene, and Pliocene periods have been made +to comprehend certain sets of strata of which the fossils do not always conform +strictly in the proportion of recent to extinct species with the definitions +first given by me, or which are implied in the etymology of those terms. + + +CHAPTER X. + +RECENT AND POST-PLIOCENE PERIODS. + +Recent and Post-pliocene Periods. +Terms defined. +Formations of the Recent Period. +Modern littoral Deposits containing Works of Art near Naples. +Danish Peat and Shell-mounds. +Swiss Lake-dwellings. +Periods of Stone, Bronze, and Iron. +Post-pliocene Formations. +Coexistence of Man with extinct Mammalia. +Reindeer Period of South of France. +Alluvial Deposits of Paleolithic Age. +Higher and Lower-level Valley-gravels. +Loess or Inundation-mud of the Nile, Rhine, etc. +Origin of Caverns. +Remains of Man and extinct Quadrupeds in Cavern Deposits. +Cave of Kirkdale. +Australian Cave-breccias. +Geographical Relationship of the Provinces of living Vertebrata and those of +extinct Post-pliocene Species. +Extinct struthious Birds of New Zealand. +Climate of the Post-pliocene Period. +Comparative Longevity of Species in the Mammalia and Testacea. +Teeth of Recent and Post-pliocene Mammalia. + +We have seen in the last chapter that the uppermost or newest strata are called +Post-tertiary, as being more modern than the Tertiary. It will also be observed +that the Post-tertiary formations are divided into two subordinate groups: the +Recent, and Post-pliocene. In the former, or the Recent, the mammalia as well as +the shells are identical with species now living: whereas in the Post-pliocene, +the shells being all of living forms, a part, and often a considerable part, of +the mammalia belonged to extinct species. To this nomenclature it may be +objected that the term Post-pliocene should in strictness include all geological +monuments posterior in date to the Pliocene; but when I have occasion to speak +of the whole collectively, I shall call them Post-tertiary, and reserve the term +Post-pliocene for the older Post-tertiary formations, while the Upper or newer +ones will be called "Recent." + +Cases will occur where it may be scarcely possible to draw the boundary line +between the Recent and Post-pliocene deposits; and we must expect these +difficulties to increase rather than diminish with every advance in our +knowledge, and in proportion as gaps are filled up in the series of records. + +RECENT PERIOD. + +It was stated in the sixth chapter, when I treated of denudation, that the dry +land, or that part of the earth's surface which is not covered by the waters of +lakes or seas, is generally wasting away by the incessant action of rain and +rivers, and in some cases by the undermining and removing power of waves and +tides on the sea-coast. But the rate of waste is very unequal, since the level +and gently sloping lands, where they are protected by a continuous covering of +vegetation, escape nearly all wear and tear, so that they may remain for ages in +a stationary condition, while the removal of matter is constantly widening and +deepening the intervening ravines and valleys. + +The materials, both fine and coarse, carried down annually by rivers from the +higher regions to the lower, and deposited in successive strata in the basins of +seas and lakes, must be of enormous volume. We are always liable to underrate +their magnitude, because the accumulation of strata is going on out of sight. + +There are, however, causes at work which, in the course of centuries, tend to +render visible these modern formations, whether of marine or lacustrine origin. +For a large portion of the earth's crust is always undergoing a change of level, +some areas rising and others sinking at the rate of a few inches, or a few feet, +perhaps sometimes yards, in a century; so that spaces which were once subaqueous +are gradually converted into land, and others which were high and dry become +submerged. In consequence of such movements we find in certain regions, as in +Cashmere, for example, where the mountains are often shaken by earthquakes, +deposits which were formed in lakes in the historical period, but through which +rivers have now cut deep and wide channels. In lacustrine strata thus +intersected, works of art and fresh-water shells are seen. In other districts on +the borders of the sea, usually at very moderate elevations above its level, +raised beaches occur, or marine littoral deposits, such as those in which, on +the borders of the Bay of Baiae, near Naples, the well-known temple of Serapis +was imbedded. In that case the date of the monument buried in the marine strata +is ascertainable, but in many other instances the exact age of the remains of +human workmanship is uncertain, as in the estuary of the Clyde at Glasgow, where +many canoes have been exhumed, with other works of art, all assignable to some +part of the Recent Period. + +DANISH PEAT AND SHELL-MOUNDS OR KITCHEN-MIDDENS. + +Sometimes we obtain evidence, without the aid of a change of level, of events +which took place in pre-historic times. The combined labours, for example, of +the antiquary, zoologist, and botanist have brought to light many monuments of +the early inhabitants buried in peat-mosses in Denmark. Their geological age is +determined by the fact that, not only the contemporaneous fresh-water and land +shells, but all the quadrupeds, found in the peat, agree specifically with those +now inhabiting the same districts, or which are known to have been indigenous in +Denmark within the memory of man. In the lower beds of peat (a deposit varying +from 20 to 30 feet in thickness), weapons of stone accompany trunks of the +Scotch fir, Pinus sylvestris. This peat may be referred to that part of the +stone period for which Sir John Lubbock proposed the name of "Neolithic" in +contradistinction to a still older era, termed by him "Paleolithic," and which +will be described in the sequel. (Sir John Lubbock Pre-historic Times page 3 +1865.) In the higher portions of the same Danish bogs, bronze implements are +associated with trunks and acorns of the common oak. It appears that the pine +has never been a native of Denmark in historical times, and it seems to have +given place to the oak about the time when articles and instruments of bronze +superseded those of stone. It also appears that, at a still later period, the +oak itself became scarce, and was nearly supplanted by the beech, a tree which +now flourishes luxuriantly in Denmark. Again, at the still later epoch when the +beech-tree abounded, tools of iron were introduced, and were gradually +substituted for those of bronze. + +On the coasts of the Danish islands in the Baltic, certain mounds, called in +those countries "Kjokken-modding," or "kitchen-middens," occur, consisting +chiefly of the castaway shells of the oyster, cockle, periwinkle, and other +eatable kinds of molluscs. The mounds are from three to ten feet high, and from +100 to 1000 feet in their longest diameter. They greatly resemble heaps of +shells formed by the Red Indians of North America along the eastern shores of +the United States. In the old refuse-heaps, recently studied by the Danish +antiquaries and naturalists with great skill and diligence, no implements of +metal have ever been detected. All the knives, hatchets, and other tools, are of +stone, horn, bone, or wood. With them are often intermixed fragments of rude +pottery, charcoal and cinders, and the bones of quadrupeds on which the rude +people fed. These bones belong to wild species still living in Europe, though +some of them, like the beaver, have long been extirpated in Denmark. The only +animal which they seem to have domesticated was the dog. + +As there is an entire absence of metallic tools, these refuse-heaps are referred +to the Neolithic division of the age of stone, which immediately preceded in +Denmark the age of bronze. It appears that a race more advanced in civilisation, +armed with weapons of that mixed metal, invaded Scandinavia, and ousted the +aborigines. + +LACUSTRINE HABITATIONS OF SWITZERLAND. + +In Switzerland a different class of monuments, illustrating the successive ages +of stone, bronze, and iron, has been of late years investigated with great +success, and especially since 1854, in which year Dr. F. Keller explored near +the shore at Meilen, in the bottom of the lake of Zurich, the ruins of an old +village, originally built on numerous wooden piles, driven, at some unknown +period, into the muddy bed of the lake. Since then a great many other +localities, more than a hundred and fifty in all, have been detected of similar +pile-dwellings, situated near the borders of the Swiss lakes, at points where +the depth of water does not exceed 15 feet. (Bulletin de la Societie Vaudoise +des Sciences Nat. tome 6 Lausanne 1860; and Antiquity of Man by the author +chapter 2.) The superficial mud in such cases is filled with various articles, +many hundreds of them being often dredged up from a very limited area. Thousands +of piles, decayed at their upper extremities, are often met with still firmly +fixed in the mud. + +As the ages of stone, bronze, and iron merely indicate successive stages of +civilisation, they may all have coexisted at once in different parts of the +globe, and even in contiguous regions, among nations having little intercourse +with each other. To make out, therefore, a distinct chronological series of +monuments is only possible when our observations are confined to a limited +district, such as Switzerland. + +The relative antiquity of the pile-dwellings, which belong respectively to the +ages of stone and bronze, is clearly illustrated by the associations of the +tools with certain groups of animal remains. Where the tools are of stone, the +castaway bones which served for the food of the ancient people are those of +deer, the wild boar, and wild ox, which abounded when society was in the hunter +state. But the bones of the later or bronze epoch were chiefly those of the +domestic ox, goat, and pig, indicating progress in civilisation. Some villages +of the stone age are of later date than others, and exhibit signs of an improved +state of the arts. Among their relics are discovered carbonised grains of wheat +and barley, and pieces of bread, proving that the cultivation of cereals had +begun. In the same settlements, also, cloth, made of woven flax and straw, has +been detected. + +The pottery of the bronze age in Switzerland is of a finer texture, and more +elegant in form, than that of the age of stone. At Nidau, on the lake of Bienne, +articles of iron have also been discovered, so that this settlement was +evidently not abandoned till that metal had come into use. + +At La Thene, in the northern angle of the lake of Neufchatel, a great many +articles of iron have been obtained, which in form and ornamentation are +entirely different both from those of the bronze period and from those used by +the Romans. Gaulish and Celtic coins have also been found there by MM. Schwab +and Desor. They agree in character with remains, including many iron swords, +which have been found at Tiefenau, near Berne, in ground supposed to have been a +battle-field; and their date appears to have been anterior to the great Roman +invasion of Northern Europe, though perhaps not long before that event. (Sir J. +Lubbock's Lecture, Royal Institution February 27, 1863.) Coins, which sometimes +occur in deposits of the age of iron, have never yet been found in formations of +the ages of bronze or stone. + +The period of bronze must have been one of foreign commerce, as tin, which +enters into this metallic mixture in the proportion of about ten per cent to the +copper, was obtained by the ancients chiefly from Cornwall. (Diodorus 5, 21, 22 +and Sir H. James Note on Block of Tin dredged up in Falmouth Harbour. Royal +Institution of Cornwall 1863.) Very few human bones of the bronze period have +been met with in the Danish peat, or in the Swiss lake-dwellings, and this +scarcity is generally attributed by archaeologists to the custom of burning the +dead, which prevailed in the age of bronze. + +POST-PLIOCENE PERIOD. + +From the foregoing observations we may infer that the ages of iron and bronze in +Northern and Central Europe were preceded by a stone age, the Neolithic, +referable to that division of the post-tertiary epoch which I have called +Recent, when the mammalia as well as the other organic remains accompanying the +stone implements were of living species. But memorials have of late been brought +to light of a still older age of stone, for which, as above stated, the name +Paleolithic has been proposed, when man was contemporary in Europe with the +elephant and rhinoceros, and various other animals, of which many of the most +conspicuous have long since died out. + +REINDEER PERIOD IN SOUTH OF FRANCE. + +In the larger number of the caves of Europe, as for example in those of England, +Belgium, Germany, and many parts of France, the animal remains agree +specifically with the fauna of this oldest division of the age of stone, or that +to which belongs the drift of Amiens and Abbeville presently to be mentioned, +containing flint implements of a very antique type. But there are some caves in +the departments of Dordogne, Aude, and other parts of the south of France, which +are believed by M. Lartet to be of intermediate date between the Paleolithic and +Neolithic periods. To this intermediate era M. Lartet gave, in 1863, the name of +the "reindeer period," because vast quantities of the bones and horns of that +deer have been met with in the French caverns. In some cases separate plates of +molars of the mammoth, and several teeth of the great Irish deer, Cervus +megaceros, and of the cave-lion, Felis spelaea, have been found mixed up with +cut and carved bones of reindeer. On one of these sculptured bones in the cave +of Perigord, a rude representation of the mammoth, with its long curved tusks +and covering of wool, occurs, which is regarded by M. Lartet as placing beyond +all doubt the fact that the early inhabitants of these caves must have seen this +species of elephant still living in France. The presence of the marmot, as well +as the reindeer and some other northern animals, in these caverns seems to imply +a colder climate than that of the Swiss lake-dwellings, in which no remains of +reindeer have as yet been discovered. The absence of this last in the old +lacustrine habitations of Switzerland is the more significant, because in a cave +in the neighbourhood of the lake of Geneva, namely, that of Mont Saleve, bones +of the reindeer occur with flint implements similar to those of the caverns of +Dordogne and Perigord. + +The state of the arts, as exemplified by the instruments found in these caverns +of the reindeer period, is somewhat more advanced than that which characterises +the tools of the Amiens drift, but is nevertheless more rude than that of the +Swiss lake-dwellings. No metallic articles occur, and the stone hatchets are not +ground after the fashion of celts; the needles of bone are shaped in a +workmanlike style, having their eyes drilled with consummate skill. + +The formations above alluded to, which are as yet but imperfectly known, may be +classed as belonging to the close of the Paleolithic era, of the monuments of +which I am now about to treat. + +ALLUVIAL DEPOSITS OF THE PALEOLITHIC AGE. + +(FIGURE 87. Recent and Post-pliocene alluvial deposits. +1. Peat of the recent period. +2. Gravel of modern river. +2'. Loam of brick-earth (loess) of same age as 2, formed by inundations of the +river. +3. Lower-level valley-gravel with extinct mammalia (Post-pliocene). +3'. Loam of same age. +4. Higher-level valley-gravel (Post-pliocene). +4'. Loam of same age. +5. Upland gravel of various kinds and periods, consisting in some places of +unstratified boulder clay or glacial drift. +6. Older rocks.) + +The alluvial and marine deposits of the Paleolithic age, the earliest to which +any vestiges of man have yet been traced back, belong to a time when the +physical geography of Europe differed in a marked degree from that now +prevailing. In the Neolithic period, the valleys and rivers coincided almost +entirely with those by which the present drainage of the land is effected, and +the peat-mosses were the same as those now growing. The situation of the shell- +mounds and lake-dwellings above alluded to is such as to imply that the +topography of the districts where they are observed has not subsequently +undergone any material alteration. Whereas we no sooner examine the Post- +pliocene formations, in which the remains of so many extinct mammalia are found, +than we at once perceive a more decided discrepancy between the former and +present outline of the surface. Since those deposits originated, changes of +considerable magnitude have been effected in the depth and width of many +valleys, as also in the direction of the superficial and subterranean drainage, +and, as is manifest near the sea-coast, in the relative position of land and +water. In Figure 87 an ideal section is given, illustrating the different +position which the Recent and Post-pliocene alluvial deposits occupy in many +European valleys. + +The peat, No. 1, has been formed in a low part of the modern alluvial plain, in +parts of which gravel No. 2 of the recent period is seen. Over this gravel the +loam or fine sediment 2' has in many places been deposited by the river during +floods which covered nearly the whole alluvial plain. + +No. 3 represents an older alluvium, composed of sand and gravel, formed before +the valley had been excavated to its present depth. It contains the remains of +fluviatile shells of living species associated with the bones of mammalia, in +part of recent, and in part of extinct species. Among the latter, the mammoth +(E. primigenius) and the Siberian rhinoceros (R. tichorhinus) are the most +common in Europe. No. 3' is a remnant of the loam or brick-earth by which No. 3 +was overspread. No. 4 is a still older and more elevated terrace, similar in its +composition and organic remains to No. 3, and covered in like manner with its +inundation-mud, 4'. Sometimes the valley-gravels of older date are entirely +missing, or there is only one, and occasionally there are more than two, marking +as many successive stages in the excavation of the valley. They usually occur at +heights varying from 10 to 100 feet, sometimes on the right and sometimes on the +left side of the existing river-plain, but rarely in great strength on exactly +opposite sides of the valley. + +Among the genera of extinct quadrupeds most frequently met with in England, +France, Germany, and other parts of Europe, are the elephant, rhinoceros, +hippopotamus, horse, great Irish deer, bear, tiger, and hyaena. In the peat, No. +1 (Figure 87), and in the more modern gravel and silt (No. 2), works of art of +the ages of iron and bronze, and of the later or Neolithic stone period, already +described, are met with. In the more ancient or Paleolithic gravels, 3 and 4, +there have been found of late years in several valleys in France and England-- +as, for example, in those of the Seine and Somme, and of the Thames and Ouse, +near Bedford-- stone implements of a rude type, showing that man coexisted in +those districts with the mammoth and other extinct quadrupeds of the genera +above enumerated. In 1847, M. Boucher de Perthes observed in an ancient alluvium +at Abbeville, in Picardy, the bones of extinct mammalia associated in such a +manner with flint implements of a rude type as to lead him to infer that both +the organic remains and the works of art were referable to one and the same +period. This inference was soon after confirmed by Mr. Prestwich, who found in +1859 a flint tool in situ in the same stratum at Amiens that contained the +remains of extinct mammalia. + +The flint implements found at Abbeville and Amiens are most of them considered +to be hatchets and spear-heads, and are different from those commonly called +"celts." These celts, so often found in the recent formations, have a more +regular oblong shape, the result of grinding, by which also a sharp edge has +been given to them. The Abbeville tools found in gravel at different levels, as +in Nos. 3 and 4, Figure 87, in which bones of the elephant, rhinoceros, and +other extinct mammalia occur, are always unground, having evidently been brought +into their present form simply by the chipping off of fragments of flint by +repeated blows, such as could be given by a stone hammer. + +Some of them are oval, others of a spear-headed form, no two exactly alike, and +yet the greater number of each kind are obviously fashioned after the same +general pattern. Their outer surface is often white, the original black flint +having been discoloured and bleached by exposure to the air, or by the action of +acids, as they lay in the gravel. They are most commonly stained of the same +ochreous colour as the flints of the gravel in which they are imbedded. +Occasionally their antiquity is indicated not only by their colour but by +superficial incrustations of carbonate of lime, or by dendrites formed of oxide +of iron and manganese. The edges also of most of them are worn, sometimes by +having been used as tools, or sometimes by having been rolled in the old river's +bed. They are met with not only in the lower-level gravels, as in No. 3, Figure +87, but also in No. 4, or the higher gravels, as at St. Acheul, in the suburbs +of Amiens, where the old alluvium lies at an elevation of about 100 feet above +the level of the river Somme. At both levels fluviatile and land-shells are met +with in the loam as well as in the gravel, but there are no marine shells +associated, except at Abbeville, in the lowest part of the gravel, near the sea, +and a few feet only above the present high-water mark. Here with fossil shells +of living species are mingled the bones of Elephas primigenius and E. antiquus, +Rhinoceros tichorhinus, Hippopotamus, Felis spelaea, Hyaena spelaea, reindeer, +and many others, the bones accompanying the flint implements in such a manner as +to show that both were buried in the old alluvium at the same period. + +Nearly the entire skeleton of a rhinoceros was found at one point, namely, in +the Menchecourt drift at Abbeville, the bones being in such juxtaposition as to +show that the cartilage must have held them together at the time of their +inhumation. + +The general absence here and elsewhere of human bones from gravel and sand in +which flint tools are discovered, may in some degree be due to the present +limited extent of our researches. But it may also be presumed that when a hunter +population, always scanty in numbers, ranged over this region, they were too +wary to allow themselves to be overtaken by the floods which swept away many +herbivorous animals from the low river-plains where they may have been pasturing +or sleeping. Beasts of prey prowling about the same alluvial flats in search of +food may also have been surprised more readily than the human tenant of the same +region, to whom the signs of a coming tempest were better known. + +INUNDATION-MUD OF RIVERS.-- BRICK-EARTH.-- FLUVIATILE LOAM, OR LOESS. + +As a general rule, the fluviatile alluvia of different ages (Nos. 2, 3, 4, +Figure 87) are severally made up of coarse materials in their lower portions, +and of fine silt or loam in their upper parts. For rivers are constantly +shifting their position in the valley-plain, encroaching gradually on one bank, +near which there is deep water, and deserting the other or opposite side, where +the channel is growing shallower, being destined eventually to be converted into +land. Where the current runs strongest, coarse gravel is swept along, and where +its velocity is slackened, first sand, and then only the finest mud, is thrown +down. A thin film of this fine sediment is spread, during floods, over a wide +area, on one, or sometimes on both sides, of the main stream, often reaching as +far as the base of the bluffs or higher grounds which bound the valley. Of such +a description are the well-known annual deposits of the Nile, to which Egypt +owes its fertility. So thin are they, that the aggregate amount accumulated in a +century is said rarely to exceed five inches, although in the course of +thousands of years it has attained a vast thickness, the bottom not having been +reached by borings extending to a depth of 60 feet towards the central parts of +the valley. Everywhere it consists of the same homogeneous mud, destitute of +stratification-- the only signs of successive accumulation being where the Nile +has silted up its channel, or where the blown sands of the Libyan desert have +invaded the plain, and given rise to alternate layers of sand and mud. + +In European river-loams we occasionally observe isolated pebbles and angular +pieces of stone which have been floated by ice to the places where they now +occur; but no such coarse materials are met with in the plains of Egypt. + +In some parts of the valley of the Rhine the accumulation of similar loam, +called in Germany "loess," has taken place on an enormous scale. Its colour is +yellowish-grey, and very homogeneous; and Professor Bischoff has ascertained, by +analysis, that it agrees in composition with the mud of the Nile. Although for +the most part unstratified, it betrays in some places marks of stratification, +especially where it contains calcareous concretions, or in its lower part where +it rests on subjacent gravel and sand which alternate with each other near the +junction. About a sixth part of the whole mass is composed of carbonate of lime, +and there is usually an intermixture of fine quartzose and micaceous sand. + +(FIGURE 88. Succinea elongata.) + +Although this loam of the Rhine is unsolidified, it usually terminates where it +has been undermined by running water in a vertical cliff, from the face of which +shells of terrestrial, fresh-water and amphibious mollusks project in relief. +These shells do not imply the permanent sojourn of a body of fresh water on the +spot, for the most aquatic of them, the Succinea, inhabits marshes and wet +grassy meadows. The Succinea elongata (or S. oblongata), Figure 88, is very +characteristic both of the loess of the Rhine and of some other European river- +loams. + +(FIGURE 89. Pupa muscorum (Linn.).) + +(FIGURE 90. Helix hispida (Linn.) (plebeia).) + +Among the land-shells of the Rhenish loess, Helix hispida, Figure 90, and Pupa +muscorum, Figure 89, are very common. Both the terrestrial and aquatic shells +are of most fragile and delicate structure, and yet they are almost invariably +perfect and uninjured. They must have been broken to pieces had they been swept +along by a violent inundation. Even the colour of some of the land-shells, as +that of Helix nemoralis, is occasionally preserved. + +In parts of the valley of the Rhine, between Bingen and Basle, the fluviatile +loam or loess now under consideration is several hundred feet thick, and +contains here and there throughout that thickness land and amphibious shells. As +it is seen in masses fringing both sides of the great plain, and as occasionally +remnants of it occur in the centre of the valley, forming hills several hundred +feet in height, it seems necessary to suppose, first, a time when it slowly +accumulated; and secondly, a later period, when large portions of it were +removed, or when the original valley, which had been partially filled up with +it, was re-excavated. + +Such changes may have been brought about by a great movement of oscillation, +consisting first of a general depression of the land, and then of a gradual re- +elevation of the same. The amount of continental depression which first took +place in the interior, must be imagined to have exceeded that of the region near +the sea, in which case the higher part of the great valley would have its +alluvial plain gradually raised by an accumulation of sediment, which would only +cease when the subsidence of the land was at an end. If the direction of the +movement was then reversed, and, during the re-elevation of the continent, the +inland region nearest the mountains should rise more rapidly than that near the +coast, the river would acquire a denuding power sufficient to enable it to sweep +away gradually nearly all the loam and gravel with which parts of its basin had +been filled up. Terraces and hillocks of mud and sand would then alone remain to +attest the various levels at which the river had thrown down and afterwards +removed alluvial matter. + +CAVERN DEPOSITS CONTAINING HUMAN REMAINS AND BONES OF EXTINCT ANIMALS. + +In England, and in almost all countries where limestone rocks abound, caverns +are found, usually consisting of cavities of large dimensions, connected +together by low, narrow, and sometimes torturous galleries or tunnels. These +subterranean vaults are usually filled in part with mud, pebbles, and breccia, +in which bones occur belonging to the same assemblage of animals as those +characterising the Post-pliocene alluvia above described. Some of these bones +are referable to extinct and others to living species, and they are occasionally +intermingled, as in the valley-gravels, with implements of one or other of the +great divisions of the stone age, and these are not unfrequently accompanied by +human bones, which are much more common in cavern deposits than in valley- +alluvium. + +Each suite of caverns, and the passages by which they communicate the one with +the other, afford memorials to the geologist of successive phases through which +they must have passed. First, there was a period when the carbonate of lime was +carried out gradually by springs; secondly, an era when engulfed rivers or +occasional floods swept organic and inorganic debris into the subterranean +hollows previously formed; and thirdly, there were such changes in the +configuration of the region as caused the engulfed rivers to be turned into new +channels, and springs to be dried up, after which the cave-mud, breccia, gravel, +and fossil bones would bear the same kind of relation to the existing drainage +of the country as the older valley-drifts with their extinct mammalian remains +and works of art bear to the present rivers and alluvial plains. + +The quarrying away of large masses of Carboniferous and Devonian limestone, near +Liege, in Belgium, has afforded the geologist magnificent sections of some of +these caverns, and the former communication of cavities in the interior of the +rocks with the old surface of the country by means of vertical or oblique +fissures, has been demonstrated in places where it would not otherwise have been +suspected, so completely have the upper extremities of these fissures been +concealed by superficial drift, while their lower ends, which extended into the +roofs of the caves, are masked by stalactitic incrustations. + +The origin of the stalactite is thus explained by the eminent chemist Liebig. +Mould or humus, being acted on by moisture and air, evolves carbonic acid, which +is dissolved by rain. The rain-water, thus impregnated, permeates the porous +limestone, dissolves a portion of it, and afterwards, when the excess of +carbonic acid evaporates in the caverns, parts with the calcareous matter, and +forms stalactite. Even while caverns are still liable to be occasionally flooded +such calcareous incrustations accumulate, but it is generally when they are no +longer in the line of drainage that a solid floor of hard stalagmite is formed +on the bottom. + +The late Dr. Schmerling examined forty caves near Liege, and found in all of +them the remains of the same fauna, comprising the mammoth, tichorhine +rhinoceros, cave-bear, cave-hyaena, cave-lion, and many others, some of extinct +and some of living species, and in all of them flint implements. In four or five +caves only parts of human skeletons were met with, comprising sometimes skulls +with a few other bones, sometimes nearly every part of the skeleton except the +skull. In one of the caves, that of Engihoul, where Schmerling had found the +remains of at least three human individuals, they were mingled in such a manner +with bones of extinct mammalia, as to leave no doubt on his mind (in 1833) of +man having co-existed with them. + +In 1860, Professor Malaise, of Liege, explored with me this same cave of +Engihoul, and beneath a hard floor of stalagmite we found mud full of bones of +extinct and recent animals, such as Schmerling had described, and my companion, +persevering in his researches after I had returned to England, extracted from +the same deposit two human lower jaw-bones retaining their teeth. The skulls +from these Belgian caverns display no marked deviation from the normal European +type of the present day. + +The careful investigations carried on by Dr. Falconer, Mr. Pengelly, and others, +in the Brixham cave near Torquay, in 1858, demonstrated that flint knives were +there imbedded in such a manner in loam underlying a floor of stalagmite as to +prove that man had been an inhabitant of that region when the cave-bear and +other members of the ancient post-pliocene fauna were also in existence. + +The absence of gnawed bones had led Dr. Schmerling to infer that none of the +Belgian caves which he explored had served as the dens of wild beasts; but there +are many caves in Germany and England which have certainly been so inhabited, +especially by the extinct hyaena and bear. + +A fine example of a hyaena's den was afforded by the cave of Kirkdale, so well +described by the late Dr. Buckland in his Reliquiae Diluvianae. In that cave, +above twenty-five miles north-north-east of York, the remains of about 300 +hyaenas, belonging to individuals of every age, were detected. The species +(Hyaena spelaea) has been considered by palaeontologists as extinct; it was +larger than the fierce Hyaena crocuta of South Africa, which it closely +resembled, and of which it is regarded by Mr. Boyd Dawkins as a variety. Dr. +Buckland, after carefully examining the spot, proved that the hyaenas must have +lived there; a fact attested by the quantity of their dung, which, as in the +case of the living hyaena, is of nearly the same composition as bone, and almost +as durable. In the cave were found the remains of the ox, young elephant, +hippopotamus, rhinoceros, horse, bear, wolf, hare, water-rat, and several birds. +All the bones have the appearance of having been broken and gnawed by the teeth +of the hyaenas; and they occur confusedly mixed in loam or mud, or dispersed +through a crust of stalagmite which covers it. In these and many other cases it +is supposed that portions of herbivorous quadrupeds have been dragged into +caverns by beasts of prey, and have served as their food-- an opinion quite +consistent with the known habits of the living hyaena. + +AUSTRALIAN CAVE-BRECCIAS. + +Ossiferous breccias are not confined to Europe, but occur in all parts of the +globe; and those discovered in fissures and caverns in Australia correspond +closely in character with what has been called the bony breccia of the +Mediterranean, in which the fragments of bone and rock are firmly bound together +by a red ochreous cement. + +Some of these caves were examined by the late Sir T. Mitchell in the Wellington +Valley, about 210 miles west of Sidney, on the river Bell, one of the principal +sources of the Macquarie, and on the Macquarie itself. The caverns often branch +off in different directions through the rock, widening and contracting their +dimensions, and the roofs and floors are covered with stalactite. The bones are +often broken, but do not seem to be water-worn. In some places they lie imbedded +in loose earth, but they are usually included in a breccia. + +The remains belong to marsupial animals. Among the most abundant are those of +the kangaroo, of which there are four species, while others belong to the genera +Phascolomys, the wombat; Dasyurus, the ursine opossum; Phalangista, the vulpine +opossum; and Hypsiprymnus, the kangaroo-rat. + +(FIGURE 91. Part of lower jaw of Macropus atlas. Owen. A young individual of an +extinct species. +a. Permanent false molar, in the alveolus.) + +(FIGURE 92. Lower jaw of largest living species of kangaroo. (Macropus major.)) + +In the fossils above enumerated, several species are larger than the largest +living ones of the same genera now known in Australia. Figure 91 of the right +side of a lower jaw of a kangaroo (Macropus atlas, Owen) will at once be seen to +exceed in magnitude the corresponding part of the largest living kangaroo, which +is represented in Figure 92. In both these specimens part of the substance of +the jaw has been broken open, so as to show the permanent false molar (a, Figure +91), concealed in the socket. From the fact of this molar not having been cut, +we learn that the individual was young, and had not shed its first teeth. + +The reader will observe that all these extinct quadrupeds of Australia belong to +the marsupial family, or, in other words, that they are referable to the same +peculiar type of organisation which now distinguishes the Australian mammalia +from those of other parts of the globe. This fact is one of many pointing to a +general law deducible from the fossil vertebrate and invertebrate animals of +times immediately antecedent to our own, namely, that the present geographical +distribution of organic FORMS dates back to a period anterior to the origin of +existing SPECIES; in other words, the limitation of particular genera or +families of quadrupeds, mollusca, etc., to certain existing provinces of land +and sea, began before the larger part of the species now contemporary with man +had been introduced into the earth. + +Professor Owen, in his excellent "History of British Fossil Mammals," has called +attention to this law, remarking that the fossil quadrupeds of Europe and Asia +differ from those of Australia or South America. We do not find, for example, in +the Europaeo-Asiatic province fossil kangaroos, or armadillos, but the elephant, +rhinoceros, horse, bear, hyaena, beaver, hare, mole, and others, which still +characterise the same continent. + +In like manner, in the Pampas of South America the skeletons of Megatherium, +Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct forms, +are analogous to the living sloth, armadillo, cavy, capybara, and llama. The +fossil quadrumana, also associated with some of these forms in the Brazilian +caves, belong to the Platyrrhine family of monkeys, now peculiar to South +America. That the extinct fauna of Buenos Ayres and Brazil was very modern has +been shown by its relation to deposits of marine shells, agreeing with those now +inhabiting the Atlantic. + +The law of geographical relationship above alluded to, between the living +vertebrata of every great zoological province and the fossils of the period +immediately antecedent, even where the fossil species are extinct, is by no +means confined to the mammalia. New Zealand, when first examined by Europeans, +was found to contain no indigenous land quadrupeds, no kangaroos, or opossums, +like Australia; but a wingless bird abounded there, the smallest living +representative of the ostrich family, called the Kiwi by the natives (Apteryx). +In the fossils of the Post-pliocene period in this same island, there is the +like absence of kangaroos, opossums, wombats, and the rest; but in their place a +prodigious number of well-preserved specimens of gigantic birds of the +struthious order, called by Owen Dinornis and Palapteryx, which are entombed in +superficial deposits. These genera comprehended many species, some of which were +four, some seven, others nine, and others eleven feet in height! It seems +doubtful whether any contemporary mammalia shared the land with this population +of gigantic feathered bipeds. + +Mr. Darwin, when describing the recent and fossil mammalia of South America, has +dwelt much on the wonderful relationship of the extinct to the living types in +that part of the world, inferring from such geographical phenomena that the +existing species are all related to the extinct ones which preceded them by a +bond of common descent. + +CLIMATE OF THE POST-PLIOCENE PERIOD. + +The evidence as to the climate of Europe during this epoch is somewhat +conflicting. The fluviatile and land-shells are all of existing species, but +their geographical range has not always been the same as at present. Some, for +example, which then lived in Britain are now only found in Norway and Finland, +probably implying that the Post-pliocene climate of Britain was colder, +especially in the winter. So also the reindeer and the musk-ox (Ovibos +moschatus), now inhabitants of the Arctic regions, occur fossil in the valleys +of the Thames and Avon, and also in France and Germany, accompanied in most +places by the mammoth and the woolly rhinoceros. At Grays in Essex, on the other +hand, another species both of elephant and rhinoceros occurs, together with a +hippopotamus and the Cyrena fluminalis, a shell now extinct in Europe but still +an inhabitant of the Nile and some Asiatic rivers. With it occurs the Unio +littoralis, now living in the Seine and Loire. In the valley of the Somme flint +tools have been found associated with Hippopotamus major and Cyrena fluminalis +in the lower-level Post-pliocene gravels; while in the higher-level (and more +ancient) gravels similar tools are more abundant, and are associated with the +bones of the mammoth and other Post-pliocene quadrupeds indicative of a colder +climate. + +It is possible that we may here have evidence of summer and winter migrations +rather than of a general change of temperature. Instead of imagining that the +hippopotamus lived all the year round with the musk-ox and lemming, we may +rather suppose that the apparently conflicting evidence may be due to the place +of our observations being near the boundary line of a northern and southern +fauna, either of which may have advanced or receded during comparatively slight +and temporary fluctuations of climate. There may then have been a continuous +land communication between England and the North of Siberia, as well as in an +opposite direction with Africa, then united to Southern Europe. + +In drift at Fisherton, near Salisbury, thirty feet above the river Wiley, the +Greenland lemming and a new species of the Arctic genus Spermophilus have been +found, along with the mammoth, reindeer, cave-hyaena, and other mammalia suited +to a cold climate. A flint implement was taken out from beneath the bones of the +mammoth. In a higher and older deposit in the vicinity, flint tools like those +of Amiens have been discovered. Nearly all the known Post-pliocene quadrupeds +have now been found accompanying flint knives or hatchets in such a way as to +imply their coexistence with man; and we have thus the concurrent testimony of +several classes of geological facts to the vast antiquity of the human race. In +the first place, the disappearance of a great variety of species of wild animals +from every part of a wide continent must have required a vast period for its +accomplishment; yet this took place while man existed upon the earth, and was +completed before that early period when the Danish shell-mounds were formed or +the oldest of the Swiss lake-dwellings constructed. Secondly, the deepening and +widening of valleys, indicated by the position of the river gravels at various +heights, implies an amount of change of which that which has occurred during the +historical period forms a scarcely perceptible part. Thirdly, the change in the +course of rivers which once flowed through caves now removed from any line of +drainage, and the formation of solid floors of stalagmite, must have required a +great lapse of time. Lastly, ages must have been required to change the climate +of wide regions to such an extent as completely to alter the geographical +distribution of many mammalia as well as land and fresh-water shells. The 3000 +or 4000 years of the historical period does not furnish us with any appreciable +measure for calculating the number of centuries which would suffice for such a +series of changes, which are by no means of a local character, but have operated +over a considerable part of Europe. + +RELATIVE LONGEVITY OF SPECIES IN THE MAMMALIA AND TESTACEA. + +I called attention in 1830 to the fact, which had not at that time attracted +notice, that the association in the Post-pliocene deposits of shells, +exclusively of living species, with many extinct quadrupeds betokened a +longevity of species in the testacea far exceeding that in the mammalia. +(Principles of Geology 1st edition volume 3 page 140.) Subsequent researches +seem to show that this greater duration of the same specific forms in the class +mollusca is dependent on a still more general law, namely, that the lower the +grade of animals, or the greater the simplicity of their structure, the more +persistent are they in general in their specific characters throughout vast +periods of time. Not only have the invertebrata, as shown by geological data, +altered at a less rapid rate than the vertebrata, but if we take one of the +classes of the former, as for example the mollusca, we find those of more simple +structure to have varied at a slower rate than those of a higher and more +complex organisation; the Brachiopoda, for example, more slowly than the +lamellibranchiate bivalves, while the latter have been more persistent than the +univalves, whether gasteropoda or cephalopoda. In like manner the specific +identity of the characters of the foraminifera, which are among the lowest types +of the invertebrata, has outlasted that of the mollusca in an equally decided +manner. + +TEETH OF POST-PLIOCENE MAMMALIA. + +To those who have never studied comparative anatomy, it may seem scarcely +credible that a single bone taken from any part of the skeleton may enable a +skilful osteologist to distinguish, in many cases, the genus, and sometimes the +species, of quadrupeds to which it belonged. Although few geologists can aspire +to such knowledge, which must be the result of long practice and study, they +will nevertheless derive great advantage from learning, what is comparatively an +easy task, to distinguish the principal divisions of the mammalia by the forms +and characters of their teeth. + +Figures 93 through 105 represent the teeth of some of the more common species +and genera found in alluvial and cavern deposits. + +(FIGURE 93. Elephas primigenius (or Mammoth ); molar of upper jaw, right side; +one-third of natural size. Post-pliocene. +a. Grinding surface. +b. Side view.) + +(FIGURE 94. Elephas antiquus, Falconer. Penultimate molar, one-third of natural +size. Post-pliocene and Pliocene.) + +(FIGURE 95. Elephas meridionalis, Nesti. Penultimate molar, one-third of natural +size. Post-pliocene and Pliocene.) + +(FIGURE 96. Rhinoceros leptorhinus, Cuvier-- Rhin. megarhinus, Christol; fossil +from fresh-water beds of Grays, Essex; penultimate molar, lower jaw, left side; +two-thirds of natural size. Post-pliocene and Newer Pliocene.) + +(FIGURE 97. Rhinoceros tichorhinus; penultimate molar, lower jaw, left side; +two-thirds of natural size. Post-pliocene.) + +(FIGURE 98. Hippopotamus; from cave near Palermo; molar tooth; two-thirds of +natural size. Post-pliocene.) + +(FIGURE 99. Horse. Equus caballus, L. (common horse); from the shell-marl, +Forfarshire; second molar, lower jaw. Recent. +a. Grinding surface, two-thirds natural size. +b. Side view of same, half natural size.) + +(FIGURE 100. Deer. +Moose (Cervus alces, L.); recent; molar of upper jaw. +a. Grinding surface. +b. Side view, two-thirds of natural size.) + +(FIGURE 101. Ox. +Ox, common, from shell-marl, Forfarshire; true molar, upper jaw; two-thirds +natural size. Recent. +c. Grinding surface. +d. Side view, fangs uppermost.) + +(FIGURE 102. Bear. +a. Canine tooth or tusk of bear (Ursus spelaeus); from cave near Liege. +b. Molar of left side, upper jaw; one-third of natural size. Post-pliocene.) + +(FIGURE 103. Tiger. +c. Canine tooth of tiger (Felis tigris); recent. +d. Outside view of posterior molar, lower jaw: one-third of natural size. +Recent.) + +(FIGURE 104. Hyaena spelaea, Goldf. (variety of H. crocuta); lower jaw. +Kent's Hole, Torquay, Devonshire; one-third natural size. Post-pliocene.) + +(FIGURE 105. Teeth of a new species of Arvicola, field-mouse; from the Norwich +Crag. Newer Pliocene. +a. Grinding surface. +b. Side view of the same. +c. Natural size of a and b.) + +On comparing the grinding surfaces of the corresponding molars of the three +species of elephants, Figures 93, 94, 95 it will be seen that the folds of +enamel are most numerous in the mammoth, fewer and wider, or more open, in E. +antiquus; and most open and fewest in E. meridionalis. It will be also seen that +the enamel in the molar of the Rhinoceros tichorhinus (Figure 97), is much +thicker than in that of the Rhinoceros leptorhinus (Figure 96). + + +CHAPTER XI. + +POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS. (As to the former excess +of cold, whether brought about by modifications in the height and distribution +of the land or by altered astronomical conditions, see Principles volume 1 10th +edition 1867 chapters 12 and 13 "Vicissitudes of Climate.") + +Geographical Distribution, Form, and Characters of Glacial Drift. +Fundamental Rocks, polished, grooved, and scratched. +Abrading and striating Action of Glaciers. +Moraines, Erratic Blocks, and "Roches Moutonnees." +Alpine Blocks on the Jura. +Continental Ice of Greenland. +Ancient Centres of the Dispersion of Erratics. +Transportation of Drift by floating Icebergs. +Bed of the Sea furrowed and polished by the running aground of floating Ice- +islands. + +CHARACTER AND DISTRIBUTION OF GLACIAL DRIFT. + +In speaking of the loose transported matter commonly found on the surface of the +land in all parts of the globe, I alluded to the exceptional character of what +has been called the boulder formation in the temperate and Arctic latitudes of +the northern hemisphere. The peculiarity of its form in Europe north of the +50th, and in North America north of the 40th parallel of latitude, is now +universally attributed to the action of ice, and the difference of opinion +respecting it is now chiefly restricted to the question whether land-ice or +floating icebergs have played the chief part in its distribution. It is wanting +in the warmer and equatorial regions, and reappears when we examine the lands +which lie south of the 40th and 50th parallels in the southern hemisphere, as, +for example, in Patagonia, Tierra del Fuego, and New Zealand. It consists of +sand and clay, sometimes stratified, but often wholly devoid of stratification +for a depth of 50, 100, or even a greater number of feet. To this unstratified +form of the deposit the name of TILL has long been applied in Scotland. It +generally contains a mixture of angular and rounded fragments of rock, some of +large size, having occasionally one or more of their sides flattened and +smoothed, or even highly polished. The smoothed surfaces usually exhibit many +scratches parallel to each other, one set of which often crosses an older set. +The till is almost everywhere wholly devoid of organic remains, except those +washed into it from older formations, though in some places it contains marine +shells, usually of northern or Arctic species, and frequently in a fragmentary +state. The bulk of the till has usually been derived from the grinding down into +mud of rocks in the immediate neighbourhood, so that it is red in a region of +Red Sandstone, as in Strathmore in Forfarshire; grey or black in a district of +coal and bituminous shale, as around Edinburgh; and white in a chalk country, as +in parts of Norfolk and Denmark. The stony fragments dispersed irregularly +through the till usually belong, especially in mountainous countries, to rocks +found in some part of the same hydrographical basin; but there are regions where +the whole of the boulder clay has come from a distance, and huge blocks, or +"erratics," as they have been called, many feet in diameter, have not +unfrequently travelled hundreds of miles from their point of departure, or from +the parent rocks from which they have evidently been detached. These are +commonly angular, and have often one or more of their sides polished and +furrowed. + +The rock on which the boulder formation reposes, if it consists of granite, +gneiss, marble, or other hard stone, capable of permanently retaining any +superficial markings which may have been imprinted upon it, is usually smoothed +or polished, like the erratics above described, and exhibits parallel striae and +furrows having a determinate direction. This direction, both in Europe and North +America, agrees generally in a marked manner with the course taken by the +erratic blocks in the same district. The boulder clay, when it was first +studied, seemed in many of its characters so singular and anomalous, that +geologists despaired of ever being able to interpret the phenomena by reference +to causes now in action. In those exceptional cases where marine shells of the +same date as the boulder clay were found, nearly all of them were recognised as +living species-- a fact conspiring with the superficial position of the drift to +indicate a comparatively modern origin. + +The term "diluvium" was for a time the most popular name of the boulder +formation, because it was referred by many to the deluge of Noah, while others +retained the name as expressive of their opinion that a series of diluvial waves +raised by hurricanes and storms, or by earthquakes, or by the sudden upheaval of +land from the bed of the sea, had swept over the continents, carrying with them +vast masses of mud and heavy stones, and forcing these stones over rocky +surfaces so as to polish and imprint upon them long furrows and striae. But +geologists were not long in seeing that the boulder formation was characteristic +of high latitudes, and that on the whole the size and number of erratic blocks +increases as we travel towards the Arctic regions. They could not fail to be +struck with the contrast which the countries bordering the Baltic presented when +compared with those surrounding the Mediterranean. The multitude of travelled +blocks and striated rocks in the one region, and the absence of such appearances +in the other, were too obvious to be overlooked. Even the great development of +the boulder formation, with large erratics so far south as the Alps, offered an +exception to the general rule favourable to the hypothesis that there was some +intimate connection between it and accumulations of snow and ice. + +TRANSPORTING AND ABRADING POWER OF GLACIERS. + +(FIGURE 106. Limestone, polished, furrowed, and scratched by the glacier of +Rosenlau in Switzerland. (Agassiz.) +a a. White streaks or scratches, caused by small grains of flint frozen into the +ice. +b b. Furrows.) + +I have described elsewhere ("Principles" volume 1 chapter 16 1867) the manner in +which the snow of the Alpine heights is prevented from accumulating indefinitely +in thickness by the constant descent of a large portion of it by gravitation. +Becoming converted into ice it forms what are termed glaciers, which glide down +the principal valleys. On their surface are seen mounds of rubbish or large +heaps of sand and mud, with angular fragments of rock which fall from the steep +slopes or precipices bounding the glaciers. When a glacier, thus laden, descends +so far as to reach a region about 3500 feet above the level of the sea, the +warmth of the air is such that it melts rapidly in summer, and all the mud, +sand, and pieces of rock are slowly deposited at its lower end, forming a +confused heap of unstratified rubbish called a MORAINE, and resembling the TILL +before described. + +Besides the blocks thus carried down on the top of the glacier, many fall +through fissures in the ice to the bottom, where some of them become firmly +frozen into the mass, and are pushed along the base of the glacier, abrading, +polishing, and grooving the rocky floor below, as a diamond cuts glass, or as +emery-powder polishes steel. The striae which are made, and the deep grooves +which are scooped out by this action, are rectilinear and parallel to an extent +never seen in those produced on loose stones or rocks, where shingle is hurried +along by a torrent, or by the waves on a sea-beach. In addition to these +polished, striated, and grooved surfaces of rock, another mark of the former +action of a glacier is the "roche moutonnee." Projecting eminences of rock so +called have been smoothed and worn into the shape of flattened domes by the +glacier as it passed over them. They have been traced in the Alps to great +heights above the present glaciers, and to great horizontal distances beyond +them. + +ALPINE BLOCKS ON THE JURA. + +The moraines, erratics, polished surfaces, domes, and striae, above described, +are observed in the great valley of Switzerland, fifty miles broad; and almost +everywhere on the Jura, a chain which lies to the north of this valley. The +average height of the Jura is about one-third that of the Alps, and it is now +entirely destitute of glaciers; yet it presents almost everywhere similar +moraines, and the same polished and grooved surfaces. The erratics, moreover, +which cover it, present a phenomenon which has astonished and perplexed the +geologist for more than half a century. No conclusion can be more incontestable +than that these angular blocks of granite, gneiss, and other crystalline +formations came from the Alps, and that they have been brought for a distance of +fifty miles and upward across one of the widest and deepest valleys in the +world; so that they are now lodged on a chain composed of limestone and other +formations, altogether distinct from those of the Alps. Their great size and +angularity, after a journey of so many leagues, has justly excited wonder; for +hundreds of them are as large as cottages; and one in particular, composed of +gneiss, celebrated under the name of Pierre a Bot, rests on the side of a hill +about 900 feet above the lake of Neufchatel, and is no less than 40 feet in +diameter. + +In the year 1821, M. Venetz first announced his opinion that the Alpine glaciers +must formerly have extended far beyond their present limits, and the proofs +appealed to by him in confirmation of this doctrine were acknowledged by all +subsequent observers, and greatly strengthened by new observations and +arguments. M. Charpentier supposed that when the glaciers extended continuously +from the Alps to the Jura, the former mountains were 2000 or 3000 feet higher +than at present. Other writers, on the contrary, conjectured that the whole +country had been submerged, and the moraines and erratic blocks transported on +floating icebergs; but a careful study of the distribution of the travelled +masses, and the total absence of marine shells from the old glacial drift of +Switzerland, have entirely disproved this last hypothesis. In addition to the +many evidences of the action of ice in the northern parts of Europe which we +have already mentioned, there occur here and there in some of these countries, +what are wanting in Switzerland, deposits of marine fossil shells, which exhibit +so arctic a character that they must have led the geologist to infer the former +prevalence of a much colder climate, even had he not encountered so many +accompanying signs of ice-action. The same marine shells demonstrate the +submergence of large areas in Scandinavia and the British Isles, during the +glacial cold. + +A characteristic feature of the deposits under consideration in all these +countries is the occurrence of large erratic blocks, and sometimes of moraine +matter, in situations remote from lofty mountains, and separated from the +nearest points where the parent rocks appear at the surface by great intervening +valleys, or arms of the sea. We also often observe striae and furrows, as in +Norway, Sweden, and Scotland, which deviate from the direction which they ought +to follow if they had been connected with the present line of drainage, and +they, therefore, imply the prevalence of a very distinct condition of things at +the time when the cold was most intense. The actual state of North Greenland +seems to afford the best explanation of such abnormal glacial markings. + +GREENLAND CONTINENTAL ICE. + +Greenland is a vast unexplored continent, buried under one continuous and +colossal mass of ice that is always moving seaward, a very small part of it in +an easterly direction, and all the rest westward, or towards Baffin's Bay. All +the minor ridges and valleys are levelled and concealed under a general covering +of snow, but here and there some steep mountains protrude abruptly from the icy +slope, and a few superficial lines of stones or moraines are visible at certain +seasons, when no snow has fallen for many months, and when evaporation, promoted +by the wind and sun, has caused much of the upper snow to disappear. The height +of this continent is unknown, but it must be very great, as the most elevated +lands of the outskirts, which are described as comparatively low, attain +altitudes of 4000 to 6000 feet. The icy slope gradually lowers itself towards +the outskirts, and then terminates abruptly in a mass about 2000 feet in +thickness, the great discharge of ice taking place through certain large friths, +which, at their upper ends, are usually about four miles across. Down these +friths the ice is protruded in huge masses, several miles wide, which continue +their course-- grating along the rocky bottom like ordinary glaciers long after +they have reached the salt water. When at last they arrive at parts of Baffin's +Bay deep enough to buoy up icebergs from 1000 to 1500 feet in vertical +thickness, broken masses of them float off, carrying with them on their surface +not only fine mud and sand but large stones. These fragments of rock are often +polished and scored on one or more sides, and as the ice melts, they drop down +to the bottom of the sea, where large quantities of mud are deposited, and this +muddy bottom is inhabited by many mollusca. + +Although the direction of the ice-streams in Greenland may coincide in the main +with that which separate glaciers would take if there were no more ice than +there is now in the Swiss Alps, yet the striation of the surface of the rocks on +an ice-clad continent would, on the whole, vary considerably in its minor +details from that which would be imprinted on rocks constituting a region of +separate glaciers. For where there is a universal covering of ice there will be +a general outward movement from the higher and more central regions towards the +circumference and lower country, and this movement will be, to a certain extent, +independent of the minor inequalities of hill and valley, when these are all +reduced to one level by the snow. The moving ice may sometimes cross even at +right angles deep narrow ravines, or the crests of buried ridges, on which last +it may afterwards seem strange to detect glacial striae and polishing after the +liquefaction of the snow and ice has taken place. + +Rink mentions that in North Greenland powerful springs of clayey water escape in +winter from under the ice, where it descends to "the outskirts," and where, as +already stated, it is often 2000 feet thick-- a fact showing how much grinding +action is going on upon the surface of the subjacent rocks. I also learn from +Dr. Torell that there are large areas in the outskirts, now no longer covered +with permanent snow or glaciers, which exhibit on their surface unmistakable +signs of ancient ice-action, so that, vast as is the power now exerted by ice in +Greenland, it must once have operated on a still grander scale. The land, though +now very elevated, may perhaps have been formerly much higher. It is well-known +that the south coast of Greenland, from latitude 60 degrees to about 70 degrees +north, has for the last four centuries been sinking at the rate of several feet +in a century. By this means a surface of rock, well scored and polished by ice, +is now slowly subsiding beneath the sea, and is becoming strewed over, as the +icebergs melt, with impalpable mud and smoothed and scratched stones. It is not +precisely known how far north this downward movement extends. + +DRIFT CARRIED BY ICEBERGS. + +An account was given so long ago as the year 1822, by Scoresby, of icebergs seen +by him in the Arctic seas drifting along in latitudes 69 and 70 degrees north, +which rose above the surface from 100 to 200 feet, and some of which measured a +mile in circumference. Many of them were loaded with beds of earth and rock, of +such thickness that the weight was conjectured to be from 50,000 to 100,000 +tons. A similar transportation of rocks is known to be in progress in the +southern hemisphere, where boulders included in ice are far more frequent than +in the north. One of these icebergs was encountered in 1839, in mid-ocean, in +the antarctic regions, many hundred miles from any known land, sailing +northward, with a large erratic block firmly frozen into it. Many of them, +carefully measured by the officers of the French exploring expedition of the +Astrolabe, were between 100 and 225 feet high above water, and from two to five +miles in length. Captain d'Urville ascertained one of them which he saw floating +in the Southern Ocean to be 13 miles long and 100 feet high, with walls +perfectly vertical. The submerged portions of such islands must, according to +the weight of ice relatively to sea-water, be from six to eight times more +considerable than the part which is visible, so that when they are once fairly +set in motion, the mechanical force which they might exert against any obstacle +standing in their way would be prodigious. + +We learn, therefore, from a study both of the arctic and antarctic regions, that +a great extent of land may be entirely covered throughout the whole year by snow +and ice, from the summits of the loftiest mountains to the sea-coast, and may +yet send down angular erratics to the ocean. We may also conclude that such land +will become in the course of ages almost everywhere scored and polished like the +rocks which underlie a glacier. The discharge of ice into the surrounding sea +will take place principally through the main valleys, although these are hidden +from our sight. Erratic blocks and moraine matter will be dispersed somewhat +irregularly after reaching the sea, for not only will prevailing winds and +marine currents govern the distribution of the drift, but the shape of the +submerged area will have its influence; inasmuch as floating ice, laden with +stones, will pass freely through deep water, while it will run a ground where +there are reefs and shallows. Some icebergs in Baffin's Bay have been seen +stranded on a bottom 1000 or even 1500 feet deep. In the course of ages such a +sea-bed may become densely covered with transported matter, from which some of +the adjoining greater depths may be free. If, as in West Greenland, the land is +slowly sinking, a large extent of the bottom of the ocean will consist of rock +polished and striated by land-ice, and then overspread by mud and boulders +detached from melting bergs. + +The mud, sand, and boulders thus let fall in still water must be exactly like +the moraines of terrestrial glaciers, devoid of stratification and organic +remains. But occasionally, on the outer side of such packs of stranded bergs, +the waves and currents may cause the detached earthy and stony materials to be +sorted according to size and weight before they reach the bottom, and to acquire +a stratified arrangement. + +I have already alluded to the large quantity of ice, containing great blocks of +stone, which is sometimes seen floating far from land, in the southern or +Antarctic seas. After the emergence, therefore, of such a submarine area, the +superficial detritus will have no necessary relation to the hills, valleys, and +river-plains over which it will be scattered. Many a water-shed may intervene +between the starting-point of each erratic or pebble and its final resting- +place, and the only means of discovering the country from which it took its +departure will consist in a careful comparison of its mineral or fossil contents +with those of the parent rocks. + + +CHAPTER XII. + +POST-PLIOCENE PERIOD, CONTINUED.-- GLACIAL CONDITIONS, CONCLUDED. + +Glaciation of Scandinavia and Russia. +Glaciation of Scotland. +Mammoth in Scotch Till. +Marine Shells in Scotch Glacial Drift. +Their Arctic Character. +Rarity of Organic Remains in Glacial Deposits. +Contorted Strata in Drift. +Glaciation of Wales, England, and Ireland. +Marine Shells of Moel Tryfaen. +Erratics near Chichester. +Glacial Formations of North America. +Many Species of Testacea and Quadrupeds survived the Glacial Cold. +Connection of the Predominance of Lakes with Glacial Action. +Action of Ice in preventing the silting up of Lake-basins. +Absence of Lakes in the Caucasus. +Equatorial Lakes of Africa. + +GLACIATION OF SCANDINAVIA AND RUSSIA. + +In large tracts of Norway and Sweden, where there have been no glaciers in +historical times, the signs of ice-action have been traced as high as 6000 feet +above the level of the sea. These signs consist chiefly of polished and furrowed +rock-surfaces, of moraines and erratic blocks. The direction of the erratics, +like that of the furrows, has usually been conformable to the course of the +principal valleys; but the lines of both sometimes radiate outward in all +directions from the highest land, in a manner which is only explicable by the +hypothesis above alluded to of a general envelope of continental ice, like that +of Greenland (Chapter 11.) Some of the far-transported blocks have been carried +from the central parts of Scandinavia towards the Polar regions; others +southward to Denmark; some south-westward, to the coast of Norfolk in England; +others south-eastward, to Germany, Poland, and Russia. + +In the immediate neighbourhood of Upsala, in Sweden, I had observed, in 1834, a +ridge of stratified sand and gravel, in the midst of which occurs a layer of +marl, evidently formed originally at the bottom of the Baltic, by the slow +growth of the mussel, cockle, and other marine shells of living species, +intermixed with some proper to fresh water. The marine shells are all of +dwarfish size, like those now inhabiting the brackish waters of the Baltic; and +the marl, in which many of them are imbedded, is now raised more than 100 feet +above the level of the Gulf of Bothnia. Upon the top of this ridge repose +several huge erratics, consisting of gneiss for the most part unrounded, from +nine to sixteen feet in diameter, and which must have been brought into their +present position since the time when the neighbouring gulf was already +characterised by its peculiar fauna. Here, therefore, we have proof that the +transport of erratics continued to take place, not merely when the sea was +inhabited by the existing testacea, but when the north of Europe had already +assumed that remarkable feature of its physical geography which separates the +Baltic from the North Sea, and causes the Gulf of Bothnia to have only one- +fourth of the saltness belonging to the ocean. In Denmark, also, recent shells +have been found in stratified beds, closely associated with the boulder clay. + +GLACIATION OF SCOTLAND. + +Mr. T.F. Jamieson, in 1858, adduced a great body of facts to prove that the +Grampians once sent down glaciers from the central regions in all directions +towards the sea. "The glacial grooves," he observed, "radiate outward from the +central heights towards all points of the compass, though they do not always +strictly conform to the actual shape and contour of the minor valleys and +ridges." + +These facts and other characteristics of the Scotch drift lead us to the +inference that when the glacial cold first set in, Scotland stood higher above +the sea than at present, and was covered for the most part with snow and ice, as +Greenland is now. This sheet of land-ice sliding down to lower levels, ground +down and polished the subjacent rocks, sweeping off nearly all superficial +deposits of older date, and leaving only till and boulders in their place. To +this continental state succeeded a period of depression and partial submergence. +The sea advanced over the lower lands, and Scotland was converted into an +archipelago, some marine sand with shells being spread over the bottom of the +sea. On this sand a great mass of boulder clay usually quite devoid of fossils +was accumulated. Lastly, the land re-emerged from the water, and, reaching a +level somewhat above its present height, became connected with the continent of +Europe, glaciers being formed once more in the higher regions, though the ice +probably never regained its former extension. (Jamieson Quarterly Geological +Journal 1860 volume 16 page 370.) After all these changes, there were some minor +oscillations in the level of the land, on which, although they have had +important geographical consequences, separating Ireland from England, for +example, and England from the Continent, we need not here enlarge. + +MAMMOTH IN SCOTCH TILL. + +Almost all remains of the terrestrial fauna of the Continent which preceded the +period of submergence have been lost; but a few patches of estuarine and fresh- +water formations escaped denudation by submergence. To these belong the peaty +clay from which several mammoths' tusks and horns of reindeer were obtained at +Kilmaurs, in Ayrshire as long ago as 1816. Mr. Bryce in 1865 ascertained that +the fresh-water formation containing these fossils rests on carboniferous +sandstone, and is covered, first by a bed of marine sand with arctic shells, and +then with a great mass of till with glaciated boulders. (Bryce Quarterly +Geological Journal volume 21 page 217 1865.) Still more recent explorations in +the neighbourhood of Kilmaurs have shown that the fresh-water formation contains +the seed of the pond-weed Potamogeton and the aquatic Ranunculus; and Mr. Young +of the Glasgow Museum washed the mud adhering to the reindeer horns of Kilmaurs +and that which filled the cracks of the associated elephants' tusks, and +detected in these fossils (which had been in the Glasgow Museum for half a +century) abundance of the same seeds. + +All doubts, therefore, as to the true position of the remains of the mammoth, a +fossil so rare in Scotland, have been set at rest, and it serves to prove that +part of the ancient continent sank beneath the sea at a period of great cold, as +the shells of the overlying sand attest. The incumbent till or boulder clay is +about 40 feet thick, but it often attains much greater thickness in the same +part of Scotland. + +MARINE SHELLS OF SCOTCH DRIFT. + +(FIGURE 107. Astarte borealis, Chem.; (A. arctica, Moll. A. compressa, Mont.) + +(FIGURE 108. Leda lanceolata (oblonga), Sowerby.) + +(FIGURE 109. Saxicava rugosa, Penn.) + +(FIGURE 110. Pecten islandicus, Moll. Northern shell common in the drift of the +Clyde, in Scotland. ) + +(FIGURE 111. Natica clausa, Bred. Northern shell common in the drift of the +Clyde, in Scotland.) + +(FIGURE 112. Trophon clathratum, Linne. Northern shell common in the drift of +the Clyde, in Scotland.) + +(FIGURE 113. Leda truncata. +a. Exterior of left valve. +b. Interior of same.) + +(FIGURE 114. Tellina calcarea, Chem. (Tellina proxima, Brown.) +a. Outside of left valve. +b. Interior of same.) + +The greatest height to which marine shells have yet been traced in this boulder +clay is at Airdie, in Lanarkshire, ten miles east of Glasgow, 524 feet above the +level of the sea. At that spot they were found imbedded in stratified clays with +till above and below them. There appears no doubt that the overlying deposit was +true glacial till, as some boulders of granite were observed in it, which must +have come from distances of sixty miles at the least. + +The shells figured in Figures 107 to 112 are only a few out of a large +assemblage of living species, which, taken as a whole, bear testimony to +conditions far more arctic than those now prevailing in the Scottish seas. But a +group of marine shells, indicating a still greater excess of cold, has been +brought to light since 1860 by the Reverend Thomas Brown, from glacial drift or +clay on the borders of the estuaries of the Forth and Tay. This clay occurs at +Elie, in Fife, and at Errol, in Perthshire; and has already afforded about 35 +shells, all of living species, and now inhabitants of arctic regions, such as +Leda truncata, Tellina proxima (see Figures 113 and 114), Pecten Groenlandicus, +Crenella laevigata, Crenella nigra, and others, some of them first brought by +Captain Sir E. Parry from the coast of Melville Island, latitude 76 degrees +north. These were all identified in 1863 by Dr. Torell, who had just returned +from a survey of the seas around Spitzbergen, where he had collected no less +than 150 species of mollusca, living chiefly on a bottom of fine mud derived +from the moraines of melting glaciers which there protrude into the sea. He +informed me that the fossil fauna of this Scotch glacial deposit exhibits not +only the species but also the peculiar varieties of mollusca now characteristic +of very high latitudes. Their large size implies that they formerly enjoyed a +colder, or, what was to them a more genial climate, than that now prevailing in +the latitude where the fossils occur. Marine shells have also been found in the +glacial drift of Caithness and Aberdeenshire at heights of 250 feet, and in +Banff of 350 feet, and stratified drift continuous with the above ascends to +heights of 500 feet. Already 75 species are enumerated from Caithness, and the +same number from Aberdeenshire and Banff, and in both cases all but six are +arctic species. + +I formerly suggested that the absence of all signs of organic life in the Scotch +drift might be connected with the severity of the cold, and also in some places +with the depth of the sea during the period of extreme submergence; but my faith +in such an hypothesis has been shaken by modern investigations, an exuberance of +life having been observed both in arctic and antarctic seas of great depth, and +where floating ice abounds. The difficulty, moreover, of accounting for the +entire dearth of marine shells in till is removed when once we have adopted the +theory of this boulder clay being the product of land-ice. For glaciers coming +down from a continental ice-sheet like that which covers Greenland may fill +friths many hundred feet below the sea-level, and even invade parts of a bay a +thousand feet deep, before they find water enough to float off their terminal +portions in the form of icebergs. In such a case till without marine shells may +first accumulate, and then, if the climate becomes warmer and the ice melts, a +marine deposit may be superimposed on the till without any change of level being +required. + +Another curious phenomenon bearing on this subject was styled by the late Hugh +Miller the "striated pavements" of the boulder clay. Where portions of the till +have been removed by the sea on the shores of the Forth, or in the interior by +railway cuttings, the boulders imbedded in what remains of the drift are seen to +have been all subjected to a process of abrasion and striation, the striae and +furrows being parallel and persistent across them all, exactly as if a glacier +or iceberg had passed over them and scored them in a manner similar to that so +often undergone by the solid rocks below the glacial drift. It is possible, as +Mr. Geikie conjectures, that this second striation of the boulders may be +referable to floating ice. (Geikie Transactions of the Geological Society of +Glasgow volume 1 part 2 page 68 1863.) + +CONTORTED STRATA IN DRIFT. + +(FIGURE 115. Section of contorted drift overlying till, seen on left bank of +South Esk, near Cortachie, in 1840. Height of section seen, from a to d, about +50 feet. +a, b. Gravel and sand. +f, g. Contorted drift. +Till.) + +In Scotland the till is often covered with stratified gravel, sand, and clay, +the beds of which are sometimes horizontal and sometimes contorted for a +thickness of several feet. Such contortions are not uncommon in Forfarshire, +where I observed them, among other places, in a vertical cutting made in 1840 +near the left bank of the South Esk, east of the bridge of Cortachie. The +convolutions of the beds of fine and coarse sand, gravel, and loam, extend +through a thickness of no less than 25 feet vertical, or from b to c, Figure +115, the horizontal stratification being resumed very abruptly at a short +distance, as to the right of f, g. The overlying coarse gravel and sand, a, is +in some places horizontal, in others it exhibits cross bedding, and does not +partake of the disturbances which the strata b, c, have undergone. The +underlying till is exposed for a depth of about 20 feet; and we may infer from +sections in the neighbourhood that it is considerably thicker. + +In some cases I have seen fragments of stratified clays and sands, bent in like +manner, in the middle of a great mass of till. Mr. Trimmer has suggested, in +explanation of such phenomena, the intercalation in the glacial period of large +irregular masses of snow or ice between layers of sand and gravel. Some of the +cliffs near Behring's Straits, in which the remains of elephants occur, consist +of ice mixed with mud and stones; and Middendorf describes the occurrence in +Siberia of masses of ice, found at various depths from the surface after digging +through drift. Whenever the intercalation of snow and ice with drift, whether +stratified or unstratified, has taken place, the melting of the ice will cause +such a failure of support as may give rise to flexures, and sometimes to the +most complicated foldings. But in many cases the strata may have been bent and +deranged by the mechanical pressure of an advancing glacier, or by the sideway +thrust of huge islands of ice running aground against sandbanks; in which case, +the position of the beds forming the foundation of the banks may not be at all +disturbed by the shock. + +There are indeed many signs in Scotland of the action of floating ice, as might +have been expected where proofs of submergence in the Glacial Period are not +wanting. Among these are the occurrence of large erratic blocks, frequently in +clusters at or near the tops of hills or ridges, places which may have formed +islets or shallows in the sea where floating ice would mostly ground and +discharge its cargo on melting. Glaciers or land-ice would, on the contrary, +chiefly discharge their cargoes at the bottom of valleys. Traces of an earlier +and independent glaciation have also been observed in some regions where the +striation, apparently produced by ice proceeding from the north-west, is not +explicable by the radiation of land-ice from a central mountainous region. +(Milne Home Transactions of the Royal Society Edinburgh volume 25 1868-9.) + +GLACIATION OF WALES AND ENGLAND. + +The mountains of North Wales were recognised, in 1842, by Dr. Buckland, as +having been an independent centre of the dispersion of erratics-- great +glaciers, long since extinct, having radiated from the Snowdonian heights in +Carnarvonshire, through seven principal valleys towards as many points of the +compass, carrying with them large stony fragments, and grooving the subjacent +rocks in as many directions. + +Besides this evidence of land-glaciers, Mr. Trimmer had previously, in 1831, +detected the signs of a great submergence in Wales in the Post-pliocene period. +He had observed stratified drift, from which he obtained about a dozen species +of marine shells, near the summit of Moel Tryfaen, a hill 1400 feet high, on the +south side of the Menai Straits. I had an opportunity of examining in the summer +of 1863, together with the Reverend W.S. Symonds, a long and deep cutting made +through this drift by the Alexandra Mining Company in search of slates. At the +top of the hill above-mentioned we saw a stratified mass of incoherent sand and +gravel 35 feet thick, from which no less than 54 species of mollusca, besides +three characteristic arctic varieties-- in all 57 forms-- have been obtained by +Mr. Darbishire. They belong without exception to species still living in British +or more northern seas; eleven of them being exclusively arctic, four common to +the arctic and British seas, and a large proportion of the remainder having a +northward range, or, if found at all in the southern seas of Britain, being +comparatively less abundant. In the lowest beds of the drift were large heavy +boulders of far-transported rocks, glacially polished and scratched on more than +one side. Underneath the whole we saw the edges of vertical slates exposed to +view, which here, like the rocks in other parts of Wales, both at greater and +less elevations, exhibit beneath the drift unequivocal marks of prolonged +glaciation. The whole deposit has much the appearance of an accumulation in +shallow water or on a beach, and it probably acquired its thickness during the +gradual subsidence of the coast-- an hypothesis which would require us to +ascribe to it a high antiquity, since we must allow time, first for its sinking, +and then for its re-elevation. + +The height reached by these fossil shells on Moel Tryfaen is no less than 1300 +feet-- a most important fact when we consider how very few instances we have on +record beyond the limits of Wales, whether in Europe or North America, of marine +shells having been found in glacial drift at half the height above indicated. A +marine molluscous fauna, however, agreeing in character with that of Moel +Tryfaen, and comprising as many species, has been found in drift at Macclesfield +and other places in central England, sometimes reaching an elevation of 1200 +feet. + +Professor Ramsay estimated the probable amount of submergence during some part +of the glacial period at about 2300 feet; for he was unable to distinguish the +superficial sands and gravel which reached that high elevation from the drift +which, at Moel Tryfaen and at lower points, contains shells of living species. +The evidence of the marine origin of the highest drift is no doubt inconclusive +in the absence of shells, so great is the resemblance of the gravel and sand of +a sea beach and of a river's bed, when organic remains are wanting; but, on the +other hand, when we consider the general rarity of shells in drift which we know +to be of marine origin, we can not suppose that, in the shelly sands of Moel +Tryfaen, we have hit upon the exact uppermost limit of marine deposition, or, in +other words, a precise measure of the submergence of the land beneath the sea +since the glacial period. + +We are gradually obtaining proofs of the larger part of England, north of a line +drawn from the mouth of the Thames to the Bristol Channel, having been under the +sea and traversed by floating ice since the commencement of the glacial epoch. +Among recent observations illustrative of this point, I may allude to the +discovery, by Mr. J.F. Bateman, near Blackpool, in Lancashire, fifty miles from +the sea, and at the height of 568 feet above its level, of till containing +rounded and angular stones and marine shells, such as Turritella communis, +Purpura lapillus, Cardium edule, and others, among which Trophon clathratum +(=Fusus Bamffius), though still surviving in North British seas, indicates a +cold climate. + +ERRATICS NEAR CHICHESTER. + +The most southern memorials of ice-action and of a Post-pliocene fauna in Great +Britain is on the coast of the county of Sussex, about 25 miles west of +Brighton, and 15 south of Chichester. A marine deposit exposed between high and +low tide occurs on both sides of the promontory called Selsea Bill, in which Mr. +Godwin-Austen found thirty-eight species of shells, and the number has since +been raised to seventy. + +This assemblage is interesting because on the whole, while all the species are +recent, they have a somewhat more southern aspect than those of the present +British Channel. It is true that about forty of them range from British to high +northern latitudes; but several of them, as, for example, Lutraria rugosa and +Pecten polymorphous, which are abundant, are not known at present to range +farther north than the coast of Portugal, and seem to indicate a warmer +temperature than now prevails on the coast where we find them fossil. What +renders this curious is the fact that the sandy loam in which they occur is +overlaid by yellow clayey gravel with large erratic blocks which must have been +drifted into their present position by ice when the climate had become much +colder. These transported fragments of granite, syenite, and greenstone, as well +as of Devonian and Silurian rocks, may have come from the coast of Normandy and +Brittany, and are many of them of such large size that we must suppose them to +have been drifted into their present site by coast-ice. I measured one of +granite, at Pagham, 21 feet in circumference. In the gravel of this drift with +erratics are a few littoral shells of living species, indicating an ancient +coast-line. + +GLACIAL FORMATIONS OF NORTH AMERICA. + +In the western hemisphere, both in Canada and as far south as the 40th and even +38th parallel of latitude in the United States, we meet with a repetition of all +the peculiarities which distinguish the European boulder formation. Fragments of +rock have travelled for great distances, especially from north to south: the +surface of the subjacent rock is smoothed, striated, and fluted; unstratified +mud or TILL containing boulders is associated with strata of loam, sand, and +clay, usually devoid of fossils. Where shells are present, they are of species +still living in northern seas, and not a few of them identical with those +belonging to European drift, including most of those already given in Figures +107 to 112. The fauna also of the glacial epoch in North America is less rich in +species than that now inhabiting the adjacent sea, whether in the Gulf of St. +Lawrence, or off the shores of Maine, or in the Bay of Massachusetts. + +The extension on the American continent of the range of erratics during the +Post-pliocene period to lower latitudes than they reached in Europe, agrees well +with the present southward deflection of the isothermal lines, or rather the +lines of equal winter temperature. It seems that formerly, as now, a more +extreme climate and a more abundant supply of ice prevailed on the western side +of the Atlantic. Another resemblance between the distribution of the drift +fossils in Europe and North America has yet to be pointed out. In Canada and the +United States, as in Europe, the marine shells are generally confined to very +moderate elevations above the sea (between 100 and 700 feet), while the erratic +blocks and the grooved and polished surfaces of rock extend to elevations of +several thousand feet. + +I have already mentioned that in Europe several quadrupeds of living, as well as +extinct, species were common to pre-glacial and post-glacial times. In like +manner there is reason to suppose that in North America much of the ancient +mammalian fauna, together with nearly all the invertebrata, lived through the +ages of intense cold. That in the United States the Mastodon giganteus was very +abundant after the drift period, is evident from the fact that entire skeletons +of this animal are met with in bogs and lacustrine deposits occupying hollows in +the glacial drift. They sometimes occur in the bottom even of small ponds +recently drained by the agriculturist for the sake of the shell-marl. In 1845 no +less than six skeletons of the same species of Mastodon were found in Warren +county, New Jersey, six feet below the surface, by a farmer who was digging out +the rich mud from a small pond which he had drained. Five of these skeletons +were lying together, and a large part of the bones crumbled to pieces as soon as +they were exposed to the air. + +It would be rash, however, to infer from such data that these quadrupeds were +mired in MODERN times, unless we use that term strictly in a geological sense. I +have shown that there is a fluviatile deposit in the valley of the Niagara, +containing shells of the genera Melania, Lymnea, Planorbis, Velvata, Cyclaz, +Unio, Helix, etc., all of recent species, from which the bones of the great +Mastodon have been taken in a very perfect state. Yet the whole excavation of +the ravine, for many miles below the Falls, has been slowly effected since that +fluviatile deposit was thrown down. Other extinct animals accompany the Mastodon +giganteus in the post-glacial deposits of the United States, and this, taken +with the fact that so few of the mollusca, even of the commencement of the cold +period, differ from species now living is important, as refuting the hypothesis, +for which some have contended, that the intensity of the glacial cold +annihilated all the species in temperate and arctic latitudes. + +CONNECTION OF THE PREDOMINANCE OF LAKES WITH GLACIAL ACTION. + +It was first pointed out by Professor Ramsay in 1862, that lakes are exceedingly +numerous in those countries where erratics, striated blocks, and other signs of +ice-action abound; and that they are comparatively rare in tropical and sub- +tropical regions. Generally in countries where the winter cold is intense, such +as Canada, Scandinavia, and Finland, even the plains and lowlands are thickly +strewn with innumerable ponds and small lakes, together with some others of a +larger size; while in more temperate regions, such as Great Britain, Central and +Southern Europe, the United States, and New Zealand, lake districts occur in all +such mountainous tracts as can be proved to have been glaciated in times +comparatively modern or since the geographical configuration of the surface bore +a considerable resemblance to that now prevailing. In the same countries, beyond +the glaciated regions, lakes abruptly cease, and in warmer and tropical +countries are either entirely absent, or consist, as in equatorial Africa, of +large sheets of water unaccompanied so far as we yet know by numerous smaller +ponds and tarns. + +The southern limits of the lake districts of the Northern Hemisphere are found +at about 40 degrees N. latitude on the American continent, and about 50 degrees +in Europe, or where the Alps intervene four degrees farther south. A large +proportion of the smaller lakes are dammed up by barriers of unstratified drift, +having the exact character of the moraines of glaciers, and are termed by +geologists "morainic," but some of them are true rock-basins, and would hold +water even if all the loose drift now resting on their margins were removed. + +In a paper read before the Geological Society of London in 1862, Professor +Ramsay maintained that the first formation of most existing lakes took place +during the glacial epoch, and was due, not to elevation or subsidence, but to +actual erosion of their basins by glaciers. M. Mortillet in the same year +advanced the theory that after the Alpine lake-basins had been filled up with +loose fluviatile deposits, they were re-excavated by the great glaciers which +passed down the valleys at the time of the greatest cold, a doctrine which would +attribute to moving ice almost as great a capacity of erosion as that which +assumed that the original basins were scooped out of solid rock by glaciers. It +is impossible to deny that the mere geographical distribution of lakes points to +the intimate connection of their origin with the abundance of ice during a +former excess of cold, but how far the erosive action of moving ice has been the +sole or even the principal cause of lake-basins, is a question still open to +discussion. + +The lakes of Switzerland and the north of Italy are some of them twenty and +thirty miles in length, and so deep that their bottoms are in some cases from +1000 to 2000 feet beneath the level of the sea. It is admitted on all hands that +they were once filled with ice, and as the existing glaciers polish and grind +down, as before stated, the surface of the rocks, we are prepared to find that +every lake-basin in countries once covered by ice should bear the marks of +superficial glaciation, and also that the ice during its advance and retreat +should have left behind it much transported matter as well as some evidence of +its having enlarged the pre-existing cavity. But much more than this is demanded +by the advocates of glacial erosion. They suggest that as the old extinct +glaciers were several thousand feet thick, they were able in some places +gradually to scoop out of the solid rock cavities twenty or thirty miles in +length, and as in the case of Lago Maggiore from a thousand to two thousand six +hundred feet below the previous level of the river-channel, and also that the +ice had the power to remove from the cavity formed by its grinding action all +the materials of the missing rocks. A constant supply, it is argued, of fine mud +issues from the termination of every glacier in the stream which is produced by +the melting of the ice, and this result of friction is exhibited both during +winter and summer, affording evidence of the continual deepening and widening of +the valleys through which glaciers pass. As the fine mud is carried away by a +river from the deep pool which is formed from the base of every cataract, so it +seems to be imagined that lake-basins may be gradually emptied of the mud formed +by abrasion during the glacial period. + +I am by no means disposed to object to this theory on the ground of the +insufficiency of the time during which the extreme cold endured, but we must +carefully consider whether that same time is not so vast as to make it probable +that other forces, besides the motion of glaciers, must have cooperated in +converting some parts of the ancient valley courses into lake-basins. They who +have formed the most exalted conceptions of the erosive energy of moving ice do +not deny that during the period termed "Glacial" there have been movements of +the earth's crust sufficient to produce oscillations of level in Europe +amounting to 1000 feet or more in both directions. M. Charpentier, indeed, +attributed some of the principal changes of climate in Switzerland, during the +glacial period, to a depression of the central Alps to the extent of 3000 feet, +and Swiss geologists have long been accustomed to attribute their lake basins, +in part, to those convulsions by which the shape and course of the valleys may +have been modified. Our experience, in the lifetime of the present generation, +of the changes of level witnessed in New Zealand during great earthquakes is +entirely opposed to the notion that the movements, whether upward or downward, +are uniform in amount or direction throughout areas of indefinite extent. On the +contrary, the land has been permanently raised in one region several feet or +yards, and the rise has been found gradually to die out, so as to be +imperceptible at a distance of twenty miles, and in some areas is even exchanged +for a simultaneous downward movement of several feet. + +But, it is asked, if such inequality of movement can have contributed towards +the production of lake basins, does it not leave unexplained the comparative +rarity of lakes in tropical and subtropical countries. In reply to this question +it may be observed that in our endeavour to estimate the effects of subterranean +movements in modifying the superficial geography of a country we must remember +that each convulsion effects a very slight change. If it interferes with the +drainage, whether by raising the lower or sinking the higher portion of a +hydrographical basin, the upheaval or depression will only amount to a few feet +at a time, and there may be an interval of years or centuries before any further +movement takes place in the same region. In the mean time an incipient lake if +produced may be filled up with sediment, and the recently-formed barrier will +then be cut through by the river, whereas in a country where glacial conditions +prevail no such obliteration of the temporary lake-basin would take place; for +however deep it became by repeated sinking of the upper or rising of the lower +extremity, being always filled with ice it might remain, throughout the greater +part of its extent, free from sediment or drift until the ice melted at the +close of the glacial period. + +One of the most serious objections to the exclusive origin by ice-erosion of +wide and deep lake-basins arises from their capricious distribution, as for +example in Piedmont, both to the eastward and westward of Turin, where great +lakes are wanting (Antiquity of Man page 313.), although some of the largest +extinct glaciers descending from Mont Blanc and Monte Rosa came down from the +Alps, leaving their gigantic moraines in the low country. Here, therefore, we +might have expected to find lakes of the first magnitude rivalling the +contiguous Lago Maggiore in importance. + +A still more striking illustration of the same absence of lakes where large +glaciers abound is afforded by the Caucasus, a chain more than 300 miles long, +and the loftiest peaks of which attain heights from 16,000 to 18,000 feet. This +greatest altitude is reached by Elbruz, a mountain in latitude 43 degrees north +three degrees south of Mont Blanc, but on the other hand 3000 feet higher. The +present Caucasian glaciers are equal or superior in dimensions to those of +Switzerland, and like them give rise occasionally to temporary lakes by +obstructing the course of rivers, and causing great floods when the icy barriers +give way. Mr. Freshfield, a careful observer, writing in 1869, says: "A total +absence of lakes on both sides of the chains is the most marked feature. Not +only are there no great subalpine sheets of water, like Como or Geneva, but +mountain tarns, such as the Dauben See on the Gemmi, or the Klonthal See near +Glarus, are equally wanting." (Travels in Central Caucasus 1869 page 452.) The +same author states on the authority of the eminent Swiss geologist, Mons. E. +Favre, who also explored the Caucasus in 1868, that moraines of great height and +huge erratics of granite and other rocks "justify the assertion that the present +glaciers of the Caucasus, like those of the Alps, are only the shadows of their +former selves." + +It seems safe to assume that the chain of lakes, of which the Albert Nyanza +forms one in equatorial Africa, was due to causes other than glacial. Yet if we +could imagine a glacial period to visit that region filling the lakes with ice +and scoring the rocks which form their sides and bottoms, we should be unable to +decide how much the capacity of the basins had been enlarged and the surface +modified by glacial erosion. The same may be true of the Lago Maggiore and Lake +Superior, although the present basins of both of them afford abundant +superficial markings due to ice-action. + +But to whatever combination of causes we attribute the great Alpine lakes one +thing is clear, namely, that they are, geologically speaking, of modern origin. +Every one must admit that the upper valley of the Rhone has been chiefly caused +by fluviatile denudation, and it is obvious that the quantity of matter removed +from that valley previous to the glacial period would have been amply sufficient +to fill up with sediment the basin of the Lake of Geneva, supposing it to have +been in existence, even if its capacity had been many times greater than it is +now. (See Principles volume 1 page 420 10th edition 1867.) + +On the whole, it appears to me, in accordance with the views of Professor +Ramsay, M. Mortillet, Mr. Geikie, and others, that the abrading action of ice +has formed some mountain tarns and many morainic lakes; but when it is a +question of the origin of larger and deeper lakes, like those of Switzerland or +the north of Italy, or inland fresh-water seas, like those of Canada, it will +probably be found that ice has played a subordinate part in comparison with +those movements by which changes of level in the earth's crust are gradually +brought about. + + +TERTIARY OR CAINOZOIC PERIOD. + + +CHAPTER XIII. + +PLIOCENE PERIOD. + +Glacial Formations of Pliocene Age. +Bridlington Beds. +Glacial Drifts of Ireland. +Drift of Norfolk Cliffs. +Cromer Forest-bed. +Aldeby and Chillesford Beds. +Norwich Crag. +Older Pliocene Strata. +Red Crag of Suffolk. +Coprolitic Bed of Red Crag. +White or Coralline Crag. +Relative Age, Origin, and Climate of the Crag Deposits. +Antwerp Crag. +Newer Pliocene Strata of Sicily. +Newer Pliocene Strata of the Upper Val d'Arno. +Older Pliocene of Italy. +Subapennine Strata. +Older Pliocene Flora of Italy. + +It will be seen in the description given in the last chapter of the Post- +pliocene formations of the British Isles that they comprise a large proportion +of those commonly termed glacial, characterised by shells which, although +referable to living species, usually indicate a colder climate than that now +belonging to the latitudes where they occur fossil. But in parts of England, +more especially in Yorkshire, Norfolk, and Suffolk, there are superficial +formations of clay with glaciated boulders, and of sand and pebbles, containing +occasional, though rare, patches of shells, in which the marine fauna begins to +depart from that now inhabiting the neighbouring sea, and comprises some species +of mollusca not yet known as living, as well as extinct varieties of others, +entitling us to class them as Newer Pliocene, although belonging to the close of +that period and chronologically on the verge of the later or Post-pliocene +epoch. + +BRIDLINGTON DRIFT. + +To this era belongs the well-known locality of Bridlington, near the mouth of +the Humber, in Yorkshire, where about seventy species or well-marked varieties +of shells have been found on the coast, near the sea-level, in a bed of sand +several feet thick resting on glacial clay with much chalk debris, and covered +by a deposit of purple clay with glaciated boulders. More than a third of the +species in this drift are now inhabitants of arctic regions, none of them +extending southward to the British seas; which is the more remarkable as +Bridlington is situated in latitude 54 degrees north. Fifteen species are +British and Arctic, a very few belong to those species which range south of our +British seas. Five species or well-marked varieties are not known living, +namely, the variety of Astarte borealis (called A. Withami); A. mutabilis; the +sinistral form of Tritonium carinatum, Cardita analis, and Tellina obliqua, +Figure 120. Mr. Searles Wood also inclines to consider Nucula Cobboldiae, Figure +119, now absent from the European seas and the Atlantic, as specifically +distinct from a closely-allied shell now living in the seas surrounding +Vancouver's Island, which some conchologists regard as a variety. Tellina +obliqua also approaches very near to a shell now living in Japan. + +GLACIAL DRIFT OF IRELAND. + +Marine drift containing the last-mentioned Nucula and other glacial shells +reaches a height of from 1000 to 1200 feet in the county of Wexford, south of +Dublin. More than eighty species have already been obtained from this formation, +of which two, Conovulus pyramidalis and Nassa monensis, are not known as living; +while Turritella incrassata and Cypraea lucida no longer inhabit the British +seas, but occur in the Mediterranean. The great elevation of these shells, and +the still greater height to which the surface of the rocks in the mountainous +regions of Ireland have been smoothed and striated by ice-action, has led +geologists to the opinion that that island, like the greater part of England and +Scotland, after having been united with the continent of Europe, from whence it +received the plants and animals now inhabiting it, was in great part submerged. +The conversion of this and other parts of Great Britain into an archipelago was +followed by a re-elevation of land and a second continental period. After all +these changes the final separation of Ireland from Great Britain took place, and +this event has been supposed to have preceded the opening of the straits of +Dover. (See Antiquity of Man chapter 14.) + +DRIFT OF NORFOLK CLIFFS. + +(FIGURE 116. Tellina balthica (T. solidula).) + +There are deposits of boulder clay and till in the Norfolk cliffs principally +made up of the waste of white chalk and flints which, in the opinion of Mr. +Searles Wood, jun., and others, are older than the Bridlington drift, and +contain a larger proportion of shells common to the Norwich and Red Crag, +including a certain number of extinct forms, but also abounding in Tellina +balthica (T. solidula, Figure 116), which is found fossil at Bridlington, and +living in our British seas, but wanting in all the formations, even the newest, +afterwards to be described as Crag. As the greater part of these drifts are +barren of organic remains, their classification is at present a matter of great +uncertainty. + +They can nowhere be so advantageously studied as on the coast between +Happisburgh and Cromer. Here we may see vertical cliffs, sometimes 300 feet and +more in height, exposed for a distance of fifty miles, at the base of which the +chalk with flints crops out in nearly horizontal strata. Beds of gravel and sand +repose on this undisturbed chalk. They are often strangely contorted, and +envelop huge masses or erratics of chalk with layers of vertical flint. I +measured one of these fragments in 1839 at Sherringham, and found it to be +eighty feet in its longest diameter. It has been since entirely removed by the +waves of the sea. In the floor of the chalk beneath it the layers of flint were +horizontal. Such erratics have evidently been moved bodily from their original +site, probably by the same glacial action which has polished and striated some +of the accompanying granitic and other boulders, occasionally six feet in +diameter, which are imbedded in the drift. + +CROMER FOREST-BED. + +Intervening between these glacial formations and the subjacent chalk lies what +has been called the Cromer Forest-bed. This buried forest has been traced from +Cromer to near Kessingland, a distance of more than forty miles, being exposed +at certain seasons between high and low water mark. It is the remains of an old +land and estuarine deposit, containing the submerged stumps of trees standing +erect with their roots in the ancient soil. Associated with the stumps and +overlying them, are lignite beds with fresh-water shells of recent species, and +laminated clay without fossils. Through the lignite and forest-bed are scattered +cones of the Scotch and spruce firs with the seeds of recent plants, and the +bones of at least twenty species of terrestrial mammalia. Among these are two +species of elephant, E. meridionalis, Nesti, and E. antiquus, the former found +in the Newer Pliocene beds of the Val d'Arno, near Florence. In the same bed +occur Hippopotamus major, Rhinoceros etruscus, both of them also Val d'Arno +species, many species of deer considered by Mr. Boyd Dawkins to be +characteristic of warmer countries, and also a horse, beaver, and field-mouse. +Half of these mammalia are extinct, and the rest still survive in Europe. The +vegetation taken alone does not imply a temperature higher than that now +prevailing in the British Isles. There must have been a subsidence of the forest +to the amount of 400 or 500 feet, and a re-elevation of the same to an equal +extent in order to allow the ancient surface of the chalk or covering of soil, +on which the forest grew, to be first covered with several hundred feet of +drift, and then upheaved so that the trees should reach their present level. +Although the relative antiquity of the forest-bed to the overlying glacial till +is clear, there is some difference of opinion as to its relation to the crag +presently to be described. + +CHILLESFORD AND ALDEBY BEDS. + +(FIGURE 117. Natica helicoides, Johnson.) + +It is in the counties of Norfolk, Suffolk, and Essex, that we obtain our most +valuable information respecting the British Pliocene strata, whether newer or +older. They have obtained in those counties the provincial name of "Crag," +applied particularly to masses of shelly sand which have long been used in +agriculture to fertilise soils deficient in calcareous matter. At Chillesford, +between Woodbridge and Aldborough in Suffolk, and Aldeby, near Beccles, in the +same county, there occur stratified deposits, apparently older than any of the +preceding drifts of Yorkshire, Norfolk, and Suffolk. They are composed at +Chillesford of yellow sands and clays, with much mica, forming horizontal beds +about twenty feet thick. Messrs. Prestwich and Searles Wood, senior, who first +described these beds, point out that the shells indicate on the whole a colder +climate than the Red Crag; two-thirds of them being characteristic of high +latitudes. Among these are Cardium Groenlandicum, Leda limatula, Tritonium +carinatum, and Scalaria Groenlandica. In the upper part of the laminated clays a +skeleton of a whale was found associated with casts of the characteristic +shells, Nucula Cobboldiae and Tellina obliqua, already referred to as no longer +inhabiting our seas, and as being extinct varieties if not species. The same +shells occur in a perfect state in the lower part of the formation. Natica +helicoides (Figure 117) is an example of a species formerly known only as +fossil, but which has now been found living in our seas. + +At Aldeby, where beds occur decidedly similar in mineral character as well as +fossil remains, Messrs. Crowfoot and Dowson have now obtained sixty-six species +of mollusca, comprising the Chillesford species and some others. Of these about +nine-tenths are recent. They are in a perfect state, clearly indicating a cold +climate; as two-thirds of them are now met with in arctic regions. As a rule, +the lamellibranchiate molluscs have both valves united, and many of them, such +as Mya arenaria, stand with the siphonal end upward, as when in a living state. +Tellina balthica, before mentioned (Figure 116) as so characteristic of the +glacial beds, including the drift of Bridlington, has not yet been found in +deposits of Chillesford and Aldeby age, whether at Sudbourn, East Bavent, +Horstead, Coltishall, Burgh, or in the highest beds overlying the Norwich Crag +proper at Bramerton and Thorpe. + +NORWICH OR FLUVIO-MARINE CRAG. + +(FIGURE 118. Mastodon arvernensis, third milk molar, left side, upper jaw; +grinding surface, natural size. Norwich Crag, Postwick, also found in Red Crag, +see below.) + +The beds above alluded to ought, perhaps, to be regarded as beds of passage +between the glacial formations and those called from a provincial name "Crag," +the newest member of which has been commonly called the "Norwich Crag." It is +chiefly seen in the neighbourhood of Norwich, and consists of beds of incoherent +sand, loam, and gravel, which are exposed to view on both banks of the Yare, as +at Bramerton and Thorpe. As they contain a mixture of marine, land, and fresh- +water shells, with bones of fish and mammalia, it is clear that these beds have +been accumulated at the bottom of a sea near the mouth of a river. They form +patches rarely exceeding twenty feet in thickness, resting on white chalk. At +their junction with the chalk there invariably intervenes a bed called the +"Stone-bed," composed of unrolled chalk-flints, commonly of large size, mingled +with the remains of a land fauna comprising Mastodon arvernensis, Elephas +meridionalis, and an extinct species of deer. The mastodon, which is a species +characteristic of the Pliocene strata of Italy and France, is the most abundant +fossil, and one not found in the Cromer forest before mentioned. When these +flints, probably long exposed in the atmosphere, became submerged, they were +covered with barnacles, and the surface of the chalk became perforated by the +Pholas crispata, each fossil shell still remaining at the bottom of its +cylindrical cavity, now filled up with loose sand from the incumbent crag. This +species of Pholas still exists, and drills the rocks between high and low water +on the British coast. The name of "Fluvio-marine" has often been given to this +formation, as no less than twenty species of land and fresh-water shells have +been found in it. They are all of living species; at least only one univalve, +Paludina lenta, has any, and that a very doubtful, claim to be regarded as +extinct. + +(FIGURE 119. Nucula Cobboldiae.) + +(FIGURE 120. Tellina obliqua.) + +Of the marine shells, 124 in number, about 18 per cent are extinct, according to +the latest estimate given me by Mr. Searles Wood; but, for reasons presently to +be mentioned, this percentage must be only regarded as provisional. It must also +be borne in mind that the proportion of recent shells would be augmented if the +uppermost beds at Bramerton, near Norwich, which belong to the most modern or +Chillesford division of the Crag, had been included, as they were formerly, by +Mr. Woodward and myself, in the Norwich series. Arctic shells, which formed so +large a proportion in the Chillesford and Aldeby beds, are more rare in the +Norwich Crag, though many northern species-- such as Rhynchonella psittacea, +Scalaria Groenlandica, Astarte borealis, Panopaea Norvegia, and others-- still +occur. The Nucula Cobboldiae and Tellina obliqua, Figures 119 and 120, before +mentioned, are frequent in these beds, as are also Littorina littorea, Cardium +edule, and Turritella communis, of our seas, proving the littoral origin of the +beds. + +OLDER PLIOCENE STRATA. + +RED CRAG. + +(FIGURE 121. Section through (left) sea, Red Crag, London Clay and Chalk +(right).) + +Among the English Pliocene beds the next in antiquity is the Red Crag, which +often rests immediately on the London Clay, as in the county of Essex, +illustrated in Figure 121. + +It is chiefly in the county of Suffolk that it is found, rarely exceeding twenty +feet in thickness, and sometimes overlying another Pliocene deposit, the +Coralline Crag, to be mentioned in the sequel. It has yielded-- exclusive of 25 +species regarded by Mr. Wood as derivative-- 256 species of mollusca, of which +65, or 25 per cent, are extinct. Thus, apart from its order of superposition, +its greater antiquity than the Norwich and glacial beds, already described, is +proved by the greater departure from the fauna of our seas. It may also be +observed that in most of the deposits of this Red Crag, the northern forms of +the Norwich Crag, and of such glacial formations as Bridlington, are less +numerous, while those having a more southern aspect begin to make their +appearance. Both the quartzose sand, of which it chiefly consists, and the +included shells, are most commonly distinguished by a deep ferruginous or +ochreous colour, whence its name. The shells are often rolled, sometimes +comminuted, and the beds have much the appearance of having been shifting sand- +banks, like those now forming on the Dogger-bank, in the sea, sixty miles east +of the coast of Northumberland. Cross stratification is almost always present, +the planes of the strata being sometimes directed towards one point of the +compass, sometimes to the opposite, in beds immediately overlying. That such a +structure is not deceptive or due to any subsequent concretionary rearrangement +of particles, or to mere bands of colour produced by the iron, is proved by each +bed being made up of flat pieces of shell which lie parallel to the planes of +the smaller strata. + +(FIGURE 122. Purpura tetragona, Sowerby; natural size.) + +(FIGURE 123. Voluta Lamberti, Sowerby. Variety characteristic of Suffolk Crag. +Pliocene.) + +(FIGURE 124. Voluta Lamberti, young individual, Cor. and Red Crag.) + +It has long been suspected that the different patches of Red Crag are not all of +the same age, although their chronological relation can not be decided by +superposition. Separate masses are characterised by shells specifically distinct +or greatly varying in relative abundance, in a manner implying that the deposits +containing them were separated by intervals of time. At Butley, Tunstall, +Sudbourn, and in the Red Crag of Chillesford, the mollusca appear to assume +their most modern aspect when the climate was colder than when the earliest +deposits of the same period were formed. At Butley, Nucula Cobboldiae, so common +in the Norwich and certain glacial beds, is found, and Purpura tetragona (Figure +122) is very abundant. On the other hand, at Walton-on-the-Naze, in Essex, we +seem to have an exhibition of the oldest phase of the Red Crag; and a warmer +climate seems indicated, not only by the absence of many northern forms, but +also by the abundance of some now living in the British seas and the +Mediterranean. Voluta Lamberti (see Figures 123 and 124), an extinct form, which +seems to have flourished chiefly in the antecedent Coralline Crag period, is +still represented here by individuals of every age. + +(FIGURE 125. Trophon antiquum, Muller. (Fusus contrarius) half natural size.) + +The reversed whelk (Figure 125) is common at Walton, where the dextral form of +that shell is unknown. Here also we find most frequently specimens of +lamellibranchiate molluscs, with both the valves united, showing that they +belonged to this sea of the Upper Crag, and were not washed in from an older +bed, such as the Coralline, in which case the ligament would not have held +together the valves in strata so often showing signs of the boisterous action of +the waves. No less than forty species of lamellibranchiate molluscs, with double +valves, have been collected by Mr. Bell from the various localities of the Red +Crag. + +At and near the base of the Red Crag is a loose bed of brown nodules, first +noticed by Professor Henslow as containing a large percentage of earthy +phosphates. This bed of coprolites (as it is called, because they were +originally supposed to be the faeces of animals) does not always occur at one +level, but is generally in largest quantity at the junction of the Crag and the +underlying formation. In thickness it usually varies from six to eighteen +inches, and in some rare cases amounts to many feet. It has been much used in +agriculture for manure, as not only the nodules, but many of the separate bones +associated with them, are largely impregnated with phosphate of lime, of which +there is sometimes as much as sixty per cent. They are not unfrequently covered +with barnacles, showing that they were not formed as concretions in the stratum +where they now lie buried, but had been previously consolidated. The phosphatic +nodules often collect fossil crabs and fishes from the London Clay, together +with the teeth of gigantic sharks. In the same bed have been found many ear- +bones of whales, and the teeth of Mastodon arvernensis, Rhinoceros +Schleiermacheri, Tapirus priscus, and Hipparion (a quadruped of the horse +family), and antlers of a stag, Cervus anoceros. Organic remains also of the +older chalk and Lias are met with, showing how great was the denudation of +previous formations during the Pliocene period. As the older White Crag, +presently to be mentioned, contains similar phosphatic nodules near its base, +those of the Red Crag may be partly derived from this source. + +WHITE OR CORALLINE CRAG. + +The lower or Coralline Crag is of very limited extent, ranging over an area +about twenty miles in length, and three or four in breadth, between the rivers +Stour and Alde, in Suffolk. It is generally calcareous and marly-- often a mass +of comminuted shells, and the remains of bryozoa (or polyzoa), passing +occasionally into a soft building-stone. (Ehrenberg proposed in 1831 the term +Bryozoum, or "Moss-animal," for the molluscous or ascidian form of polyp, +characterised by having two openings to the digestive sack, as in Eschara, +Flustra, Retepora, and other zoophytes popularly included in the corals, but now +classed by naturalists as mollusca. The term Polyzoum, synonymous with Bryozoum, +was, it seems, proposed in 1830, or the year before, by Mr. J.O. Thompson.) At +Sudbourn and Gedgrave, near Orford, this building-stone has been largely +quarried. At some places in the neighbourhood the softer mass is divided by thin +flags of hard limestone, and bryozoa placed in the upright position in which +they grew. From the abundance of these coralloid mollusca the lowest or White +Crag obtained its popular name, but true corals, as now defined, or zoantharia, +are very rare in this formation. + +The Coralline Crag rarely, if ever, attains a thickness of thirty feet in any +one section. Mr. Prestwich imagines that if the beds found at different +localities were united in the probable order of their succession, they might +exceed eighty feet in thickness, but Mr. Searles Wood does not believe in the +possibility of establishing such a chronological succession by aid of the +organic remains, and questions whether proof could be obtained of more than +forty feet. I was unable to come to any satisfactory opinion on the subject, +although at Orford, especially at Gedgrave, in the neighbourhood of that place, +I saw many sections in pits, where this crag is cut through. These pits are so +unconnected, and of such limited extent, that no continuous section of any +length can be obtained, so that speculations as to the thickness of the whole +deposit must be very vague. At the base of the formation at Sutton a bed of +phosphatic nodules, very similar to that before alluded to in the Red Crag, with +remains of mammalia, has been met with. + +(FIGURE 126. Section near Woodbridge, in Suffolk. +Through Sutton (left), Shottisham Creek, Ramsholt (right) and R. Deben. +a. Red Crag. +b. Coralline Crag. +c. London Clay.) + +Whenever the Red and Coralline Crag occur in the same district, the Red Crag +lies uppermost; and in some cases, as in the section represented in Figure 126, +which I had an opportunity of seeing exposed to view in 1839, it is clear that +the older deposit, or Coralline Crag, b, had suffered denudation, before the +newer formation, a, was thrown down upon it. At D there was not only seen a +distinct cliff, eight or ten feet high, of Coralline Crag, running in a +direction N.E. and S.W., against which the Red Crag abuts with its horizontal +layers, but this cliff occasionally overhangs. The rock composing it is drilled +everywhere by Pholades, the holes which they perforated having been afterwards +filled with sand, and covered over when the newer beds were thrown down. The +older formation is shown by its fossils to have accumulated in a deeper sea, and +contains none of those littoral forms such as the limpet, Patella, found in the +Red Crag. So great an amount of denudation could scarcely take place, in such +incoherent materials, without some of the fossils of the inferior beds becoming +mixed up with the overlying crag, so that considerable difficulty must be +occasionally experienced by the palaeontologist in deciding which species belong +severally to each group. + +(FIGURE 127. Fascicularia aurantium, Milne Edwards. Family, Tubuliporidae, of +same author. Bryozoan of extinct genus, from the inferior or Coralline Crag, +Suffolk. +a. Exterior. +b. Vertical section of interior. +c. Portion of exterior magnified. +d. Portion of interior magnified, showing that it is made up of long, thin, +straight tubes, united in conical bundles.) + +(FIGURE 128. Astarte omalii, laj.; species common to Upper and lower crag.) + +Mr. Searles Wood estimates the total number of marine testaceous mollusca of the +Coralline Crag at 350, of which 110 are not known as living, being in the +proportion of thirty-one per cent extinct. No less than 130 species of bryozoa +have been found in the Coralline Crag, and some belong to genera unknown in the +living creation, and of a very peculiar structure; as, for example, that +represented in Figure 127, which is one of several species having a globular +form. Among the testacea the genus Astarte (see Figure 128) is largely +represented, no less than fourteen species being known, and many of these being +rich in individuals. There is an absence of genera peculiar to hot climates, +such as Conus, Oliva, Fasciolaria, Crassatella, and others. The absence also of +large cowries (Cyprea), those found belonging exclusively to the section Trivia, +is remarkable. The large volute, called Voluta Lamberti (Figure 123), may seem +an exception; but it differs in form from the volutes of the torrid zone, and, +like the living Voluta Magellanica, must have been fitted for an extra-tropical +climate. + +(FIGURE 129. Lingula Dumortieri, Nyst; Suffolk and Antwerp Crag.) + +(FIGURE 130. Pyrula reticulata, Lam.; Coralline Crag, Ramsholt.) + +(FIGURE 131. Temnechinus excavatus, Forbes; Temnopleurus excavatus, Wood; +Coralline Crag, Ramsholt.) + +The occurrence of a species of Lingula at Sutton (see Figure 129) is worthy of +remark, as these Brachiopoda seem now confined to more equatorial latitudes; and +the same may be said still more decidedly of a species of Pyrula, supposed by +Mr. Wood to be identical with P. reticulata (Figure 130), now living in the +Indian Ocean. A genus also of echinoderms, called by Professor Forbes +Temnechinus (Figure 131), occurs in the Red and Coralline Crag of Suffolk, and +until lately was unknown in a living state, but it has been brought to light as +an existing form by the deep-sea dredgings, both of the United States survey, +off Florida, at a depth of from 180 to 480 feet, and more recently (1869), in +the British seas, during the explorations of the "Porcupine." + +CLIMATE OF THE CRAG DEPOSITS. + +One of the most interesting conclusions deduced from a careful comparison of the +shells of the British Pliocene strata and the fauna of our present seas has been +pointed out by Professor E. Forbes. It appears that, during the Glacial period, +a period intermediate, as we have seen, between that of the Crag and our own +time, many shells, previously established in the temperate zone, retreated +southward to avoid an uncongenial climate, and they have been found fossil in +the Newer Pliocene strata of Sicily, Southern Italy, and the Grecian +Archipelago, where they may have enjoyed, during the era of floating icebergs, a +climate resembling that now prevailing in higher European latitudes. (E. Forbes +Mem. Geological Survey of Great Britain volume 1 page 386.) The Professor gave a +list of fifty shells which inhabited the British seas while the Coralline and +Red Crag were forming, and which, though now living in our seas, were wanting, +as far as was then known, in the glacial deposits. Some few of these species +have subsequently been found in the glacial drift, but the general conclusion of +Forbes remains unshaken. + +The transport of blocks by ice, when the Red Crag was being deposited, appears +to me evident from the large size of some huge, irregular, quite unrounded chalk +flints, retaining their white coating, and 2 feet long by 18 inches broad, in +beds worked for phosphatic nodules at Foxhall, four miles south-east of Ipswich. +These must have been tranquilly drifted to the spot by floating ice. Mr. +Prestwich also mentions the occurrence of a large block of porphyry in the base +of the Coralline Crag at Sutton, which would imply that the ice-action had begun +in our seas even in this older period. The cold seems to have gone on increasing +from the time of the Coralline to that of the Norwich Crag, and became more and +more severe, not perhaps without some oscillations of temperature, until it +reached its maximum in what has been called the Glacial period, or at the close +of the Newer Pliocene, and in the Post-pliocene periods. + +RELATION OF THE FAUNA OF THE CRAG TO THAT OF THE RECENT SEAS. + +By far the greater number of the recent marine species occurring in the several +Crag formations are still inhabitants of the British seas; but even these differ +considerably in their relative abundance, some of the commonest of the Crag +shells being now extremely scarce-- as, for example, Buccinum Dalei-- while +others, rarely met with in a fossil state, are now very common, as Murex +erinaceus and Cardium echinatum. Some of the species also, the identity of which +with the living would not be disputed by any conchologist, are nevertheless +distinguishable as varieties, whether by slight deviations in form or a +difference in average dimensions. Since Mr. Searles Wood first described the +marine testacea of the Crags, the additions made to that fossil fauna have not +been considerable, whereas we have made in the same period immense progress in +our knowledge of the living testacea of the British and arctic seas, and of the +Mediterranean. By this means the naturalist has been enabled to identify with +existing species many forms previously supposed to be extinct. + +In the forthcoming supplement to the invaluable monograph communicated by Mr. +Wood to the Palaeontographical Society, in which he has completed his figures +and descriptions of the British crag shells of every age, list will be found of +all the fossil shells, of which a summary is given in the table below. + +TABLE OF NUMBER OF KNOWN SPECIES OF MARINE TESTACEA IN THE CRAG. + +COLUMN 1: KNOWN SPECIES. +COLUMN 2: TOTAL NUMBER OF KNOWN SPECIES. +COLUMN 3: NUMBER OF SPECIES NOT KNOWN AS LIVING. + +CHILLESFORD AND ALDEBY BEDS: +Bivalves: 61 : 4. +Univalves: 33 : 5. +Brachiopods: 0 : 0. +PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 9.5. + +NORWICH OR FLUVIO-MARINE CRAG: +Bivalves: 61 : 10. +Univalves: 64 : 12. +Brachiopods: 1 : 0. +PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 17.5. + +RED CRAG (Exclusive of many derivative shells): +Bivalves: 128 : 31. +Univalves: 127 : 33. +Brachiopods: 1 : 1. +PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 25.0. + +CORALLINE CRAG: +Bivalves: 161 : 47. +Univalves: 184 : 60 +Brachiopods: 5 : 3 +PERCENTAGE OF SHELLS NOT KNOWN AS LIVING : 31.5 + +To begin with the uppermost or Chillesford beds, it will be seen that about 9 +per cent only are extinct, or not known as living, whereas in the Norwich, which +succeeds in the descending order, seventeen in a hundred are extinct. Formerly, +when the Norwich or Fluvio-marine Crag was spoken of, both these formations were +included under the same head, for both at Bramerton and Thorpe, the chief +localities where the Norwich Crag was studied, an overlying deposit occurs +referable to the Chillesford age. If now the two were fused together as of old, +their shells would, according to Mr. Wood, yield a percentage of fifteen in a +hundred of species extinct or not known as living. + +To come next to the Red Crag, the reader will observe that a percentage of 25 is +given of shells unknown as living, and this increases to 31 in the antecedent +Coralline Crag. But the gap between these two stages of our Pliocene deposits is +really wider than these numbers would indicate, for several reasons. In the +first place, the Coralline Crag is more strictly the product of a single period, +the Red Crag, as we have seen, consisting of separate and independent patches, +slightly varying in age, of which the newest is probably not much anterior to +the Norwich Crag. Secondly, there was a great change of conditions, both as to +the depth of the sea and climate, between the periods of the Coralline and Red +Crag, causing the fauna in each to differ far more widely than would appear from +the above numerical results. + +The value of the analysis given in the above table of the shells of the Red and +Coralline Crags is in no small degree enhanced by the fact that they were all +either collected by Mr. Wood himself, or obtained by him direct from their +discoverers, so that he was enabled in each case to test their authenticity, and +as far as possible to avoid those errors which arise from confounding together +shells belonging to the sea of a newer deposit, and those washed into it from a +formation of older date. The danger of this confusion may be conceived when we +remember that the number of species rejected from the Red Crag as derivative by +Mr. Wood is no less than 25. Some geologists have held that on the same grounds +it is necessary to exclude as spurious some of the species found in the Norwich +Crag proper; but Mr. Wood does not entertain this view, believing that the +spurious shells which have sometimes found their way into the lists of this crag +have been introduced by want of care from strata of Red Crag. + +There can be no doubt, on the other hand, that conchologists have occasionally +rejected from the Red and Norwich Crags, as derivative, shells which really +belonged to the seas of those periods, because they were extinct or unknown as +living, which in their eyes afforded sufficient ground for suspecting them to be +intruders. The derivative origin of a species may sometimes be indicated by the +extreme scarcity of the individuals, their colour, and worn condition; whereas +an opposite conclusion may be arrived at by the integrity of the shells, +especially when they are of delicate and tender structure, or their abundance, +and, in the case of the lamellibranchiata, by their being held together by the +ligament, which often happens when the shells have been so broken that little +more than the hinges of the two valves are preserved. As to the univalves, I +have seen from a pit of Red Crag, near Woodbridge, a large individual of the +extinct Voluta Lamberti, seven inches in length, of which the lip, then perfect, +had in former stages of its growth been frequently broken, and as often +repaired. It had evidently lived in the sea of the Red Crag, where it had been +exposed to rough usage, and sustained injuries like those which the reversed +whelk, Trophon antiquum, so characteristic of the same formation, often +exhibits. Additional proofs, however, have lately been obtained by Mr. Searles +Wood that this shell had not died out in the era of the Red Crag by the +discovery of the same fossil near Southwold, in beds of the later Norwich Crag. + +ANTWERP CRAG. + +Strata of the same age as the Red and Coralline Crag of Suffolk have been long +known in the country round Antwerp, and on the banks of the Scheldt, below that +city; and the lowest division, or Black Crag, there found, is shown by the +shells to be somewhat more ancient than any of our British series, and probably +forms the first links of a downward passage from the strata of the Pliocene to +those of the Upper Miocene period. + +NEWER PLIOCENE STRATA OF SICILY. + +(FIGURE 132. Murex vaginatus, Phil.) + +At several points north of Catania, on the eastern sea-coast of Sicily-- as at +Aci-Castello, for example, Trezza, and Nizzeti-- marine strata, associated with +volcanic tuffs and basaltic lavas, are seen, which belong to a period when the +first igneous eruptions of Mount Etna were taking place in a shallow bay of the +Mediterranean. They contain numerous fossil shells, and out of 142 species that +have been collected all but eleven are identical with species now living. Some +few of these eleven shells may possibly still linger in the depths of the +Mediterranean, like Murex vaginatus, see Figure 132. The last-mentioned shell +had already become rare when the associated marine and volcanic strata above +alluded to were formed. On the whole, the modern character of the testaceous +fauna under consideration is expressed not only by the small proportion of +extinct species, but by the relative number of individuals by which most of the +other species are represented, for the proportion agrees with that observed in +the present fauna of the Mediterranean. The rarity of individuals in the extinct +species is such as to imply that they were already on the point of dying out, +having flourished chiefly in the earlier Pliocene times, when the Subapennine +strata were in progress. + +Yet since the accumulation of these Newer Pliocene sands and clays, the whole +cone of Etna, 11,000 feet in height and about 90 miles in circumference at its +base, has been slowly built up; an operation requiring many tens of thousands of +years for its accomplishment, and to estimate the magnitude of which it is +necessary to study in detail the internal structure of the mountain, and to see +the proofs of its double axis, or the evidence of the lavas of the present great +centre of eruption having gradually overwhelmed and enveloped a more ancient +cone, situated 3 1/2 miles to the east of the present one. (See a Memoir on the +Lavas and Mode of Origin of Mount Etna by the Author in Philosophical +Transactions 1858.) + +It appears that while Etna was increasing in bulk by a series of eruptions, its +whole mass, comprising the foundations of subaqueous origin above alluded to, +was undergoing a slow upheaval, by which those marine strata were raised to the +height of 1200 feet above the sea, as seen at Catera, and perhaps to greater +heights, for we can not trace their extension westward, owing to the dense and +continuous covering of modern lava under which they are buried. During the +gradual rise of these Newer Pliocene formations (consisting of clays, sands, and +basalts) other strata of Post-pliocene date, marine as well as fluviatile, +accumulated round the base of the mountain, and these, in their turn, partook of +the upward movement, so that several inland cliffs and terraces at low levels, +due partly to the action of the sea and partly to the river Simeto, originated +in succession. Fossil remains of the elephant, and other extinct quadrupeds, +have been found in these Post-Pliocene strata, associated with recent shells. + +There is probably no part of Europe where the Newer Pliocene formations enter so +largely into the structure of the earth's crust, or rise to such heights above +the level of the sea, as Sicily. They cover nearly half the island, and near its +centre, at Castrogiovanni, reach an elevation of 3000 feet. They consist +principally of two divisions, the upper calcareous and the lower argillaceous, +both of which may be seen at Syracuse, Girgenti, and Castrogiovanni. According +to Philippi, to whom we are indebted for the best account of the tertiary shells +of this island, thirty-five species out of one hundred and twenty-four obtained +from the beds in central Sicily are extinct. + +A geologist, accustomed to see nearly all the Newer Pliocene formations in the +north of Europe occupying low grounds and very incoherent in texture, is +naturally surprised to behold formations of the same age so solid and stony, of +such thickness, and attaining so great an elevation above the level of the sea. +The upper or calcareous member of this group in Sicily consists in some places +of a yellowish-white stone, like the Calcaire Grossier of Paris; in others, of a +rock nearly as compact as marble. Its aggregate thickness amounts sometimes to +700 or 800 feet. It usually occurs in regular horizontal beds, and is +occasionally intersected by deep valleys, such as those of Sortino and +Pentalica, in which are numerous caverns. The fossils are in every stage of +preservation, from shells retaining portions of their animal matter and colour +to others which are mere casts. The limestone passes downward into a sandstone +and conglomerate, below which is clay and blue marl, from which perfect shells +and corals may be disengaged. The clay sometimes alternates with yellow sand. + +South of the plain of Catania is a region in which the tertiary beds are +intermixed with volcanic matter, which has been for the most part the product of +submarine eruptions. It appears that, while the clay, sand, and yellow limestone +before mentioned were in course of deposition at the bottom of the sea, +volcanoes burst out beneath the waters, like that of Graham Island, in 1831, and +these explosions recurred again and again at distant intervals of time. Volcanic +ashes and sand were showered down and spread by the waves and currents so as to +form strata of tuff, which are found intercalated between beds of limestone and +clay containing marine shells, the thickness of the whole mass exceeding 2000 +feet. The fissures through which the lava rose may be seen in many places, +forming what are called DIKES. + +(FIGURE 133. Pecten jacobaeus; half natural size.) + +No shell is more conspicuous in these Sicilian strata than the great scallop, +Pecten jacobaeus (Figure 133), now so common in the neighbouring seas. The more +we reflect on the preponderating number of this and other recent shells, the +more we are surprised at the great thickness, solidity, and height above the sea +of the rocky masses in which they are entombed, and the vast amount of +geographical change which has taken place since their origin. It must be +remembered that, before they began to emerge, the uppermost strata of the whole +must have been deposited under water. In order, therefore, to form a just +conception of their antiquity, we must first examine singly the innumerable +minute parts of which the whole is made up, the successive beds of shells, +corals, volcanic ashes, conglomerates, and sheets of lava; and we must +afterwards contemplate the time required for the gradual upheaval of the rocks, +and the excavation of the valleys. The historical period seems scarcely to form +an appreciable unit in this computation, for we find ancient Greek temples, like +those of Girgenti (Agrigentum), built of the modern limestone of which we are +speaking, and resting on a hill composed of the same; the site having remained +to all appearances unaltered since the Greeks first colonised the island. + +It follows, from the modern geological date of these rocks, that the fauna and +flora of a large part of Sicily are of higher antiquity than the country itself. +The greater part of the island has been raised above the sea since the epoch of +existing species, and the animals and plants now inhabiting it must have +migrated from adjacent countries, with whose productions the species are now +identical. The average duration of species would seem to be so great that they +are destined to outlive many important changes in the configuration of the +earth's surface, and hence the necessity for those innumerable contrivances by +which they are enabled to extend their range to new lands as they are formed, +and to escape from those which sink beneath the sea. + +NEWER PLIOCENE STRATA OF THE UPPER VAL D'ARNO. + +When we ascend the Arno for about ten miles above Florence, we arrive at a deep +narrow valley called the Upper Val d'Arno, which appears once to have been a +lake, at a time when the valley below Florence was an arm of the sea. The +horizontal lacustrine strata of this upper basin are twelve miles long and two +broad. The depression which they fill has been excavated out of Eocene and +Cretaceous rocks, which form everywhere the sides of the valley in highly +inclined stratification. The thickness of the more modern and unconformable beds +is about 750 feet, of which the upper 200 feet consist of Newer Pliocene strata, +while the lower are Older Pliocene. The newer series are made up of sands and a +conglomerate called "sansino." Among the imbedded fossil mammalia are Mastodon +arvernensis, Elephas meridionalis, Rhinoceros etruscus, Hippopotamus major, and +remains of the genera bear, hyaena, and felis, nearly all of which occur in the +Cromer forest-bed (see Chapter 13). + +In the same upper strata are found, according to M. Gaudin, the leaves and cones +of Glyptostrobus europaeus, a plant closely allied to G. heterophyllus, now +inhabiting the north of China and Japan. This conifer had a wide range in time, +having been traced back to the Lower Miocene strata of Switzerland, and being +common at Oeningen in the Upper Miocene, as we shall see in the sequel (Chapter +14.) + +OLDER PLIOCENE OF ITALY.-- SUBAPENNINE STRATA. + +The Apennines, it is well-known, are composed chiefly of Secondary or Mesozoic +rocks, forming a chain which branches off from the Ligurian Alps and passes down +the middle of the Italian peninsula. At the foot of these mountains, on the side +both of the Adriatic and the Mediterranean, are found a series of tertiary +strata, which form, for the most part, a line of low hills occupying the space +between the older chain and the sea. Brocchi was the first Italian geologist who +described this newer group in detail, giving it the name of the Subapennine. +Though chiefly composed of Older Pliocene strata, it belongs, nevertheless, in +part, both to older and newer members of the tertiary series. The strata, for +example, of the Superga, near Turin, are Miocene; those of Asti and Parma Older +Pliocene, as is the blue marl of Sienna; while the shells of the incumbent +yellow sand of the same territory approach more nearly to the recent fauna of +the Mediterranean, and may be Newer Pliocene. + +We have seen that most of the fossil shells of the Older Pliocene strata of +Suffolk which are of recent species are identical with testacea now living in +British seas, yet some of them belong to Mediterranean species, and a few even +of the genera are those of warmer climates. We might therefore expect, in +studying the fossils of corresponding age in countries bordering the +Mediterranean, to find among them some species and genera of warmer latitudes. +Accordingly, in the marls belonging to this period at Asti, Parma, Sienna, and +parts of the Tuscan and Roman territories, we observe the genera Conus, Cypraea, +Strombus, Pyrula, Mitra, Fasciolaria, Sigaretus, Delphinula, Ancillaria, Oliva, +Terebellum, Terebra, Perna, Plicatula, and Corbis, some characteristic of +tropical seas, others represented by species more numerous or of larger size +than those now proper to the Mediterranean. + +OLDER PLIOCENE FLORA OF ITALY. + +(FIGURE 134. Oreodaphne Heerii. +Leaf half natural size. (Feuilles fossiles de la Toscane.)) + +I have already alluded to the Newer Pliocene deposits of the Upper Val d'Arno +above Florence, and stated that below those sands and conglomerates, containing +the remains of the Elephas meridionalis and other associated quadrupeds, lie an +older horizontal and conformable series of beds, which may be classed as Older +Pliocene. They consist of blue clays with some subordinate layers of lignite, +and exhibit a richer flora than the overlying Newer Pliocene beds, and one +receding farther from the existing vegetation of Europe. They also comprise more +species common to the antecedent Miocene period. Among the genera of flowering +plants, M. Gaudin enumerates pine, oak, evergreen oak, plum, plane, alder, elm, +fig, laurel, maple, walnut, birch, buckthorn, hickory, sumach, sarsaparilla, +sassafras, cinnamon, Glyptostrobus, Taxodium, Sequoia, Persea, Oreodaphne +(Figure 134), Cassia, and Psoralea, and some others. This assemblage of plants +indicates a warm climate, but not so subtropical an one as that of the Upper +Miocene period, which will presently be considered. + +(FIGURE 135. Liquidambar europaeum, var. trilobatum, A. Br. (sometimes four- +lobed, and more commonly five-lobed). +a. Leaf, half natural size. +b. Part of same, natural size. +c. Fruit, natural size. +d. Seed, natural size. Oeningen.) + +M. Gaudin, jointly with the Marquis Strozzi, has thrown much light on the botany +of beds of the same age in another part of Tuscany, at a place called Montajone, +between the rivers Elsa and Evola, where, among other plants, is found the +Oreodaphne Heerii, Gaud. (See Figure 134), which is probably only a variety of +Oreodaphne foetens, or the laurel called the Til in Madeira, where, as in the +Canaries, it constitutes a large portion of the native woods, but can not now +endure the climate of Europe. In the fossil specimens the same glands or +protuberances are preserved (see Figure 134) as those which are seen in the +axils of the primary veins of the leaves in the recent Til. (Contributions a la +Flore fossile Italienne. Gaudin and Strozzi. Plate 11 Figure 3. Gaudin page 22.) +Another plant also indicating a warmer climate is the Liquidambar europaeum, +Brong. (see Figure 135), a species nearly allied to L. styracifluum, L., which +flourishes in most places in the Southern States of North America, on the +borders of the Gulf of Mexico. + + +CHAPTER XIV. + +MIOCENE PERIOD.-- UPPER MIOCENE. + +Upper Miocene Strata of France.-- faluns of Touraine. +Tropical Climate implied by Testacea. +Proportion of recent Species of Shells. +faluns more ancient than the Suffolk Crag. +Upper Miocene of Bordeaux and the South of France. +Upper Miocene of Oeningen, in Switzerland. +Plants of the Upper Fresh-water Molasse. +Fossil Fruit and Flowers as well as Leaves. +Insects of the Upper Molasse. +Middle or Marine Molasse of Switzerland. +Upper Miocene Beds of the Bolderberg, in Belgium. +Vienna Basin. +Upper Miocene of Italy and Greece. +Upper Miocene of India; Siwalik Hills. +Older Pliocene and Miocene of the United States. + +UPPER MIOCENE STRATA OF FRANCE.-- FALUNS OF TOURAINE. + +The strata which we meet with next in the descending order are those called by +many geologists "Middle Tertiary," for which in 1833 I proposed the name of +Miocene, selecting the "faluns" of the valley of the Loire, in France, as my +example or type. I shall now call these falunian deposits Upper Miocene, to +distinguish them from others to which the name of Lower Miocene will be given. + +No British strata have a distinct claim to be regarded as Upper Miocene, and as +the Lower Miocene are also but feebly represented in the British Isles, we must +refer to foreign examples in illustration of this important period in the +earth's history. The term "faluns" is given provincially by French +agriculturists to shelly sand and marl spread over the land in Touraine, just as +similar shelly deposits were formerly much used in Suffolk to fertilise the +soil, before the coprolitic or phosphatic nodules came into use. Isolated masses +of such faluns occur from near the mouth of the Loire, in the neighbourhood of +Nantes, to as far inland as a district south of Tours. They are also found at +Pontlevoy, on the Cher, about seventy miles above the junction of that river +with the Loire, and thirty miles south-east of Tours. Deposits of the same age +also appear under new mineral conditions near the towns of Dinan and Rennes, in +Brittany. I have visited all the localities above enumerated, and found the beds +on the Loire to consist principally of sand and marl, in which are shells and +corals, some entire, some rolled, and others in minute fragments. In certain +districts, as at Doue, in the Department of Maine and Loire, ten miles south- +west of Saumur, they form a soft building-stone, chiefly composed of an +aggregate of broken shells, bryozoa, corals, and echinoderms, united by a +calcareous cement; the whole mass being very like the Coralline Crag near +Aldborough, and Sudbourn in Suffolk. The scattered patches of faluns are of +slight thickness, rarely exceeding fifty feet; and between the district called +Sologne and the sea they repose on a great variety of older rocks; being seen to +rest successively upon gneiss, clay-slate, various secondary formations, +including the chalk; and, lastly, upon the upper fresh-water limestone of the +Parisian tertiary series, which, as before mentioned (Chapter 9), stretches +continuously from the basin of the Seine to that of the Loire. + +(FIGURE 136. Dinotherium giganteum, Kaup.) + +At some points, as at Louans, south of Tours, the shells are stained of a +ferruginous colour, not unlike that of the Red Crag of Suffolk. The species are, +for the most part, marine, but a few of them belong to land and fluviatile +genera. Among the former, Helix turonensis (Figure 38, Chapter 3) is the most +abundant. Remains of terrestrial quadrupeds are here and there intermixed, +belonging to the genera Dinotherium (Figure 136), Mastodon, Rhinoceros, +Hippopotamus, Chaeropotamus, Dichobune, Deer, and others, and these are +accompanied by cetacea, such as the Lamantin, Morse, Sea-calf, and Dolphin, all +of extinct species. + +The fossil testacea of the faluns of the Loire imply, according to the late +Edward Forbes, that the beds were formed partly on the shore itself at the level +of low water, and partly at very moderate depths, not exceeding ten fathoms +below that level. The molluscan fauna is, on the whole, much more littoral than +that of the Pliocene Red and Coralline Crag of Suffolk, and implies a shallower +sea. It is, moreover, contrasted with the Suffolk Crag by the indications it +affords of an extra-European climate. Thus it contains seven species of Cypraea, +some larger than any existing cowry of the Mediterranean, several species of +Oliva, Ancillaria, Mitra, Terebra, Pyrula, Fasciolaria, and Conus. Of the cones +there are no less than eight species, some very large, whereas the only European +cone now living is of diminutive size. The genus Nerita, and many others, are +also represented by individuals of a type now characteristic of equatorial seas, +and wholly unlike any Mediterranean forms. These proofs of a more elevated +temperature seem to imply the higher antiquity of the faluns as compared with +the Suffolk Crag, and are in perfect accordance with the fact of the smaller +proportion of testacea of recent species found in the faluns. + +Out of 290 species of shells, collected by myself in 1840 at Pontlevoy, Louans, +Bossee, and other villages twenty miles south of Tours, and at Savigne, about +fifteen miles north-west of that place, seventy-two only could be identified +with recent species, which is in the proportion of twenty-five per cent. A large +number of the 290 species are common to all the localities, those peculiar to +each not being more numerous than we might expect to find in different bays of +the same sea. + +The total number of species of testaceous mollusca from the faluns in my +possession is 302, of which forty-five only, or fourteen per cent, were found by +Mr. Wood to be common to the Suffolk Crag. The number of corals, including +bryozoa and zoantharia, obtained by me at Doue and other localities before +adverted to, amounts to forty-three, as determined by Mr. Lonsdale, of which +seven (one of them a zoantharian) agree specifically with those of the Suffolk +Crag. Some of the genera occurring fossil in Touraine, as the corals Astrea and +Dendrophyllia, and the bryozoan Lunulites, have not been found in European seas +north of the Mediterranean; nevertheless, the zoantharia of the faluns do not +seem to indicate, on the whole, so warm a climate as would be inferred from the +shells. + +It was stated that, on comparing about 300 species of Touraine shells with about +450 from the Suffolk Crag, forty-five only were found to be common to both, +which is in the proportion of only fifteen per cent. The same small amount of +agreement is found in the corals also. I formerly endeavoured to reconcile this +marked difference in species with the supposed co-existence of the two faunas, +by imagining them to have severally belonged to distinct zoological provinces or +two seas, the one opening to the north and the other to the south, with a +barrier of land between them, like the Isthmus of Suez, now separating the Red +Sea and the Mediterranean. But I now abandon that idea for several reasons; +among others, because I succeeded in 1841 in tracing the Crag fauna southward in +Normandy to within seventy miles of the Falunian type, near Dinan, yet found +that both assemblages of fossils retained their distinctive characters, showing +no signs of any blending of species or transition of climate. + +The principal grounds, however, for referring the English Crag to the older +Pliocene and the French faluns to the Upper Miocene epochs, consist in the +predominance of fossil shells in the British strata identifiable with species +not only still living, but which are now inhabitants of neighbouring seas, while +the accompanying extinct species are of genera such as characterise Europe. In +the faluns, on the contrary, the recent species are in a decided minority; and +most of them are now inhabitants of the Mediterranean, the coast of Africa, and +the Indian Ocean; in a word, less northern in character, and pointing to the +prevalence of a warmer climate. They indicate a state of things receding farther +from the present condition of Central Europe in physical geography and climate, +and doubtless, therefore, receding farther from our era in time. + +(FIGURE 137. Voluta Lamberti, Sowerby. Variety characteristic of Faluns of +Touraine. Miocene.) + +Among the conspicuous fossils common to the faluns of the Loire and the Suffolk +Crag is a variety of the Voluta Lamberti, a shell already alluded to (Figure +123). The specimens of this shell which I have myself collected in Touraine, or +have seen in museums, are thicker and heavier than British individuals of the +same species, and shorter in proportion to their width, and have the folds on +the columella less oblique, as represented in Figure 137. + +UPPER MIOCENE OF BORDEAUX AND THE SOUTH OF FRANCE. + +A great extent of country between the Pyrenees and the Gironde is overspread by +tertiary deposits of various ages, and chiefly of Miocene date. Some of these, +near Bordeaux, coincide in age with the faluns of Touraine, already mentioned, +but many of the species of shells are peculiar to the south. The succession of +beds in the basin of the Gironde implies several oscillations of level by which +the same wide area was alternately converted into sea and land and into +brackish-water lagoons, and finally into fresh-water ponds and lakes. + +Among the fresh-water strata of this age near the base of the Pyrenees are +marls, limestones and sands, in which the eminent comparative anatomist, M. +Lartet, has obtained a great number of fossil mammalia common to the faluns of +the Loire and the Upper Miocene beds of Switzerland, such as Dinotherium +giganteum and Mastodon angustidens; also the bones of quadrumana, or of the ape +and monkey tribe, which were discovered in 1837, the first of that order of +quadrupeds detected in Europe. They were found near Auch, in the Department of +Gers, in latitude 43 degrees 39' N. About forty miles west of Toulouse. They +were referred by MM. Lartet and Blainville to a genus closely allied to the +Gibbon, to which they gave the name of Pliopithecus. Subsequently, in 1856, M. +Lartet described another species of the same family of long-armed apes +(Hylobates), which he obtained from strata of the same age at Saint-Gaudens, in +the Haute Garonne. The fossil remains of this animal consisted of a portion of a +lower jaw with teeth and the shaft of a humerus. It is supposed to have been a +tree-climbing frugivorous ape, equalling man in stature. As the trunks of oaks +are common in the lignite beds in which it lay, it has received the generic name +of Dryopithecus. The angle formed by the ascending ramus of the jaw and the +alveolar border is less open, and therefore more like the human subject, than in +the Chimpanzee, and what is still more remarkable, the fossil, a young but adult +individual, had all its milk teeth replaced by the second set, while its last +true molar (or wisdom-tooth) was still undeveloped, or only existed as a germ in +the jaw-bone. In the mode, therefore, of the succession of its teeth (which, as +in all the old-World apes, exactly agree in number with those in man) it +differed from the Gorilla and Chimpanzee, and corresponded with the human +species. + +UPPER MIOCENE BEDS OF OENINGEN, IN SWITZERLAND. + +The faluns of the Loire first served, as already stated, as the type of the +Miocene formations in Europe. They yielded a plentiful harvest of marine fossil +shells and corals, but were entirely barren of plants and insects. In +Switzerland, on the other hand, deposits of the same age have been discovered, +remarkable for their botanical and entomological treasures. We are indebted to +Professor Heer, of Zurich, for the description, restoration, and classification +of several hundred species and varieties of these fossil plants, the whole of +which he has illustrated by excellent figures in his "Flora Tertiaria +Helvetiae." This great work, and those of Adolphe Brongniart, Unger, Goppert and +others, show that this class of fossils is beginning to play the same important +part in the classification of the tertiary strata containing lignite or brown +coal as an older flora has long played in enabling us to understand the ancient +coal or carboniferous formation. No small skepticism has always prevailed among +botanists as to whether the leaves alone and the wood of plants could ever +afford sufficient data for determining even genera and families in the vegetable +kingdom. In truth, before such remains could be rendered available a new science +had to be created. It was necessary to study the outlines, nervation, and +microscopic structure of the leaves, with a degree of care which had never been +called for in the classification of living plants, where the flower and fruit +afforded characters so much more definite and satisfactory. As geologists, we +can not be too grateful to those who, instead of despairing when so difficult a +task was presented to them, or being discouraged when men of the highest +scientific attainments treated the fossil leaves as worthless, entered with full +faith and enthusiasm into this new and unexplored field. That they should +frequently have fallen into errors was unavoidable, but it is remarkable, +especially if we inquire into the history of Professor Heer's researches, how +often early conjectures as to the genus and family founded on the leaves alone +were afterwards confirmed when fuller information was obtained. As examples to +be found on comparing Heer's earlier and later works, I may instance the +chestnut, elm, maple, cinnamon, magnolia, buckbean or Menyanthes, vine, +buckthorn (Rhamnus), Andromeda and Myrica, and among the conifers Sequoia and +Taxodium. In all these cases the plants were first recognised by their leaves, +and the accuracy of the determination was afterwards confirmed when the fruit, +and in some instances both fruit and flower, were found attached to the same +stem as the leaves. + +But let us suppose that no fruit, seed, or flower had ever been met with in a +fossil state, we should still have been indebted to the persevering labours of +botanical palaeontologists for one of the grandest scientific discoveries for +which the present century is remarkable-- namely, the proofs now established of +the prevalence of a mild climate and a rich arborescent flora in the arctic +regions in that Miocene epoch on the history of which we are now entering. It +may be useful if I endeavour to give the reader in a few words some idea of the +nature of the evidence of these important conclusions, to show how far they may +be safely based on fossil leaves alone. When we begin by studying the fossils of +the Newer Pliocene deposits, such as those of the Upper Val d'Arno, before +alluded to, we perceive that the fossil foliage agrees almost entirely with the +trees and shrubs of a modern European forest. In the plants of the Older +Pliocene strata of the same region we observe a larger proportion of species and +genera which, although they may agree with well-known Asiatic or other foreign +types, are at present wanting in Italy. If we then examine the Miocene +formations of the same country, exotic forms become more abundant, especially +the palms, whether they belong to the European or American fan-palms, Chamaerops +and Sabal, or to the more tropical family of the date-palms or Phoenicites, +which last are conspicuous in the Lower Miocene beds of Central Europe. Although +we have not found the fruit or flower of these palms in a fossil state, the +leaves are so characteristic that no one doubts the family to which they belong, +or hesitates to accept them as indications of a warm and sub-tropical climate. + +When the Miocene formations are traced to the northward of the 50th degree of +latitude, the fossil palms fail us, but the greater proportion of the leaves, +whether identical with those of existing European trees or of forms now unknown +in Europe, which had accompanied the Miocene palms, still continue to +characterise rocks of the same age, until we meet with them not only in Iceland, +but in Greenland, in latitude 70 degrees N., and in Spitzbergen, latitude 78 +degrees 56', or within about 11 degrees of the pole, and under circumstances +which clearly show them to have been indigenous in those regions, and not to +have been drifted from the south (see Chapter 15). Not only, therefore, has the +botanist afforded the geologist much palaeontological assistance in identifying +distinct tertiary formations in distant places by his power of accurately +discriminating the forms, veining, and microscopic structure of leaves or wood, +but, independently of that exact knowledge derivable from the organs of +fructification, we are indebted to him for one of the most novel, unexpected +results of modern scientific inquiry. + +The Miocene formations of Switzerland have been called MOLASSE, a term derived +from the French MOL, and applied to a SOFT, incoherent, greenish sandstone, +occupying the country between the Alps and the Jura. This molasse comprises +three divisions, of which the middle one is marine, and being closely related by +its shells to the faluns of Touraine, may be classed as Upper Miocene. The two +others are fresh-water, the upper of which may be also grouped with the faluns, +while the lower must be referred to the Lower Miocene, as defined in the next +chapter. + +UPPER FRESH-WATER MOLASSE. + +This formation is best seen at Oeningen, in the valley of the Rhine, between +Constance and Schaffhausen, a locality celebrated for having produced in the +year 1700 the supposed human skeleton called by Scheuchzer "homo diluvii +testis," a fossil afterwards demonstrated by Cuvier to be a reptile, or aquatic +salamander, of larger dimensions than even its great living representative, the +salamander of Japan. + +The Oeningen strata consist of a series of marls and limestones, many of them +thinly laminated, and which appear to have slowly accumulated in a lake probably +fed by springs holding carbonate of lime in solution. The elliptical area over +which this fresh-water formation has been traced extends, according to Sir +Roderick Murchison, for a distance of ten miles east and west from Berlingen, on +the right bank of the river to Wangen, and to Oeningen, near Stein, on the left +bank. The organic remains have been chiefly derived from two quarries, the lower +of which is about 550 feet above the level of the Lake of Constance, while the +upper quarry is 150 feet higher. In this last, a section thirty feet deep +displays a great succession of beds, most of them splitting into slabs and some +into very thin laminae. Twenty-one beds are enumerated by Professor Heer, the +uppermost a bluish-grey marl seven feet thick, with organic remains, resting on +a limestone with fossil plants, including leaves of poplar, cinnamon, and pond- +weed (Potamogeton), together with some insects; while in the bed No. 4, below, +is a bituminous rock, in which the Mastodon tapiroides, a characteristic Upper +Miocene quadruped, has been met with. The 5th bed, two or three inches thick, +contains fossil fish, e.g., Leuciscus (roach), and the larvae of dragon-flies, +with plants such as the elm (Ulmus), and the aquatic Chara. Below this are other +plant-beds; and then, in No. 9, the stone in which the great salamander (Andrias +Scheuchzeri) and some fish were found. Below this other strata occur with fish, +tortoises, the great salamander before alluded to, fresh-water mussels, and +plants. In No. 16 the fossil fox of Oeningen, galecynus Oeningensis, Owen, was +obtained by Sir R. Murchison. To this succeed other beds with mammalia +(Lagomys), reptiles, (Emys), fish, and plants, such as walnut, maple, and +poplar. In the 19th bed are numerous fish, insects, and plants, below which are +marls of a blue indigo colour. + +In the lower quarry eleven beds are mentioned, in which, as in the upper, both +land and fresh-water plants and many insects occur. In the 6th, reckoning from +the top, many plants have been obtained, such as Liquidambar, Daphnogene, +Podogonium, and Ulmus, together with tortoises, besides the bones and teeth of a +ruminant quadruped, named by H. von Meyer Palaeomeryx eminens. No. 9 is called +the insect-bed, a layer only a few inches thick, which, when exposed to the +frost, splits into leaves as thin as paper. In these thin laminae plants such as +Liquidambar, Daphnogene, and Glyptostrobus, occur, with innumerable insects in a +wonderful state of preservation, usually found singly. Below this is an indigo- +blue marl, like that at the bottom of the higher quarry, resting on yellow marl +ascertained to be at least thirty feet thick. + +(FIGURE 138. Cinnamomum polymorphum, Ad. Brong. Upper and Lower Miocene. +a. Leaf. +b. Flower, natural size; Heer Plate 93 Figure 28. +c. Ripe fruit of Cinnamomum polymorphum, from Oeningen; Heer, Plate 94 Figure +14. +d. Fruit of recent Cinnamomum camphorum of Japan; Heer, Plate 152 Figure 18.) + +All the above fossil-bearing strata were evidently formed with extreme slowness. +Although the fossiliferous beds are, in the aggregate, no more than a few yards +in thickness, and have only been examined in the small area comprised in the two +quarries just alluded to, they give us an insight into the state of animal and +vegetable life in part of the Upper Miocene period, such as no other region in +the world has elsewhere supplied. In the year 1859, Professor Heer had already +determined no less than 475 species of plants and more than 800 insects from +these Oeningen beds. He supposes that a river entering a lake floated into it +some of the leaves and land insects, together with the carcasses of quadrupeds, +among others a great Mastodon. Occasionally, during tempests, twigs and even +boughs of trees with their leaves were torn off and carried for some distance so +as to reach the lake. Springs, containing carbonate of lime, seem at some points +to have supplied calcareous matter in solution, giving origin locally to a kind +of travertin, in which organic bodies sinking to the bottom became hermetically +sealed up. The laminae, says Heer, which immediately succeed each other were not +all formed at the same season, for it can be shown that, when some of them +originated, certain plants were in flower, whereas, when the next of these +layers was produced, the same plants had ripened their fruit. This inference is +confirmed by independent proofs derived from insects. The principal insect-bed +is rarely two inches thick, and is composed, says Heer, of about 250 leaf-like +laminae, some of which were deposited in the spring, when the Cinnamomum +polymorphum (Figure 138) was in flower, others in summer, when winged ants were +numerous, and when the poplar and willow had matured their seed; others, again, +in autumn, when the same Cinnamomum polymorphum (Figure 138) was in fruit, as +well as the liquidambar, oak, clematis, and many other plants. The ancient lake +seems to have had a belt of poplars and willows round its borders, countless +leaves of which were imbedded in mud, and together with them, at some points, a +species of reed, Arundo, which was very common. + +One of the most characteristic shrubs is a papilionaceous and leguminous plant +of an extinct genus, called by Heer Podogonium, of which two species are known. +Entire twigs have been found with flowers, and always without leaves, as the +flowers evidently came out, as in the poplar and willow tribe, before any leaves +made their appearance. Other specimens have been obtained with ripe fruits +accompanied by leaves, which resemble those of the tamarind, to which it was +evidently allied, being of the family Caesalpineae, now proper to warmer +regions. + +(FIGURE 139. Acer trilobatum, normal form; Heer, Flora Tert. Helv. Plate 114 +Figure 2. Size 1/2 diam. (Part only of the long stalk of the original fossil +specimen is here given ). Upper Miocene, Oeningen; also found in Lower Miocene +of Switzerland.) + +(FIGURE 140. Acer trilobatum. +a. Abnormal variety of leaf; Heer, Plate 110 Figure 16. +b. Flower and bracts, normal form; Heer, Plate 111 Figure 21. +c. Half a seed-vessel; Heer, Plate 111 Figure 5.) + +(FIGURE 141. Platanus aceroides, Gopp.; Heer, Plate 88 Figures 5-8. Size 2/3 +diam. Upper Miocene, Oeningen. +a. Leaf. +b. The core of a bundle of pericarps. +c. Single fruit or pericarp, natural size.) + +The Upper Miocene flora of Oeningen is peculiarly important, in consequence of +the number of genera of which not merely the leaves, but, as in the case of the +Podogonium just mentioned, the fruit also and even the flower are known. Thus +there are nineteen species of maple, ten of which have already been found with +fruit. Although in no one region of the globe do so many maples now flourish, we +need not suspect Professor Heer of having made too many species in this genus +when we consider the manner in which he has dealt with one of them, Acer +trilobatum, Figures 139 and 140. Of this plant the number of marked varieties +figured and named is very great, and no less than three of them had been +considered as distinct species by other botanists, while six of the others might +have laid claim, with nearly equal propriety, to a like distinction. The common +form, called Acer trilobatum, Figure 139, may be taken as a normal +representative of the Oeningen fossil, and Figure 140, as one of the most +divergent varieties, having almost four lobes in the leaf instead of three. + +(FIGURE 142. Smilax sagittifera; Heer, Plate 30 Figure 7. Size 1/2 diameter. +a. Leaf. +b. Flower magnified, one of the six petals wanting at d. Upper Miocene, +Oeningen. +c. Smilax obtusifolia; Heer, Plate 30 Figure 9; natural size. Upper Miocene, +Oeningen.) + +(FIGURE 143. Fruit of the fossil and recent species of Hakea, a genus of +Proteaceae. +a. Leaf of fossil species, Hakea salicina. Upper Miocene, Oeningen; Heer Plate +97 Figure 29. 1/3 diameter. +b. Impression of woody fruit of same, showing thick stalk. 2/3 diameter. +c. Seed of same, natural size. +d. Fruit of living Australian species, Hakea saligna, R. Brown. 1/2 diameter. +e. Seed of same, natural size.) + +Among the conspicuous genera which abounded in the Miocene period in Europe is +the plane-tree, Platanus, the fossil species being considered by Heer to come +nearer to the American P. occidentalis than to P. orientalis of Greece and Asia +Minor. In some of the fossil specimens the male flowers are preserved. Among +other points of resemblance with the living plane-trees, as we see them in the +parks and squares of London, fossil fragments of the trunk are met with, having +pieces of their bark peeling off. + +The vine of Oeningen, Vitis teutonica, Ad. Brong, is of a North American type. +Both the leaves and seeds have been found at Oeningen, and bunches of compressed +grapes of the same species have been met with in the brown coal of Wetteravia in +Germany. No less than eight species of smilax, a monocotyledonous genus, occur +at Oeningen and in other Upper Miocene localities, the flowers of some of them, +as well as the leaves, being preserved; as in the case of the very common +fossil, S. sagittifera, Figure 142, a. + +Leaves of plants supposed to belong to the order Proteaceae have been obtained +partly from Oeningen and partly from the lacustrine formation of the same age at +Locle in the Jura. They have been referred to the genera Banksia, Grevillea, +Hakea, and Persoonia. Of Hakea there is the impression of a supposed seed- +vessel, with its characteristic thick stalk and seeds, but as the fruit is +without structure, and has not yet been found attached to the same stem as the +leaf, the proof is incomplete. + +To whatever family the foliage hitherto regarded as proteaceous by many able +palaeontologists may eventually be shown to belong, we must be careful not to +question their affinity to that order of plants on those geographical +considerations which have influenced some botanists. The nearest living +Proteaceae now feel the in Abyssinia in latitude 20 degrees N., but the greatest +number are confined to the Cape and Australia. The ancestors, however, of the +Oeningen fossils ought not to be looked for in such distant regions, but from +that European land which in Lower Miocene times bore trees with similar foliage, +and these had doubtless an Eocene source, for cones admitted by all botanists to +be proteaceous have been met with in one division of that older Tertiary group +(see Figure 206 Chapter 16). The source of these last, again, must not be sought +in the antipodes, for in the white chalk of Aix-la-Chapelle leaves like those of +Grevillea and other proteaceous genera have been found in abundance, and, as we +shall see in Chapter 17, in a most perfect state of preservation. All geologists +agree that the distribution of the Cretaceous land and sea had scarcely any +connection with the present geography of the globe. + +(FIGURE 144. Glyptostrobus Europaeus. +Branch with ripe fruit; Heer, Plate 20 Figure 1. Upper Miocene, Oeningen.) + +In the same beds with the supposed Proteaceae there occurs at Locle a fan-palm +of the American type Sabal (for genus see Figure 151), a genus which ranges +throughout the low country near the sea from the Carolinas to Florida and +Louisiana. Among the Coniferae of Upper Miocene age is found a deciduous cypress +nearly allied to the Taxodium distichum of North America, and a Glyptostrobus +(Figure 144), very like the Japanese G. heterophyllus, now common in our +shrubberies. + +Before the appearance of Heer's work on the Miocene Flora of Switzerland, Unger +and Goppert had already pointed out the large proportion of living North +American genera which distinguished the vegetation of the Miocene period in +Central Europe. Next in number, says Heer, to these American forms at Oeningen +the European genera preponderate, the Asiatic ranking in the third, the African +in the fourth, and the Australian in the fifth degree. The American forms are +more numerous than in the Italian Pliocene flora, and the whole vegetation +indicates a warmer climate than the Pliocene, though not so high a temperature +as that of the older or Lower Miocene period. + +The conclusions drawn from the insects are for the most part in perfect harmony +with those derived from the plants, but they have a somewhat less tropical and +less American aspect, the South European types being more numerous. On the +whole, the insect fauna is richer than that now inhabiting any part of Europe. +No less than 844 species are reckoned by Heer from the Oeningen beds alone, the +number of specimens which he has examined being 5080. The entire list of Swiss +species from the Upper and Lower Miocene together amount to 1322. Almost all the +living families of Coleoptera are represented, but, as we might have anticipated +from the preponderance of arborescent and ligneous plants, the wood-eating +beetles play the most conspicuous part, the Buprestidae and other long-horned +beetles being particularly abundant. + +(FIGURE 145. Harpactor maculipes, Heer. Upper Miocene, Oeningen.) + +The patterns and some remains of the colours both of Coleoptera and Hemiptera +are preserved at Oeningen, as, for example in Harpactor (Figure 145), in which +the antennae, one of the eyes, and the legs and wings are retained. The +characters, indeed, of many of the insects are so well defined as to incline us +to believe that if this class of the invertebrata were not so rare and local, +they might be more useful than even the plants and shells in settling +chronological points in geology. + +MIDDLE OR MARINE MOLASSE (UPPER MIOCENE) OF SWITZERLAND. + +It was before stated that the Miocene formation of Switzerland consisted of, +first, the upper fresh-water molasse, comprising the lacustrine marls of +Oeningen; secondly, the marine molasse, corresponding in age to the faluns of +Touraine; and thirdly, the lower fresh-water molasse. Some of the beds of the +marine or middle series reach a height of 2470 feet above the sea. A large +number of the shells are common to the faluns of Touraine, the Vienna basin, and +other Upper Miocene localities. The terrestrial plants play a subordinate part +in the fossiliferous beds, yet more than ninety of them are enumerated by Heer +as belonging to this falunian division, and of these more than half are common +to subjacent Lower Miocene beds, while a proportion of about forty-five in one +hundred are common to the overlying Oeningen flora. Twenty-six of the ninety-two +species are peculiar. + +UPPER MIOCENE OF THE BOLDERBERG, IN BELGIUM. + +(FIGURE 146. Oliva Dufresnii, Bast. Bolderberg, Belgium; natural size. +a. Front view. +b. Back view.) + +In a small hill or ridge called the Bolderberg, which I visited in 1851, +situated near Hasselt, about forty miles E.N.E. of Brussels, strata of sand and +gravel occur, to which M. Dumont first called attention as appearing to +constitute a northern representative of the faluns of Touraine. On the whole, +they are very distinct in their fossils from the two upper divisions of the +Antwerp Crag before mentioned (Chapter 13), and contain shells of the genera +Oliva, Conus, Ancillaria, Pleurotoma, and Cancellaria in abundance. The most +common shell is an Olive (Figure 146), called by Nyst Oliva Dufresnii; and +constituting, as M. Bosquet observes, a smaller and shorter variety of the +Bordeaux species. + +So far as the shells of the Bolderberg are known, the proportion of recent +species agrees with that in the faluns of Touraine, and the climate must have +been warmer than that of the Coralline Crag of England. + +UPPER MIOCENE BEDS OF THE VIENNA BASIN. + +In South Germany the general resemblance of the shells of the Vienna tertiary +basin with those of the faluns of Touraine has long been acknowledged. In the +late Dr. Hornes's excellent work on the fossil mollusca of that formation, we +see accurate figures of many shells, clearly of the same species as those found +in the falunian sands of Touraine. + +According to Professor Suess, the most ancient and purely marine of the Miocene +strata in this basin consist of sands, conglomerates, limestones, and clays, and +they are inclined inward, or from the borders of the trough towards the centre, +their outcropping edges rising much higher than the newer beds, whether Miocene +or Pliocene, which overlie them, and which occupy a smaller area at an inferior +elevation above the sea. M. Hornes has described no less than 500 species of +gasteropods, of which he identifies one-fifth with living species of the +Mediterranean, Indian, or African seas, but the proportion of existing species +among the lamellibranchiate bivalves exceeds this average. Among many univalves +agreeing with those of Africa on the eastern side of the Atlantic are Cypraea +sanguinolenta, Buccinum lyratum, and Oliva flammulata. In the lowest marine beds +of the Vienna basin the remains of several mammalia have been found, and among +them a species of Dinotherium, a Mastodon of the Trilophodon family, a +Rhinoceros (allied to R. megarhinus, Christol), also an animal of the hog tribe, +Listriodon, von Meyer, and a carnivorous animal of the canine family. The Helix +turonensis (Figure 38 Chapter 3), the most common land shell of the French +faluns, accompanies the above land animals. In a higher member of the Vienna +Miocene series are found Dinotherium giganteum (Figure 136 Chapter 14), Mastodon +longirostris, Rhinoceros Schleiermacheri, Acerotherium incisivum, and +Hippotherium gracile, all of them equally characteristic of an Upper Miocene +deposit occurring at Eppelsheim, in Hesse Darmstadt; a locality also remarkable +as having furnished in latitude 49 degrees 50 north the bone of a large ape of +the Gibbon kind, the most northerly example yet discovered of a quadrumanous +animal. + +(FIGURE 147. Amphistegina Hauerina, d'Orbigny. Upper Miocene strata, Vienna.) + +M. Alcide d'Orbigny has shown that the foraminifera of the Vienna basin differ +alike from the Eocene and Pliocene species, and agree with those of the faluns, +so far as the latter are known. Among the Vienna foraminifera, the genus +Amphistegina (Figure 147) is very characteristic, and is supposed by d'Archiac +to take the same place among the Rhizopods of the Upper Miocene era which the +Nummulites occupy in the Eocene period. + +The flora of the Vienna basin exhibits some species which have a general range +through the whole Miocene period, such as Cinnamomum polymorphum (Figure 138 +Chapter 14), and C. Scheuchzeri, also Planera Richardi, Mich., Liquidambar +europaeum (Figure 135 Chapter 13) Juglans bilinica, Cassia ambigua, and C. +lignitum. Among the plants common to the Upper Miocene beds of Oeningen, in +Switzerland, are Platanus aceroides (Figure 141 Chapter 14), Myrica +vindobonensis, and others. + +UPPER MIOCENE STRATA OF ITALY. + +We are indebted to Signor Michelotti for a valuable work on the Miocene shells +of Northern Italy. Those found in the hill called the Superga, near Turin, have +long been known to correspond in age with the faluns of Touraine, and they +contain so many species common to the Upper Miocene strata of Bordeaux as to +lead to the conclusion that there was a free communication between the northern +part of the Mediterranean and the Bay of Biscay in the Upper Miocene period. + +UPPER MIOCENE FORMATIONS OF GREECE. + +At Pikerme, near Athens, MM. Wagner and Roth have described a deposit in which +they found the remains of the genera Mastodon, Dinotherium, Hipparion, two +species of Giraffe, Antelope, and others, some living and some extinct. With +them were also associated fossil bones of the Semnopithecus, showing that here, +as in the south of France, the quadrumana were characteristic of this period. +The whole fauna attests the former extension of a vast expanse of grassy plains +where we have now the broken and mountainous country of Greece; plains, which +were probably united with Asia Minor, spreading over the area where the deep +Aegean Sea and its numerous islands are now situated. We are indebted to M. +Gaudry, who visited Pikerme, for a treatise on these fossil bones, showing how +many data they contribute to the theory of a transition from the mammalia of the +Upper Miocene through the Pliocene and Post-pliocene forms to those of living +genera and species. + +UPPER MIOCENE OF INDIA. SIWALIK HILLS. + +The Siwalik Hills lie at the southern foot of the Himalayan chain, rising to the +height of 2000 and 3000 feet. Between the Jumna and the Ganges they consist of +inclined strata of sandstone, shingle, clay, and marl. We are indebted to the +indefatigable researches of Dr. Falconer and Sir Proby Cautley, continued for +fifteen years, for the discovery in these marls and sandstones of a great +variety of fossil mammalia and reptiles, together with many fresh-water shells. +Out of fifteen species of shells of the genera Paludina, Melania, Ampullaria, +and Unio, all are extinct or unknown species with the exception of four, which +are still inhabitants of Indian rivers. Such a proportion of living to extinct +mollusca agrees well with the usual character of an Upper Miocene or Falunian +fauna, as observed in Touraine, or in the basin of Vienna and elsewhere. + +The genera of mammalia point in the same direction. One of them, of the genus +Chalicotherium (or Anisodon of Lartet), is a pachyderm intermediate between the +Rhinoceros and Anoplothere, and characteristic of the Upper Miocene strata of +Eppelsheim, and of the south of France. With it occurs also an extinct form of +Hippopotamus, called Hexaprotodon, and a species of Hippotherium and pig, also +two species of Mastodon, two of elephant, and three other elephantine +proboscidians; none of them agreeing with any fossil forms of Europe, and being +intermediate between the genera Elephas and Mastodon, constituting the sub-genus +Stegodon of Falconer. With these are associated a monkey, allied to the +Semnopithecus entellus, now living in the Himalaya, and many ruminants. Among +these last, besides the giraffe, camel, antelope, stag, and others, we find a +remarkable new type, the Sivatherium, like a gigantic four-horned deer. There +are also new forms of carnivora, both feline and canine, the Machairodus among +the former, also hyaenas, and a subursine form called the Hyaenarctos, and a +genus allied to the otter (Enhydriodon), of formidable size. + +The giraffe, camel, and a large ostrich may be cited as proofs that there were +formerly extensive plains where now a steep chain of hills, with deep ravines, +runs for many hundred miles east and west. Among the accompanying reptiles are +several crocodiles, some of huge dimensions, and one not distinguishable, says +Dr. Falconer, from a species now living in the Ganges (C. Gangeticus); and there +is still another saurian which the same anatomist has identified with a species +now inhabiting India. There was also an extinct species of tortoise of gigantic +proportions (Colossochelys Atlas), the curved shell of which was twelve feet +three inches long and eight feet in diameter, the entire length of the animal +being estimated at eighteen feet, and its probable height seven feet. + +Numerous fossils of the Siwalik type have also been found in Perim Island, in +the Gulf of Cambay, and among these a species of Dinotherium, a genus so +characteristic of the Upper Miocene period in Europe. + +OLDER PLIOCENE AND MIOCENE FORMATIONS IN THE UNITED STATES. + +Between the Alleghany Mountains, formed of older rocks, and the Atlantic, there +intervenes, in the United States, a low region occupied principally by beds of +marl, clay, and sand, consisting of the cretaceous and tertiary formations, and +chiefly of the latter. The general elevation of this plain bordering the +Atlantic does not exceed 100 feet, although it is sometimes several hundred feet +high. Its width in the middle and southern states is very commonly from 100 to +150 miles. It consists, in the South, as in Georgia, Alabama, and South +Carolina, almost exclusively of Eocene deposits; but in North Carolina, +Maryland, Virginia, Delaware, more modern strata predominate, of the age of the +English Crag and faluns of Touraine. (Proceedings of the Geological Society +volume 4 part 3 1845 page 547.) + +(FIGURE 148. Fulgur canaliculatus. Maryland.) + +(FIGURE 149. Fusus quadricostatus, Say. Maryland.) + +In the Virginian sands, we find in great abundance a species of Astarte (A. +undulata, Conrad), which resembles closely, and may possibly be a variety of, +one of the commonest fossils of the Suffolk Crag (A. Omalii); the other shells +also, of the genera Natica, Fissurella, Artemis, Lucina, Chama, Pectunculus, and +Pecten, are analagous to shells both of the English Crag and French faluns, +although the species are almost all distinct. Out of 147 of these American +fossils I could only find thirteen species common to Europe, and these occur +partly in the Suffolk Crag, and partly in the faluns of Touraine; but it is an +important characteristic of the American group, that it not only contains many +peculiar extinct forms, such as Fusus quadricostatus, Say (see Figure 149), and +Venus tridacnoides, abundant in these same formations, but also some shells +which, like Fulgur carica of Say and F. canaliculatus (see Figure 148), +Calyptraea costata, Venus mercenaria, Lam., Modiola glandula, Totten, and Pecten +magellanicus, Lam., are recent species, yet of forms now confined to the western +side of the Atlantic-- a fact implying that some traces of the beginning of the +present geographical distribution of mollusca date back to a period as remote as +that of the Miocene strata. + +Of ten species of corals which I procured on the banks of the James River, one +agrees generically with a coral now living on the coast of the United States. +Mr. Lonsdale regarded these corals as indicating a temperature exceeding that of +the Mediterranean, and the shells would lead to similar conclusions. Those +occurring on the James River are in the 37th degree of N. latitude, while the +French faluns are in the 47th; yet the forms of the American fossils would +scarcely imply so warm a climate as must have prevailed in France when the +Miocene strata of Touraine originated. + +(FIGURE 150. Astrangia lineata, Lonsdale. Syn. Anthophyllum lineatum. +Williamsburg, Virginia.) + +Among the remains of fish in these post-eocene strata of the United States are +several large teeth of the shark family, not distinguishable specifically from +fossils of the faluns of Touraine. + + +CHAPTER XV. + +LOWER MIOCENE (OLIGOCENE OF BEYRICH). + +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. + +LINE BETWEEN MIOCENE AND EOCENE FORMATIONS. + +The marine faluns of the valley of the Loire have been already described as +resting in some places on a fresh-water tertiary limestone, fragments of which +have been broken off and rolled on the shores and in the bed of the Miocene sea. +Such pebbles are frequent at Pontlevoy on the Cher, with hollows drilled in them +in which the perforating marine shells of the Falunian period still remain. Such +a mode of superposition implies an interval of time between the origin of the +fresh-water limestone and its submergence beneath the waters of the Upper +Miocene sea. The limestone in question forms a part of the formation called the +Calcaire de la Beauce, which constitutes a large table-land between the basins +of the Loire and the Seine. It is associated with marls and other deposits, such +as may have been formed in marshes and shallow lakes in the newest part of a +great delta. Beds of flint, continuous or in nodules, accumulated in these +lakes, and aquatic plants called Charae, left their stems and seed-vessels +imbedded both in the marl and flint, together with fresh-water and land shells. +Some of the siliceous rocks of this formation are used extensively for mill- +stones. The flat summits or platforms of the hills round Paris, and large areas +in the forest of Fontainebleau, as well as the Plateau de la Beauce, already +alluded to, are chiefly composed of these fresh-water strata. Next to these in +the descending order are marine sands and sandstone, commonly called the Gres de +Fontainebleau, from which a considerable number of shells, very distinct from +those of the faluns, have been obtained at Etampes, south of Paris, and at +Montmartre and other hills in Paris itself, or in its suburbs. At the bottom of +these sands a green clay occurs, containing a small oyster, Ostrea cyathula, +Lam., which, although of slight thickness, is spread over a wide area. This clay +rests immediately on the Paris gypsum, or that series of beds of gypsum and +gypseous marl from which Cuvier first obtained several species of Palaeotherium +and other extinct mammalia. (Bulletin 1856 Journal volume 12 page 768.) + +At this junction of the clay and the gypsum the majority of French geologists +have always drawn the line between the Middle and Lower Tertiary, or between the +Miocene and Eocene formations, regarding the Fontainebleau sands and the Ostrea +cyathula clay as the base of the Miocene, and the gypsum, with its mammalia, as +the top of the Eocene group. I formerly dissented from this division, but I now +find that I must admit it to be the only one which will agree with the +distribution of the Miocene mammalia, while even the mollusca of the +Fontainebleau sands, which were formerly supposed to present at preponderance of +affinities to an Eocene fauna, have since been shown to agree more closely with +the fossils of certain deposits always regarded as Middle Tertiary at Mayence +and in Belgium. In fact, we are now arriving at that stage of progress when the +line, wherever it be drawn between Miocene and Eocene, will be an arbitrary one, +or one of mere convenience, as I shall have an opportunity of showing when the +Upper Eocene formations in the Isle of Wight are described in the sixteenth +chapter. + +LOWER MIOCENE OF CENTRAL FRANCE. + +Lacustrine strata, belonging, for the most part, to the same Miocene system as +the Calcaire de la Beauce, are again met with farther south in Auvergne, Cantal, +and Velay. They appear to be the monuments of ancient lakes, which, like some of +those now existing in Switzerland, once occupied the depressions in a +mountainous region, and have been each fed by one or more rivers and torrents. +The country where they occur is almost entirely composed of granite and +different varieties of granitic schist, with here and there a few patches of +Secondary strata, much dislocated, and which have suffered great denudation. +There are also some vast piles of volcanic matter, the greater part of which is +newer than the fresh-water strata, and is sometimes seen to rest upon them, +while a small part has evidently been of contemporaneous origin. Of these +igneous rocks I shall treat more particularly in the sequel. + +The study of these regions possesses a peculiar interest very distinct in kind +from that derivable from the investigation either of the Parisian or English +Tertiary areas. For we are presented in Auvergne with the evidence of a series +of events of astonishing magnitude and grandeur, by which the original form and +features of the country have been greatly changed, yet never so far obliterated +but that they may still, in part at least, be restored in imagination. Great +lakes have disappeared-- lofty mountains have been formed, by the reiterated +emission of lava, preceded and followed by showers of sand and scoriae-- deep +valleys have been subsequently furrowed out through masses of lacustrine and +volcanic origin-- at a still later date, new cones have been thrown up in these +valleys-- new lakes have been formed by the damming up of rivers-- and more than +one assemblage of quadrupeds, birds, and plants, Eocene, Miocene, and Pliocene, +have followed in succession; yet the region has preserved from first to last its +geographical identity; and we can still recall to our thoughts its external +condition and physical structure before these wonderful vicissitudes began, or +while a part only of the whole had been completed. There was first a period when +the spacious lakes, of which we still may trace the boundaries, lay at the foot +of mountains of moderate elevation, unbroken by the bold peaks and precipices of +Mont Dor, and unadorned by the picturesque outline of the Puy de Dome, or of the +volcanic cones and craters now covering the granitic platform. During this +earlier scene of repose deltas were slowly formed; beds of marl and sand, +several hundred feet thick, deposited; siliceous and calcareous rocks +precipitated from the waters of mineral springs; shells and insects imbedded, +together with the remains of the crocodile and tortoise, the eggs and bones of +water-birds, and the skeletons of quadrupeds, most of them of genera and species +characteristic of the Miocene period. To this tranquil condition of the surface +succeeded the era of volcanic eruptions, when the lakes were drained, and when +the fertility of the mountainous district was probably enhanced by the igneous +matter ejected from below, and poured down upon the more sterile granite. During +these eruptions, which appear to have taken place towards the close of the +Miocene epoch, and which continued during the Pliocene, various assemblages of +quadrupeds successively inhabited the district, among which are found the genera +mastodon, rhinoceros, elephant, tapir, hippopotamus, together with the ox, +various kinds of deer, the bear, hyaena, and many beasts of prey which ranged +the forest or pastured on the plain, and were occasionally overtaken by a fall +of burning cinders, or buried in flows of mud, such as accompany volcanic +eruptions. Lastly, these quadrupeds became extinct, and gave place in their turn +to the species now existing. There are no signs, during the whole time required +for this series of events, of the sea having intervened, nor of any denudation +which may not have been accomplished by currents in the different lakes, or by +rivers and floods accompanying repeated earthquakes, or subterranean movements, +during which the levels of the district have in some places been materially +modified, and perhaps the whole upraised relatively to the surrounding parts of +France. + +AUVERGNE. + +The most northern of the fresh-water groups is situated in the valley-plain of +the Allier, which lies within the department of the Puy de Dome, being the tract +which went formerly by the name of the Limagne d'Auvergne. The average breadth +of this tract is about twenty miles; and it is for the most part composed of +nearly horizontal strata of sand, sandstone, calcareous marl, clay, and +limestone, none of which observe a fixed and invariable order of superposition. +The ancient borders of the lake wherein the fresh-water strata were accumulated +may generally be traced with precision, the granite and other ancient rocks +rising up boldly from the level country. The actual junction, however, of the +lacustrine beds and the granite is rarely seen, as a small valley usually +intervenes between them. The fresh-water strata may sometimes be seen to retain +their horizontality within a very slight distance of the border-rocks, while in +some places they are inclined, and in few instances vertical. The principal +divisions into which the lacustrine series may be separated are the following:-- +first, Sandstone, grit, and conglomerate, including red marl and red sandstone; +secondly, Green and white foliated marls; thirdly, Limestone, or travertin, +often oolitic in structure; fourthly, Gypseous marls. + +The relations of these different groups can not be learnt by the study of any +one section; and the geologist who sets out with the expectation of finding a +fixed order of succession may perhaps complain that the different parts of the +basin give contradictory results. The arenaceous division, the marls, and the +limestone may all be seen in some places to alternate with each other; yet it +can by no means be affirmed that there is no order of arrangement. The sands, +sandstone, and conglomerate constitute in general a littoral group; the foliated +white and green marl, a contemporaneous central deposit more than 700 feet +thick, and thinly foliated, a character which often arises from the innumerable +thin shells or carapace valves shed by the small crustacean called Cypris in the +ancient lakes of Auvergne; and lastly the limestone is for the most part +subordinate to the newer portions of both the above formations. + +It seems that, when the ancient lake of the Limagne first began to be filled +with sediment, no volcanic action had yet produced lava and scoriae on any part +of the surface of Auvergne. No pebbles, therefore, of lava were transported into +the lake-- no fragments of volcanic rocks imbedded in the conglomerate. But at a +later period, when a considerable thickness of sandstone and marl had +accumulated, eruptions broke out, and lava and tuff were deposited, at some +spots, alternately with the lacustrine strata. It is not improbable that cold +and thermal springs, holding different mineral ingredients in solution, became +more numerous during the successive convulsions attending this development of +volcanic agency, and thus deposits of carbonate and sulphate of lime, silex, and +other minerals were produced. Hence these minerals predominate in the uppermost +strata. The subterranean movements may then have continued until they altered +the relative levels of the country, and caused the waters of the lakes to be +drained off, and the further accumulation of regular fresh-water strata to +cease. + +LOWER MIOCENE MAMMALIA OF THE LIMAGNE. + +It is scarcely possible to determine the age of the oldest part of the fresh- +water series of the Limagne, large masses both of the sandy and marly strata +being devoid of fossils. Some of the lowest beds may be of Upper Eocene date, +although, according to M. Pomel, only one bone of a Palaeotherium has been +discovered in Auvergne. But in Velay, in strata containing some species of +fossil mammalia common to the Limagne, no less than four species of Palaeothere +have been found by M. Aymard, and one of these is generally supposed to be +identical with Palaeotherium magnum, an undoubted Upper Eocene fossil, of the +Paris gypsum, the other three being peculiar. + +Not a few of the other mammalia of the Limagne belong undoubtedly to genera and +species elsewhere proper to the Lower Miocene. Thus, for example, the +Cainotherium of Bravard, a genus not far removed from the Anoplotherium, is +represented by several species, one of which, as I learn from Mr. Waterhouse, +agrees with Microtherium Renggeri of the Mayence basin. In like manner, the +Amphitragulus elegans of Pomel, an Auvergne fossil, is identified by Waterhouse +with Dorcatherium nanum of Kaup, a Rhenish species from Weissenau, near Mayence. +A small species, also, of rodent, of the genus Titanomys of H. von Meyer, is +common to the Lower Miocene of Mayence and the Limagne d'Auvergne, and there are +many other points of agreement which the discordance of nomenclature tends to +conceal. A remarkable carnivorous genus, the Hyaenodon of Laizer, is represented +by more than one species. The same genus has also been found in the Upper Eocene +marls of Hordwell Cliff, Hampshire, just below the level of the Bembridge +Limestone, and therefore a formation older than the Gypsum of Paris. Several +species of opossum (Didelphis) are met with in the same strata of the Limagne. +The total number of mammalia enumerated by M. Pomel as appertaining to the Lower +Miocene fauna of the Limagne and Velay falls little short of a hundred, and with +them are associated some large crocodiles and tortoises, and some Ophidian and +Batrachian reptiles. + +LOWER MOLASSE OF SWITZERLAND. + +The two upper divisions of the Swiss Molasse-- the one fresh-water, the other +marine-- have already been described in the preceding chapter. I shall now +proceed to treat of the third division, which is of Lower Miocene age. Nearly +the whole of this Lower Molasse is fresh-water, yet some of the inferior beds +contain a mixture of marine and fluviatile shells, the Cerithium margaritaceum, +a well-known Lower Miocene fossil, being one of the marine species. +Notwithstanding, therefore, that some of these Lower Miocene strata consist of +old shingle-beds several thousand feet in thickness, as in the Rigi, near +Lucerne, and in the Speer, near Wesen, mountains 5000 and 7000 feet above the +sea, the deposition of the whole series must have begun at or below the sea- +level. + +The conglomerates, as might be expected, are often very unequal in thickness, in +closely adjoining districts, since in a littoral formation accumulations of +pebbles would swell out in certain places where rivers entered the sea, and +would thin out to comparatively small dimensions where no streams or only small +ones came down to the coast. For ages, in spite of a gradual depression of the +land and adjacent sea-bottom, the rivers continued to cover the sinking area +with their deltas; until finally, the subsidence being in excess, the sea of the +Middle Molasse gained upon the land, and marine beds were thrown down over the +dense mass of fresh-water and brackish-water deposit, called the Lower Molasse, +which had previously accumulated. + +FLORA OF THE LOWER MOLASSE. + +In part of the Swiss Molasse, which belongs exclusively to the Lower Miocene +period, the number of plants has been estimated at more than 500 species, +somewhat exceeding those which were before enumerated as occurring in the two +upper divisions. The Swiss Lower Miocene may best be studied on the northern +borders of the Lake of Geneva, between Lausanne and Vevay, where the contiguous +villages of Monod and Rivaz are situated. The strata there, which I have myself +examined, consist of alternations of conglomerate, sandstone, and finely +laminated marls with fossil plants. A small stream falls in a succession of +cascades over the harder beds of pudding-stone, which resist, while the +sandstone and plant-bearing shales and marls give way. From the latter no less +than 193 species of plants have been obtained by the exertions of MM. Heer and +Gaudin, and they are considered to afford a true type of the vegetation of the +Lower Miocene formations of Switzerland-- a vegetation departing farther in its +character from that now flourishing in Europe than any of the higher members of +the series before alluded to, and yet displaying so much affinity to the flora +of Oeningen as to make it natural for the botanist to refer the whole to one and +the same Miocene period. There are, indeed, no less than 81 species of these +Older Miocene plants which pass up into the flora of Oeningen. + +This fact is important as bearing on the propriety of classing the Lower Molasse +of Switzerland as belonging to the Miocene rather than to the latter part of the +Eocene period. There are, indeed, so many types among the fossils, both specific +and generic, which have a wide range through the whole of the Molasse, that a +unity of character is thereby stamped on the whole flora, in spite of the +contrast between the plants of the uppermost and lowest formations, or between +Oeningen and Monod. The proofs of a warmer climate, and the excess of +arborescent over herbaceous plants, and of evergreen trees over deciduous +species, are characters common to the whole flora, but which are intensified as +we descend to the inferior deposits. + +(FIGURE 151. Sabal major, Unger sp. Vevay. Lower Miocene; Heer, Plate 41.) + +Nearly all the plants at Monod are contained in three layers of marl separated +by two of soft sandstone. The thickness of the marls is ten feet, and vegetable +matter predominates so much in some layers as to form an imperfect lignite. One +bed is filled with large leaves of a species of fig (Ficus populina), and of a +hornbeam (Carpinus grandis), the strength of the wind having probably been great +when they were blown into the lake; whereas another contiguous layer contains +almost exclusively smaller leaves, indicating, apparently, a diminished strength +in the wind. Some of the upper beds at Monod abound in leaves of Proteaceae, +Cyperaceae, and ferns, while in some of the lower ones Sequoia, Cinnamomum, and +Sparganium are common. In one bed of sandstone the trunk of a large palm-tree +was found unaccompanied by other fossils, and near Vevay, in the same series of +Lower Miocene strata, the leaves of a palm of the genus Sabal (Figure 151), a +genus now proper to America, were obtained. + +Among other genera of the same class is a Flabellaria occurring near Lausanne, +and a magnificent Phoenicites allied to the date palm. When these plants +flourished the climate must have been much hotter than now. The Alps were no +doubt much lower, and the palms now found fossil in strata elevated 2000 feet +above the sea grew nearly at the sea-level, as is demonstrated by the brackish- +water character of some of the beds into which they were carried by winds or +rivers from the adjoining coast. + +(FIGURE 152. Banksia. +a. Fruit of fossil Banksia. +b. Leaf of Banksia Deekiana.) + +In the same plant-bearing deposits of the Lower Molasse in Switzerland leaves +have been found which have been ascribed to the order Proteaceae already spoken +of as well represented in the Oeningen beds (see Chapter 14). The Proteas and +other plants of this family now flourish at the Cape of Good Hope; while the +Banksias, and a set of genera distinct from those of Africa, grow most +luxuriantly in the southern and temperate parts of Australia. They were probably +inhabitants, says Heer, of dry hilly ground, and the stiff leathery character of +their leaves must have been favourable to their preservation, allowing them to +float on a river for great distances without being injured, and then to sink, +when water-logged, to the bottom. It has been objected that the fruit of the +Proteaceae is of so tough and enduring a texture that it ought to have been more +commonly met with; but in the first place we must not forget the numerous cones +found in the Eocene strata of Sheppey, which all admit to be proteaceous and to +belong to at least two species (see Chapter 14). Secondly, besides the fruit of +Hakea before mentioned (Chapter 14), Heer found associated with fossil leaves, +having the exact form and nervation of Banksia, fruit precisely such as may have +come from a cone of that plant, and lately he has received another similar fruit +from the Lower Miocene strata of Lucerne. They may have fallen out of a decayed +cone in the same way as often happens to the seeds of the spruce fir, Pinus +abies, found scattered over the ground in our woods. It is a known fact that +among the living Proteaceae the cones are very firmly attached to the branches, +so that the seeds drop out without the cone itself falling to the ground, and +this may perhaps be the reason why, in some instances in which fossil seeds have +been found, no traces of the cone have been observed. + +(FIGURE 153. Sequoia Langsdorfii. Ad. Brong., 1/3 natural size. Rivaz, near +Lausanne; Heer, Plate 21 Figure 4. Upper and Lower Miocene and Lower Pliocene, +Val d'Arno. +a. Branch with leaves. +b. Young cone.) + +Among the Coniferae the Sequoia here figured is common at Rivaz, and is one of +the most universal plants in the Lowest Miocene of Switzerland, while it also +characterises the Miocene Brown Coals of Germany and certain beds of the Val +d'Arno, which I have called Older Pliocene, Chapter 13. + +(FIGURE 154. Lastraea stiriaca, Unger; Heer's Flora, Plate 143 Figure 8. Natural +size. Lower and Upper Miocene, Switzerland. +a. Specimen from Monod, showing the position of the sori on the middle of the +tertiary nerves. +b. More common appearance, where the sori remain and the nerves are +obliterated.) + +Among the ferns met with in profusion at Monod is the Lastraea stiriaca, Unger, +which has a wide range in the Miocene period from strata of the age of Oeningen +to the lowest part of the Swiss Molasse. In some specimens, as shown in Figure +154, the fructification is distinctly seen. + +(FIGURE 155. Cinnamomum Rossmassleri, Heer. Daphnogene cinnamomifolia, Unger. +Upper and Lower Miocene, Switzerland and Germany.) + +Among the laurels several species of Cinnamomum are very conspicuous. Besides +the C. polymorphum, before figured, Chapter 14, another species also ranges from +the Lower to the Upper Molasse of Switzerland, and is very characteristic of +different deposits of Brown Coal in Germany. It has been called Cinnamomum +Rossmassleri by Heer (see Figure 155). The leaves are easily recognised as +having two side veins, which run up uninterruptedly to their point. + +AMERICAN CHARACTER OF THE FLORA. + +If we consider not merely the number of species but those plants which +constitute the mass of the Lower Miocene vegetation, we find the European part +of the fossil flora very much less prominent than in the Oeningen beds, while +the foreground is occupied by American forms, by evergreen oaks, maples, +poplars, planes, Liquidambar, Robinia, Sequoia, Taxodium, and ternate-leaved +pines. There is also a much greater fusion of the characters now belonging to +distinct botanical provinces than in the Upper Miocene flora, and we shall find +this fusion still more strikingly exemplified as we go back to the antecedent +Eocene and Cretaceous periods. + +Professor Heer has advocated the doctrine, first advanced by Unger to explain +the large number of American genera in the Miocene flora of Europe, that the +present basin of the Atlantic was occupied by land over which the Miocene flora +could pass freely. But other able botanists have shown that it is far more +probable that the American plants came from the east and not from the west, and +instead of reaching Europe by the shortest route over an imaginary Atlantis, +migrated in an opposite direction, crossing the whole of Asia. + +ARCTIC MIOCENE FLORA. + +But when we indulge in speculations as to the geographical origin of the Miocene +plants of Central Europe, we must take into account the discoveries recently +made of a rich terrestrial flora having flourished in the Arctic Regions in the +Miocene period from which many species may have migrated from a common centre so +as to reach the present continents of Europe, Asia, and America. Professor Heer +has examined the various collections of fossil plants that have been obtained in +North Greenland (latitude 70 degrees), Iceland, Spitzbergen, and other parts of +the Arctic regions, and has determined that they are of Miocene age and indicate +a temperate climate. (Heer "Miocene baltische Flora" and "Fossil-flora von +Alaska" 1869.) Including the collections recently brought from Greenland by Mr. +Whymper, the Arctic Miocene flora now comprises 194 species, and that of +Greenland 137 species, of which 46, or exactly one-third, are identical with +plants found in the Miocene beds of Central Europe. Considerably more than half +the number are trees, which is the more remarkable since, at the present day, +trees do not exist in any part of Greenland even 10 degrees farther south. + +More than thirty species of Coniferae have been found, including several +Sequoias (allied to the gigantic Wellingtonia of California), with species of +Thujopsis and Salisburia now peculiar to Japan. There are also beeches, oaks, +planes, poplars, maples, walnuts, limes, and even a magnolia, two cones of which +have recently been obtained, proving that this splendid evergreen not only lived +but ripened its fruit within the Arctic circle. Many of the limes, planes, and +oaks were large-leaved species, and both flowers and fruit, besides immense +quantities of leaves, are in many cases preserved. Among the shrubs were many +evergreens, as Andromeda, and two extinct genera, Daphnogene and M'Clintockia, +with fine leathery leaves, together with hazel, blackthorn, holly, logwood, and +hawthorn. A species of Zamia (Zamites) grew in the swamps, with Potamogeton, +Sparganium, and Menyanthes, while ivy and vines twined around the forest trees +and broad-leaved ferns grew beneath their shade. Even in Spitzbergen, as far +north as latitude 78 degrees 56', no less than ninety-five species of fossil +plants have been obtained, including Taxodium of two species, hazel, poplar, +alder, beech, plane-tree, and lime. Such a vigorous growth of trees within 12 +degrees of the pole, where now a dwarf willow and a few herbaceous plants form +the only vegetation, and where the ground is covered with almost perpetual snow +and ice, is truly remarkable. + +The identity of so many of the fossils with Miocene species of Central Europe +and Italy not only proves that the climate of Greenland was much warmer than it +is now, but also renders it probable that a much more uniform climate prevailed +over the entire northern hemisphere. This is also indicated by the whole +character of the Upper Miocene flora of Central Europe, which does not +necessitate a mean temperature very much greater than exists at present, if we +suppose such absence of winter cold as is proper to insular climates. Professor +Heer believes that the mean temperature of North Greenland must have been at +least 30 degrees higher than at present, while an addition of 10 degrees to the +mean temperature of Central Europe would probably be as much as was required. +The chief locality where this wonderful flora is preserved is at Atanekerdluk in +North Greenland (latitude 70 degrees), on a hill at an elevation of about 1200 +feet above the sea. There is here a considerable succession of sedimentary +strata pierced by volcanic rocks. Fossil plants occur in all the beds, and the +erect trunks as thick as a man's body which are sometimes found, together with +the abundance of specimens of flowers and fruit in good preservation, +sufficiently prove that the plants grew where they are now found. At Disco +island and other localities on the same part of the coast, good coal is +abundant, interstratified with beds of sandstone, in some of which fossil plants +have also been found, similar to those at Atanekerdluk. + +LOWER MIOCENE, BELGIUM. + +(FIGURE 156. Leda (Nucula) Deshayesiana, Nyst.) + +(FIGURE 157. Vanessa pluto; natural size. Lower Miocene, Radaboj, Croatia.) + +The Upper Miocene Bolderberg beds, mentioned in Chapter 14, rest on a Lower +Miocene formation called the Rupelian of Dumont. This formation is best seen at +the villages of Rupelmonde and Boom, ten miles south of Antwerp, on the banks of +the Scheldt and near the junction with it of a small stream called the Rupel. A +stiff clay abounding in fossils is extensively worked at the above localities +for making tiles. It attains a thickness of about 100 feet, and though very +different in age, much resembles in mineral character the "London clay," +containing, like it, septaria or concretions of argillaceous limestone traversed +by cracks in the interior, which are filled with calc-spar. The shells, +referable to about forty species, have been described by MM. Nyst and De +Koninck. Among them Leda (or Nucula) Deshayesiana (see Figure 156) is by far the +most abundant; a fossil unknown as yet in the English tertiary strata, but when +young much resembling Leda amygdaloides of the London Clay proper (see Figure +213 Chapter 16). Among other characteristic shells are Pecten Hoeninghausii, and +a species of Cassidaria, and several of the genus Pleurotoma. Not a few of these +testacea agree with English Eocene species, such as Actaeon simulatus, Sowb, +Cancellaria evulsa, Brander, Corbula pisum (Figure 157), and Nautilus (Aturia) +ziczac. They are accompanied by many teeth of sharks, as Lamna contortidens, +Ag., Oxyrhinaxiphodon, Ag., Carcharodon angustidens (see Figure 196 Chapter 16), +Ag., and other fish, some of them common to the Middle Eocene strata. + +KLEYN SPAWEN BEDS. + +The succession of the Lower Miocene strata of Belgium can be best studied in the +environs of Kleyn Spawen, a village situated about seven miles west of +Maestricht, in the old province of Limburg in Belgium. In that region, about 200 +species of testacea, marine and fresh-water, have been obtained, with many +foraminifera and remains of fish. In none of the Belgian Lower Miocene strata +could I find any nummulites; and M. d'Archiac had previously observed that these +foraminifera characterise his "Lower Tertiary Series," as contrasted with the +Middle, and they therefore serve as a good test of age between Eocene and +Miocene, at least in Belgium and the North of France. (D'Archiac Monograph pages +79, 100.) Between the Bolderberg beds and the Rupelian clay there is a great gap +in Belgium, which seems, according to M. Beyrich, to be filled up in the North +of Germany by what he calls the Sternberg beds, and which, had Dumont found them +in Belgium, he might probably have termed Upper Rupelian. + +LOWER MIOCENE OF GERMANY. + +RUPELIAN CLAY OF HERMSDORF, NEAR BERLIN. + +Professor Beyrich has described a mass of clay, used for making tiles, within +seven miles of the gates of Berlin, near the village of Hermsdorf, rising up +from beneath the sands with which that country is chiefly overspread. This clay +is more than forty feet thick, of a dark bluish-grey colour, and, like that of +Rupelmonde, contains septaria. Among other shells, the Leda Deshayesiana, before +mentioned (Figure 156), abounds, together with many species of Pleurotoma, +Voluta, etc., a certain proportion of the fossils being identical in species +with those of Rupelmonde. + +MAYENCE BASIN. + +An elaborate description has been published by Dr. F. Sandberger of the Mayence +tertiary area, which occupies a tract from five to twelve miles in breadth, +extending for a great distance along the left bank of the Rhine from Mayence to +the neighbourhood of Manheim, and which is also found to the east, north, and +south-west of Frankfort. M. De Koninck, of Liege, first pointed out to me that +the purely marine portion of the deposit contained many species of shells common +to the Kleyn Spawen beds, and to the clay of Rupelmonde, near Antwerp. Among +these he mentioned Cassidaria depressa, Tritonium argutum, Brander (T. +flandricum, De Koninck), Tornatella simulata, Aporrhais Sowbyi, Leda +Deshayesiana (Figure 156), Corbula pisum, (Figure 158) and others. + +LOWER MIOCENE BEDS OF CROATIA. + +The Brown Coal of Radaboj, near Angram in Croatia, not far from the borders of +Styria, is covered, says Von Buch, by beds containing the marine shells of the +Vienna basin, or, in other words, by Upper Miocene or Falunian strata. They +appear to correspond in age to the Mayence basin, or to the Rupelian strata of +Belgium. They have yielded more than 200 species of fossil plants, described by +the late Professor Unger. These plants are well preserved in a hard marlstone, +and contain several palms; among them the Sabal, Figure 151, and another genus +allied to the date-palm Phoenicites spectabilis. The only abundant plant among +the Radaboj fossils which is characteristic of the Upper Miocene period is the +Populus mutabilis, whereas no less than fifty of the Radaboj species are common +to the more ancient flora of the Lower Molasse of Switzerland. + +The insect fauna is very rich, and, like the plants, indicates a more tropical +climate than do the fossils of Oeningen presently to be mentioned. There are ten +species of Termites, or white ants, some of gigantic size, and large dragon- +flies with speckled wings, like those of the Southern States in North America; +there are also grasshoppers of considerable size, and even the Lepidoptera are +not unrepresented. In one instance, the pattern of a butterfly's wing has +escaped obliteration in the marl-stone of Radaboj; and when we reflect on the +remoteness of the time from which it has been faithfully transmitted to us, this +fact may inspire the reader with some confidence as to the reliable nature of +the characters which other insects of a more durable texture, such as the +beetles, may afford for specific determination. The Vanessa above figured +retains, says Heer, some of its colours, and corresponds with Vanessa Hadena of +India. + +Professor Beyrich has made known to us the existence of a long succession of +marine strata in North Germany, which lead by an almost gradual transition from +beds of Upper Miocene age to others of the age of the base of the Lower Miocene. +Although some of the German lignites called Brown Coal belong to the upper parts +of this series, the most important of them are of Lower Miocene date, as, for +example, those of the Siebengebirge, near Bonn, which are associated with +volcanic rocks. + +Professor Beyrich confines the term "Miocene" to those strata which agree in age +with the faluns of Touraine, and he has proposed the term "Oligocene" for those +older formations called Lower Miocene in this work. + +LOWER MIOCENE OF ITALY. + +In the hills of which the Superga forms a part there is a great series of +Tertiary strata which pass downward into the Lower Miocene. Even in the Superga +itself there are some fossil plants which, according to Heer, have never been +found in Switzerland so high as the marine Molasse, such as Banksia longifolia, +and Carpinus grandis. In several parts of the Ligurian Apennines, as at Dego and +Carcare, the Lower Miocene appears, containing some nummulites, and at Cadibona, +north of Savona, fresh-water strata of the same age occur, with dense beds of +lignite inclosing remains of the Anthracotherium magnum and Anthracotherium +minimum, besides other mammalia enumerated by Gastaldi. In these beds a great +number of the Lower Miocene plants of Switzerland have been discovered. + +LOWER MIOCENE OF ENGLAND-- HEMPSTEAD BEDS. + +We have already stated that the Upper Miocene formation is nowhere represented +in the British Isles; but strata referable to the Lower Miocene period are found +both in England, Scotland, and Ireland. In the Hampshire basin these occupy a +very small superficial area, having been discovered by the late Edward Forbes at +Hempstead near Yarmouth, in the northern part of the Isle of Wight, where they +are 170 feet thick, and rich in characteristic marine shells. They overlie the +uppermost of an extensive series of Eocene deposits of marine, brackish, and +fresh-water formations, which rest on the Chalk and terminate upward in strata +corresponding in age to the Paris gypsum, and containing the same extinct genera +of quadrupeds, Palaeotherium, Anoplotherium, and others which Cuvier first +described. The following is the succession of these Lower Miocene strata, most +of them exposed in a cliff east of Yarmouth: + +(FIGURE 158. Corbula pisum. Hempstead Beds, Isle of Wight.) + +(FIGURE 159. Cyrena semistriata. Hempstead Beds.) + +1. The uppermost or Corbula beds, consisting of marine sands and clays, contain +Voluta Rathieri, a characteristic Lower Miocene shell; Corbula pisum (Figure +158), a species common to the Upper Eocene clay of Barton; Cyrena semistriata +(Figure 159), several Cerithia, and other shells peculiar to this series. + +(FIGURE 160. Cerithium plicatum, Lam., Hempstead.) + +(FIGURE 161. Cerithium elegans. Hempstead.) + +(FIGURE 162. Rissoa Chastelii, Nyst, sp. Hempstead, Isle of Wight.) + +(FIGURE 163. Paludina lenta. Hempstead Bed.) + +2. Next below are fresh-water and estuary marls and carbonaceous clays in the +brackish-water portion of which are found abundantly Cerithium plicatum, Lam. +(Figure 160), Cerithium elegans (Figure 161), and Cerithium tricinctum; also +Rissoa Chastelii (Figure 162), a very common Kleyn Spawen shell, and which +occurs in each of the four subdivisions of the Hempstead series down to its +base, where it passes into the Bembridge beds. In the fresh-water portion of the +same beds Paludina lenta (Figure 163) occurs; a shell identified by some +conchologists with a species now living, Paludina unicolor; also several species +of Lymneus, Planorbis, and Unio. + +3. The next series, or middle fresh-water and estuary marls, are distinguished +by the presence of Melania fasciata, Paludina lenta, and clays with Cypris; the +lowest bed contains Cyrena semistriata (Figure 159), mingled with Cerithia and a +panopaea. + +4. The lower fresh-water and estuary marls contain Melania costata, Sowerby, +Melanopsis, etc. The bottom bed is carbonaceous, and called the "Black band," in +which Rissoa Chastelii (Figure 162), before alluded to, is common. This bed +contains a mixture of Hempstead shells with those of the underlying Upper Eocene +or Bembridge series. The mammalia, among which is Hyopotamus bovinus, differ, so +far as they are known, from those of the Bembridge beds. Among the plants, +Professor Heer has recognised four species common to the lignite of Bovey +Tracey, a Lower Miocene formation presently to be described: namely, Sequoia +Couttsiae, Heer; Andromeda reticulata, Ettings.; Nelumbium (Nymphoea) doris, +Heer; and Carpolithes Websteri, Brong. (Pengelly, preface to The Lignite +Formation of Bovey Tracey page 17. London 1863.) The seed-vessels of Chara +medicaginula, Brong, and Chara helicteres are characteristic of the Hempstead +beds generally. + +The Hyopotamus belongs to the hog tribe, or the same family as the +Anthracotherium, of which seven species, varying in size from the hippopotamus +to the wild boar, have been found in Italy and other part of Europe associated +with the lignites of the Lower Miocene period. + +LIGNITES AND CLAYS OF BOVEY TRACEY, DEVONSHIRE. + +Surrounded by the granite and other rocks of the Dartmoor hills in Devonshire, +is a formation of clay, sand, and lignite, long known to geologists as the Bovey +Coal formation, respecting the age of which, until the year 1861, opinions were +very unsettled. This deposit is situated at Bovey Tracey, a village distant +eleven miles from Exeter in a south-west, and about as far from Torquay in a +north-west direction. The strata extend over a plain nine miles long, and they +consist of the materials of decomposed and worn-down granite and vegetable +matter, and have evidently filled up an ancient hollow or lake-like expansion of +the valleys of the Bovey and Teign. + +The lignite is of bad quality for economical purposes, as there is a great +admixture in it of iron pyrites, and it emits a sulphurous odour, but it has +been successfully applied to the baking of pottery, for which some of the fine +clays are well adapted. Mr. Pengelly has confirmed Sir H. De la Beche's opinion +that much of the upper portion of this old lacustrine formation has been removed +by denudation. (Philosophical Transactions 1863. Paper by W. Pengelly F.R.S. and +Dr. Oswald Heer.) + +At the surface is a dense covering of clay and gravel with angular stones +probably of the Post-pliocene period, for in the clay are three species of +willow and the dwarf birch, Betula nana, indicating a climate colder than that +of Devonshire at the present day. + +Below this are Lower Miocene strata about 300 feet in thickness, in the upper +part of which are twenty-six beds of lignite, clay, and sand, and at their base +a ferruginous quartzose sand, varying in thickness from two to twenty-seven +feet. Below this sand are forty-five beds of alternating lignite and clay. No +shells or bones of mammalia, and no insect, with the exception of one fragment +of a beetle (Buprestis); in a word, no organic remains, except plants, have as +yet been found. These plants occur in fourteen of the beds-- namely, in two of +the clays, and the rest in the lignites. One of the beds is a perfect mat of the +debris of a coniferous tree, called by Heer Sequoia Couttsiae, intermixed with +leaves of ferns. The same Sequoia (before mentioned as a Hempstead fossil) is +spread through all parts of the formation, its cones, and seeds, and branches of +every age being preserved. It is a species supplying a link between Sequoia +Langsdorfii (see Figure 153) and Sequoia Sternbergi, the widely spread fossil +representatives of the two living trees Sequoia sempervirens and Sequoia +gigantea (or Wellingtonia), both now confined to California. Another bed is full +of the large rhizomes of ferns, while two others are rich in dicotyledonous +leaves. In all, Professor Heer enumerates forty-nine species of plants, twenty +of which are common to the Miocene beds of the Continent, a majority of them +being characteristic of the Lower Miocene. The new species, also of Bovey, are +allied to plants of the older Miocene deposits of Switzerland, Germany, and +other Continental countries. The grape-stones of two species of vine occur in +the clays, and leaves of the fig and seeds of a water-lily. The oak and laurel +have supplied many leaves. Of the triple-nerved laurels several are referred to +Cinnamomum. There are leaves also of a palm of which the genus is not +determined. Leaves also of proteaceous forms, like some of the Continental +fossils before mentioned, occur, and ferns like the well-known Lastraea stiriaca +(Figure 154), displaying at Bovey, as in Switzerland, its fructification. + +The croziers of some of the young ferns are very perfect, and were at first +mistaken by collectors for shells of the genus Planorbis. On the whole, the +vegetation of Bovey implies the existence of a sub-tropical climate in +Devonshire, in the Lower Miocene period. + +SCOTLAND: ISLE OF MULL. + +In the sea-cliffs forming the headland of Ardtun, on the west coast of Mull, in +the Hebrides, several bands of tertiary strata containing leaves of +dicotyledonous plants were discovered in 1851 by the Duke of Argyll. (Quarterly +Geological Journal 1851 page 19.) From his description it appears that there are +three leaf-beds, varying in thickness from 1 1/2 to 5 1/2 feet, which are +interstratified with volcanic tuff and trap, the whole mass being about 130 feet +in thickness. A sheet of basalt 40 feet thick covers the whole; and another +columnar bed of the same rock, ten feet thick, is exposed at the bottom of the +cliff. One of the leaf-beds consists of a compressed mass of leaves +unaccompanied by any stems, as if they had been blown into a marsh where a +species of Equisetum grew, of which the remains are plentifully imbedded in +clay. + +It is supposed by the Duke of Argyll that this formation was accumulated in a +shallow lake or marsh in the neighbourhood of a volcano, which emitted showers +of ashes and streams of lava. The tufaceous envelope of the fossils may have +fallen into the lake from the air as volcanic dust, or have been washed down +into it as mud from the adjoining land. Even without the aid of organic remains +we might have decided that the deposit was newer than the chalk, for chalk- +flints containing cretaceous fossils were detected by the duke in the principal +mass of volcanic ashes or tuff. (Quarterly Geological Journal 1851 page 90.) + +The late Edward Forbes observed that some of the plants of this formation +resembled those of Croatia, described by Unger, and his opinion has been +confirmed by Professor Heer, who found that the conifer most prevalent was the +Sequoia Langsdorfii (Figure 153), also Corylus grossedentata, a Lower Miocene +species of Switzerland and of Menat in Auvergne. There is likewise a plane-tree, +the leaves of which seem to agree with those of Platanus aceroides (Figure 141 +Chapter 14), and a fern which is as yet peculiar to Mull, Filicites hebridica, +Forbes. + +These interesting discoveries in Mull led geologists to suspect that the basalt +of Antrim, in Ireland, and of the celebrated Giant's Causeway, might be of the +same age. The volcanic rocks that overlie the chalk, and some of the strata +associated with and interstratified between masses of basalt, contain leaves of +dicotyledonous plants, somewhat imperfect, but resembling the beech, oak, and +plane, and also some coniferae of the genera pine and Sequoia. The general +dearth of strata in the British Isles, intermediate in age between the formation +of the Eocene and Pliocene periods, may arise, says Professor Forbes, from the +extent of dry land which prevailed in that vast interval of time. If land +predominated, the only monuments we are likely ever to find of Miocene date are +those of lacustrine and volcanic origin, such as the Bovey Coal in Devonshire, +the Ardtun beds in Mull, or the lignites and associated basalts in Antrim. + +LOWER MIOCENE, UNITED STATES: NEBRASKA. + +In the territory of Nebraska, on the Upper Missouri, near the Platte River, +latitude 42 degrees N., a tertiary formation occurs, consisting of white +limestone, marls, and siliceous clay, described by Dr. D. Dale Owen (David Dale +Owen Geological Survey of Wisconsin etc. Philadelphia 1852.), in which many +bones of extinct quadrupeds, and of chelonians of land or fresh-water forms, are +met with. Among these, Dr. Leidy describes a gigantic quadruped, called by him +Titanotherium, nearly allied to the Palaeotherium, but larger than any of the +species found in the Paris gypsum. With these are several species of the genus +Oreodon, Leidy, uniting the characters of pachyderms and ruminants also; +Eucrotaphus, another new genus of the same mixed character; two species of +rhinoceros of the sub-genus Acerotherium, a Lower Miocene form of Europe before +mentioned; two species of Archaeotherium, a pachyderm allied to Chaeropotamus +and Hyracotherium; also Paebrotherium, an extinct ruminant allied to +Dorcatherium, Kaup; also Agriochoerus, of Leidy, a ruminant allied to +Merycopotamus of Falconer and Cautley; and, lastly, a large carnivorous animal +of the genus Machairodus, the most ancient example of which in Europe occurs in +the Lower Miocene strata of Auvergne, but of which some species are found in +Pliocene deposits. The turtles are referred to the genus Testudo, but have some +affinity to Emys. On the whole, the Nebraska formation is probably newer than +the Paris gypsum, and referable to the Lower Miocene period, as above defined. + + +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. + +EOCENE AREAS OF THE NORTH OF EUROPE. + +(FIGURE 164. Map of the principal Eocene areas of North-western Europe, showing: +Shaded dotted: Hypogene rocks and strata older than the Devonian. +Shaded horizontal lines: Eocene formations. +NB.-- the space left blank is occupied by fossiliferous formations from the +Devonian to the chalk inclusive.) + +The strata next in order in the descending series are those which I term Eocene. + +In the map in Figure 164, the position of several Eocene areas in the north of +Europe is pointed out. When this map was constructed I classed as the newer part +of the Eocene those Tertiary strata which have been described in the last +chapter as Lower Miocene, and to which M. Beyrich has given the name of +Oligocene. None of these occur in the London Basin, and they occupy in that of +Hampshire, as we have seen in Chapter 15, too insignificant a superficial area +to be noticed in a map on this scale. They fill a larger space in the Paris +Basin between the Seine and the Loire, and constitute also part of the northern +limits of the area of the Netherlands which are shaded in the map. + +TABLE 16.1. TABLE OF ENGLISH AND FRENCH EOCENE STRATA. + +COLUMN 1: NAME OF STRATA. + +COLUMN 2: ENGLISH SUBDIVISIONS. + +COLUMN 3: FRENCH EQUIVALENTS. + +UPPER EOCENE. + +A.1: Bembridge series, Isle of Wight: Gypseous series of Montmartre. + +A.2: Osborne or St. Helen's series, Isle of Wight: Calcaire siliceux, or +Travertin Inferieur. + +A.3: Headon series, Isle of Wight: Calcaire siliceux, or Travertin Inferieur. + +A.4: Barton series. Sands and clays of Barton Cliff, Hants: Gres de Beauchamp, +or Sables Moyens. + +MIDDLE EOCENE. + +B.1: Bracklesham series: Calcaire Grossier. + +B.2: Alum Bay and Bournemouth beds: Wanting in France? + +B.2: Wanting in England?: Soissonnais Sands, or Lits Coquilliers. + +LOWER EOCENE. + +C.1: London Clay: Argile de Londres, Cassel, near Dunkirk. + +C.2: Woolwich and Reading series: Argile plastique and lignite. + +C.3: Thanet sands: Sables de Bracheux. + + +It is in the northern part of the Isle of Wight that we have the uppermost beds +of the true Eocene best exhibited-- namely, those which correspond in their +fossils with the celebrated gypsum of the Paris basin before alluded to in +Chapter 15 (see Table 16.1). That gypsum has been selected by almost all +Continental geologists as affording the best line of demarkation between the +Middle and Lower Tertiary, or, in other words, between the Lower Miocene and +Eocene formations. + +In reference to Table 16.1 I may observe, that the correlation of the French and +English subdivisions here laid down is often a matter of great doubt and +difficulty, notwithstanding their geographical proximity. This arises from +various circumstances, partly from the former prevalence of marine conditions in +one basin simultaneously with fluviatile or lacustrine in the other, and +sometimes from the existence of land in one area causing a break or absence of +all records during a period when deposits may have been in progress in the other +basin. As bearing on this subject, it may be stated that we have unquestionable +evidence of oscillations of level shown by the superposition of salt or +brackish-water strata to fluviatile beds; and those of deep-sea origin to strata +formed in shallow water. Even if the upward and downward movements were uniform +in amount and direction, which is very improbable, their effect in producing the +conversion of sea into land or land into sea would be different, according to +the previous shape and varying elevation of the land and bottom of the sea. +Lastly, denudation, marine and subaerial, has frequently caused the absence of +deposits in one basin of corresponding age to those in the other, and this +destructive agency has been more than ordinarily effective on account of the +loose and unconsolidated nature of the sands and clays. + +UPPER EOCENE OF ENGLAND. + +BEMBRIDGE SERIES, A.1. + +These beds are about 120 feet thick, and, as stated in Chapter 15, lie +immediately under the Hempstead beds, near Yarmouth, in the Isle of Wight, being +conformable with those Lower Miocene strata. They consist of marls, clays, and +limestones of fresh-water, brackish, and marine origin. Some of the most +abundant shells, as Cyrena semistriata var., and Paludina lenta, Figure 163 +Chapter 15, are common to this and to the overlying Hempstead series; but the +majority of the species are distinct. The following are the subdivisions +described by the late Professor Forbes: + +(FIGURE 165. Melania turritissima, Forbes. Bembridge.) + +a. Upper marls, distinguished by the abundance of Melania turritissima, Forbes +(Figure 165). + +(FIGURE 166. Fragment of carapace of Trionyx. Bembridge Beds, Isle of Wight.) + +b. Lower marls, characterised by Cerithium mutabile, Cyrena pulchra, etc., and +by the remains of Trionyx (see Figure 166). + +c. Green marls, often abounding in a peculiar species of oyster, and accompanied +by Cerithium, Mytilus, Arca, nucula, etc.) + +(FIGURE 167. Bulimus ellipticus, Sowerby. Bembridge Limestone. 1/2 natural +size.) + +(FIGURE 168. Helix occlusa, Edwards. Bembridge Limestone, Isle of Wight.) + +(FIGURE 169. Paludina orbicularis. Bembridge.) + +(FIGURE 170. Planorbis discus, Edwards. Bembridge. 1/2 diameter.) + +(FIGURE 171. Lymnea longiscata, Brand. Natural size.) + +(FIGURE 172. Chara tuberculata, seed-vessel. Bembridge Limestone, Isle of +Wight.) + +d. Bembridge limestones, compact cream-coloured limestones alternating with +shales and marls, in all of which land-shells are common, especially at Sconce, +near Yarmouth, as described by Mr. F. Edwards. The Bulimus ellipticus, Figure +167, and Helix occlusa, Figure 168, are among its best known land-shells. +Paludina orbicularis, Figure 169, is also of frequent occurrence. One of the +bands is filled with a little globular Paludina. Among the fresh-water +pulmonifera, Lymnea longiscata (Figure 171) and Planorbis discus (Figure 170) +are the most generally distributed: the latter represents or takes the place of +the Planorbis euomphalus (see Figure 175) of the more ancient Headon series. +Chara tuberculata (Figure 172) is the characteristic Bembridge gyrogonite or +seed-vessel. + +(FIGURE 173. Anoplotherium commune. Binstead, Isle of Wight. +Lower molar tooth, natural size.) + +(FIGURE 174. Palaeotherium magnum, Cuvier.) + +(FIGURE 175. Planorbis euomphalus, Sowerby. Headon Hill. 1/2 diameter.) + +From this formation on the shores of Whitecliff Bay, Dr. Mantell obtained a fine +specimen of a fan palm, Flabellaria Lamanonis, Brong., a plant first obtained +from beds of corresponding age in the suburbs of Paris. The well-known building- +stone of Binstead, near Ryde, a limestone with numerous hollows caused by +Cyrenae which have disappeared and left the moulds of their shells, belongs to +this subdivision of the Bembridge series. In the same Binstead stone Mr. Pratt +and the Reverend Darwin Fox first discovered the remains of mammalia +characteristic of the gypseous series of Paris, as Palaeotherium magnum (Figure +174), Palaeotherium medium, Palaeotherium minus, Palaeotherium minimum, +Palaeotherium curtum, Palaeotherium crassum; also Anoplotherium commune (Figure +173), Anoplotherium secundarium, Dichobune cervinum, and Chaeropotamus Cuvieri. +The Palaeothere above alluded to resembled the living tapir in the form of the +head, and in having a short proboscis, but its molar teeth were more like those +of the rhinoceros. Palaeotherium magnum was of the size of a horse, three or +four feet high. The woodcut, Figure 174, is one of the restorations which Cuvier +attempted of the outline of the living animal, derived from the study of the +entire skeleton. As the vertical range of particular species of quadrupeds, so +far as our knowledge extends, is far more limited than that of the testacea, the +occurrence of so many species at Binstead, agreeing with fossils of the Paris +gypsum, strengthens the evidence derived from shells and plants of the +synchronism of the two formations. + +OSBORNE OR ST. HELEN'S SERIES, A.2. + +This group is of fresh and brackish-water origin, and very variable in mineral +character and thickness. Near Ryde, it supplies a freestone much used for +building, and called by Professor Forbes the Nettlestone grit. In one part +ripple-marked flagstones occur, and rocks with fucoidal markings. The Osborne +beds are distinguished by peculiar species of Paludina, Melania, and Melanopsis, +as also of Cypris and the seeds of Chara. + +HEADON SERIES A.3. + +These beds are seen both in Whitecliff Bay, Headon Hill, and Alum Bay, or at the +east and west extremities of the Isle of Wight. The upper and lower portions are +fresh-water, and the middle of mixed origin, sometimes brackish and marine. +Everywhere Planorbis euomphalus, Figure 175, characterises the fresh-water +deposits, just as the allied form, Planorbis discus, Figure 170, does the +Bembridge limestone. The brackish-water beds contain Potamomya plana, Cerithium +mutabile, and Potamides cinctus (Figure 37 Chapter 3), and the marine beds Venus +(or Cytherea) incrassata, a species common to the Limburg beds and Gres de +Fontainebleau, or the Lower Miocene series. The prevalence of salt-water remains +is most conspicuous in some of the central parts of the formation. + +(FIGURE 176. Helix labyrinthica, Say. Headon Hill, Isle of Wight; and Hordwell +Cliff, Hants-- also recent.) + +(FIGURE 177. Neritina concava, Sowerby. Headon series.) + +(FIGURE 178. Lymnea caudata, Edw. Headon series.) + +(FIGURE 179. Cerithium concavum, Sowerby. Headon series.) + +Among the shells which are widely distributed through the Headon series are +Neritina concava (Figure 177), Lymnea caudata (Figure 178), and Cerithium +concavum (Figure 179). Helix labyrinthica, Say (Figure 176), a land-shell now +inhabiting the United States, was discovered in this series by Mr. Searles Wood +in Hordwell Cliff. It is also met with in Headon Hill, in the same beds. At +Sconce, in the Isle of Wight, it occurs in the Bembridge series, and affords a +rare example of an Eocene fossil of a species still living, though, as usual in +such cases, having no local connection with the actual geographical range of the +species. The lower and middle portion of the Headon series is also met with in +Hordwell Cliff (or Hordle, as it is often spelt), near Lymington, Hants. Among +the shells which abound in this cliff are Paludina lenta and various species of +Lymnea, Planorbis, Melania, Cyclas, Unio, Potamomya, Dreissena, etc. + +Among the chelonians we find a species of Emys, and no less than six species of +Trionyx; among the saurians an alligator and a crocodile; among the ophidians +two species of land-snakes (Paleryx, Owen); and among the fish Sir P. Egerton +and Mr. Wood have found the jaws, teeth, and hard shining scales of the genus +Lepidosteus, or bony pike of the American rivers. This same genus of fresh-water +ganoids has also been met with in the Hempstead beds in the Isle of Wight. The +bones of several birds have been obtained from Hordwell, and the remains of +quadrupeds of the genera Palaeotherium (Palaeotherium minus), Anoplotherium, +Anthracotherium, Dichodon, Dichobune, Spalacodon, and Hyaenodon. The latter +offers, I believe, the oldest known example of a true carnivorous animal in the +series of British fossils, although I attach very little theoretical importance +to the fact, because herbivorous species are those most easily met with in a +fossil state in all save cavern deposits. In another point of view, however, +this fauna deserves notice. Its geological position is considerably lower than +that of the Bembridge or Montmartre beds, from which it differs almost as much +in species as it does from the still more ancient fauna of the Lower Eocene beds +to be mentioned in the sequel. It therefore teaches us what a grand succession +of distinct assemblages of mammalia flourished on the earth during the Eocene +period. + +Many of the marine shells of the brackish-water beds of the above series, both +in the Isle of Wight and Hordwell Cliff, are common to the underlying Barton +Clay: and, on the other hand, there are some fresh-water shells, such as Cyrena +obovata, which are common to the Bembridge beds, notwithstanding the +intervention of the St. Helen's series. The white and green marls of the Headon +series, and some of the accompanying limestones, often resemble the Eocene +strata of France in mineral character and colour in so striking a manner as to +suggest the idea that the sediment was derived from the same region or produced +contemporaneously under very similar geographical circumstances. + +(FIGURE 180. Solenastraea cellulosa, Duncan. Brockenhurst.) + +At Brockenhurst, near Lyndhurst, in the New Forest, marine strata have recently +been found containing fifty-nine shells, of which many have been described by +Mr. Edwards. These beds rest on the Lower Headon, and are considered as the +equivalent of the middle part of the Headon series, many of the shells being +common to the brackish-water or Middle Headon beds of Colwell and Whitecliff +Bays, such as Cancellaria muricata, Sowerby, Fusus labiatus, Sowerby, etc. In +these beds at Brockenhurst, corals, ably described by Dr. Duncan, have recently +been found in abundance and perfection; see Figure 180, Solenastraea cellulosa. + +Baron von Konen has pointed out that no less than forty-six out of the fifty- +nine Brockenhurst shells, or a proportion of 78 per cent, agree with species +occurring in Dumont's Lower Tongrian formation in Belgium. (Quarterly Geological +Journal volume 20 page 97 1864.) This being the case, we might fairly expect +that if we had a marine equivalent of the Bembridge series or of the +contemporaneous Paris gypsum, we should find it to contain a still greater +number of shells common to the Tongrian beds of Belgium, but the exact +correlation of these fresh-water groups of France, Belgium, and Britain has not +yet been fully made out. It is possible that the Tongrian of Dumont may be newer +than the Bembridge series, and therefore referable to the Lower Miocene. If ever +the whole series should be complete, we must be prepared to find the marine +equivalent of the Bembridge beds, or the uppermost Eocene, passing by +imperceptible shades into the inferior beds of the overlying Miocene strata. + +Among the fossils found in the Middle Headon are Cytherea incrassata and +Cerithium plicatum (Figure 160 Chapter 15). These shells, especially the latter, +are very characteristic of the Lower Miocene, and their occurrence in the Headon +series has been cited as an objection to the line proposed to be drawn between +Miocene and Eocene. But if we were to attach importance to such occasional +passages, we should soon find that no lines of division could be drawn anywhere, +for in the present state of our knowledge of the Tertiary series there will +always be species common to beds above and below our boundary-lines. + +BARTON SERIES (SANDS AND CLAYS), A.4 TABLE 16.1.) + +(FIGURE 181. Chama squamosa, Eichw. Barton.) + +Both in the Isle of Wight, and in Hordwell Cliff, Hants, the Headon beds, above- +mentioned, rest on white sands usually devoid of fossils, and used in the Isle +of Wight for making glass. In one of these sands Dr. Wright found Chama +squamosa, a Barton Clay shell, in great plenty, and certain impressions of +marine shells have been found in sands supposed to be of the same age in +Whitecliff Bay. These sands have been called Upper Bagshot in the maps of our +Government Survey, but this identification of a fossiliferous series in the Isle +of Wight with an unfossiliferous formation in the London Basin can scarcely be +depended upon. The Barton Clay, which immediately underlies these sands, is seen +vertical in Alum Bay, Isle of Wight, and nearly horizontal in the cliffs of the +mainland near Lymington. This clay, together with the Bracklesham beds, +presently to be described, has been termed Middle Bagshot by the Survey. In +Barton Cliff, where it attains a thickness of about 300 feet, it is rich in +marine fossils. + +It was formerly confounded with the London Clay, an older Eocene deposit of very +similar mineral character, to be mentioned below, which contains many shells in +common, but not more than one-fourth of the whole. In other words, there are +known at present 247 species in the London Clay and 321 in that of Barton, and +only 70 common to the two formations. Fifty-six of these have been found in the +intermediate Bracklesham beds, and the reappearance of the other 14 may imply a +return of similar conditions, whether of temperature or depth or of a muddy +argillaceous bottom, common to the two periods of the London and Barton Clays. +According to M. Hebert, the most characteristic Barton Clay fossils correspond +to those of the Gres de Beauchamp, or Sables Moyens, of the Paris Basin, but it +also contains many common to the older Calcaire Grossier. + +SHELLS OF THE BARTON CLAY. + +(FIGURE 182. Mitra scabra, Sowerby.) + +(FIGURE 183. Voluta ambigua, Sol.) + +(FIGURE 184. Typhis pungens, Brand.) + +(FIGURE 185. Voluta athleta, Sol. Barton and Bracklesham.) + +(FIGURE 186. Terebellum fusiforme, Lam. Barton and Bracklesham.) + +(FIGURE 187. Terebellum sopita, Brand.) + +(FIGURE 188. Cardita sulcata, Brand. Barton.) + +(FIGURE 189. Crassatella sulcata, Sowerby. Bracklesham and Barton.) + +(FIGURE 190. Nummulites variolaria, Lam. Var. of Nummulites radiata, Sowerby. +Middle Eocene, Bracklesham Bay. +a. Natural size. +b. Magnified.) + +Certain foraminifera called Nummulites begin, when we study the Tertiary +formations in a descending order, to make their first appearance in these beds. +A small species called Nummulites variolaria, Figure 190, is found both on the +Hampshire coast and in beds of the same age in Whitecliff Bay, in the Isle of +Wight. Several marine shells, such as Corbula pisum (Figure 158), are common to +the Barton beds and the Hempstead or Lower Miocene series, and a still greater +number, as before stated, are common to the Headon series. + +MIDDLE EOCENE, ENGLAND. + +BRACKLESHAM BEDS AND BAGSHOT SANDS (B.1, TABLE 16.1). + +(FIGURE 191. Cardita (Venericardia) planicosta, Lam.) + +(FIGURE 192. Nummulites (Nummularia) laevigata. Bracklesham. Dixon's Fossils of +Sussex, Plate 8. +a. Section of nummulite. +b. Group, with an individual showing the exterior of the shell.) + +Beneath the Barton Clay we find in the north of the Isle of Wight, both in Alum +and Whitecliff Bays, a great series of various coloured sands and clays for the +most part unfossiliferous, and probably of estuarine origin. As some of these +beds contain Cardita planicosta (Figure 191) they have been identified with the +marine beds much richer in fossils seen in the coast section in Bracklesham Bay +near Chichester in Sussex, where the strata consist chiefly of green clayey +sands with some lignite. Among the Bracklesham fossils besides the Cardita, the +huge Cerithium giganteum is seen, so conspicuous in the Calcaire Grossier of +Paris, where it is sometimes two feet in length. The Nummulites laevigata (see +Figure 192), so characteristic of the lower beds of the Calcaire Grossier in +France, where it sometimes forms stony layers, as near Compiegne, is very common +in these beds, together with Nummulites scabra and Nummulites variolaria. Out of +193 species of testacea procured from the Bagshot and Bracklesham beds in +England, 126 occur in the Calcaire Grossier in France. It was clearly, +therefore, coeval with that part of the Parisian series more nearly than with +any other. + +(FIGURE 193. Palaeophis typhoeus, Owen; an Eocene sea-serpent. Bracklesham. +a, b. Vertebra, with long neural spine preserved. +c. Two vertebrae articulated together.) + +(FIGURE 194. Defensive spine of Ostracion. Bracklesham.) + +(FIGURE 195. Dental plates of Myliobates Edwardsi. Bracklesham Bay. +Dixon's Fossils of Sussex, Plate 8.) + +According to tables compiled from the best authorities by Mr. Etheridge, the +number of mollusca now known from the Bracklesham beds in Great Britain is 393, +of which no less than 240 are peculiar to this subdivision of the British Eocene +series, while 70 are common to the Older London Clay, and 140 to the Newer +Barton Clay. The volutes and cowries of this formation, as well as the lunulites +and corals, favour the idea of a warm climate having prevailed, which is borne +out by the discovery of a serpent, Palaeophis typhoeus (see Figure 193), +exceeding, according to Professor Owen, twenty feet in length, and allied in its +osteology to the Boa, Python, Coluber, and Hydrus. The compressed form and +diminutive size of certain caudal vertebrae indicate so much analogy with Hydrus +as to induce Professor Owen to pronounce this extinct ophidian to have been +marine. (Palaeontological Society Monograph Reptiles part 2 page 61.) Among the +companions of the sea-snake of Bracklesham was an extinct crocodile (Gavialis +Dixoni, Owen), and numerous fish, such as now frequent the seas of warm +latitudes, as the Ostracion of the family Balistidae, of which a dorsal spine is +figured (see Figure 194), and gigantic rays of the genus Myliobates (see Figure +195). + +(FIGURE 196. Carcharodon angustidens, Agassiz.) + +(FIGURE 197. Otodus obliquus, Agassiz.) + +(FIGURE 198. Lamna elegans, Agassiz.) + +(FIGURE 199. Galeocerdo latidens, Agassiz.) + +The teeth of sharks also, of the genera Carcharodon, Otodus, Lamna, Galeocerdo, +and others, are abundant. (See Figures 196, 197, 198, 199.) + +MARINE SHELLS OF BRACKLESHAM BEDS. + +ALUM BAY AND BOURNEMOUTH BEDS. (LOWER BAGSHOT OF ENGLISH SURVEY), B.2, TABLE +16.1.) + +(FIGURE 200. Pleurotoma attenuata, Sowerby.) + +(FIGURE 201. Voluta Selseiensis, Edwards.) + +(FIGURE 202. Turritella multisulcata, Lam.) + +(FIGURE 203. Lucina serrata, Sowerby. Magnified.) + +(FIGURE 204. Conus deperditus, Brug.) + +To that great series of sands and clays which intervene between the equivalents +of the Bracklesham Beds and the London Clay or Lower Eocene, our Government +Survey has given the name of the Lower Bagshot sands, for they are supposed to +agree in age with the inferior unfossiliferous sands of the country round +Bagshot in the London Basin. This part of the series is finely exposed in the +vertical beds of Alum bay, in the Isle of Wight, and east and west of +Bournemouth, on the south coast of Hampshire. In some of the close and white +compact clays of this locality, there are not only dicotyledonous leaves, but +numerous fronds of ferns allied to Gleichenia which are well preserved with +their fruit. + +None of the beds are of great horizontal extent, and there is much cross- +stratification in the sands, and in some places black carbonaceous seams and +lignite. In the midst of these leaf-beds in Studland Bay, Purbeck shells of the +genus Unio attest the fresh-water origin of the white clay. + +No less than forty species of plants are mentioned by MM. de la Harpe and Gaudin +from this formation in Hampshire, among which the Proteaceae (Dryandra, etc.) +and the fig tribe are abundant, as well as the cinnamon and several other +laurineae, with some papilionaceous plants. On the whole, they remind the +botanist of the types of subtropical India and Australia. (Heer Climat et +Vegetation du Pays Tertiaire page 172.) + +Heer has mentioned several species which are common to this Alum Bay flora and +that of Monte Bolca, near Verona, so celebrated for its fossil fish, and where +the strata contain nummulites and other Middle Eocene fossils. He has +particularly alluded to Aralia primigenia (of which genus a fruit has since been +found by Mr. Mitchell at Bournemouth), Daphnogene Veronensis, and Ficus +granadilla, as among the species common to and characteristic of the Isle of +Wight and Italian Eocene beds; and he observes that in the flora of this period +these forms of a temperate climate which constitute a marked feature in the +European Miocene formations, such as the willow, poplar, birch, alder, elm, +hornbeam, oak, fir, and pine, are wanting. The American types are also absent, +or much more feebly represented than in the Miocene period, although fine +specimens of the fan-palm (Sabal) have been found in these Eocene clays at +Studland. The number of exotic forms which are common to the Eocene and Miocene +strata of Europe, like those to be alluded to in the sequel which are common to +the Eocene and Cretaceous fauna, demonstrate the remoteness of the times in +which the geographical distribution of living plants originated. A great +majority of the Eocene genera have disappeared from our temperate climates, but +not the whole of them; and they must all have exerted some influence on the +assemblages of species which succeeded them. Many of these last occurring in the +Upper Miocene are indeed so closely allied to the flora now surviving as to make +it questionable, even in the opinion of naturalists opposed to the doctrine of +transmutation, whether they are not genealogically related the one to the other. + +LOWER EOCENE FORMATIONS, ENGLAND. + +LONDON CLAY (C.1, TABLE 16.1). + +This formation underlies the preceding, and sometimes attains a thickness of 500 +feet. It consists of tenacious brown and bluish-grey clay, with layers of +concretions called septaria, which abound chiefly in the brown clay, and are +obtained in sufficient numbers from sea-cliffs near Harwich, and from shoals off +the coast of Essex and the Isle of Sheppey, to be used for making Roman cement. +The total number of British fossil mollusca known at present (January, 1870) in +this formation are 254, of which 166 are peculiar, or not found in other Eocene +beds in this country. The principal localities of fossils in the London clay are +Highgate Hill, near London, the Island of Sheppey at the mouth of the Thames, +and Bognor on the Sussex coast. Out of 133 fossil shells, Mr. Prestwich found +only 20 to be common to the Calcaire Grossier (from which 600 species have been +obtained), while 33 are common to the "Lits Coquilliers" (see below), in which +200 species are known in France. + +In the Island of Sheppey near the mouth of the Thames, the thickness of the +London Clay is estimated by Mr. Prestwich to be more than 500 feet, and it is in +the uppermost 50 feet that a great number of fossil fruits were obtained, being +chiefly found on the beach when the sea has washed away the clay of the rapidly +wasting cliffs. + +(FIGURE 205. Nipadites ellipticus, Bowerbank. Fossil fruit of palm, from +Sheppey.) + +Mr. Bowerbank, in a valuable publication on these fossil fruits and seeds, has +described no less than thirteen fruits of palms of the recent type Nipa, now +only found in the Molucca and Philippine Islands, and in Bengal (see Figure +205). In the delta of the Ganges, Dr. Hooker observed the large nuts of Nipa +fruticans floating in such numbers in the various arms of that great river, as +to obstruct the paddle-wheels of steamboats. These plants are allied to the +cocoanut tribe on the one side, and on the other to the Pandanus, or screw-pine. +There are also met with three species of Anona, or custard-apple; and +cucurbitaceous fruits (of the gourd and melon family), and fruits of various +species of Acacia. + +Besides fir-cones or fruit of true Coniferae there are cones of Proteaceae in +abundance, and the celebrated botanist the late Robert Brown pointed out the +affinity of these to the New Holland types Petrophila and Isopogon. Of the first +there are about fifty, and of the second thirty described species now living in +Australia. + +(FIGURE 206. Eocene Proteaceous Fruit. +Petrophiloides Richardsoni. London Clay, Sheppey. Natural size. +a. Cone. +b. Section of cone showing the position of the seeds.) + +Ettingshausen remarked in 1851 that five of the fossil species from Sheppey, +named by Bowerbank (Fossil Fruits and Seeds of London Clay Plates 9 and 10.) +were specimens of the same fruit (see Figure 206), in different states of +preservation; and Mr. Carruthers, having examined the original specimens now in +the British Museum, tells me that all these cones from Sheppey may be reduced to +two species, which have an undoubted affinity to the two existing Australian +genera above mentioned, although their perfect identity in structure can not be +made out. + +The contiguity of land may be inferred not only from these vegetable +productions, but also from the teeth and bones of crocodiles and turtles, since +these creatures, as Dean Conybeare remarked, must have resorted to some shore to +lay their eggs. Of turtles there were numerous species referred to extinct +genera. These are, for the most part, not equal in size to the largest living +tropical turtles. A sea-snake, which must have been thirteen feet long, of the +genus Palaeophis before mentioned, has also been described by Professor Owen +from Sheppey, of a different species from that of Bracklesham, and called +Palaeophis toliapicus. A true crocodile, also, Crocodilus toliapicus, and +another saurian more nearly allied to the gavial, accompany the above fossils; +also the relics of several birds and quadrupeds. One of these last belongs to +the new genus Hyracotherium of Owen, of the hog tribe, allied to Chaeropotamus, +another is a Lophiodon; a third a pachyderm called Coryphodon eocaenus by Owen, +larger than any existing tapir. All these animals seem to have inhabited the +banks of the great river which floated down the Sheppey fruits. They imply the +existence of a mammiferous fauna antecedent to the period when nummulites +flourished in Europe and Asia, and therefore before the Alps, Pyrenees, and +other mountain-chains now forming the backbones of great continents, were raised +from the deep; nay, even before a part of the constituent rocky masses now +entering into the central ridges of these chains had been deposited in the sea. + +SHELLS OF THE LONDON CLAY. + +(FIGURE 207. Voluta nodosa, Sowerby. Highgate.) + +(FIGURE 208. Phorus extensus, Sowerby. Highgate.) + +(FIGURE 209. Rostellaria (Hippocrenes) ampla, Brander. 1/3 of natural size; also +found in the Barton clay.) + +(FIGURE 210. Nautilus centralis, Sowerby. Highgate.) + +(FIGURE 211. Aturia ziczac, Bronn. Syn. Nautilus ziczac, Sowerby. London clay. +Sheppey.) + +(FIGURE 212. Belosepia sepioidea, De Blainv. London clay. Sheppey.) + +(FIGURE 213. Leda amygdaloides, Sowerby. Highgate.) + +(FIGURE 214. Cyptodon (Axinus) angulatum, Sowerby. London clay. Hornsey.) + +(FIGURE 215. Astropecten crispatus, E. Forbes. Sheppey.) + +The marine shells of the London Clay confirm the inference derivable from the +plants and reptiles in favour of a high temperature. Thus many species of Conus +and Voluta occur, a large Cypraea, C. oviformis, a very large Rostellaria +(Figure 209), a species of Cancellaria, six species of Nautilus (Figure 211), +besides other Cephalopoda of extinct genera, one of the most remarkable of which +is the Belosepia (Figure 212). Among many characteristic bivalve shells are Leda +amygdaloides (Figure 213) and Cryptodon angulatum (Figure 214), and among the +Radiata a star-fish, Astropecten (Figure 215.) + +These fossils are accompanied by a sword-fish (Tetrapterus priscus, Agassiz), +about eight feet long, and a saw-fish (Pristis bisulcatus, Agassiz), about ten +feet in length; genera now foreign to the British seas. On the whole, about +eighty species of fish have been described by M. Agassiz from these beds of +Sheppey, and they indicate, in his opinion, a warm climate. + +In the lower part of the London clay at Kyson, a few miles east of Woodbridge, +the remains of mammalia have been detected. Some of these have been referred by +Professor Owen to an opossum, and others to the genus Hyracotherium. The teeth +of this last-mentioned pachyderm were at first, in 1840, supposed to belong to a +monkey, an opinion afterwards abandoned by Owen when more ample materials for +comparison were obtained. + +WOOLWICH AND READING SERIES (C.2, TABLE 16.1.) + +This formation was formerly called the Plastic Clay, as it agrees with a similar +clay used in pottery which occupies the same position in the French series, and +it has been used for the like purposes in England. (Prestwich Quarterly +Geological Journal volume 10.) + +No formations can be more dissimilar, on the whole, in mineral character than +the Eocene deposits of England and Paris; those of our own island being almost +exclusively of mechanical origin-- accumulations of mud, sand, and pebbles; +while in the neighbourhood of Paris we find a great succession of strata +composed of limestones, some of them siliceous, and of crystalline gypsum and +siliceous sandstone, and sometimes of pure flint used for millstones. Hence it +is often impossible, as before stated, to institute an exact comparison between +the various members of the English and French series, and to settle their +respective ages. But in regard to the division which we have now under +consideration, whether we study it in the basins of London, Hampshire, or Paris, +we recognise as a general rule the same mineral character, the beds consisting +over a large area of mottled clays and sand, with lignite, and with some strata +of well-rolled flint pebbles, derived from the chalk, varying in size, but +occasionally several inches in diameter. These strata may be seen in the Isle of +Wight in contact with the chalk, or in the London basin, at Reading, Blackheath, +and Woolwich. In some of the lowest of them, banks of oysters are observed, +consisting of Ostrea bellovacina, so common in France in the same relative +position. In these beds at Bromley, Dr. Buckland found a large pebble to which +five full-grown oysters were affixed, in such a manner as to show that they had +commenced their first growth upon it, and remained attached to it through life. + +(FIGURE 216. Cyrena cuneiformis, Sowerby. Natural size. Woolwich clays.) + +(FIGURE 217. Melania (Melanatria) inquinata, Des. Syn. Cerithium melanoides, +Sowerby. Woolwich clays.) + +In several places, as at Woolwich on the Thames, at Newhaven in Sussex, and +elsewhere, a mixture of marine and fresh-water testacea distinguishes this +member of the series. Among the latter, Cyrena cuneiformis (see Figure 216) and +Melania inquinata (see Figure 217) are very common, as in beds of corresponding +age in France. They clearly indicate points where rivers entered the Eocene sea. +Usually there is a mixture of brackish, fresh-water, and marine shells, and +sometimes, as at Woolwich, proofs of the river and the sea having successively +prevailed on the same spot. At New Charlton, in the suburbs of Woolwich, Mr. de +la Condamine discovered in 1849, and pointed out to me, a layer of sand +associated with well-rounded flint pebbles in which numerous individuals of the +Cyrena tellinella were seen standing endwise with both their valves united, the +siphonal extremity of each shell being uppermost, as would happen if the +mollusks had died in their natural position. I have described a bank of sandy +mud, in the delta of the Alabama River at Mobile, on the borders of the Gulf of +Mexico, where in 1846 I dug out at low tide specimens of living species of +Cyrena and of a Gnathodon, which were similarly placed with their shells erect, +or in a posture which enables the animal to protrude its siphon upward, and draw +in or reject water at pleasure. (Second Visit to the United States volume 2 page +104.) The water at Mobile is usually fresh, but sometimes brackish. At Woolwich +a body of river-water must have flowed permanently into the sea where the +Cyrenae lived, and they may have been killed suddenly by an influx of pure salt- +water, which invaded the spot when the river was low, or when a subsidence of +land took place. Traced in one direction, or eastward towards Herne Bay, the +Woolwich beds assume more and more of a marine character; while in an opposite, +or south-western direction, they become, as near Chelsea and other places, more +fresh-water, and contain Unio, Paludina, and layers of lignite, so that the land +drained by the ancient river seems clearly to have been to the south-west of the +present site of the metropolis. + +FLUVIATILE BEDS UNDERLYING DEEP-SEA STRATA. + +Before the minds of geologists had become familiar with the theory of the +gradual sinking of land, and its conversion into sea at different periods, and +the consequent change from shallow to deep water, the fluviatile and littoral +character of this inferior group appeared strange and anomalous. After passing +through hundreds of feet of London clay, proved by its fossils to have been +deposited in deep salt-water, we arrive at beds of fluviatile origin, and +associated with them masses of shingle, attaining at Blackheath, near London, a +thickness of 50 feet. These shingle banks are probably of marine origin, but +they indicate the proximity of land, and the existence of a shore where the +flints of the chalk were rolled into sand and pebbles, and spread over a wide +space. We have, therefore, first, as before stated, evidence of oscillations of +level during the accumulation of the Woolwich series, then of a great +submergence, which allowed a marine deposit 500 thick to be laid over the +antecedent beds of fresh and brackish water origin. + +THANET SANDS (C.3 TABLE 16.1). + +The Woolwich or plastic clay above described may often be seen in the Hampshire +basin in actual contact with the chalk, constituting in such places the lowest +member of the British Eocene series. But at other points another formation of +marine origin, characterised by a somewhat different assemblage of organic +remains, has been shown by Mr. Prestwich to intervene between the chalk and the +Woolwich series. For these beds he has proposed the name of "Thanet Sands," +because they are well seen in the Isle of Thanet, in the northern part of Kent, +and on the sea-coast between Herne Bay and the Reculvers, where they consist of +sands with a few concretionary masses of sandstone, and contain, among other +fossils, Pholadomya cuneata, Cyprina morrisii, Corbula longirostris, Scalaria +Bowerbankii, etc. The greatest thickness of these beds is 90 feet. + +UPPER EOCENE FORMATIONS OF FRANCE. + +The tertiary formations in the neighbourhood of Paris consist of a series of +marine and fresh-water strata, alternating with each other, and filling up a +depression in the chalk. The area which they occupy has been called the Paris +Basin, and is about 180 miles in its greatest length from north to south, and +about 90 miles in breadth from east to west. MM. Cuvier and Brongniart +attempted, in 1810, to distinguish five different groups, comprising three +fresh-water and two marine, which were supposed to imply that the waters of the +ocean, and of rivers and lakes, had been by turns admitted into and excluded +from the same area. Investigations since made in the Hampshire and London basins +have rather tended to confirm these views, at least so far as to show that since +the commencement of the Eocene period there have been great movements of the bed +of the sea, and of the adjoining lands, and that the superposition of deep-sea +to shallow-water deposits (the London Clay, for example, to the Woolwich beds) +can only be explained by referring to such movements. It appears, +notwithstanding, from the researches of M. Constant Prevost, that some of the +minor alternations and intermixtures of fresh-water and marine deposits, in the +Paris basin, may be accounted for without such changes of level, by imagining +both to have been simultaneously in progress, in the same bay of the same sea, +or a gulf into which many rivers entered. + +GYPSEOUS SERIES OF MONTMARTRE (A.1, TABLE 16.1). + +To enlarge on the numerous subdivisions of the Parisian strata would lead me +beyond my present limits; I shall therefore give some examples only of the most +important formations. Beneath the Gres de Fontainebleau, belonging to the Lower +Miocene period, as before stated, we find, in the neighbourhood of Paris, a +series of white and green marls, with subordinate beds of gypsum. These are most +largely developed in the central parts of the Paris basin, and, among other +places, in the hill of Montmartre, where its fossils were first studied by +Cuvier. + +The gypsum quarried there for the manufacture of plaster of Paris occurs as a +granular crystalline rock, and, together with the associated marls, contains +land and fluviatile shells, together with the bones and skeletons of birds and +quadrupeds. Several land-plants are also met with, among which are fine +specimens of the fan-palm or palmetto tribe (Flabellaria). The remains also of +fresh-water fish, and of crocodiles and other reptiles, occur in the gypsum. The +skeletons of mammalia are usually isolated, often entire, the most delicate +extremities being preserved; as if the carcasses, clothed with their flesh and +skin, had been floated down soon after death, and while they were still swollen +by the gases generated by their first decomposition. The few accompanying shells +are of those light kinds which frequently float on the surface of rivers, +together with wood. + +In this formation the relics of about fifty species of quadrupeds, including the +genera Palaeotherium (see Figure 174), Anoplotherium (see Figure 218), and +others, have been found, all extinct, and nearly four-fifths of them belonging +to the Perissodactyle or odd-toed division of the order Pachydermata, which now +contains only four living genera, namely, rhinoceros, tapir, horse, and hyrax. +With them a few carnivorous animals are associated, among which are the +Hyaenodon dasyuroides, a species of dog, Canis Parisiensis, and a weasel, +Cynodon Parisiensis. Of the Rodentia are found a squirrel; of the Cheiroptera, a +bat; while the Marsupalia (an order now confined to America, Australia, and some +contiguous islands) are represented by an opossum. + +Of birds, about ten species have been ascertained, the skeletons of some of +which are entire. None of them are referable to existing species. (Cuvier, Oss. +Foss. tome 3 page 255.) The same remark, according to MM. Cuvier and Agassiz, +applies both to the reptiles and fish. Among the last are crocodiles and +tortoises of the genera Emys and Trionyx. + +(FIGURE 218. Xiphodon gracile, or Anoplotherium gracile, Cuvier. Restored +outline.) + +The tribe of land quadrupeds most abundant in this formation is such as now +inhabits alluvial plains and marshes, and the banks of rivers and lakes, a class +most exposed to suffer by river inundations. Among these were several species of +Palaeotherium, a genus before alluded to. These were associated with the +Anoplotherium, a tribe intermediate between pachyderms and ruminants. One of the +three divisions of this family was called by Cuvier Xiphodon. Their forms were +slender and elegant, and one, named Xiphodon gracile (Figure 218), was about the +size of the chamois; and Cuvier inferred from the skeleton that it was as light, +graceful, and agile as the gazelle. + +FOSSIL FOOTPRINTS. + +There are three superimposed masses of gypsum in the neighbourhood of Paris, +separated by intervening deposits of laminated marl. In the uppermost of the +three, in the valley of Montmorency, M. Desnoyers discovered in 1859 many +footprints of animals occurring at no less than six different levels. (Sur des +Empreintes de Pas d'Animaux par M. J. Desnoyers. Compte rendu de l'Institut +1859.) The gypsum to which they belong varies from thirty to fifty feet in +thickness, and is that which has yielded to the naturalist the largest number of +bones and skeletons of mammalia, birds, and reptiles. I visited the quarries, +soon after the discovery was made known, with M. Desnoyers, who also showed me +large slabs in the Museum at Paris, where, on the upper planes of +stratification, the indented foot-marks were seen, while corresponding casts in +relief appeared on the lower surfaces of the strata of gypsum which were +immediately superimposed. A thin film of marl, which before it was dried and +condensed by pressure must have represented a much thicker layer of soft mud, +intervened between the beds of solid gypsum. On this mud the animals had +trodden, and made impressions which had penetrated to the gypseous mass below, +then evidently unconsolidated. Tracks of the Anoplotherium with its bisulcate +hoof, and the trilobed footprints of Palaeotherium, were seen of different +sizes, corresponding to those of several species of these genera which Cuvier +had reconstructed, while in the same beds were foot-marks of carnivorous +mammalia. The tracks also of fluviatile, lacustrine, and terrestrial tortoises +(Emys, Trionyx, etc.) were discovered, also those of crocodiles, iguanas, +geckos, and great batrachians, and the footprints of a huge bird, apparently a +wader, of the size of the gastornis, to be mentioned in the sequel. There were +likewise the impressions of the feet of other creatures, some of them clearly +distinguishable from any of the fifty extinct types of mammalia of which the +bones have been found in the Paris gypsum. The whole assemblage, says Desnoyers, +indicate the shores of a lake, or several small lakes communicating with each +other, on the borders of which many species of pachyderms wandered, and beasts +of prey which occasionally devoured them. The tooth-marks of these last had been +detected by palaeontologists long before on the bones and skulls of Paleotheres +entombed in the gypsum. + +IMPERFECTION OF THE RECORD. + +These foot-marks have revealed to us new and unexpected proofs that the air- +breathing fauna of the Upper Eocene period in Europe far surpassed in the number +and variety of its species the largest estimate which had previously been formed +of it. We may now feel sure that the mammalia, reptiles, and birds which have +left portions of their skeletons as memorials of their existence in the solid +gypsum constituted but a part of the then living creation. Similar inferences +may be drawn from the study of the whole succession of geological records. In +each district the monuments of periods embracing thousands, and probably in some +instances hundreds of thousands of years, are totally wanting. Even in the +volumes which are extant the greater number of the pages are missing in any +given region, and where they are found they contain but few and casual entries +of the physical events or living beings of the times to which they relate. It +may also be remarked that the subordinate formations met with in two +neighbouring countries, such as France and England (the minor Tertiary groups +above enumerated), commonly classed as equivalents and referred to corresponding +periods, may nevertheless have been by no means strictly coincident in date. +Though called contemporaneous, it is probable that they were often separated by +intervals of many thousands of years. We may compare them to double stars, which +appear single to the naked eye because seen from a vast distance in space, and +which really belong to one and the same stellar system, though occupying places +in space extremely remote if estimated by our ordinary standard of terrestrial +measurements. + +CALCAIRE SILICIEUX, OR TRAVERTIN INFERIEUR (A.2 AND 3 TABLE 16.1). + +This compact siliceous limestone extends over a wide area. It resembles a +precipitate from the waters of mineral springs, and is often traversed by small +empty sinuous cavities. It is, for the most part, devoid of organic remains, but +in some places contains fresh-water and land species, and never any marine +fossils. The calcaire siliceux and the calcaire grossier usually occupy distinct +parts of the Paris basin, the one attaining its fullest development in those +places where the other is of slight thickness. They are described by some +writers as alternating with each other towards the centre of the basin, as at +Sergy and Osny. + +The gypsum, with its associated marls before described, is in greatest force +towards the centre of the basin, where the calcaire grossier and calcaire +silicieux are less fully developed. + +GRES DE BEAUCHAMP, OR SABLES MOYENS (A.4 TABLE 16.1). + +In some parts of the Paris basin, sands and marls, called the Gres de Beauchamp, +or Sables moyens, divide the gypseous beds from the calcaire grossier proper. +These sands, in which a small nummulite (N. variolaria) is very abundant, +contain more than 300 species of marine shells, many of them peculiar, but +others common to the next division. + +MIDDLE EOCENE FORMATIONS OF FRANCE. + +CALCAIRE GROSSIER, UPPER AND MIDDLE (B.1 TABLE 16.1). + +The upper division of this group consists in great part of beds of compact, +fragile limestone, with some intercalated green marls. The shells in some parts +are a mixture of Cerithium, Cyclostoma, and Corbula; in others Limnea, +Cerithium, Paludina, etc. In the latter, the bones of reptiles and mammalia, +Palaeotherium and Lophiodon, have been found. The middle division, or calcaire +grossier proper, consists of a coarse limestone, often passing into sand. It +contains the greater number of the fossil shells which characterise the Paris +basin. No less than 400 distinct species have been procured from a single spot +near Grignon, where they are imbedded in a calcareous sand, chiefly formed of +comminuted shells, in which, nevertheless, individuals in a perfect state of +preservation, both of marine, terrestrial, and fresh-water species, are mingled +together. Some of the marine shells may have lived on the spot; but the +Cyclostoma and Limnea, being land and fresh-water shells, must have been brought +thither by rivers and currents, and the quantity of triturated shells implies +considerable movement in the waters. + +Nothing is more striking in this assemblage of fossil testacea than the great +proportion of species referable to the genus Cerithium (Figures 160 and 161 +Chapter 15). There occur no less than 137 species of this genus in the Paris +basin, and almost all of them in the calcaire grossier. Most of the living +Cerithia inhabit the sea near the mouths of rivers, where the waters are +brackish; so that their abundance in the marine strata now under consideration +is in harmony with the hypothesis that the Paris basin formed a gulf into which +several rivers flowed. + +EOCENE FORAMINIFERA. + +(FIGURE 219. Calcarina rarispina, Desh. +a. Natural size. +b. Magnified.) + +(FIGURE 220. Spirolina stenostoma, Desh. +a. Natural size. +b. Magnified.) + +(FIGURE 221. Triloculina inflata, Desh. +a. Natural size. +b. Magnified.) + +In some parts of the calcaire grossier round Paris, certain beds occur of a +stone used in building, and called by the French geologists "Miliolite +limestone." It is almost entirely made up of millions of microscopic shells, of +the size of minute grains of sand, which all belong to the class Foraminifera. +Figures of some of these are given in Figures 219 to 221. As this miliolitic +stone never occurs in the Faluns, or Upper Miocene strata of Brittany and +Touraine, it often furnishes the geologist with a useful criterion for +distinguishing the detached Eocene and Upper Miocene formations scattered over +those and other adjoining provinces. The discovery of the remains of +Palaeotherium and other mammalia in some of the upper beds of the calcaire +grossier shows that these land animals began to exist before the deposition of +the overlying gypseous series had commenced. + +LOWER CALCAIRE GROSSIER, OR GLAUCONIE GROSSIERE (B.1 TABLE 16.1). + +The lower part of the calcaire grossier, which often contains much green earth, +is characterised at Auvers, near Pontoise, to the north of Paris, and still more +in the environs of Compiegne, by the abundance of nummulites, consisting chiefly +of N. laevigata, N. scabra, and N. Lamarcki, which constitute a large proportion +of some of the stony strata, though these same foraminifera are wanting in beds +of similar age in the immediate environs of Paris. + +SOISSONNAIS SANDS, OR LITS COQUILLIERS (B.2 TABLE 16.1). + +(FIGURE 222. Nerita conoidea, Lam. Syn. N. Schmidelliana, Chemnitz.) + +Below the preceding formation, shelly sands are seen, of considerable thickness, +especially at Cuisse-Lamotte, near Compiegne, and other localities in the +Soissonnais, about fifty miles N.E. of Paris, from which about 300 species of +shells have been obtained, many of them common to the calcaire grossier and the +Bracklesham beds of England, and many peculiar. The Nummulites planulata is very +abundant, and the most characteristic shell is the Nerita conoidea, Lam., a +fossil which has a very wide geographical range; for, as M. d'Archiac remarks, +it accompanies the nummulitic formation from Europe to India, having been found +in Cutch, near the mouths of the Indus, associated with Nummulites scabra. No +less than 33 shells of this group are said to be identical with shells of the +London clay proper, yet, after visiting Cuisse-Lamotte and other localities of +the "Sables inferieurs" of Archiac, I agree with Mr. Prestwich, that the latter +are probably newer than the London clay, and perhaps older than the Bracklesham +beds of England. The London clay seems to be unrepresented in the Paris basin, +unless partially so, by these sands. (d'Archiac Bulletin tome 10 and Prestwich +Quarterly Geological Journal 1847 page 377.) + +LOWER EOCENE FORMATIONS OF FRANCE. + +ARGILE PLASTIQUE (C.2 TABLE 16.1). + +At the base of the tertiary system in France are extensive deposits of sands, +with occasional beds of clay used for pottery, and called "argile plastique." +Fossil oysters (Ostrea bellovacina) abound in some places, and in others there +is a mixture of fluviatile shells, such as Cyrena cuneiformis (Figure 216), +Melania inquinata (Figure 217), and others, frequently met with in beds +occupying the same position in the London Basin. Layers of lignite also +accompany the inferior clays and sands. + +Immediately upon the chalk at the bottom of all the tertiary strata in France +there generally is a conglomerate or breccia of rolled and angular chalk-flints, +cemented by siliceous sand. These beds appear to be of littoral origin, and +imply the previous emergence of the chalk, and its waste by denudation. In the +year 1855, the tibia and femur of a large bird equalling at least the ostrich in +size were found at Meudon, near Paris, at the base of the Plastic clay. This +bird, to which the name of Gastornis Parisiensis has been assigned, appears, +from the Memoirs of MM. Hebert, Lartet, and Owen, to belong to an extinct genus. +Professor Owen refers it to the class of wading land birds rather than to an +aquatic species. (Quarterly Geological Journal volume 12 page 204 1856.) + +That a formation so much explored for economical purposes as the Argile +plastique around Paris, and the clays and sands of corresponding age near +London, should never have afforded any vestige of a feathered biped previously +to the year 1855, shows what diligent search and what skill in osteological +interpretation are required before the existence of birds of remote ages can be +established. + +SABLES DE BRACHEUX (C.3 TABLE 16.1). + +The marine sands called the Sables de Bracheux (a place near Beauvais), are +considered by M. Hebert to be older than the Lignites and Plastic clay, and to +coincide in age with the Thanet Sands of England. At La Fere, in the Department +of Aisne, in a deposit of this age, a fossil skull has been found of a quadruped +called by Blainville Arctocyon primaevus, and supposed by him to be related both +to the bear and to the Kinkajou (Cercoleptes). This creature appears to be the +oldest known tertiary mammifer. + +NUMMULITIC FORMATIONS OF EUROPE, ASIA, ETC. + +Of all the rocks of the Eocene period, no formations are of such great +geographical importance as the Upper and Middle Eocene, as above defined, +assuming that the older tertiary formation, commonly called nummulitic, is +correctly ascribed to this group. It appears that of more than fifty species of +these foraminifera described by D'Archiac, one or two species only are found in +other tertiary formations whether of older or newer date. Nummulites intermedia, +a Middle Eocene form, ascends into the Lower Miocene, but it seems doubtful +whether any species descends to the level of the London clay, still less to the +Argile plastique or Woolwich beds. Separate groups of strata are often +characterised by distinct species of nummulite; thus the beds between the lower +Miocene and the lower Eocene may be divided into three sections, distinguished +by three different species of nummulites, N. variolaria in the upper, N. +laevigata in the middle, and N. planulata in the lower beds. The nummulitic +limestone of the Swiss Alps rises to more than 10,000 feet above the level of +the sea, and attains here and in other mountain chains a thickness of several +thousand feet. It may be said to play a far more conspicuous part than any other +tertiary group in the solid framework of the earth's crust, whether in Europe, +Asia, or Africa. It occurs in Algeria and Morocco, and has been traced from +Egypt, where it was largely quarried of old for the building of the Pyramids, +into Asia Minor, and across Persia by Bagdad to the mouths of the Indus. It has +been observed not only in Cutch, but in the mountain ranges which separate +Scinde from Persia, and which form the passes leading to Caboul; and it has been +followed still farther eastward into India, as far as eastern Bengal and the +frontiers of China. + +(FIGURE 223. Nummulites Puschi, D'Archiac. Peyrehorade, Pyrenees. +a. External surface of one of the nummulites, of which longitudinal sections are +seen in the limestone. +b. Transverse section of same.) + +Dr. T. Thompson found nummulites at an elevation of no less than 16,500 feet +above the level of the sea, in Western Thibet. One of the species, which I +myself found very abundant on the flanks of the Pyrenees, in a compact +crystalline marble (Figure 223) is called by M. D'Archiac Nummulites Puschi. The +same is also very common in rocks of the same age in the Carpathians. In many +distant countries, in Cutch, for example, some of the same shells, such as +Nerita conoidea (Figure 222), accompany the nummulites, as in France. The +opinion of many observers, that the Nummulitic formation belongs partly to the +cretaceous era, seems chiefly to have arisen from confounding an allied genus, +Orbitoides, with the true Nummulite. + +When we have once arrived at the conviction that the nummulitic formation +occupies a middle and upper place in the Eocene series, we are struck with the +comparatively modern date to which some of the greatest revolutions in the +physical geography of Europe, Asia, and Northern Africa must be referred. All +the mountain-chains, such as the Alps, Pyrenees, Carpathians, and Himalayas, +into the composition of whose central and loftiest parts the nummulitic strata +enter bodily, could have had no existence till after the Middle Eocene period. +During that period the sea prevailed where these chains now rise, for nummulites +and their accompanying testacea were unquestionably inhabitants of salt water. +Before these events, comprising the conversion of a wide area from a sea to a +continent, England had been peopled, as I before pointed out, by various +quadrupeds, by herbivorous pachyderms, by insectivorous bats, and by opossums. + +Almost all the volcanoes which preserve any remains of their original form, or +from the craters of which lava streams can be traced, are more modern than the +Eocene fauna now under consideration; and besides these superficial monuments of +the action of heat, Plutonic influences have worked vast changes in the texture +of rocks within the same period. Some members of the nummulitic and overlying +tertiary strata called flysch have actually been converted in the central Alps +into crystalline rocks, and transformed into marble, quartz-rock, micha-schist, +and gneiss. (Murchison Quarterly Journal of Geological Society volume 5 and +Lyell volume 6 1850 Anniversary Address.) + +EOCENE STRATA IN THE UNITED STATES. + +In North America the Eocene formations occupy a large area bordering the +Atlantic, which increases in breadth and importance as it is traced southward +from Delaware and Maryland to Georgia and Alabama. They also occur in Louisiana +and other States both east and west of the valley of the Mississippi. At +Claiborne, in Alabama, no less than 400 species of marine shells, with many +echinoderms and teeth of fish, characterise one member of this system. Among the +shells, the Cardita planicosta, before mentioned (Figure 191), is in abundance; +and this fossil and some others identical with European species, or very nearly +allied to them, make it highly probable that the Claiborne beds agree in age +with the central or Bracklesham group of England, and with the calcaire +grossiere of Paris. (See paper by the Author Quarterly Journal of Geological +Society volume 4 page 12 and Second Visit to the United States volume 2 page +59.) + +Higher in the series is a remarkable calcareous rock, formerly called "the +nummulite limestone," from the great number of discoid bodies resembling +nummulites which it contains, fossils now referred by A. d'Orbigny to the genus +Orbitoides, which has been demonstrated by Dr. Carpenter to belong to the +foraminifera. (Quarterly Journal of Geological Society volume 6 page 32.) That +naturalist, moreover, is of opinion that the Orbitoides alluded to (O. Mantelli) +is of the same species as one found in Cutch, in the Middle Eocene or nummulitic +formation of India. + +Above the orbitoidal limestone is a white limestone, sometimes soft and +argillaceous, but in parts very compact and calcareous. It contains several +peculiar corals, and a large Nautilus allied to N. ziczac; also in its upper bed +a gigantic cetacean, called Zeuglodon by Owen. (See Memoir by R.W. Gibbes +Journal of Academy of Natural Science Philadelphia volume 1 1847.) + +The colossal bones of this cetacean are so plentiful in the interior of Clarke +County, Alabama, as to be characteristic of the formation. The vertebral column +of one skeleton found by Dr. Buckley at a spot visited by me, extended to the +length of nearly seventy feet, and not far off part of another backbone nearly +fifty feet long was dug up. I obtained evidence, during a short excursion, of so +many localities of this fossil animal within a distance of ten miles, as to lead +me to conclude that they must have belonged to at least forty distinct +individuals. + +(FIGURE 224. Zeuglodon cetoides, Owen. Basilosaurus, Harlan. +Molar tooth, natural size.) + +(FIGURE 225. Zeuglodon cetoides, Owen. Basilosaurus, Harlan. +Vertebra, reduced.) + +Professor Owen first pointed out that this huge animal was not reptilian, since +each tooth was furnished with double roots (Figure 224), implanted in +corresponding double sockets; and his opinion of the cetacean nature of the +fossil was afterwards confirmed by Dr. Wyman and Dr. R.W. Gibbes. That it was an +extinct mammal of the whale tribe has since been placed beyond all doubt by +discovery of the entire skull of another fossil species of the same family, +having the double occipital condyles only met with in mammals, and the +convoluted tympanic bones which are characteristic of cetaceans. + + +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. + +We have treated in the preceding chapters of the Tertiary or Cainozoic strata, +and have next to speak of the Secondary or Mesozoic formations. The uppermost of +these last is commonly called the chalk or the cretaceous formation, from creta, +the latin name for that remarkable white earthy limestone, which constitutes an +upper member of the group in those parts of Europe where it was first studied. +The marked discordance in the fossils of the tertiary, as compared with the +cretaceous formations, has long induced many geologists to suspect that an +indefinite series of ages elapsed between the respective periods of their +origin. Measured, indeed, by such a standard, that is to say, by the amount of +change in the Fauna and Flora of the earth effected in the interval, the time +between the Cretaceous and Eocene may have been as great as that between the +Eocene and Recent periods, to the history of which the last seven chapters have +been devoted. Several deposits have been met with here and there, in the course +of the last half century, of an age intermediate between the white chalk and the +plastic clays and sands of the Paris and London districts, monuments which have +the same kind of interest to a geologist which certain medieval records excite +when we study the history of nations. For both of them throw light on ages of +darkness, preceded and followed by others of which the annals are comparatively +well-known to us. But these newly-discovered records do not fill up the wide +gap, some of them being closely allied to the Eocene, and others to the +Cretaceous type, while none appear as yet to possess so distinct and +characteristic a fauna as may entitle them to hold an independent place in the +great chronological series. + +Among the formations alluded to, the Thanet Sands of Prestwich have been +sufficiently described in the last chapter, and classed as Lower Eocene. To the +same tertiary series belong the Belgian formations, called by Professor Dumont, +Landenian. On the other hand, the Maestricht and Faxoe limestones are very +closely connected with the chalk, to which also the Pisolitic limestone of +France is referable. + +CLASSIFICATION OF THE CRETACEOUS ROCKS. + +TABLE 17.1. + +UPPER CRETACEOUS OR CHALK PERIOD. + +1. Maestricht Beds and Faxoe Limestone. +2. Upper White Chalk, with flints. +3. Lower White Chalk, without flints. +4. Chalk Marl. +5. Chloritic series (or Upper Greensand). +6. Gault. + +LOWER CRETACEOUS OR NEOCOMIAN. + +1. Marine: Upper Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper +part). +2. Marine: Middle Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper +part). +3. Marine: Lower Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper +part). + +The cretaceous group has generally been divided into an Upper and a Lower +series, the Upper called familiarly THE CHALK, and the Lower THE GREENSAND; the +one deriving its name from the predominance of white earthy limestone and marl, +of which it consists in a great part of France and England, the other or lower +series from the plentiful mixture of green or chloritic grains contained in some +of the sands and cherts of which it largely consists in the same countries. But +these mineral characters often fail, even when we attempt to follow out the same +continuous subdivisions throughout a small portion of the north of Europe, and +are worse than valueless when we desire to apply them to more distant regions. +It is only by aid of the organic remains which characterise the successive +marine subdivisions of the formation that we are able to recognise in remote +countries, such as the south of Europe or North America, the formations which +were there contemporaneously in progress. To the English student of geology it +will be sufficient to begin by enumerating those groups which characterise the +series in this country and others immediately contiguous, alluding but slightly +to those of more distant regions. In Table 17.1 it will be seen that I have used +the term Neocomian for that commonly called "Lower Greensand;" as this latter +term is peculiarly objectionable, since the green grains are an exception to the +rule in many of the members of this group even in districts where it was first +studied and named. + +MAESTRICHT BEDS. + +(FIGURE 226. Belemnitella mucronata, Maestricht, Faxoe, and White Chalk. +a. Entire specimen, showing vascular impression on outer surface, and +characteristic slit. +b. Section of same, showing place of phragmocone. (For particulars of structure +see Chapter 18.)) + +On the banks of the Meuse, at Maestricht, reposing on ordinary white chalk with +flints, we find an upper calcareous formation about 100 feet thick, the fossils +of which are, on the whole, very peculiar, and all distinct from tertiary +species. Some few are of species common to the inferior white chalk, among which +may be mentioned Belemnitella mucronata (Figure 226) and Pecten quadricostatus, +a shell regarded by many as a mere variety of Pecten quinquecostatus (see Figure +270). Besides the Belemnite there are other genera, such as Baculites and +Hamites, never found in strata newer than the cretaceous, but frequently met +with in these Maestricht beds. On the other hand, Voluta, Fasciolaria, and other +genera of univalve shells, usually met with only in tertiary strata, occur. + +The upper part of the rock, about 20 feet thick, as seen in St. Peter's Mount, +in the suburbs of Maestricht, abounds in corals and Bryozoa, often detachable +from the matrix; and these beds are succeeded by a soft yellowish limestone 50 +feet thick, extensively quarried from time immemorial for building. The stone +below is whiter, and contains occasional nodules of grey chert or chalcedony. + +(FIGURE 227. Mosasaurus Camperi. Original more than three feet long.) + +(FIGURE 228. Hemipneustes radiatus, Ag. Spatangus radiatus, Lam. +Chalk of Maestricht and white chalk.) + +M. Bosquet, with whom I examined this formation (August, 1850), pointed out to +me a layer of chalk from two to four inches thick, containing green earth and +numerous encrinital stems, which forms the line of demarkation between the +strata containing the fossils peculiar to Maestricht and the white chalk below. +The latter is distinguished by regular layers of black flint in nodules, and by +several shells, such as Terebratula carnea (see Figure 246), wholly wanting in +beds higher than the green band. Some of the organic remains, however, for which +St. Peter's Mount is celebrated, occur both above and below that parting layer, +and, among others, the great marine reptile called Mosasaurus (see Figure 227), +a saurian supposed to have been 24 feet in length, of which the entire skull and +a great part of the skeleton have been found. Such remains are chiefly met with +in the soft freestone, the principal member of the Maestricht beds. Among the +fossils common to the Maestricht and white chalk may be instanced the +echinoderm, Figure 228. + +I saw proofs of the previous denudation of the white chalk exhibited in the +lower bed of the Maestricht formation in Belgium, about 30 miles S.W. of +Maestricht, at the village of Jendrain, where the base of the newer deposit +consisted chiefly of a layer of well-rolled, black chalk-flint pebbles, in the +midst of which perfect specimens of Thecidea papillata and Belemnitella +mucronata are imbedded. To a geologist accustomed in England to regard rolled +pebbles of chalk-flint as a common and distinctive feature of tertiary beds of +different ages, it is a new and surprising phenomenon to behold strata made up +of such materials, and yet to feel no doubt that they were accumulated in a sea +in which the belemnite and other cretaceous mollusca flourished. + +PISOLITIC LIMESTONE OF FRANCE. + +Geologists were for many years at variance respecting the chronological +relations of this rock, which is met with in the neighbourhood of Paris, and at +places north, south, east, and west of that metropolis, as between Vertus and +Laversines, Meudon and Montereau. By many able palaeontologists the species of +fossils, more than fifty in number, were declared to be more Eocene in their +appearance than Cretaceous. But M. Hebert found in this formation at Montereau, +near Paris, the Pecten quadricostatus, a well-known Cretaceous species, together +with some other fossils common to the Maestricht chalk and to the Baculite +limestone of the Cotentin, in Normandy. He therefore, as well as M. Alcide +d'Orbigny, who had carefully studied the fossils, came to the opinion that it +was an upper member of the Cretaceous group. It is usually in the form of a +coarse yellowish or whitish limestone, and the total thickness of the series of +beds already known is about 100 feet. Its geographical range, according to M. +Hebert, is not less than 45 leagues from east to west, and 35 from north to +south. Within these limits it occurs in small patches only, resting +unconformably on the white chalk. + +(FIGURE 229. Portion of Baculites Faujasii. +Maestricht and Faxoe beds and white chalk.) + +(FIGURE 230. Nautilus Danicus, Schl. Faxoe, Denmark.) + +The Nautilus Danicus, Figure 230, and two or three other species found in this +rock, are frequent in that of Faxoe, in Denmark, but as yet no Ammonites, +Hamites, Scaphites, Turrilites, Baculites, or Hippurites have been met with. The +proportion of peculiar species, many of them of tertiary aspect, is confessedly +large; and great aqueous erosion suffered by the white chalk, before the +pisolitic limestone was formed, affords an additional indication of the two +deposits being widely separated in time. The pisolitic formation, therefore, may +eventually prove to be somewhat more intermediate in date between the secondary +and tertiary epochs than the Maestricht rock. + +CHALK OF FAXOE. + +In the island of Seeland, in Denmark, the newest member of the chalk series, +seen in the sea-cliffs at Stevensklint resting on white chalk with flints, is a +yellow limestone, a portion of which, at Faxoe, where it is used as a building +stone, is composed of corals, even more conspicuously than is usually observed +in recent coral reefs. It has been quarried to the depth of more than 40 feet, +but its thickness is unknown. The imbedded shells are chiefly casts, many of +them of univalve mollusca, which are usually very rare in the white chalk of +Europe. Thus, there are two species of Cypraea, one of Oliva, two of Mitra, four +of the genus Cerithium, six of Fusus, two of Trochus, one of Patella, one of +Emarginula, etc.; on the whole, more than thirty univalves, spiral or +patelliform. At the same time, some of the accompanying bivalve shells, +echinoderms, and zoophytes, are specifically identical with fossils of the true +Cretaceous series. Among the cephalopoda of Faxoe may be mentioned Baculites +Faujasii (Figure 229), and Belemnitella mucronata (Figure 226), shells of the +white chalk. The Nautilus Danicus (see Figure 230) is characteristic of this +formation; and it also occurs in France in the calcaire pisolitique of Laversin +(Department of Oise). The claws and entire skull of a small crab, Brachyurus +rugosus (Schlott.), are scattered through the Faxoe stone, reminding us of +similar crustaceans inclosed in the rocks of modern coral reefs. Some small +portions of this coralline formation consist of white earthy chalk. + +COMPOSITION, EXTENT AND ORIGIN OF THE WHITE CHALK. + +(FIGURE 231. Diagrammatic section from Hertfordshire, in England, to Sens, in +France. +Through London (left), Hythe, Boulogne, Valley of Bray, Paris and Sens (right).) + +The highest beds of chalk in England and France consist of a pure, white, +calcareous mass, usually too soft for a building-stone, but sometimes passing +into a more solid state. It consists, almost purely, of carbonate of lime; the +stratification is often obscure, except where rendered distinct by +interstratified layers of flint, a few inches thick, occasionally in continuous +beds, but oftener in nodules, and recurring at intervals generally from two to +four feet distant from each other. This upper chalk is usually succeeded, in the +descending order, by a great mass of white chalk without flints, below which +comes the chalk marl, in which there is a slight admixture of argillaceous +matter. The united thickness of the three divisions in the south of England +equals, in some places, 1000 feet. The section in Figure 231 will show the +manner in which the white chalk extends from England into France, covered by the +tertiary strata described in former chapters, and reposing on lower cretaceous +beds. + +The area over which the white chalk preserves a nearly homogeneous aspect is so +vast, that the earlier geologists despaired of discovering any analogous +deposits of recent date. Pure chalk, of nearly uniform aspect and composition, +is met with in a north-west and south-east direction, from the north of Ireland +to the Crimea, a distance of about 1140 geographical miles, and in an opposite +direction it extends from the south of Sweden to the south of Bordeaux, a +distance of about 840 geographical miles. In Southern Russia, according to Sir +R. Murchison, it is sometimes 600 feet thick, and retains the same mineral +character as in France and England, with the same fossils, including Inoceramus +Cuvieri, Belemnitella mucronata, and Ostrea vesicularis (Figure 251). + +(Figures 232 to 236.-- Organic bodies forming the ooze of the bed of the +Atlantic at great depths. + +(FIGURE 232. Globigerina bulloides. Calcareous Rhizopod.) + +(FIGURE 233. Actinocyclus. Siliceous Diatomaceae. ) + +(FIGURE 234. Pinnularia. Siliceous Diatomaceae.) + +(FIGURE 235. Eunotia bidens. Siliceous Diatomaceae.) + +(FIGURE 236. Spicula of sponge. Siliceous sponge.)) + +Great light has recently been thrown upon the origin of the unconsolidated white +chalk by the deep soundings made in the North Atlantic, previous to laying down, +in 1858, the electric telegraph between Ireland and Newfoundland. At depths +sometimes exceeding two miles, the mud forming the floor of the ocean was found, +by Professor Huxley, to be almost entirely composed (more than nineteen- +twentieths of the whole) of minute Rhizopods, or foraminiferous shells of the +genus Globigerina, especially the species Globigerina bulloides (see Figure +232.) the organic bodies next in quantity were the siliceous shells called +Polycystineae, and next to them the siliceous skeletons of plants called +Diatomaceae (Figures 233, 234, 235), and occasionally some siliceous spiculae of +sponges (Figure 236) were intermixed. These were connected by a mass of living +gelatinous matter to which he has given the name of Bathybius, and which +contains abundance of very minute bodies termed Coccoliths and Coccospheres, +which have also been detected fossil in chalk. + +Sir Leopold MacClintock and Dr. Wallich have ascertained that 95 per cent of the +mud of a large part of the North Atlantic consists of Globigerina shells. But +Captain Bullock, R.N., lately brought up from the enormous depth of 16,860 feet +a white, viscid, chalky mud, wholly devoid of Globigerinae. This mud was +perfectly homogeneous in composition, and contained no organic remains visible +to the naked eye. Mr. Etheridge, however, has ascertained by microscopical +examination that it is made up of Coccoliths, Discoliths, and other minute +fossils like those of the Chalk classed by Huxley as Bathybius, when this term +is used in its widest sense. This mud, more than three miles deep, was dredged +up in latitude 20 degrees 19' N., longitude 4 degrees 36' E., or about midway +between Madeira and the Cape of Good Hope. + +The recent deep-sea dredgings in the Atlantic conducted by Dr. Wyville Thomson, +Dr. Carpenter, Mr. Gwyn Jeffreys, and others, have shown that on the same white +mud there sometimes flourish Mollusca, Crustacea, and Echinoderms, besides +abundance of siliceous sponges, forming, on the whole, a marine fauna bearing a +striking resemblance in its general character to that of the ancient chalk. + +POPULAR ERROR AS TO THE GEOLOGICAL CONTINUITY OF THE CRETACEOUS PERIOD. + +We must be careful, however, not to overrate the points of resemblance which the +deep-sea investigations have placed in a strong light. They have been supposed +by some naturalists to warrant a conclusion expressed in these words: "We are +still living in the Cretaceous epoch;" a doctrine which has led to much popular +delusion as to the bearing of the new facts on geological reasoning and +classification. The reader should be reminded that in geology we have been in +the habit of founding our great chronological divisions, not on foraminifera and +sponges, nor even on echinoderms and corals, but on the remains of the most +highly organised beings available to us, such as the mollusca; these being met +with, as explained in Chapter 9, in stratified rocks of almost every age. In +dealing with the mollusca, it is those of the highest or most specialised +organisation, which afford us the best characters in proportion as their +vertical range is the most limited. Thus the Cephalopoda are the most valuable, +as having a more restricted range in time than the Gasteropoda; and these, +again, are more characteristic of the particular stratigraphical subdivisions +than are the Lamellibranchiate Bivalves, while these last, again, are more +serviceable in classification than the Brachiopoda, a still lower class of +shell-fish, which are the most enduring of all. + +When told that the new dredgings prove that "we are still living in the Chalk +Period," we naturally ask whether some cuttle-fish has been found with a +Belemnite forming part of its internal framework; or have Ammonites, Baculites, +Hamites, Turrilites, with four or five other Cephalopodous genera characteristic +of the chalk and unknown as tertiary, been met with in the abysses of the ocean? +Or, in the absence of these long-extinct forms, has a single spiral univalve, or +species of Cretaceous Gasteropod, been found living? Or, to descend still lower +in the scale, has some characteristic Cretaceous genus of Lamellibranchiate +Bivalve, such as the Inoceramus, or Hippurite, foreign to the Tertiary seas, +been proved to have survived down to our time? Or, of the numerous genera of +lamellibranchiates common to the Cretaceous and Recent seas, has one species +been found living? The answer to all these questions is-- not one has been +found. Even of the humblest shell-fish, the Brachiopods, no new species common +to the Cretaceous and recent seas has yet been met with. It has been very +generally admitted by conchologists that out of a hundred species of this tribe +occurring fossil in the Upper Chalk-- one, and one only, Terebratulina striata, +is still living, being thought to be identical with Terebratula caput-serpentis. +Although this identity is still questioned by some naturalists of authority, it +would certainly not surprise us if another lamp-shell of equal antiquity should +be met with in the deep sea. + +Had it been declared that we are living in the Eocene epoch, the idea would not +be so extravagant, for the great reptiles of the Upper Chalk, the Mosasaurus, +Pliosaurus, and Pterodactyle, and many others, as well as so many genera of +chambered univalves, had already disappeared from the earth, and the marine +fauna had made a greater approach to our own by nearly the entire difference +which separates it from the fauna of the Cretaceous seas. The Eocene nummulitic +limestone of Egypt is a rock mainly composed, like the more ancient white chalk, +of globigerine mud; and if the reader will refer to what we have said of the +extent to which the nummulitic marine strata, formed originally at the bottom of +the sea, now enter into the framework of mountain chains of the principal +continents, he will at once perceive that the present Atlantic, Pacific, and +Indian Oceans are geographical terms, which must be wholly without meaning when +applied to the Eocene, and still more to the Cretaceous Period; so that to talk +of the chalk having been uninterruptedly forming in the Atlantic from the +Cretaceous Period to our own, is as inadmissible in a geographical as in a +geological sense. + +CHALK-FLINTS. + +The origin of the layers of flint, whether in the form of nodules, or continuous +sheets, or in veins or cracks not parallel to the stratification, has always +been more difficult to explain than that of the white chalk. But here, again, +the late deep-sea soundings have suggested a possible source of such mineral +matter. During the cruise of the "Bulldog," already alluded to, it was +ascertained that while the calcareous Globigerinae had almost exclusive +possession of certain tracts of the sea-bottom, they were wholly wanting in +others, as between Greenland and Labrador. According to Dr. Wallich, they may +flourish in those spaces where they derive nutriment from organic and other +matter, brought from the south by the warm waters of the Gulf Stream, and they +may be absent where the effects of that great current are not felt. Now, in +several of the spaces where the calcareous Rhizopods are wanting, certain +microscopic plants, called Diatomaceae, above-mentioned (Figures 233-235), the +solid parts of which are siliceous, monopolise the ground at a depth of nearly +400 fathoms, or 2400 feet. + +The large quantities of silex in solution required for the formation of these +plants may probably arise from the disintegration of feldspathic rocks, which +are universally distributed. As more than half of their bulk is formed of +siliceous earth, they may afford an endless supply of silica to all the great +rivers which flow into the ocean. We may imagine that, after a lapse of many +years or centuries, changes took place in the direction of the marine currents, +favouring at one time a supply in the same area of siliceous, and at another of +calcareous matter in excess, giving rise in the one case to a preponderance of +Globigerinae, and in the other of Diatomaceae. These last, and certain sponges, +may by their decomposition have furnished the silex, which, separating from the +chalky mud, collected round organic bodies, or formed nodules, or filled +shrinkage cracks. + +POT-STONES. + +(FIGURE 237. View of a chalk-pit at Horstead, near Norwich, showing the position +of the pot-stones. From a drawing by Mrs. Gunn.) + +A more difficult enigma is presented by the occurrence of certain huge flints, +or pot-stones, as they are called in Norfolk, occurring singly, or arranged in +nearly continuous columns at right angles to the ordinary and horizontal layers +of small flints. I visited in the year 1825 an extensive range of quarries then +open on the river Bure, near Horstead, about six miles from Norwich, which +afforded a continuous section, a quarter of a mile in length, of white chalk, +exposed to the depth of about twenty-six feet, and covered by a bed of gravel. +The pot-stones, many of them pear-shaped, were usually about three feet in +height and one foot in their transverse diameter, placed in vertical rows, like +pillars, at irregular distances from each other, but usually from twenty to +thirty feet apart, though sometimes nearer together, as in Figure 237. These +rows did not terminate downward in any instance which I could examine, nor +upward, except at the point where they were cut off abruptly by the bed of +gravel. On breaking open the pot-stones, I found an internal cylindrical nucleus +of pure chalk, much harder than the ordinary surrounding chalk, and not +crumbling to pieces like it, when exposed to the winter's frost. At the distance +of half a mile, the vertical piles of pot-stones were much farther apart from +each other. Dr. Buckland has described very similar phenomena as characterising +the white chalk on the north coast of Antrim, in Ireland. (Geological +Transactions 1st Series volume 4 page 413.) + +VITREOUS SPONGES OF THE CHALK. + +These pear-shaped masses of flint often resemble in shape and size the large +sponges called Neptune's Cups (Spongia patera, Hardw.), which grow in the seas +of Sumatra; and if we could suppose a series of such gigantic sponges to be +separated from each other, like trees in a forest, and the individuals of each +successive generation to grow on the exact spot where the parent sponge died and +was enveloped in calcareous mud, so that they should become piled one above the +other in a vertical column, their growth keeping pace with the accumulation of +the enveloping calcareous mud, a counterpart of the phenomena of the Horstead +pot-stones might be obtained. + +(FIGURE 238. Ventriculites radiatus, Mantell. Syn. Ocellaria radiata. D'Orbigny. +White chalk.) + +Professor Wyville Thomson, describing the modern soundings in 1869 off the north +coast of Scotland, speaks of the ooze or chalk mud brought from a depth of about +3000 feet, and states that at one haul they obtained forty specimens of vitreous +sponges buried in the mud. He suggests that the Ventriculites of the chalk were +nearly allied to these sponges, and that when the silica of their spicules was +removed, and was dissolved out of the calcareous matrix, it set into flint. + +BOULDERS AND GROUPS OF PEBBLES IN CHALK. + +The occurrence here and there, in the white chalk of the south of England, of +isolated pebbles of quartz and green schist has justly excited much wonder. It +was at first supposed that they had been dropped from the roots of some floating +tree, by which means stones are carried to some of the small coral islands of +the Pacific. But the discovery in 1857 of a group of stones in the white chalk +near Croydon, the largest of which was syenite and weighed about forty pounds, +accompanied by pebbles and fine sand like that of a beach, has been shown by Mr. +Godwin Austen to be inexplicable except by the agency of floating ice. If we +consider that icebergs now reach 40 degrees north latitude in the Atlantic, and +several degrees nearer the equator in the southern hemisphere, we can the more +easily believe that even during the Cretaceous epoch, assuming that the climate +was milder, fragments of coast ice may have floated occasionally as far as the +south of England. + +DISTINCTNESS OF MINERAL CHARACTER IN CONTEMPORANEOUS ROCKS OF THE CRETACEOUS +PERIOD. + +But we must not imagine that because pebbles are so rare in the white chalk of +England and France there are no proofs of sand, shingle, and clay having been +accumulated contemporaneously even in European seas. The siliceous sandstone +called "upper quader" by the Germans overlies white argillaceous chalk or +"planer-kalk," a deposit resembling in composition and organic remains the chalk +marl of the English series. This sandstone contains as many fossil shells common +to our white chalk as could be expected in a sea-bottom formed of such different +materials. It sometimes attains a thickness of 600 feet, and, by its jointed +structure and vertical precipices, plays a conspicuous part in the picturesque +scenery of Saxon Switzerland, near Dresden. It demonstrates that in the +Cretaceous sea, as in our own, distinct mineral deposits were simultaneously in +progress. The quartzose sandstone alluded to, derived from the detritus of the +neighbouring granite, is absolutely devoid of carbonate of lime, yet it was +formed at the distance only of four hundred miles from a sea-bottom now +constituting part of France, where the purely calcareous white chalk was +forming. In the North American continent, on the other hand, where the Upper +Cretaceous formations are so widely developed, true white chalk, in the ordinary +sense of that term, does not exist. + +FOSSILS OF THE WHITE CHALK. + +(FIGURE 239. Ananchytes ovatus, Leske. White chalk, upper and lower. +a. Side view. +b. Base of the shell, on which both the oral and anal apertures are placed; the +anal being more round, and at the smaller end.) + +(FIGURE 240. Micraster cor-angumum, Leske. White chalk.) + +(FIGURE 241. Galerites albogalerus, Lam. White chalk.) + +(FIGURE 242. Marsupites Milleri. Mant. White chalk.) + +Among the fossils of the white chalk, echinoderms are very numerous; and some of +the genera, like Ananchytes (see Figure 239), are exclusively cretaceous. Among +the Crinoidea, the Marsupites (Figure 242) is a characteristic genus. Among the +mollusca, the cephalopoda are represented by Ammonites, Baculites (Figure 229), +and Belemnites (Figure 226). Although there are eight or more species of +Ammonites and six of them peculiar to it, this genus is much less fully +represented than in each of the other subdivisions of the Upper Cretaceous +group. + +(FIGURE 243. Terebratulina striata, Wahlenb. Upper white chalk.) + +(FIGURE 244. Rhynchonella octoplicata, Sowerby. (Var. of R. plicatilis). Upper +white chalk. + +(FIGURE 245. Magas pumila, Sowerby. Upper white chalk.) + +(FIGURE 246. Terebratula carnea, Sowerby. Upper white chalk.) + +(FIGURE 247. Terebratula biplicata, Brocch. Upper cretaceous.) + +(FIGURE 248. Crania Parisiensis, Duf. Inferior or attached valve. Upper white +chalk.) + +(FIGURE 249. Pecten Beaveri, Sowerby. Reduced to one-third diameter. Lower white +chalk and chalk marl. Maidstone.) + +(FIGURE 250. Lima spinosa, Sowerby. Syn. Spondylus spinosus. Upper white chalk.) + +(FIGURE 251. Ostrea vesicularis. Syn. Gryphaea convexa. Upper chalk and upper +greensand.) + +Among the brachiopoda in the white chalk, the Terebratulae are very abundant +(see Figures 243-247). With these are associated some forms of oyster (see +Figure 251), and other bivalves (Figures 249, 250). + +(FIGURE 252. Inoceramus Lamarckii. Syn. Catillus Lamarckii. White chalk (Dixon's +Geology Sussex Table 28 Figure 29).) + +Among the bivalve mollusca, no form marks the Cretaceous era in Europe, America, +and India in a more striking manner than the extinct genus Inoceramus (Catillus +of Lam.; see Figure 252), the shells of which are distinguished by a fibrous +texture, and are often met with in fragments, having probably been extremely +friable. + +(Figures 253 to 256. Radiolites Mortoni. Mantell. Houghton, Sussex. White chalk. +Diameter one-seventh natural size. On the side where the shell is thinnest, +there is one external furrow and corresponding internal ridge, a, b, Figures +253, 254; but they are usually less prominent than in these figures. The upper +or opercular valve is wanting. + +(FIGURE 253. Two individuals deprived of their upper valves, adhering together.) + +(FIGURE 254. Same seen from above.) + +(FIGURE 255. Transverse section of part of the wall of the shell, magnified to +show the structure.) + +(FIGURE 256. Vertical section of the same.)) + +Of the singular family called Rudistes by Lamarck, hereafter to be mentioned as +extremely characteristic of the chalk of southern Europe, a single +representative only (Figure 253) has been discovered in the white chalk of +England. + +(FIGURE 257. Eschara disticha. White chalk. +a. Natural size. +b. Portion magnified.) + +(FIGURE 258. Escharina oceani. +a. Natural size. +b. Part of the same magnified. +White chalk.) + +(FIGURE 259. A branching sponge in a flint, from the white chalk. From the +collection of Mr. Bowerbank.) + +The general absence of univalve mollusca in the white chalk is very marked. Of +bryozoa there is an abundance, such as Eschara and Escharina (Figures 257, 258). +These and other organic bodies, especially sponges, such as Ventriculites +(Figure 238), are dispersed indifferently through the soft chalk and hard flint, +and some of the flinty nodules owe their irregular forms to inclosed sponges, +such as Figure 259, a, where the hollows in the exterior are caused by the +branches of a sponge (Figure 259, b), seen on breaking open the flint. + +(FIGURE 260. Palatal tooth of Ptychodus decurrens. Lower white chalk. +Maidstone.) + +(FIGURE 261. Cestracion Phillippi; recent. Port Jackson. Buckland, Bridgewater +Treatise Plate 27 d.)) + +The remains of fishes of the Upper Cretaceous formations consist chiefly of +teeth belonging to the shark family. Some of the genera are common to the +Tertiary formations, and some are distinct. To the latter belongs the genus +Ptychodus (Figure 260), which is allied to the living Port Jackson shark, +Cestracion Phillippi, the anterior teeth of which (see Figure 261, a) are sharp +and cutting, while the posterior or palatal teeth (b) are flat (Figure 260). But +we meet with no bones of land-animals, nor any terrestrial or fluviatile shells, +nor any plants, except sea-weeds, and here and there a piece of drift-wood. All +the appearances concur in leading us to conclude that the white chalk was the +product of an open sea of considerable depth. + +The existence of turtles and oviparous saurians, and of a Pterodactyl or winged +lizard, found in the white chalk of Maidstone, implies, no doubt, some +neighbouring land; but a few small islets in mid-ocean, like Ascension, formerly +so much frequented by migratory droves of turtle, might perhaps have afforded +the required retreat where these creatures laid their eggs in the sand, or from +which the flying species may have been blown out to sea. Of the vegetation of +such islands we have scarcely any indication, but it consisted partly of +cycadaceous plants; for a fragment of one of these was found by Captain Ibbetson +in the Chalk Marl of the Isle of Wight, and is referred by A. Brongniart to +Clathraria Lyellii, Mantell, a species common to the antecedent Wealden period. +The fossil plants, however, of beds corresponding in age to the white chalk at +Aix-la-Chapelle, presently to be described, like the sandy beds of Saxony, +before alluded to, afford such evidence of land as to prove how vague must be +any efforts of ours to restore the geography of that period. + +The Pterodactyl of the Kentish chalk, above alluded to, was of gigantic +dimensions, measuring 16 feet 6 inches from tip to tip of its outstretched +wings. Some of its elongated bones were at first mistaken by able anatomists for +those of birds; of which class no osseous remains have as yet been derived from +the white chalk, although they have been found (as will be seen) in the +Chloritic sand. + +(FIGURE 262. Coprolites of fish, from the chalk.) + +The collector of fossils from the white chalk was formerly puzzled by meeting +with certain bodies which they call larch-cones, which were afterwards +recognised by Dr. Buckland to be the excrement of fish (see Figure 262). They +are composed in great part of phosphate of lime. + +LOWER WHITE CHALK. + +(FIGURE 263. Baculites anceps, Lam. Lower chalk.) + +The Lower White Chalk, which is several hundred feet thick, without flints, has +yielded 25 species of Ammonites, of which half are peculiar to it. The genera +Baculite, Hamite, Scaphite, Turrilite, Nautilus, Belemnite, and Belemnitella, +are also represented. + +CHALK MARL. + +(FIGURE 264. Ammonites Rhotomagensis. Chalk marl. Back and side view.) + +(FIGURE 265. Turrilites costatus, Lam. Lower chalk and chalk marl. +a. Section, showing the foliated border of the sutures of the chambers.) + +(FIGURE 266. Scaphites aequalis. Chloritic marl and sand, Dorsetshire.) + +The lower chalk without flints passes gradually downward, in the south of +England, into an argillaceous limestone, "the chalk marl," already alluded to. +It contains 32 species of Ammonites, seven of which are peculiar to it, while +eleven pass up into the overlying lower white chalk. A. Rhotomagensis is +characteristic of this formation. Among the British cephalopods of other genera +may be mentioned Scaphites aequalis (Figure 266) and Turrilites costatus (Figure +265). + +CHLORITIC SERIES (OR UPPER GREENSAND). + +According to the old nomenclature, this subdivision of the chalk was called +Upper Greensand, in order to distinguish it from those members of the Neocomian +or Lower Cretaceous series below the Gault to which the name of Greensand had +been applied. Besides the reasons before given for abandoning this nomenclature, +it is objectionable in this instance as leading the uninitiated to suppose that +the divisions thus named Upper and Lower Greensand are of co-ordinate value, +instead of which the chloritic sand is quite a subordinate member of the Upper +Cretaceous group, and the term Greensand has very commonly been used for the +whole of the Lower Cretaceous rocks, which are almost comparable in importance +to the entire Upper Cretaceous series. The higher portion of the Chloritic +series in some districts has been called chloritic marl, from its consisting of +a chalky marl with chloritic grains. In parts of Surrey, where calcareous matter +is largely intermixed with sand, it forms a stone called malm-rock or firestone. +In the cliffs of the southern coast of the Isle of Wight it contains bands of +calcareous limestone with nodules of chert. + +COPROLITE BED. + +The so-called coprolite bed, found near Farnham, in Surrey, and near Cambridge, +contains nodules of phosphate of lime in such abundance as to be largely worked +for the manufacture of artificial manure. It belongs to the upper part of the +Chloritic series, and is doubtless chiefly of animal origin, and may perhaps be +partly coprolitic, derived from the excrement of fish and reptiles. The late Mr. +Barrett discovered in it, near Cambridge, in 1858, the remains of a bird, which +was rather larger than the common pigeon, and probably of the order Natatores, +and which, like most of the Gull tribe, had well-developed wings. Portions of +the metacarpus, metatarsus, tibia, and femur have been detected, and the +determinations of Mr. Barrett have been confirmed by Professor Owen. + +This phosphatic bed in the suburbs of Cambridge must have been formed partly by +the denudation of pre-existing rocks, mostly of Cretaceous age. The fossil +shells and bones of animals washed out of these denuded strata, now forming a +layer only a few feet thick, have yielded a rich harvest to the collector. A +large Rudist of the genus Radiolite, no less than two feet in height, may be +seen in the Cambridge Museum, obtained from this bed. The number of reptilian +remains, all apparently of Cretaceous age, is truly surprising; more than ten +species of Pterodactyl, five or six of Ichthyosaurus, one of Pliosaurus, one of +Dinosaurus, eight of Chelonians, besides other forms, having been recognised. + +The chloritic sand is regarded by many geologists as a littoral deposit of the +Chalk Ocean, and therefore contemporaneous with part of the chalk marl, and +even, perhaps, with some part of the white chalk. For, as the land went on +sinking, and the cretaceous sea widened its area, white mud and chloritic sand +were always forming somewhere, but the line of sea-shore was perpetually +shifting its position. Hence, though both sand and mud originated +simultaneously, the one near the land, the other far from it, the sands in every +locality where a shore became submerged might constitute the underlying deposit. + +(FIGURE 267. Ostrea columba. Syn. Gryphaea columba. Chloritic sand.) + +(FIGURE 268. Ostrea carinata. Chalk marl and chloritic sand. Neocomian.) + +(FIGURE 269. Terebrirostra lyra, Sowerby. Chloritic sand.) + +(FIGURE 270. Pecten 5-costatus. White chalk and chloritic sand. Neocomian.) + +(FIGURE 271. Plagiostoma Hoperi, Sowerby. Syn. Lima Hoperi. White chalk and +chloritic sand.) + +Among the characteristic mollusca of the chloritic sand may be mentioned +Terebrirostra lyra (Figure 269), Plagiostoma Hoperi (Figure 271), Pecten +quinque-costatus (Figure 270), and Ostrea columba (Figure 267). + +The Cephalopoda are abundant, among which 40 species of Ammonites are now known, +10 being peculiar to this subdivision, and the rest common to the beds +immediately above or below. + +GAULT. + +(FIGURE 272. Ancyloceras spinigerum, d'Orb. Syn. Hamites spiniger, Sowerby. Near +Folkestone. Gault.) + +The lowest member of the Upper Cretaceous group, usually about 100 feet thick in +the S.E. of England, is provincially termed Gault. It consists of a dark blue +marl, sometimes intermixed with green sand. Many peculiar forms of cephalopoda, +such as the Hamite (Figure 272), and Scaphite, with other fossils, characterise +this formation, which, small as is its thickness, can be traced by its organic +remains to distant parts of Europe, as, for example, to the Alps. + +Twenty-one species of British Ammonites are recorded as found in the Gault, of +which only eight are peculiar to it, ten being common to the overlying Chloritic +series. + +CONNECTION BETWEEN UPPER AND LOWER CRETACEOUS STRATA.-- BLACKDOWN BEDS. + +The break between the Upper and Lower Cretaceous formations will be appreciated +when it is stated that, although the Neocomian contains 31 species of Ammonite, +and the Gault, as we have seen, 21, there are only three of those common to both +divisions. Nevertheless, we may expect the discovery in England, and still more +when we extend our survey to the Continent, of beds of passage intermediate +between the Upper and Lower Cretaceous. Even now the Blackdown beds in +Devonshire, which rest immediately on Triassic strata, and which evidently +belong to some part of the Cretaceous series, have been referred by some +geologists to the Upper group, by others to the Lower or Neocomian. They +resemble the Folkestone beds of the latter series in mineral character, and 59 +out of 156 of their fossil mollusca are common to them; but they have also 16 +species common to the Gault, and 20 to the overlying Chloritic series; and what +is very important, out of seven Ammonites six are found also in the Gault and +Chloritic series, only one being peculiar to the Blackdown beds. + +Professor Ramsay has remarked that there is a stratigraphical break; for in +Kent, Surrey, and Sussex, at those few points where there are exposures of +junctions of the Gault and Neocomian, the surface of the latter has been much +eroded or denuded, while to the westward of the great chalk escarpment the +unconformability of the two groups is equally striking. At Blackdown this +unconformability is still more marked, for though distant only 100 miles from +Kent and Surrey, no formation intervenes between these beds and the Trias; all +intermediate groups, such as the Lower Neocomian and Oolite, having either not +been deposited or destroyed by denudation. + +FLORA OF THE UPPER CRETACEOUS PERIOD. + +As the Upper Cretaceous rocks of Europe are, for the most part, of purely marine +origin, and formed in deep water usually far from the nearest shore, land-plants +of this period, as we might naturally have anticipated, are very rarely met +with. In the neighbourhood of Aix-la-Chapelle, however, an important exception +occurs, for there certain white sands and laminated clays, 400 feet in +thickness, contain the remains of terrestrial plants in a beautiful state of +preservation. These beds are the equivalents of the white chalk and chalk marl +of England, or Senonien of d'Orbigny, although the white siliceous sands of the +lower beds, and the green grains in the upper part of the formation, cause it to +differ in mineral character from our white chalk. + +Beds of fine clay, with fossil plants, and with seams of lignite, and even +perfect coal, are intercalated. Floating wood, containing perforating shells, +such as Pholas and Gastrochoena, occur. There are likewise a few beds of a +yellowish-brown limestone, with marine shells, which enable us to prove that the +lowest and highest plant-beds belong to one group. Among these shells are Pecten +quadricostatus, and several others which are common to the upper and lower part +of the series, and Trigonia limbata, d'Orbigny, a shell of the white chalk. On +the whole, the organic remains and the geological position of the strata prove +distinctly that in the neighbourhood of Aix-la-Chapelle a gulf of the ancient +Cretaceous sea was bounded by land composed of Devonian rocks. These rocks +consisted of quartzose and schistose beds, the first of which supplied white +sand and the other argillaceous mud to a river which entered the sea at this +point, carrying down in its turbid waters much drift-wood and the leaves of +plants. Occasionally, when the force of the river abated, marine shells of the +genera Trigonia, Turritella, Pecten, etc., established themselves in the same +area, and plants allied to Zostera and Fucus grew on the bottom. + +The fossil plants of this member of the upper chalk at Aix have been diligently +collected and studied by Dr. Debey, and as they afford the only example yet +known of a terrestrial flora older than the Eocene, in which the great divisions +of the vegetable kingdom are represented in nearly the same proportions as in +our own times, they deserve particular attention. Dr. Debey estimates the number +of species as amounting to more than two hundred, of which sixty-seven are +cryptogamous, chiefly ferns, twenty species of which can be well determined, +most of them being in fructification. The scars on the bark of one or two are +supposed to indicate tree-ferns. Of thirteen genera three are still existing, +namely, Gleichenia, now inhabiting the Cape of Good Hope, and New Holland; +Lygodium, now spread extensively through tropical regions, but having some +species which live in Japan and North America; and Asplenium, a cosmopolite +form. Among the phaenogamous plants, the Conifers are abundant, the most common +belonging to a genus called Cycadopteris by Debey, and hardly separable from +Sequoia (or Wellingtonia), of which both the cones and branches are preserved. +When I visited Aix, I found the silicified wood of this plant very plentifully +dispersed through the white sands in the pits near that city. In one silicified +trunk 200 rings of annual growth could be counted. Species of Araucaria like +those of Australia are also found. Cycads are extremely rare, and of +Monocotyledons there are but few. No palms have been recognised with certainty, +but the genus Pandanus, or screw pine, has been distinctly made out. The number +of the Dicotyledonous Angiosperms is the most striking feature in so ancient a +flora. + +(In this and subsequent remarks on fossil plants I shall often use Dr. Lindley's +terms, as most familiar in this country; but as those of M. A. Brongniart are +much cited, it may be useful to geologists to give a table explaining the +corresponding names of groups so much spoken of in palaeontology. + +COLUMN 1. BRONGNIART. + +COLUMN 2. LINDLEY. + +COLUMN 3. EXAMPLES. + +CRYPTOGAMIC. + +1. Cryptogamous amphigens, or cellular cryptogamic: Thallogens: Lichens, sea- +weeds, fungi. + +2. Cryptogamous acrogens: Acrogens: Mosses, equisetums, ferns, lycopodiums-- +Lepidodendra. + +PHAENEROGAMIC. + +3. Dicotyledonous gymnosperms: Gymnogens: Conifers and Cycads. + +4. Dicotyledonous Angiosperms: Exogens: Compositae, leguminosae, umbelliferae, +cruciferae, heaths, etc. All native European trees except conifers. + +5. Monocotyledons: Endogens. Palms, lilies, aloes, rushes, grasses, etc.) + +Among them we find the familiar forms of the Oak, Fig, and Walnut (Quercus, +Ficus, and Juglans), of the last both the nuts and leaves; also several genera +of the Myrtaceae. But the predominant order is the Proteaceae, of which there +are between sixty and seventy supposed species, many of extinct genera, but some +referred to the following living forms-- Dryandra, Grevillea, Hakea, Banksia, +Persoonia-- all now belonging to Australia, and Leucospermum, species of which +form small bushes at the Cape. + +The epidermis of the leaves of many of these Aix plants, especially of the +Proteaceae, is so perfectly preserved in an envelope of fine clay, that under +the microscope the stomata, or polygonal cellules, can be detected, and their +peculiar arrangement is identical with that known to characterise some living +Proteaceae (Grevillea, for example). Although this peculiarity of the structure +of stomata is also found in plants of widely distant orders, it is, on the +whole, but rarely met with, and being thus observed to characterise a foliage +previously suspected to be proteaceous, it adds to the probability that the +botanical evidence had been correctly interpreted. + +An occasional admixture at Aix-la-Chapelle of Fucoids and Zosterites attests, +like the shells, the presence of salt-water. Of insects, Dr. Debey has obtained +about ten species of the families Curculionidae and Carabidae. + +The resemblance of the flora of Aix-la-Chapelle to the tertiary and living +floras in the proportional number of dicotyledonous angiosperms as compared to +the gymnogens, is a subject of no small theoretical interest, because we can now +affirm that these Aix plants flourished before the rich reptilian fauna of the +secondary rocks had ceased to exist. The Ichthyosaurus, Pterodactyl, and +Mosasaurus were of coeval date with the oak, the walnut, and the fig. +Speculations have often been hazarded respecting a connection between the rarity +of Exogens in the older rocks and a peculiar state of the atmosphere. A denser +air, it was suggested, had in earlier times been alike adverse to the well-being +of the higher order of flowering plants, and of the quick-breathing animals, +such as mammalia and birds, while it was favourable to a cryptogamic and +gymnospermous flora, and to a predominance of reptile life. But we now learn +that there is no incompatibility in the co-existence of a vegetation like that +of the present globe, and some of the most remarkable forms of the extinct +reptiles of the age of gymnosperms. + +If the passage seem at present to be somewhat sudden from the flora of the Lower +or Neocomian to that of the Upper Cretaceous period, the abruptness of the +change will probably disappear when we are better acquainted with the fossil +vegetation of the uppermost beds of the Neocomian and that of the lowest strata +of the Gault or true Cretaceous series. + +HIPPURITE LIMESTONE.-- DIFFERENCE BETWEEN THE CHALK OF THE NORTH AND SOUTH OF +EUROPE. + +(FIGURE 273. Map of part of S.W. France, from the Loire river to the Pyrenees.) + +By the aid of the three tests, superposition, mineral character, and fossils, +the geologist has been enabled to refer to the same Cretaceous period certain +rocks in the north and south of Europe, which differ greatly both in their +fossil contents and in their mineral composition and structure. + +If we attempt to trace the cretaceous deposits from England and France to the +countries bordering the Mediterranean, we perceive, in the first place, that in +the neighbourhood of London and Paris they form one great continuous mass, the +Straits of Dover being a trifling interruption, a mere valley with chalk cliffs +on both sides. We then observe that the main body of the chalk which surrounds +Paris stretches from Tours to near Poitiers (see Figure 273, in which the shaded +part represents chalk). + +Between Poitiers and La Rochelle, the space marked A on the map separates two +regions of chalk. This space is occupied by the Oolite and certain other +formations older than the Chalk and Neocomian, and has been supposed by M. E. de +Beaumont to have formed an island in the Cretaceous sea. South of this space we +again meet with rocks which we at once recognise to be cretaceous, partly from +the chalky matrix and partly from the fossils being very similar to those of the +white chalk of the north: especially certain species of the genera Spatangus, +Ananchytes, Cidarites, Nucula, Ostrea, Gryphaea (Exogyra), Pecten, Plagiostoma +(Lima), Trigonia, Catillus (Inoceramus), and Terebratula. (d'Archiac, Sur la +form. Cretacee du S.-O. de la France Mem. de la Soc. Geol. de France tome 2.) +But Ammonites, as M. d'Archiac observes, of which so many species are met with +in the chalk of the north of France, are scarcely ever found in the southern +region; while the genera Hamite, Turrilite, and Scaphite, and perhaps Belemnite, +are entirely wanting. + +(FIGURE 274. Radiolites radiosa, d'Orbigny. White chalk of France. +b. Upper valve of same.) + +(FIGURE 275. Radiolites foliaceus, d'Orbigny. Syn. Sphaerulites agarici-formis, +Blainv. White chalk of France.) + +(FIGURE 276. Hippurites organisans, Desmoulins. Upper chalk:-- chalk marl of +Pyrenees? (d'Orbigny's Palaeontologie francaise plate 533.) +a. Young individual; when full grown they occur in groups adhering laterally to +each other. +b. Upper side of the upper valve, showing a reticulated structure in those +parts, b, where the external coating is worn off. +c. Upper end or opening of the lower and cylindrical valve. +d. Cast of the interior of the lower conical valve.) + +On the other hand, certain forms are common in the south which are rare or +wholly unknown in the north of France. Among these may be mentioned many +Hippurites, Sphaerulites, and other members of that great family of mollusca +called Rudistes by Lamarck, to which nothing analogous has been discovered in +the living creation, but which is quite characteristic of rocks of the +Cretaceous era in the south of France, Spain, Sicily, Greece, and other +countries bordering the Mediterranean. The species called Hippurites organisans +(Figure 276) is more abundant than any other in the south of Europe; and the +geologist should make himself well acquainted with the cast of the interior, d, +which is often the only part preserved in many compact marbles of the Upper +Cretaceous period. The flutings on the interior of the Hippurite, which are +represented on the cast by smooth, rounded longitudinal ribs, and in some +individuals attain a great size and length, are wholly unlike the markings on +the exterior of the shell. + +CRETACEOUS ROCKS IN THE UNITED STATES. + +If we pass to the American continent, we find in the State of New Jersey a +series of sandy and argillaceous beds wholly unlike in mineral character to our +Upper Cretaceous system; which we can, nevertheless, recognise as referable, +palaeontologically, to the same division. + +That they were about the same age generally as the European chalk and Neocomian, +was the conclusion to which Dr. Morton and Mr. Conrad came after their +investigation of the fossils in 1834. The strata consist chiefly of green sand +and green marl, with an overlying coralline limestone of a pale yellow colour, +and the fossils, on the whole, agree most nearly with those of the Upper +European series, from the Maestricht beds to the Gault inclusive. I collected +sixty shells from the New Jersey deposits in 1841, five of which were identical +with European species-- Ostrea larva, O. vesicularis, Gryphaea costata, Pecten +quinque-costatus, Belemnitella mucronata. As some of these have the greatest +vertical range in Europe, they might be expected more than any others to recur +in distant parts of the globe. Even where the species were different, the +generic forms, such as the Baculite and certain sections of Ammonites, as also +the Inoceramus (see above, Figure 252) and other bivalves, have a decidedly +cretaceous aspect. Fifteen out of the sixty shells above alluded to were +regarded by Professor Forbes as good geographical representatives of well-known +cretaceous fossils of Europe. The correspondence, therefore, is not small, when +we reflect that the part of the United States where these strata occur is +between 3000 and 4000 miles distant from the chalk of Central and Northern +Europe, and that there is a difference of ten degrees in the latitude of the +places compared on opposite sides of the Atlantic. Fish of the genera Lamna, +Galeus, and Carcharodon are common to New Jersey and the European cretaceous +rocks. So also is the genus Mosasaurus among reptiles. + +It appears from the labours of Dr. Newberry and others, that the Cretaceous +strata of the United States east and west of the Appalachians are characterised +by a flora decidedly analogous to that of Aix-la-Chapelle above-mentioned, and +therefore having considerable resemblance to the vegetation of the Tertiary and +Recent Periods. + + +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. + +We now come to the Lower Cretaceous Formation which was formerly called Lower +Greensand, and for which it will be useful for reasons before explained (Chapter +17) to use the term "Neocomian." + +TABLE 18.1. LOWER CRETACEOUS OR NEOCOMIAN GROUP. + +COLUMN 1: MARINE. +COLUMN 2: FRESH-WATER. + +1. Upper Neocomian-- Greensand of Folkestone, Sandgate, and Hythe, Atherfield +clay, upper part of Speeton clay: Part of Wealden beds of Kent, Surrey, Sussex, +Hants, and Dorset. + +2. Middle Neocomian-- Punfield Marine bed, Tealby beds, middle part of Speeton +clay: Part of Wealden beds of Kent, Surrey, Sussex, Hants, and Dorset. + +3. Lower Neocomian-- Lower part of Speeton clay: Part of Wealden beds of Kent, +Surrey, Sussex, Hants, and Dorset. + +In Western France, the Alps, the Carpathians, Northern Italy, and the Apennines, +an extensive series of rocks has been described by Continental geologists under +the name of Tithonian. These beds, which are without any marine equivalent in +this country, appear completely to bridge over the interval between the +Neocomian and the Oolites. They may, perhaps, as suggested by Mr. Judd, be of +the same age as part of the Wealden series. + +UPPER NEOCOMIAN. + +FOLKSTONE AND HYTHE BEDS. + +(FIGURE 277. Nautilus plicatus, Sowerby, in Fitton's Monog.) + +(FIGURE 278. Ancyloceras gigas, d'Orbigny.) + +(FIGURE 279. Gervillia anceps, Desh. Upper Neocomian, Surrey.) + +(FIGURE 280. Trigonia caudata, Agassiz. Upper Neocomian.) + +(FIGURE 281. Terebratula sella, Sowerby. Upper Neocomian, Hythe.) + +(FIGURE 282. Diceras Lonsdalii. Upper Neocomian, Wilts. +a. The bivalve shell. +b. Cast of one of the valves enlarged.) + +The sands which crop out beneath the Gault in Wiltshire, Surrey, and Sussex are +sometimes in the uppermost part pure white, at others of a yellow and +ferruginous colour, and some of the beds contain much green matter. At +Folkestone they contain layers of calcareous matter and chert, and at Hythe, in +the neighbourhood, as also at Maidstone and other parts of Kent, the limestone +called Kentish Rag is intercalated. This somewhat clayey and calcareous stone +forms strata two feet thick, alternating with quartzose sand. The total +thickness of these Folkestone and Hythe beds is less than 300 feet, and they are +seen to rest immediately on a grey clay, to which we shall presently allude as +the Atherfield clay. Among the fossils of the Folkestone and Hythe beds we may +mention Nautilus plicatus (Figure 277), Ancyloceras (Scaphites) gigas (Figure +278), which has been aptly described as an Ammonite more or less uncoiled; +Trigonia caudata (Figure 280), Gervillia anceps (Figure 279), a bivalve genus +allied to Avicula, and Terebratula sella (Figure 281). In ferruginous beds of +the same age in Wiltshire is found a remarkable shell called Diceras Lonsdalii +(Figure 282), which abounds in the Upper and Middle Neocomian of Southern +Europe. This genus is closely allied to Chama, and the cast of the interior has +been compared to the horns of a goat. + +ATHERFIELD CLAY. + +We mentioned before that the Folkstone and Hythe series rest on a grey clay. +This clay is only of slight thickness in Kent and Surrey, but acquires great +dimensions at Atherfield, in the Isle of Wight. The difference, indeed, in +mineral character and thickness of the Upper Neocomian formation near +Folkestone, and the corresponding beds in the south of the Isle of Wight, about +100 miles distant, is truly remarkable. In the latter place we find no limestone +answering to the Kentish Rag, and the entire thickness from the bottom of the +Atherfield clay to the top of the Neocomian, instead of being less than 300 feet +as in Kent, is given by the late Professor E. Forbes as 843 feet, which he +divides into sixty-three strata, forming three groups. The uppermost of these +consists of ferruginous sands, the second of sands and clay, and the third or +lowest of a brown clay, abounding in fossils. + +Pebbles of quartzose sandstone, jasper, and flinty slate, together with grains +of chlorite and mica, and, as Mr. Godwin-Austen has shown, fragments and water- +worn fossils of the oolitic rocks, speak plainly of the nature of the pre- +existing formations, by the wearing down of which the Neocomian beds were +formed. The land, consisting of such rocks, was doubtless submerged before the +origin of the white chalk, a deposit which was formed in a more open sea, and in +clearer waters. + +(FIGURE 283. Perna Mulleti, Desh. One-eighth natural size. +a. Exterior. +b. Part of hinge-line of upper or right valve.) + +Among the shells of the Atherfield clay the biggest and most abundant shell is +the large Perna Mulleti, of which a reduced figure is given in Figure 283. + +SIMILARITY OF CONDITIONS CAUSING REAPPEARANCE OF SPECIES. + +Some species of mollusca and other fossils range through the whole series, while +others are confined to particular subdivisions, and Forbes laid down a law which +has since been found of very general application in regard to estimating the +chronological relations of consecutive strata. Whenever similar conditions, he +says, are repeated, the same species reappear, provided too great a lapse of +time has not intervened; whereas if the length of the interval has been +geologically great, the same genera will reappear represented by distinct +species. Changes of depth, or of the mineral nature of the sea-bottom, the +presence or absence of lime or of peroxide of iron, the occurrence of a muddy, +or a sandy, or a gravelly bottom, are marked by the banishment of certain +species and the predominance of others. But these differences of conditions +being mineral, chemical, and local in their nature, have no necessary connection +with the extinction, throughout a large area, of certain animals or plants. When +the forms proper to loose sand or soft clay, or to perfectly clear water, or to +a sea of moderate or great depth, recur with all the same species, we may infer +that the interval of time has been, geologically speaking, small, however dense +the mass of matter accumulated. But if, the genera remaining the same, the +species are changed, we have entered upon a new period; and no similarity of +climate, or of geographical and local conditions, can then recall the old +species which a long series of destructive causes in the animate and inanimate +world has gradually annihilated. + +SPEETON CLAY, UPPER DIVISION. + +(FIGURE 284. Ammonites Deshayesii, Leym. Upper Neocomian.) + +On the coast, beneath the white chalk of Flamborough Head, in Yorkshire, an +argillaceous formation crops out, called the Speeton clay, several hundred feet +in thickness, the palaeontological relations of which have been ably worked out +by Mr. John W. Judd, and he has shown that it is separable into three divisions, +the uppermost of which, 150 feet thick, and containing 87 species of mollusca, +decidedly belongs to the Atherfield clay and associated strata of Hythe and +Folkestone, already described. (Judd, Speeton clay, Quarterly Geological Journal +volume 24 1868 page 218.) It is characterised by the Perna Mulleti (Figure 283) +and Terebratula sella (Figure 281), and by Ammonites Deshayesii (Figure 284), a +well-known Hythe fossil. Fine skeletons of reptiles of the genera Pliosaurus and +Teleosaurus have been obtained from this clay. At the base of this upper +division of the Speeton clay there occurs a layer of large Septaria, formerly +worked for the manufacture of cement. This bed is crowded with fossils, +especially Ammonites, one species of which, three feet in diameter, was observed +by Mr. Judd. + +MIDDLE NEOCOMIAN. + +TEALBY SERIES. + +(FIGURE 285. Pecten cinctus, Sowerby. (P. crassitesta, Rom.) Middle Neocomian, +England; Middle and Lower Neocomian, Germany. One-fifth natural size.) + +(FIGURE 286. Ancyloceras (Crioceras) Duvallei, Leveille. Middle and Lower +Neocomian. One-fifth natural size.) + +At Tealby, a village in the Lincolnshire Wolds, there crop out beneath the white +chalk some non-fossiliferous ferruginous sands about twenty-feet thick, beneath +which are beds of clay and limestone, about fifty feet thick, with an +interesting suite of fossils, among which are Pecten cinctus (Figure 285), from +9 to 12 inches in diameter, Ancyloceras Duvallei (Figure 286), and some forty +other shells, many of them common to the Middle Speeton clay, about to be +mentioned. Mr. Judd remarks that as Ammonites clypei-formis and Terebratula +hippopus characterise the Middle Neocomian of the Continent, it is to this stage +that the Tealby series containing the same fossils may be assigned. (Judd +Quarterly Geological Journal 1867 volume 23 page 249.) + +The middle division of the Speeton clay, occurring at Speeton below the cement- +bed, before alluded to, is 150 feet thick, and contains about 39 species of +mollusca, half of which are common to the overlying clay. Among the peculiar +shells, Pecten cinctus (Figure 285) and Ancyloceras (Crioceras) Duvallei (Figure +286) occur. + +LOWER NEOCOMIAN. + +(FIGURE 287. Ammonites Noricus, Schloth. Lower Neocomian, Speeton.) + +In the lower division of the Speeton clay, 200 feet thick, 46 species of +mollusca have been found, and three divisions, each characterised by its +peculiar ammonite, have been noticed by Mr. Judd. The central zone is marked by +Ammonites Noricus (see Figure 287). On the Continent these beds are well-known +by their corresponding fossils, the Hils clay and conglomerate of the north of +Germany agreeing with the Middle and Lower Speeton, the latter of which, with +the same mineral characters and fossils as in Yorkshire, is also found in the +little island of Heligoland. Yellow limestone, which I have myself seen near +Neuchatel, in Switzerland, represents the Lower Neocomian at Speeton. + +WEALDEN FORMATION. + +Beneath the Atherfield clay or Upper Neocomian of the S.E. of England, a fresh- +water formation is found, called the Wealden, which, although it occupies a +small horizontal area in Europe, as compared to the White Chalk and the marine +Neocomian beds, is nevertheless of great geological interest, since the imbedded +remains give us some insight into the nature of the terrestrial fauna and flora +of the Lower Cretaceous epoch. The name of Wealden was given to this group +because it was first studied in parts of Kent, Surrey, and Sussex, called the +Weald; and we are indebted to Dr. Mantell for having shown, in 1822, in his +"Geology of Sussex," that the whole group was of fluviatile origin. In proof of +this he called attention to the entire absence of Ammonites, Belemnites, +Brachiopoda, Echinodermata, Corals, and other marine fossils, so characteristic +of the Cretaceous rocks above, and of the Oolitic strata below, and to the +presence in the Weald of Paludinae, Melaniae, Cyrenae, and various fluviatile +shells, as well as the bones of terrestrial reptiles and the trunks and leaves +of land-plants. + +(FIGURE 288. Section from (left) W.S.W. through Brixton bay, Isle of Wight, +Solent and South Downs to E.N.E. (right). +1. Tertiary. +2. Chalk and Gault. +3. Upper Neocomian (or Lower Greensand). +4. Wealden (Weald Clay and Hastings Sands).) + +The evidence of so unexpected a fact as that of a dense mass of purely fresh- +water origin underlying a deep-sea deposit (a phenomenon with which we have +since become familiar) was received, at first, with no small doubt and +incredulity. But the relative position of the beds is unequivocal; the Weald +Clay being distinctly seen to pass beneath the Atherfield Clay in various parts +of Surrey, Kent, and Sussex, and to reappear in the Isle of Wight at the base of +the Cretaceous series, being, no doubt, continuous far beneath the surface, as +indicated by the dotted lines in Figure 288. They are also found occupying the +same relative position below the chalk in the peninsula of Purbeck, Dorsetshire, +where, as we shall see in the sequel, they repose on strata referable to the +Upper Oolite. + +WEALD CLAY. + +The Upper division, or Weald Clay, is, in great part, of fresh-water origin, but +in its highest portion contains beds of oysters and other marine shells which +indicate fluvio-marine conditions. The uppermost beds are not only conformable, +as Dr. Fitton observes, to the inferior strata of the overlying Neocomian, but +of similar mineral composition. To explain this, we may suppose that, as the +delta of a great river was tranquilly subsiding, so as to allow the sea to +encroach upon the space previously occupied by fresh-water, the river still +continued to carry down the same sediment into the sea. In confirmation of this +view it may be stated that the remains of the Iguanodon Mantelli, a gigantic +terrestrial reptile, very characteristic of the Wealden, has been discovered +near Maidstone, in the overlying Kentish Rag, or marine limestone of the Upper +Neocomian. Hence we may infer that some of the saurians which inhabited the +country of the great river continued to live when part of the district had +become submerged beneath the sea. Thus, in our own times, we may suppose the +bones of large alligators to be frequently entombed in recent fresh-water strata +in the delta of the Ganges. But if part of that delta should sink down so as to +be covered by the sea, marine formations might begin to accumulate in the same +space where fresh-water beds had previously been formed; and yet the Ganges +might still pour down its turbid waters in the same direction, and carry seaward +the carcasses of the same species of alligator, in which case their bones might +be included in marine as well as in subjacent fresh-water strata. + +(FIGURES 289 AND 290. Tooth of Iguanodon Mantelli. + +(FIGURE 289. a, and b.) + +(FIGURE 290. A. Partially worn tooth of young individual of the same. +b. Crown of tooth in adult worn down. (Mantell.))) + +The Iguanodon, first discovered by Dr. Mantell, was an herbivorous reptile, of +which the teeth, though bearing a great analogy, in their general form and +crenated edges (see Figure 289 a and b), to the modern Iguanas which now +frequent the tropical woods of America and the West Indies, exhibit many +important differences. It appears that they have often been worn by the process +of mastication; whereas the existing herbivorous reptiles clip and gnaw off the +vegetable productions on which they feed, but do not chew them. Their teeth +frequently present an appearance of having been chipped off, but never, like the +fossil teeth of the Iguanodon, have a flat ground surface (see Figure 290, b) +resembling the grinders of herbivorous mammalia. Dr. Mantell computes that the +teeth and bones of this species which passed under his examination during twenty +years must have belonged to no less than seventy-one distinct individuals, +varying in age and magnitude from the reptile just burst from the egg, to one of +which the femur measured twenty-four inches in circumference. Yet, +notwithstanding that the teeth were more numerous than any other bones, it is +remarkable that it was not until the relics of all these individuals had been +found, that a solitary example of part of a jaw-bone was obtained. Soon +afterwards remains both of the upper and lower jaw were met with in the Hastings +beds in Tilgate Forest, near Cuckfield. In the same sands at Hastings, Mr. +Beckles found large tridactyle impressions which it is conjectured were made by +the hind feet of this animal, on which it is ascertained that there were only +three well-developed toes. + +(FIGURE 291. Cypris spinigera, Fitton.) + +(FIGURE 292. Weald clay with Cyprides.) + +Occasionally bands of limestone, called Sussex Marble, occur in the Weald Clay, +almost entirely composed of a species of Paludina, closely resembling the common +P. vivipara of English rivers. Shells of the Cypris, a genus of Crustaceans +mentioned in Chapter 3 as abounding in lakes and ponds, are also plentifully +scattered through the clays of the Wealden, sometimes producing, like plates of +mica, a thin lamination (see Figure 292). + +HASTINGS SANDS. + +This lower division of the Wealden consists of sand, sandstone, calciferous +grit, clay, and shale; the argillaceous strata, notwithstanding the name, +predominating somewhat over the arenaceous, as will be seen by reference to the +following table, drawn up by Messrs. Drew and Foster, of the Geological Survey +of Great Britain: + +TABLE 18.1. SUBORDINATE FORMATIONS IN THE HASTINGS SAND. + +COLUMN 1: NAME OF SUBORDINATE FORMATION. +COLUMN 2: MINERAL COMPOSITION OF THE STRATA. +COLUMN 3: THICKNESS IN FEET. + +Tunbridge Wells Sand: Sandstone and loam: 150. + +Wadhurst Clay: Blue and brown shale and clay, with a little calc-grit: 100. + +Ashdown Sand: Hard sand, with some beds of calc-grit: 160. + +Ashburnham Beds: Mottled white and red clay, with some sandstone: 330. + +The picturesque scenery of the "High Rocks" and other places in the +neighbourhood of Tunbridge Wells is caused by the steep natural cliffs, to which +a hard bed of white sand, occurring in the upper part of the Tunbridge Wells +Sand, mentioned in the above table, gives rise. This bed of "rock-sand" varies +in thickness from 25 to 48 feet. Large masses of it, which were by no means hard +or capable of making a good building-stone, form, nevertheless, projecting rocks +with perpendicular faces, and resist the degrading action of the river because, +says Mr. Drew, they present a solid mass without planes of division. The +calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in which the +remains of the Iguanodon and Hylaeosaurus were first found by Dr. Mantell, +constitute an upper member of the Tunbridge Wells Sand, while the "sand-rock" of +the Hastings cliffs, about 100 feet thick, is one of the lower members of the +same. The reptiles, which are very abundant in this division, consist partly of +saurians, referred by Owen and Mantell to eight genera, among which, besides +those already enumerated, we find the Megalosaurus and Plesiosaurus. The +Pterodactyl also, a flying reptile, is met with in the same strata, and many +remains of Chelonians of the genera Trionyx and Emys, now confined to tropical +regions. + +(FIGURE 293. Lepidotus Mantelli, Agassiz. Wealden. +a. Palate and teeth. +b. Side view of teeth. +c. Scale.) + +The fishes of the Wealden are chiefly referable to the Ganoid and Placoid +orders. Among them the teeth and scales of Lepidotus are most widely diffused +(see Figure 293). These ganoids were allied to the Lepidosteus, or Gar-pike, of +the American rivers. The whole body was covered with large rhomboidal scales, +very thick, and having the exposed part coated with enamel. Most of the species +of this genus are supposed to have been either river-fish, or inhabitants of the +sea at the mouth of estuaries. + +(FIGURE 294. Unio Valdensis, Mant. Isle of Wight and Dorsetshire; in the lower +beds of the Hastings Sands. a, b.) + +(FIGURE 295. Underside of slab of sandstone about one yard in diameter. +Stammerham, Sussex.) + +At different heights in the Hastings Sands, we find again and again slabs of +sandstone with a strong ripple-mark, and between these slabs beds of clay many +yards thick. In some places, as at Stammerham, Horsham, near there, are +indications of this clay having been exposed so as to dry and crack before the +next layer was thrown down upon it. The open cracks in the clay have served as +moulds, of which casts have been taken in relief, and which are, therefore, seen +on the lower surface of the sandstone (see Figure 295). + +(FIGURE 296. Sphenopteris gracilis, Fitton. From the Hastings Sands near +Tunbridge Wells. +a. A portion of the same magnified.) + +Near the same place a reddish sandstone occurs in which are innumerable traces +of a fossil vegetable, apparently Sphenopteris, the stems and branches of which +are disposed as if the plants were standing erect on the spot where they +originally grew, the sand having been gently deposited upon and around them; and +similar appearances have been remarked in other places in this formation. +(Mantell Geology of S.E. of England page 244.) In the same division also of the +Wealden, at Cuckfield, is a bed of gravel or conglomerate, consisting of water- +worn pebbles of quartz and jasper, with rolled bones of reptiles. These must +have been drifted by a current, probably in water of no great depth. + +From such facts we may infer that, notwithstanding the great thickness of this +division of the Wealden, the whole of it was a deposit in water of a moderate +depth, and often extremely shallow. This idea may seem startling at first, yet +such would be the natural consequence of a gradual and continuous sinking of the +ground in an estuary or bay, into which a great river discharged its turbid +waters. By each foot of subsidence, the fundamental rock would be depressed one +foot farther from the surface; but the bay would not be deepened, if newly- +deposited mud and sand should raise the bottom one foot. On the contrary, such +new strata of sand and mud might be frequently laid dry at low water, or +overgrown for a season by a vegetation proper to marshes. + +PUNFIELD BEDS, BRACKISH AND MARINE. + +(FIGURE 297. Vicarya Lujani, De Verneuil (Foss. de Utrillas.) Wealden, Punfield. +a. Nearly perfect shell. +b. Vertical section of smaller specimen, showing continuous ridges as in +Nerinaea.) + +The shells of the Wealden beds belong to the genera Melanopsis, Melania, +Paludina, Cyrena, Cyclas, Unio (see Figure 294), and others, which inhabit +rivers or lakes; but one band has been found at Punfield, in Dorsetshire, +indicating a brackish state of the water, where the genera Corbula, Mytilus, and +Ostrea occur; and in some places this bed becomes purely marine, containing some +well-known Neocomian fossils, among which Ammonites Deshayesii (Figure 284) may +be mentioned. Others are peculiar as British, but very characteristic of the +Upper and Middle Neocomian of Spain, and among these the Vicarya Lujani (Figure +297), a shell allied to Nerinea, is conspicuous. + +By reference to Table 18.1 it will be seen that the Wealden beds are given as +the fresh-water equivalents of the Marine Neocomian. The highest part of them in +England may, for reasons just given, be regarded as Upper Neocomian, while some +of the inferior portions may correspond in age to the Middle and Lower divisions +of that group. In favour of this latter view, M. Marcou mentions that a fish +called Asteracanthus granulosus, occurring in the Tilgate beds, is +characteristic of the lowest beds of the Neocomian of the Jura, and it is well +known that Corbula alata, common in the Ashburnham beds, is found also at the +base of the Neocomian of the Continent. + +AREA OF THE WEALDEN. + +In regard to the geographical extent of the Wealden, it can not be accurately +laid down, because so much of it is concealed beneath the newer marine +formations. It has been traced about 320 English miles from west to east, from +the coast of Dorsetshire to near Boulogne, in France; and nearly 200 miles from +north-west to south-east, from Surrey and Hampshire to Vassy, in France. If the +formation be continuous throughout this space, which is very doubtful, it does +not follow that the whole was contemporaneous; because, in all likelihood, the +physical geography of the region underwent frequent changes throughout the whole +period, and the estuary may have altered its form, and even shifted its place. +Dr. Dunker, of Cassel, and H. von Meyer, in an excellent monograph on the +Wealdens of Hanover and Westphalia, have shown that they correspond so closely, +not only in their fossils, but also in their mineral characters, with the +English series, that we can scarcely hesitate to refer the whole to one great +delta. Even then, the magnitude of the deposit may not exceed that of many +modern rivers. Thus, the delta of the Quorra or Niger, in Africa, stretches into +the interior for more than 170 miles, and occupies, it is supposed, a space of +more than 300 miles along the coast, thus forming a surface of more than 25,000 +square miles, or equal to about one-half of England. (Fitton Geology of Hastings +page 58, who cites Lander's Travels.) Besides, we know not, in such cases, how +far the fluviatile sediment and organic remains of the river and the land may be +carried out from the coast, and spread over the bed of the sea. I have shown, +when treating of the Mississippi, that a more ancient delta, including species +of shells such as now inhabit Louisiana, has been upraised, and made to occupy a +wide geographical area, while a newer delta is forming; and the possibility of +such movements and their effects must not be lost sight of when we speculate on +the origin of the Wealden. (See Chapter 6 and Second Visit to the United States +volume 2 chapter 34.) + +It may be asked where the continent was placed, from the ruins of which the +Wealden strata were derived, and by the drainage of which a great river was fed. +If the Wealden was gradually going downward 1000 feet or more perpendicularly, a +large body of fresh-water would not continue to be poured into the sea at the +same point. The adjoining land, if it participated in the movement, could not +escape being submerged. But we may suppose such land to have been stationary, or +even undergoing contemporaneous slow upheaval. There may have been an ascending +movement in one region, and a descending one in a contiguous parallel zone of +country. But even if that were the case, it is clear that finally an extensive +depression took place in that part of Europe where the deep sea of the +Cretaceous period was afterwards brought in. + +THICKNESS OF THE WEALDEN. + +In the Weald area itself, between the North and South Downs, fresh-water beds to +the thickness of 1600 feet are known, the base not being reached. Probably the +thickness of the whole Wealden series, as seen in Swanage Bay, can not be +estimated as less than 2000 feet. + +WEALDEN FLORA. + +The flora of the Wealden is characterised by a great abundance of Coniferae, +Cycadeae, anD Ferns, and by the absence of leaves and fruits of Dicotyledonous +Angiosperms. The discovery in 1855, in the Hastings beds of the Isle of Wight, +of Gyrogonites, or spore-vessels of the Chara, was the first example of that +genus of plants, so common in the tertiary strata, being found in a Secondary or +Mesozoic rock. + + +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. + +CLASSIFICATION OF THE OOLITE. + +Immediately below the Hastings Sands we find in Dorsetshire another remarkable +fresh-water formation, called THE PURBECK, because it was first studied in the +sea-cliffs of the peninsula of Purbeck in that county. These beds are for the +most part of fresh-water origin, but the organic remains of some few +intercalated beds are marine, and show that the Purbeck series has a closer +affinity to the Oolitic group, of which it may be considered as the newest or +uppermost member. + +In England generally, and in the greater part of Europe, both the Wealden and +Purbeck beds are wanting, and the marine cretaceous group is followed +immediately, in the descending order, by another series called the Jurassic. In +this term, the formations commonly designated as "the Oolite and Lias" are +included, both being found in the Jura Mountains. The Oolite was so named +because in the countries where it was first examined the limestones belonging to +it had an Oolitic structure (see Chapter 3). These rocks occupy in England a +zone nearly thirty miles in average breadth, which extends across the island, +from Yorkshire in the north-east, to Dorsetshire in the south-west. Their +mineral characters are not uniform throughout this region; but the following are +the names of the principal subdivisions observed in the central and south- +eastern parts of England. + +TABLE 19.1. OOLITE. + +UPPER OOLITE: +a. Purbeck beds. +b. Portland stone and sand. +c. Kimmeridge clay. + +MIDDLE OOLITE: +d. Coral rag. +e. Oxford clay, and Kelloway rock. + +LOWER OOLITE: +f. Cornbrash and Forest marble. +g. Great Oolite and Stonesfield slate. +h. Fuller's earth. +i. Inferior Oolite. + +The Upper Oolitic system of the Table 19.1 has usually the Kimmeridge clay for +its base; the Middle Oolitic system, the Oxford clay. The Lower system reposes +on the Lias, an argillo-calcareous formation, which some include in the Lower +Oolite, but which will be treated of separately in the next chapter. Many of +these subdivisions are distinguished by peculiar organic remains; and, though +varying in thickness, may be traced in certain directions for great distances, +especially if we compare the part of England to which the above-mentioned type +refers with the north-east of France and the Jura Mountains adjoining. In that +country, distant above 400 geographical miles, the analogy to the accepted +English type, notwithstanding the thinness or occasional absence of the clays, +is more perfect than in Yorkshire or Normandy. + +PHYSICAL GEOGRAPHY. + +The alternation, on a grand scale, of distinct formations of clay and limestone +has caused the oolitic and liassic series to give rise to some marked features +in the physical outline of parts of England and France. Wide valleys can usually +be traced throughout the long bands of country where the argillaceous strata +crop out; and between these valleys the limestones are observed, forming ranges +of hills or more elevated grounds. These ranges terminate abruptly on the side +on which the several clays rise up from beneath the calcareous strata. + +(FIGURE 298. Section through Lias (left), Lower Oolite, Oxford Clay, Middle +Oolite, Kim. Clay. Upper Oolite. Gault, Chalk and London Clay (right).) + +Figure 298 will give the reader an idea of the configuration of the surface now +alluded to, such as may be seen in passing from London to Cheltenham, or in +other parallel lines, from east to west, in the southern part of England. It has +been necessary, however, in this drawing, greatly to exaggerate the inclination +of the beds, and the height of the several formations, as compared to their +horizontal extent. It will be remarked, that the lines of steep slope, or +escarpment, face towards the west in the great calcareous eminences formed by +the chalk and the Upper, Middle, and Lower Oolites; and at the base of which we +have respectively the Gault, Kimmeridge clay, Oxford clay, and Lias. This last +forms, generally, a broad vale at the foot of the escarpment of inferior Oolite, +but where it acquires considerable thickness, and contains solid beds of +marlstone, it occupies the lower part of the escarpment. + +The external outline of the country which the geologist observes in travelling +eastward from Paris to Metz, is precisely analogous, and is caused by a similar +succession of rocks intervening between the tertiary strata and the Lias; with +this difference, however, that the escarpments of Chalk, Upper, Middle, and +Lower Oolites face towards the east instead of the west. It is evident, +therefore, that the denuding causes (see Chapter 6) have acted similarly over an +area several hundred miles in diameter, removing the softer clays more +extensively than the limestones, and causing these last to form steep slopes or +escarpments wherever the harder calcareous rock was based upon a more yielding +and destructible formation. + +UPPER OOLITE. + +PURBECK BEDS. + +These strata, which we class as the uppermost member of the Oolite, are of +limited geographical extent in Europe, as already stated, but they acquire +importance when we consider the succession of three distinct sets of fossil +remains which they contain. Such repeated changes in organic life must have +reference to the history of a vast lapse of ages. The Purbeck beds are finely +exposed to view in Durdlestone Bay, near Swanage, Dorsetshire, and at Lulworth +Cove and the neighbouring bays between Weymouth and Swanage. At Meup's Bay, in +particular, Professor E. Forbes examined minutely, in 1850, the organic remains +of this group, displayed in a continuous sea-cliff section, and it appears from +his researches that the Upper, Middle, and Lower Purbecks are each marked by +peculiar species of organic remains, these again being different, so far as a +comparison has yet been instituted, from the fossils of the overlying Hastings +Sands and Weald Clay. + +UPPER PURBECK. + +(FIGURE 299. Cyprides from the Upper Purbeck. +a. Cypris gibbosa, E. Forbes. +b. Cypris tuberculata, E. Forbes. +c. Cypris leguminella, E. Forbes.) + +The highest of the three divisions is purely fresh-water, the strata, about +fifty feet in thickness, containing shells of the genera Paludina, Physa, +Limnaea, Planorbis, Valvata, Cyclas, and Unio, with Cyprides and fish. All the +species seem peculiar, and among these the Cyprides are very abundant and +characteristic (see Figure 299, a, b, c.) + +The stone called "Purbeck Marble," formerly much used in ornamental architecture +in the old English cathedrals of the southern counties, is exclusively procured +from this division. + +MIDDLE PURBECK. + +Next in succession is the Middle Purbeck, about thirty feet thick, the uppermost +part of which consists of fresh-water limestone, with cyprides, turtles, and +fish, of different species from those in the preceding strata. Below the +limestone are brackish-water beds full of Cyrena, and traversed by bands +abounding in Corbula and Melania. These are based on a purely marine deposit, +with Pecten, Modiola, Avicula, and Thracia. Below this, again, come limestones +and shales, partly of brackish and partly of fresh-water origin, in which many +fish, especially species of Lepidotus and Microdon radiatus, are found, and a +crocodilian reptile named Macrorhynchus. Among the mollusks, a remarkable ribbed +Melania, of the section Chilina, occurs. + +(FIGURE 300. Ostrea distorta, Sowerby. Cinder-bed. Middle Purbeck.) + +(FIGURE 301. Hemicidaris Purbeckensis, E. Forbes. Middle Purbeck.) + +(FIGURE 302. Cyprides from the Middle Purbecks. +a. Cypris striato-punctata, E. Forbes. +b. Cypris fasciculata, E. Forbes. +c. Cypris granulata, Sowerby.) + +(FIGURE 303. Physa Bristovii, E. Forbes. Middle Purbeck.) + +Immediately below is a great and conspicuous stratum, twelve feet thick, formed +of a vast accumulation of shells of Ostrea distorta (Figure 300), long familiar +to geologists under the local name of "Cinder-bed." In the uppermost part of +this bed Professor Forbes discovered the first echinoderm (Figure 301) as yet +known in the Purbeck series, a species of Hemicidaris, a genus characteristic of +the Oolitic period, and scarcely, if at all, distinguishable from a previously +known Oolitic fossil. It was accompanied by a species of Perna. Below the +Cinder-bed fresh-water strata are again seen, filled in many places with species +of Cypris (Figure 302, a, b, c), and with Valvata, Paludina, Planorbis, Limnaea, +Physa (Figure 303), and Cyclas, all different from any occurring higher in the +series. It will be seen that Cypris fasciculata (Figure 302, b) has tubercles at +the end only of each valve, a character by which it can be immediately +recognised. In fact, these minute crustaceans, almost as frequent in some of the +shales as plates of mica in a micaceous sandstone, enable geologists at once to +identify the Middle Purbeck in places far from the Dorsetshire cliffs, as, for +example, in the Vale of Wardour in Wiltshire. Thick beds of chert occur in the +Middle Purbeck filled with mollusca and cyprides of the genera already +enumerated, in a beautiful state of preservation, often converted into +chalcedony. Among these Professor Forbes met with gyrogonites (the spore-vessels +of Chara), plants never until 1851 discovered in rocks older than the Eocene. +About twenty feet below the "Cinder-bed" is a stratum two or three inches thick, +in which fossil mammalia presently to be mentioned occur, and beneath this a +thin band of greenish shales, with marine shells and impressions of leaves like +those of a large Zostera, forming the base of the Middle Purbeck. + +FOSSIL MAMMALIA OF THE MIDDLE PURBECK. + +In 1852, after alluding to the discovery of numerous insects and air-breathing +mollusca in the Purbeck strata, I remarked that, although no mammalia had then +been found, "it was too soon to infer their non-existence on mere negative +evidence." (Elements of Geology 4th edition.) Only two years after this remark +was in print, Mr. W.R. Brodie found in the Middle Purbeck, about twenty feet +below the "Cinder-bed" above alluded to, in Durdlestone Bay, portions of several +small jaws with teeth, which Professor Owen recognised as belonging to a small +mammifer of the insectivorous class, more closely allied in its dentition to the +Amphitherium (or Thylacotherium) than to any existing type. + +Four years later (in 1856) the remains of several other species of warm-blooded +quadrupeds were exhumed by Mr. S.H. Beckles, F.R.S., from the same thin bed of +marl near the base of the Middle Purbeck. In this marly stratum many reptiles, +several insects, and some fresh-water shells of the genera Paludina, Planorbis, +and Cyclas, were found. + +Mr. Beckles had determined thoroughly to explore the thin layer of calcareous +mud from which in the suburbs of Swanage the bones of the Spalacotherium had +already been obtained, and in three weeks he brought to light from an area forty +feet long and ten wide, and from a layer the average thickness of which was only +five inches, portions of the skeletons of six new species of mammalia, as +interpreted by Dr. Falconer, who first examined them. Before these interesting +inquiries were brought to a close, the joint labours of Professor Owen and Dr. +Falconer had made it clear that twelve or more species of mammalia characterised +this portion of the Middle Purbeck, most of them insectivorous or predaceous, +varying in size from that of a mole to that of the common polecat, Mustela +putorius. While the majority had the character of insectivorous marsupials, Dr. +Falconer selected one as differing widely from the rest, and pointed out that in +certain characters it was allied to the living Kangaroo-rat, or Hypsiprymnus, +ten species of which now inhabit the prairies and scrub-jungle of Australia, +feeding on plants, and gnawing scratched-up roots. A striking peculiarity of +their dentition, one in which they differ from all other quadrupeds, consists in +their having a single large pre-molar, the enamel of which is furrowed with +vertical grooves, usually seven in number. + +(FIGURE 304. Pre-molar of the recent Australian Hypsiprymnus Gaimardi, showing 7 +grooves, at right angles to the length of the jaw, magnified 3 1/2 diameters.) + +(FIGURE 305. Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii, +magnified 5 1/2 diameters, showing 7 diagonal grooves.) + +(FIGURE 306. Plagiaulex Becklesii, Falconer. Middle Purbeck. Right ramus of +lower jaw, magnified two diameters. +a. Incisor. +b, c. Line of vertical fracture behind the pre-molars. +pm. Three pre-molars, the third and last (much larger than the other two taken +together) being divided by a crack. +m. Sockets of two missing molars.) + +The largest pre-molar (see Figure 305) in the fossil genus exhibits in like +manner seven parallel grooves, producing by their termination a similar serrated +edge in the crown; but their direction is diagonal-- a distinction, says Dr. +Falconer, which is "trivial, not typical." As these oblique furrows form so +marked a character of the majority of the teeth, Dr. Falconer gave to the fossil +the generic name of Plagiaulax. The shape and relative size of the incisor, a, +Figure 306, exhibit a no less striking similarity to Hypsiprymnus. Nevertheless, +the more sudden upward curve of this incisor, as well as other characters of the +jaw, indicate a great deviation in the form of Plagiaulax from that of the +living kangaroo-rats. + +There are two fossil specimens of lower jaws of this genus evidently referable +to two distinct species extremely unequal in size and otherwise distinguishable. +The Plagiaulax Becklesii (Figure 306) was about as big as the English squirrel +or the flying phalanger of Australia (Petaurus Australis, Waterhouse). The +smaller fossil, having only half the linear dimensions of the other, was +probably only one-twelfth of its bulk. It is of peculiar geological interest, +because, as shown by Dr. Falconer, its two back molars bear a decided +resemblance to those of the Triassic Microlestes (Figure 389 Chapter 19), the +most ancient of known mammalia, of which an account will be given in Chapter 21. + +Up to 1857 all the mammalian remains discovered in secondary rocks had consisted +solely of single branches of the lower jaw, but in that year Mr. Beckles +obtained the upper portion of a skull, and on the same slab the lower jaw of +another quadruped with eight molars, a large canine, and a broad and thick +incisor. It has been named Triconodon from its bicuspid teeth, and is supposed +to have been a small insectivorous marsupial, about the size of a hedgehog. +Other jaws have since been found indicating a larger species of the same genus. + +Professor Owen has proposed the name of Galestes for the largest of the mammalia +discovered in 1858 in Purbeck, equalling the polecat (Mustela putorius) in size. +It is supposed to have been predaceous and marsupial. + +Between forty and fifty pieces or sides of lower jaws with teeth have been found +in oolitic strata in Purbeck; only five upper maxillaries, together with one +portion of a separate cranium, occur at Stonesfield, and it is remarkable that +with these there were no examples in Purbeck of an entire skeleton, nor of any +considerable number of bones in juxtaposition. In several portions of the matrix +there were detached bones, often much decomposed, and fragments of others +apparently mammalian; but if all of them were restored, they would scarcely +suffice to complete the five skeletons to which the five upper maxillaries above +alluded to belonged. As the average number of pieces in each mammalian skeleton +is about 250, there must be many thousands of missing bones; and when we +endeavour to account for their absence, we are almost tempted to indulge in +speculations like those once suggested to me by Dr. Buckland, when he tried to +solve the enigma in reference to Stonesfield; "The corpses," he said, "of +drowned animals, when they float in a river, distended by gases during +putrefaction, have often their lower jaw hanging loose, and sometimes it has +dropped off. The rest of the body may then be drifted elsewhere, and sometimes +may be swallowed entire by a predaceous reptile or fish, such as an ichthyosaur +or a shark." + +As all the above-mentioned Purbeck marsupials, belonging to eight or nine genera +and to about fourteen species, insectivorous, predaceous, and herbivorous, have +been obtained from an area less than 500 square yards in extent, and from a +single stratum no more than a few inches thick, we may safely conclude that the +whole lived together in the same region, and in all likelihood they constituted +a mere fraction of the mammalia which inhabited the lands drained by one river +and its tributaries. They afford the first positive proof as yet obtained of the +co-existence of a varied fauna of the highest class of vertebrata with that +ample development of reptile life which marks all the periods from the Trias to +the Lower Cretaceous inclusive, and with a gymnospermous flora, or that state of +the vegetable kingdom when cycads and conifers predominated over all kinds of +plants, except the ferns, so far, at least, as our present imperfect knowledge +of fossil botany entitles us to speak. + +TABLE 19.2. NUMBER AND DISTRIBUTION OF ALL THE KNOWN SPECIES OF FOSSIL MAMMALIA +FROM STRATA OLDER THAN THE PARIS GYPSUM, OR THAN THE BEMBRIDGE SERIES OF THE +ISLE OF WIGHT. + +TERTIARY: + +Headon Series and beds between the Paris Gypsum and the Gres de Beauchamp: 14: +10 English, 4 French. + +Barton Clay and Sables de Beauchamp: 0. + +Bagshot Beds, Calcaire Grossier, and Upper Soissonnais of Cuisse-Lamotte: 20: 16 +French, 1 English, 3 United States (I allude to several Zeuglodons found in +Alabama, and referred by some zoologists to three species.) + +London Clay, including the Kyson Sand: 7 English. + +Plastic Clay and Lignite: 9: 7 French, 2 English. + +Sables de Bracheux: 1 French. + +Thanet Sands and Lower Landenian of Belgium: 0. + +SECONDARY: + +Maestricht Chalk: 0. + +White Chalk: 0. + +Chalk Marl: 0. + +Chloritic Series (Upper Greensand): 0. + +Gault: 0. + +Neocomian (Lower Greensand): 0. + +Wealden: 0. + +Upper Purbeck Oolite : 0. + +Middle Purbeck Oolite : 14 Swanage. + +Lower Purbeck Oolite: 0. + +Portland Oolite: 0. + +Kimmeridge Clay: 0. + +Coral Rag: 0. + +Oxford Clay: 0. + +Great Oolite: 4 Stonesfield. + +Inferior Oolite: 0. + +Lias: 0. + +Upper Trias: 4 Wurtemberg, Somersetshire. N. Carolina. + +Middle Trias: 0. + +Lower Trias: 0. + +PRIMARY. + +Permian: 0. + +Carboniferous : 0. + +Devonian: 0. + +Silurian: 0. + +Cambrian: 0. + +Laruentian: 0. + +Table 19.2 will enable the reader to see at a glance how conspicuous a part, +numerically considered, the mammalian species of the Middle Purbeck now play +when compared with those of other formations more ancient than the Paris gypsum, +and, at the same time, it will help him to appreciate the enormous hiatus in the +history of fossil mammalia which at present occurs between the Eocene and +Purbeck periods, and between the latter and the Stonesfield Oolite, and between +this again and the Trias. + +The Sables de Bracheux, enumerated in the Tertiary division of the table, +supposed by Mr. Prestwich to be somewhat newer than the Thanet Sands, and by M. +Hebert to be of about that age, have yielded at La Fere the Arctocyon +(Palaeocyon) primaevus, the oldest known tertiary mammal. + +It is worthy of notice, that in the Hastings Sands there are certain layers of +clay and sandstone in which numerous footprints of quadrupeds have been found by +Mr. Beckles, and traced by him in the same set of rocks through Sussex and the +Isle of Wight. They appear to belong to three or four species of reptiles, and +no one of them to any warm-blooded quadruped. They ought, therefore, to serve as +a warning to us, when we fail in like manner to detect mammalian footprints in +older rocks (such as the New Red Sandstone), to refrain from inferring that +quadrupeds, other than reptilian, did not exist or pre-exist. + +But the most instructive lesson read to us by the Purbeck strata consists in +this: They are all, with the exception of a few intercalated brackish and marine +layers, of fresh-water origin; they are 160 feet in thickness, have been well +searched by skillful collectors, and by the late Edward Forbes in particular, +who studied them for months consecutively. They have been numbered, and the +contents of each stratum recorded separately, by the officers of the Geological +Survey of Great Britain. They have been divided into three distinct groups by +Forbes, each characterised by the same genera of pulmoniferous mollusca and +cyprides, these genera being represented in each group by different species; +they have yielded insects of many orders, and the fruits of several plants; and +lastly, they contain "dirt-beds," or old terrestrial surfaces and vegetable +soils at different levels, in some of which erect trunks and stumps of cycads +and conifers, with their roots still attached to them, are preserved. Yet when +the geologist inquires if any land-animals of a higher grade than reptiles lived +during any one of these three periods, the rocks are all silent, save one thin +layer a few inches in thickness; and this single page of the earth's history has +suddenly revealed to us in a few weeks the memorials of so many species of +fossil mammalia, that they already outnumber those of many a subdivision of the +tertiary series, and far surpass those of all the other secondary rocks put +together! + +LOWER PURBECK. + +(FIGURE 307. Cyprides from the Lower Purbeck. +a. Cypris Purbeckensis, Forbes. +b. Same magnified. +c. Cypris punctata, Forbes. +d, e. Two views magnified of the same.) + +Beneath the thin marine band mentioned above as the base of the Middle Purbeck, +some purely fresh-water marls occur, containing species of Cypris (Figure 307 a, +c), Valvata, and Limnaea, different from those of the Middle Purbeck. This is +the beginning of the inferior division, which is about 80 feet thick. Below the +marls are seen, at Meup's Bay, more than thirty feet of brackish-water strata, +abounding in a species of Serpula, allied to, if not identical with, Serpula +coacervites, found in beds of the same age in Hanover. There are also shells of +the genus Rissoa (of the subgenus Hydrobia), and a little Cardium of the +subgenus Protocardium, in these marine beds, together with Cypris. Some of the +cypris-bearing shales are strangely contorted and broken up, at the west end of +the Isle of Purbeck. The great dirt-bed or vegetable soil containing the roots +and stools of Cycadeae, which I shall presently describe, underlies these marls, +and rests upon the lowest fresh-water limestone, a rock about eight feet thick, +containing Cyclas, Valvata, and Limnaea, of the same species as those of the +uppermost part of the Lower Purbeck, or above the dirt-bed. The fresh-water +limestone in its turn rests upon the top beds of the Portland stone, which, +although it contains purely marine remains, often consists of a rock +undistinguishable in mineral character from the Lowest Purbeck limestone. + +DIRT-BED OR ANCIENT SURFACE-SOIL. + +(FIGURE 308. Mantellia nidiformis, Brongniart. The upper part shows the woody +stem, the lower part the bases of the leaves.) + +The most remarkable of all the varied succession of beds enumerated in the above +list is that called by the quarrymen "the dirt," or "black dirt," which was +evidently an ancient vegetable soil. It is from 12 to 18 inches thick, is of a +dark brown or black colour, and contains a large proportion of earthy lignite. +Through it are dispersed rounded and sub-angular fragments of stone, from 3 to 9 +inches in diameter, in such numbers that it almost deserves the name of gravel. +I also saw in 1866, in Portland, a smaller dirt-bed six feet below the principal +one, six inches thick, consisting of brown earth with upright Cycads of the same +species, Mantellia nidiformis, as those found in the upper bed, but no +Coniferae. The weight of the incumbent strata squeezing down the compressible +dirt-bed has caused the Cycads to assume that form which has led the quarrymen +to call them "petrified birds' nests," which suggested to Brongniart the +specific name of nidiformis. I am indebted to Mr. Carruthers for Figure 308 of +one of these Purbeck specimens, in which the original cylindrical figure has +been less distorted than usual by pressure. + +Many silicified trunks of coniferous trees, and the remains of plants allied to +Zamia and Cycas, are buried in this dirt-bed, and must have become fossil on the +spots where they grew. The stumps of the trees stand erect for a height of from +one to three feet, and even in one instance to six feet, with their roots +attached to the soil at about the same distances from one another as the trees +in a modern forest. The carbonaceous matter is most abundant immediately around +the stumps, and round the remains of fossil Cycadeae. + +(FIGURE 309. Section in Isle of Portland, Dorset. (Buckland and De la +Beche.)showing layers (from top to bottom): Fresh-water calcareous slate: Dirt- +bed and ancient forest: Lowest fresh-water beds of the Lower Purbeck: and +Portland stone, marine.) + +Besides the upright stumps above mentioned, the dirt-bed contains the stems of +silicified trees laid prostrate. These are partly sunk into the black earth, and +partly enveloped by a calcareous slate which covers the dirt-bed. The fragments +of the prostrate trees are rarely more than three or four feet in length; but by +joining many of them together, trunks have been restored, having a length from +the root to the branches of from 20 to 23 feet, the stems being undivided for 17 +or 20 feet, and then forked. The diameter of these near the root is about one +foot; but I measured one myself, in 1866, which was 3 1/2 feet in diameter, said +by the quarrymen to be unusually large. Root-shaped cavities were observed by +Professor Henslow to descend from the bottom of the dirt-bed into the subjacent +fresh-water stone, which, though now solid, must have been in a soft and +penetrable state when the trees grew. The thin layers of calcareous slate +(Figure 309) were evidently deposited tranquilly, and would have been horizontal +but for the protrusion of the stumps of the trees, around the top of each of +which they form hemispherical concretions. + +(FIGURE 310. Section of cliff east of Lulworth Cove. (Buckland and De la Beche.) +showing layers (from top to bottom): Fresh-water calcareous slate: Dirt-bed, +with stools of trees: Fresh-water: Portland stone, marine.) + +The dirt-bed is by no means confined to the island of Portland, where it has +been most carefully studied, but is seen in the same relative position in the +cliffs east of Lulworth Cove, in Dorsetshire, where, as the strata have been +disturbed, and are now inclined at an angle of 45 degrees, the stumps of the +trees are also inclined at the same angle in an opposite direction-- a beautiful +illustration of a change in the position of beds originally horizontal (see +Figure 310). + +From the facts above described we may infer, first, that those beds of the Upper +Oolite, called "the Portland," which are full of marine shells, were overspread +with fluviatile mud, which became dry land, and covered by a forest, throughout +a portion of the space now occupied by the south of England, the climate being +such as to permit the growth of the Zamia and Cycas. Secondly. This land at +length sank down and was submerged with its forests beneath a body of fresh- +water, from which sediment was thrown down enveloping fluviatile shells. +Thirdly. The regular and uniform preservation of this thin bed of black earth +over a distance of many miles, shows that the change from dry land to the state +of a fresh-water lake or estuary, was not accompanied by any violent denudation, +or rush of water, since the loose black earth, together with the trees which lay +prostrate on its surface, must inevitably have been swept away had any such +violent catastrophe taken place. + +The forest of the dirt-bed, as before hinted, was not everywhere the first +vegetation which grew in this region. Besides the lower bed containing upright +Cycadeae, before mentioned, another has sometimes been found above it, which +implies oscillations in the level of the same ground, and its alternate +occupation by land and water more than once. + +SUBDIVISIONS OF THE PURBECK. + +It will be observed that the division of the Purbecks into upper, middle, and +lower, was made by Professor Forbes strictly on the principle of the entire +distinctness of the species of organic remains which they include. The lines of +demarkation are not lines of disturbance, nor indicated by any striking physical +characters or mineral changes. The features which attract the eye in the +Purbecks, such as the dirt-beds, the dislocated strata at Lulworth, and the +Cinder-bed, do not indicate any breaks in the distribution of organised beings. +"The causes which led to a complete change of life three times during the +deposition of the fresh-water and brackish strata must," says this naturalist, +"be sought for, not simply in either a rapid or a sudden change of their area +into land or sea, but in the great lapse of time which intervened between the +epochs of deposition at certain periods during their formation." + +Each dirt-bed may, no doubt, be the memorial of many thousand years or +centuries, because we find that two or three feet of vegetable soil is the only +monument which many a tropical forest has left of its existence ever since the +ground on which it now stands was first covered with its shade. Yet, even if we +imagine the fossil soils of the Lower Purbeck to represent as many ages, we need +not be surprised to find that they do not constitute lines of separation between +strata characterised by different zoological types. The preservation of a layer +of vegetable soil, when in the act of being submerged, must be regarded as a +rare exception to a general rule. It is of so perishable a nature, that it must +usually be carried away by the denuding waves or currents of the sea, or by a +river; and many Purbeck dirt-beds were probably formed in succession and +annihilated, besides those few which now remain. + +The plants of the Purbeck beds, so far as our knowledge extends at present, +consist chiefly of Ferns, Coniferae, and Cycadeae (Figure 308), without any +angiosperms; the whole more allied to the Oolitic than to the Cretaceous +vegetation. The same affinity is indicated by the vertebrate and invertebrate +animals. Mr. Brodie has found the remains of beetles and several insects of the +homopterous and trichopterous orders, some of which now live on plants, while +others are of such forms as hover over the surface of our present rivers. + +PORTLAND OOLITE AND SAND (b, TABLE 19.1). + +(FIGURE 311. Cerithium Portlandicum (=Terebra) Sowerby. +a. Cast of shell known as "Portland screw." +b. The shell itself. ) + +(FIGURE 312. Isastraea oblonga, M. Edw. and J. Haime. As seen on a polished +slab of chert from the Portland Sand, Tisbury.) + +(FIGURE 313. Trigonia gibbosa. 1/2 natural size. Portland Stone, Tisbury. +a. The hinge.) + +(FIGURE 314. Cardium dissimile. 1/4 natural size. Portland Stone.) + +(FIGURE 315. Ostrea expansa. Portland Sand.) + +The Portland Oolite has already been mentioned as forming in Dorsetshire the +foundation on which the fresh-water limestone of the Lower Purbeck reposes (see +above). It supplies the well-known building-stone of which St. Paul's and so +many of the principal edifices of London are constructed. About fifty species of +mollusca occur in this formation, among which are some ammonites of large size. +The cast of a spiral univalve called by the quarrymen the "Portland screw" (a, +Figure 311), is common; the shell of the same (b) being rarely met with. Also +Trigonia gibbosa (Figure 313) and Cardium dissimile (Figure 314). This upper +member rests on a dense bed of sand, called the Portland Sand, containing +similar marine fossils, below which is the Kimmeridge Clay. In England these +Upper Oolite formations are almost wholly confined to the southern counties. But +some fragments of them occur beneath the Neocomian or Speeton Clay on the coast +of Yorkshire, containing many more fossils common to the Portlandian of the +Continent than does the same formation in Dorsetshire. Corals are rare in this +formation, although one species is found plentifully at Tisbury, Wiltshire, in +the Portland Sand, converted into flint and chert, the original calcareous +matter being replaced by silex (Figure 312). + +KIMMERIDGE CLAY. + +The Kimmeridge Clay consists, in great part, of a bituminous shale, sometimes +forming an impure coal, several hundred feet in thickness. In some places in +Wiltshire it much resembles peat; and the bituminous matter may have been, in +part at least, derived from the decomposition of vegetables. But as impressions +of plants are rare in these shales, which contain ammonites, oysters, and other +marine shells, with skeletons of fish and saurians, the bitumen may perhaps be +of animal origin. Some of the saurians (Pliosaurus) in Dorsetshire are among the +most gigantic of their kind. + +(FIGURE 316. Cardium striatulum. Kimmeridge Clay, Hartwell.) + +(FIGURE 317. Ostrea deltoidea. Kimmeridge Clay, 1/4 natural size.) + +(FIGURE 318. Gryphaea (Exogyra) virgula. Kimmeridge Clay.) + +(FIGURE 319. Trigonellites latus, Park, Kimmeridge Clay.) + +Among the fossils, amounting to nearly 100 species, may be mentioned Cardium +striatulum (Figure 316) and Ostrea deltoidea (Figure 317), the latter found in +the Kimmeridge Clay throughout England and the north of France, and also in +Scotland, near Brora. The Gryphaea virgula (Figure 318), also met with in the +Kimmeridge Clay near Oxford, is so abundant in the Upper Oolite of parts of +France as to have caused the deposit to be termed "marnes a gryphees virgules." +Near Clermont, in Argonne, a few leagues from St. Menehould, where these +indurated marls crop out from beneath the Gault, I have seen them, on +decomposing, leave the surface of every ploughed field literally strewed over +with this fossil oyster. The Trigonellites latus (Aptychus of some +authors)(Figure 319) is also widely dispersed through this clay. The real nature +of the shell, of which there are many species in oolitic rocks, is still a +matter of conjecture. Some are of opinion that the two plates have been the +gizzard of a cephalopod; others, that it may have formed a bivalve operculum of +the same. + +SOLENHOFEN STONE. + +(FIGURE 320. Skeleton of Pterodactylus crassirostris. Oolite of Pappenheim, near +Solenhofen. +a. This bone, consisting of four joints, is part of the fifth or outermost digit +elongated, as in bats, for the support of a wing.) + +The celebrated lithographic stone of Solenhofen in Bavaria, appears to be of +intermediate age between the Kimmeridge clay and the Coral Rag, presently to be +described. It affords a remarkable example of the variety of fossils which may +be preserved under favourable circumstances, and what delicate impressions of +the tender parts of certain animals and plants may be retained where the +sediment is of extreme fineness. Although the number of testacea in this slate +is small, and the plants few, and those all marine, count Munster had determined +no less than 237 species of fossils when I saw his collection in 1833; and among +them no less than seven SPECIES of flying reptiles or pterodactyls (see Figure +320), six saurians, three tortoises, sixty species of fish, forty-six of +crustacea, and twenty-six of insects. These insects, among which is a libellula, +or dragon-fly, must have been blown out to sea, probably from the same land to +which the pterodactyls, and other contemporaneous air-breathers, resorted. + +(FIGURE 321. Tail and feather of Archaeopteryx, from Solenhofen, and tail of +living bird for comparison. +A. Caudal vertebrae of Archaeopteryx macrura, Owen; with impression of tail- +feathers; one-fifth natural size. +B. Two caudal vertebrae of same; natural size. +C. Single feather, found in 1861 at Solenhofen, by Von Meyer, and called +Archaeopteryx lithographica; natural size. +D. Tail of recent vulture (Gyps Bengalensis) showing attachment of tail-feathers +in living birds; one-quarter natural size. +E. Profile of caudal vertebrae of same; one-third natural size. +e, e. Direction of tail-feathers when seen in profile. +f. Ploughshare bone or broad terminal joint (seen also in f, D.)) + +In the same slate of Solenhofen a fine example was met with in 1862 of the +skeleton of a bird almost entire, and retaining even its feathers so perfect +that the vanes as well as the shaft are preserved. The head was at first +supposed to be wanting, but Mr. Evans detected on the slab what seems to be the +impression of the cranium and beak, much resembling in size and shape that of +the jay or woodcock. This valuable specimen is now in the British Museum, and +has been called by Professor Owen Archaeopteryx macrura. Although anatomists +agree that it is a true bird, yet they also find that in the length of the bones +of the tail, and some other minor points of its anatomy, it approaches more +nearly to reptiles than any known living bird. In the living representatives of +the class Aves, the tail-feathers are attached to a coccygian bone, consisting +of several vertebrae united together, whereas in the Archaeopteryx the tail is +composed of twenty vertebrae, each of which supports a pair of quill-feathers. +The first five only of the vertebrae, as seen in A, have transverse processes, +the fifteen remaining ones become gradually longer and more tapering. The +feathers diverge outward from them at an angle of 45 degrees. + +Professor Huxley in his late memoirs on the order of reptiles called +Dinosaurians, which are largely represented in all the formations, from the +Neocomian to the Trias inclusive, has shown that they present in their structure +many remarkable affinities to birds. But a reptile about two feet long, called +Compsognathus, lately found in the Stonesfield slate, makes a much greater +approximation to the class Aves than any Dinosaur, and therefore forms a closer +link between the classes Aves and Reptilia than does the Archaeopteryx. + +It appears doubtful whether any species of British fossil, whether of the +vertebrate or invertebrate class, is common to the Oolite and Chalk. But there +is no similar break or discordance as we proceed downward, and pass from one to +another of the several leading members of the Jurassic group, the Upper, Middle, +and Lower Oolite, and the Lias, there being often a considerable proportion of +the mollusca, sometimes as much as a fourth, common to such divisions as the +Upper and Middle Oolite. + +MIDDLE OOLITE. + +CORAL RAG. + +(FIGURE 322. Thecosmilia annularis, Milne Edwards and J. Haime. Coral Rag, +Steeple Ashton.) + +(FIGURE 323. Thamnastraea. Coral Rag. Steeple Ashton.) + +(FIGURE 324. Ostrea gregaria, Coral Rag, Steeple Ashton.) + +One of the limestones of the Middle Oolite has been called the "Coral Rag," +because it consists, in part, of continuous beds of petrified corals, most of +them retaining the position in which they grew at the bottom of the sea. In +their forms they more frequently resemble the reef-building polyparia of the +Pacific than do the corals of any other member of the Oolite. They belong +chiefly to the genera Thecosmilia (Figure 322), Protoseris, and Thamnastraea, +and sometimes form masses of coral fifteen feet thick. In Figure 323 of a +Thamnastraea from this formation, it will be seen that the cup-shaped cavities +are deepest on the right-hand side, and that they grow more and more shallow, +until those on the left side are nearly filled up. The last-mentioned stars are +supposed to represent a perfected condition, and the others an immature state. +These coralline strata extend through the calcareous hills of the north-west of +Berkshire, and north of Wilts, and again recur in Yorkshire, near Scarborough. +The Ostrea gregarea (Figure 324) is very characteristic of the formation in +England and on the Continent. + +(FIGURE 325. Nerinaea Goodhallii, Fitton. Coral Rag, Weymouth. 1/4 natural +size.) + +One of the limestones of the Jura, referred to the age of the English Coral Rag, +has been called "Nerinaean limestone" (Calcaire a Nerinees) by M. Thirria; +Nerinaea being an extinct genus of univalve shells (Figure 325) much resembling +the Cerithium in external form. Figure 325 shows the curious and continuous +ridges on the columnella and whorls. + +OXFORD CLAY. + +(FIGURE 326. Belemnites hastatus. Oxford Clay.) + +(FIGURE 327. Ammonites Jason, Reinecke. (Syn. A. Elizabethae, Pratt. Oxford +Clay, Christian Malford, Wiltshire.) + +(FIGURE 328. Belemnites Puzosianus, d'Orbigny. B. Owenii, Pierce. Oxford Clay, +Christian Malford, Wiltshire. +a. Section of the shell projecting from the phragmacone. +b-c. External covering to the ink-bag and phragmacone. +c, d. Osselet, or that portion commonly called the belemnite. +e. Conical chambered body called the phragmacone. +f. Position of ink-bag beneath the shelly covering.) + +The coralline limestone, or "Coral Rag," above described, and the accompanying +sandy beds, called "calcareous grits," of the Middle Oolite, rest on a thick bed +of clay, called the "Oxford Clay," sometimes not less than 600 feet thick. In +this there are no corals, but great abundance of cephalopoda, of the genera +Ammonite and Belemnite (Figures 326 and 327). In some of the finely laminated +clays ammonites are very perfect, although somewhat compressed, and are +frequently found with the lateral lobe extended on each side of the opening of +the mouth into a horn-like projection (Figure 327). These were discovered in the +cuttings of the Great Western Railway, near Chippenham, in 1841, and have been +described by Mr. Pratt (Annals of Natural History November 1841). + +Similar elongated processes have been also observed to extend from the shells of +some Belemnites discovered by Dr. Mantell in the same clay (see Figure 328), +who, by the aid of this and other specimens, has been able to throw much light +on the structure of singular extinct forms of cuttle-fish. (See Philosophical +Transactions 1850 page 363; also Huxley Memoirs of Geological Survey 1864; +Phillips Palaeontological Society.) + +KELLOWAY ROCK. + +The arenaceous limestone which passes under this name is generally grouped as a +member of the Oxford clay, in which it forms, in the south-west of England, +lenticular masses, 8 or 10 feet thick, containing at Kelloway, in Wiltshire, +numerous casts of ammonites and other shells. But in Yorkshire this calcareo- +arenaceous formation thickens to about 30 feet, and constitutes the lower part +of the Middle Oolite, extending inland from Scarborough in a southerly +direction. The number of mollusca which it contains is, according to Mr. +Etheridge, 143, of which only 34, or 23 1/2 per cent, are common to the Oxford +clay proper. Of the 52 Cephalopoda, 15 (namely 13 species of ammonite, the +Ancyloceras Calloviense and one Belemnite) are common to the Oxford Clay, giving +a proportion of nearly 30 per cent. + +LOWER OOLITE. + +CORNBRASH AND FOREST MARBLE. + +The upper division of this series, which is more extensive than the preceding or +Middle Oolite, is called in England the Cornbrash, as being a brashy, easily +broken rock, good for corn land. It consists of clays and calcareous sandstones, +which pass downward into the Forest Marble, an argillaceous limestone, abounding +in marine fossils. In some places, as at Bradford, this limestone is replaced by +a mass of clay. The sandstones of the Forest Marble of Wiltshire are often +ripple-marked and filled with fragments of broken shells and pieces of drift- +wood, having evidently been formed on a coast. Rippled slabs of fissile oolite +are used for roofing, and have been traced over a broad band of country from +Bradford in Wilts, to Tetbury in Gloucestershire. These calcareous tile-stones +are separated from each other by thin seams of clay, which have been deposited +upon them, and have taken their form, preserving the undulating ridges and +furrows of the sand in such complete integrity, that the impressions of small +footsteps, apparently of crustaceans, which walked over the soft wet sands, are +still visible. In the same stone the claws of crabs, fragments of echini, and +other signs of a neighbouring beach, are observed. (P. Scrope Proceedings of the +Geological Society March 1831.) + +GREAT (OR BATH) OOLITE. + +(FIGURE 329. Eunomia radiata, Lamouroux. (Calamophyllia, Milne Edwards.) +a. Section transverse to the tubes. +b. Vertical section, showing the radiation of the tubes. +c. Portion of interior of tubes magnified, showing striated surface.) + +Although the name of Coral Rag has been appropriated, as we have seen, to a +member of the Middle Oolite before described, some portions of the Lower Oolite +are equally entitled in many places to be called coralline limestones. Thus the +Great Oolite near Bath contains various corals, among which the Eunomia radiata +(Figure 329) is very conspicuous, single individuals forming masses several feet +in diameter; and having probably required, like the large existing brain-coral +(Meandrina) of the tropics, many centuries before their growth was completed. + +(FIGURE 330. Apiocrinites rotundus, or Pear Encrinite; Miller. Fossil at +Bradford, Wilts. +a. Stem of Apiocrinites, and one of the articulations, natural size. +b. Section at Bradford of Great Oolite and overlying clay, containing the fossil +encrinites. (See text.) +c. Three perfect individuals of Apiocrinites, represented as they grew on the +surface of the Great Oolite. +d. Body of the Apiocrinites rotundus. Half natural size.) + +(FIGURE 331. Apiocrinus. +a. Single plate of body of Apiocrinus, overgrown with serpulae and bryozoa. +Natural size. Bradford Clay. +b. Portion of the same magnified, showing the bryozoan Diastopora diluviana +covering one of the serpulae.) + +Different species of crinoids, or stone-lilies, are also common in the same +rocks with corals; and, like them, must have enjoyed a firm bottom, where their +base of attachment remained undisturbed for years (c, Figure 330). Such fossils, +therefore, are almost confined to the limestones; but an exception occurs at +Bradford, near Bath, where they are enveloped in clay sometimes 60 feet thick. +In this case, however, it appears that the solid upper surface of the "Great +Oolite" had supported, for a time, a thick submarine forest of these beautiful +zoophytes, until the clear and still water was invaded by a current charged with +mud, which threw down the stone-lilies, and broke most of their stems short off +near the point of attachment. The stumps still remain in their original +position; but the numerous articulations, once composing the stem, arms, and +body of the encrinite, were scattered at random through the argillaceous deposit +in which some now lie prostrate. These appearances are represented in the +section b, Figure 330, where the darker strata represent the Bradford clay, +which is however a formation of such local development that in many places it +can not easily be separated from the clays of the overlying "forest-marble" and +underlying "fuller's earth." The upper surface of the calcareous stone below is +completely incrusted over with a continuous pavement, formed by the stony roots +or attachments of the Crinoidea; and besides this evidence of the length of time +they had lived on the spot, we find great numbers of single joints, or circular +plates of the stem and body of the encrinite, covered over with serpulae. Now +these serpulae could only have begun to grow after the death of some of the +stone-lilies, parts of whose skeletons had been strewed over the floor of the +ocean before the irruption of argillaceous mud. In some instances we find that, +after the parasitic serpulae were full grown, they had become incrusted over +with a bryozoan, called Diastopora diluviana (see b, Figure 331); and many +generations of these molluscoids had succeeded each other in the pure water +before they became fossil. + +We may, therefore, perceive distinctly that, as the pines and cycadeous plants +of the ancient "dirt-bed," or fossil forest, of the Lower Purbeck were killed by +submergence under fresh water, and soon buried beneath muddy sediment, so an +invasion of argillaceous matter put a sudden stop to the growth of the Bradford +Encrinites, and led to their preservation in marine strata. + +Such differences in the fossils as distinguish the calcareous and argillaceous +deposits from each other, would be described by naturalists as arising out of a +difference in the STATIONS of species; but besides these, there are variations +in the fossils of the higher, middle, and lower part of the oolitic series, +which must be ascribed to that great law of change in organic life by which +distinct assemblages of species have been adapted, at successive geological +periods, to the varying conditions of the habitable surface. In a single +district it is difficult to decide how far the limitation of species to certain +minor formations has been due to the local influence of STATIONS, or how far it +has been caused by time or the law of variation above alluded to. But we +recognise the reality of the last-mentioned influence, when we contrast the +whole oolitic series of England with that of parts of the Jura, Alps, and other +distant regions, where, although there is scarcely any lithological resemblance, +yet some of the same fossils remain peculiar in each country to the Upper, +Middle, and Lower Oolite formations respectively. Mr. Thurmann has shown how +remarkably this fact holds true in the Bernese Jura, although the argillaceous +divisions, so conspicuous in England, are feebly represented there, and some +entirely wanting. + +(FIGURE 332. Terebratula digona, Sowerby. Natural size. Bradford Clay.) + +(FIGURE 333. Purpuroidea nodulata. One-fourth natural size. Great Oolite, +Minchinhampton.) + +(FIGURE 334. Cylindrites acutus. Sowb. Syn. Actaeon acutus. Great Oolite, +Minchinhampton.) + +(FIGURE 335. Patella rugosa, Sowerby. Great Oolite.) + +(FIGURE 336. Nerita costulata, Desh. Great Oolite.) + +(FIGURE 337. Rimula (Emarginula) clathrata, Sowerby. Great Oolite.) + +The calcareous portion of the Great Oolite consists of several shelly +limestones, one of which, called the Bath Oolite, is much celebrated as a +building-stone. In parts of Gloucestershire, especially near Minchinhampton, the +Great Oolite, says Mr. Lycett, "must have been deposited in a shallow sea, where +strong currents prevailed, for there are frequent changes in the mineral +character of the deposit, and some beds exhibit false stratification. In others, +heaps of broken shells are mingled with pebbles of rocks foreign to the +neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood, +and crabs' claws. The shelly strata, also, have occasionally suffered +denudation, and the removed portions have been replaced by clay." In such +shallow-water beds shells of the genera Patella, Nerita, Rimula, Cylindrites are +common (see Figures 334 to 337); while cephalopods are rare, and instead of +ammonites and belemnites, numerous genera of carnivorous trachelipods appear. +Out of 224 species of univalves obtained from the Minchinhampton beds, Mr. +Lycett found no less than 50 to be carnivorous. They belong principally to the +genera Buccinum, Pleurotoma, Rostellaria, Murex, Purpuroidea (Figure 333), and +Fusus, and exhibit a proportion of zoophagous species not very different from +that which obtains in seas of the Recent period. These zoological results are +curious and unexpected, since it was imagined that we might look in vain for the +carnivorous trachelipods in rocks of such high antiquity as the Great Oolite, +and it was a received doctrine that they did not begin to appear in considerable +numbers till the Eocene period, when those two great families of cephalopoda, +the ammonites and belemnites, and a great number of other representatives of the +same class of chambered shells, had become extinct. + +STONESFIELD SLATE: MAMMALIA. + +(FIGURE 338. Elytron of Buprestis? Stonesfield.) + +The slate of Stonesfield has been shown by Mr. Lonsdale to lie at the base of +the Great Oolite. (Proceedings of the Geological Society volume 1 page 414.) It +is a slightly oolitic shelly limestone, forming large lenticular masses imbedded +in sand only six feet thick, but very rich in organic remains. It contains some +pebbles of a rock very similar to itself, and which may be portions of the +deposit, broken up on a shore at low water or during storms, and redeposited. +The remains of belemnites, trigoniae, and other marine shells, with fragments of +wood, are common, and impressions of ferns, cycadeae, and other plants. Several +insects, also, and, among the rest, the elytra or wing-covers of beetles, are +perfectly preserved (see Figure 338), some of them approaching nearly to the +genus Buprestis. The remains, also, of many genera of reptiles, such as +Plesiosaur, Crocodile, and Pterodactyl, have been discovered in the same +limestone. + +But the remarkable fossils for which the Stonesfield slate is most celebrated +are those referred to the mammiferous class. The student should be reminded that +in all the rocks described in the preceding chapters as older than the Eocene, +no bones of any land-quadruped, or of any cetacean, had been discovered until +the Spalacotherium of the Purbeck beds came to light in 1854. Yet we have seen +that terrestrial plants were not wanting in the Upper Cretaceous formation (see +Chapter 17), and that in the Wealden there was evidence of fresh-water sediment +on a large scale, containing various plants, and even ancient vegetable soils. +We had also in the same Wealden many land-reptiles and winged insects, which +render the absence of terrestrial quadrupeds the more striking. The want, +however, of any bones of whales, seals, dolphins, and other aquatic mammalia, +whether in the chalk or in the upper or middle oolite, is certainly still more +remarkable. + +These observations are made to prepare the reader to appreciate more justly the +interest felt by every geologist in the discovery in the Stonesfield slate of no +less than ten specimens of lower jaws of mammiferous quadrupeds, belonging to +four different species and to three distinct genera, for which the names of +Amphitherium, Phascolotherium, and Stereognathus have been adopted. + +(FIGURE 339. Tupaia tana. Right ramus of lower jaw. Natural size. A recent +insectivorous placental mammal, from Sumatra.) + +(Figures 340 and 341. Part of lower jaw of Tupaia tana. Twice natural size. + +(FIGURE 340. End view seen from behind, showing the very slight inflection of +the angle at c.) + +(FIGURE 341. Side view of same.)) + +(Figures 342 and 343. Part of lower jaw of Didelphys Azarae; recent, Brazil. +Natural size. + +(FIGURE 342. End view seen from behind, showing the inflection of the angle of +the jaw, c, d.) + +(FIGURE 343. Side view of same.)) + +(FIGURE 344. Amphitherium Prevostii, Cuvier sp. Stonesfield Slate. Syn. +Thylacotherium Prevostii, Valenc. +a. Coronoid process. +b. Condyle. +c. Angle of jaw. +d. Double-fanged molars.) + +(FIGURE 345. Amphitheriumm Broderipii, Owen. Natural size. Stonesfield Slate.) + +It is now generally admitted that these or really the remains of mammalia +(although it was at first suggested that they might be reptiles), and the only +question open to controversy is limited to this point, whether the fossil +mammalia found in the Lower Oolite of Oxfordshire ought to be referred to the +marsupial quadrupeds, or to the ordinary placental series. Cuvier had long ago +pointed out a peculiarity in the form of the angular process (c, Figures 342 and +343) of the lower jaw, as a character of the genus Didelphys; and Professor Owen +has since confirmed the doctrine of its generality in the entire marsupial +series. In all these pouched quadrupeds this process is turned inward, as at c, +d, Figure 342, in the Brazilian opossum, whereas in the placental series, as at +c, Figures 340 and 341, there is an almost entire absence of such inflection. +The Tupaia Tana of Sumatra has been selected by Mr. Waterhouse for this +illustration, because the jaws of that small insectivorous quadruped bear a +great resemblance to those of the Stonesfield Amphitherium. By clearing away the +matrix from the specimen of Amphitherium Prevostii here represented (Figure +344), Professor Owen ascertained that the angular process (c) bent inward in a +slighter degree than in any of the known marsupialia; in short, the inflection +does not exceed that of the mole or hedgehog. This fact made him doubt whether +the Amphitherium might not be an insectivorous placental, although it offered +some points of approximation in its osteology to the marsupials, especially to +the Myrmecobius, a small insectivorous quadruped of Australia, which has nine +molars on each side of the lower jaw, besides a canine and three incisors. (A +figure of this recent Myrmecobius will be found in my Principles of Geology +chapter 9.) Another species of Amphitherium has been found at Stonesfield +(Figure 345), which differs from the former (Figure 344) principally in being +larger. + +(FIGURE 346. Phascolotherium Bucklandi, Broderip, sp. +a. Natural size. +b. Molar of same, magnified.) + +The second mammiferous genus discovered in the same slates was named originally +by Mr. Broderip Didelphys Bucklandi (see Figure 346), and has since been called +Phascolotherium by Owen. It manifests a much stronger likeness to the marsupials +in the general form of the jaw, and in the extent and position of its inflected +angle, while the agreement with the living genus Didelphys in the number of the +pre-molar and molar teeth is complete. (Owen's British Fossil Mammals page 62.) + +In 1854 the remains of another mammifer, small in size, but larger than any of +those previously known, was brought to light. The generic name of Stereognathus +was given to it, and, as is usually the case in these old rocks (see above), it +consisted of part of a lower jaw, in which were implanted three double-fanged +teeth, differing in structure from those of all other known recent or extinct +mammals. + +PLANTS OF THE OOLITE. + +(FIGURE 347. Portion of a fossil fruit of Podocarya Bucklandi, Ung., magnified. +(Buckland's Bridgewater Treatise Plate 63.) Inferior Oolite, Charmouth, Dorset.) + +(FIGURE 348. Cone of fossil Araucaria sphaerocarpa, Carruthers. Inferior Oolite. +Bruton, Somersetshire. One-third diameter of original. In the collection of the +British Museum.) + +The Araucarian pines, which are now abundant in Australia and its islands, +together with marsupial quadrupeds, are found in like manner to have accompanied +the marsupials in Europe during the Oolitic period (see Figure 348). In the same +rock endogens of the most perfect structure are met with, as, for example, +fruits allied to the Pandanus, such as the Kaidacarpum ooliticum of Carruthers +in the Great Oolite, and the Podocarya of Buckland (see Figure 347) in the +Inferior Oolite. + +FULLER'S EARTH. + +(FIGURE 349. Ostrea acuminata. Fuller's Earth.) + +Between the Great and Inferior Oolite near Bath, an argillaceous deposit, called +"the fuller's earth," occurs; but it is wanting in the north of England. It +abounds in the small oyster represented in Figure 349. The number of mollusca +known in this deposit is about seventy; namely, fifty Lamellibranchiate +Bivalves, ten Brachiopods, three Gasteropods, and seven or eight Cephalopods. + +INFERIOR OOLITE. + +This formation consists of a calcareous freestone, usually of small thickness, +but attaining in some places, as in the typical area of Cheltenham and the +Western Cotswolds, a thickness of 250 feet. It sometimes rests upon yellow +sands, formerly classed as the sands of the Inferior Oolite, but now regarded as +a member of the Upper Lias. These sands repose upon the Upper Lias clays in the +south and west of England. The Collyweston slate, formerly classed with the +Great Oolite, and supposed to represent in Northamptonshire the Stonesfield +slate, is now found to belong to the Inferior Oolite, both by community of +species and position in the series. The Collyweston beds, on the whole, assume a +much more marine character than the Stonesfield slate. Nevertheless, one of the +fossil plants Aroides Stutterdi, Carruthers, remarkable, like the Pandanaceous +species before mentioned (Figure 347) as a representative of the +monocotyledonous class, is common to the Stonesfield beds in Oxfordshire. + +(FIGURE 350. Hemitelites Brownii, Goepp. Syn. Phlebopteris contigua, Lind. and +Hutt. Lower carbonaceous strata, Inferior Oolite shales. Gristhorpe, Yorkshire.) + +The Inferior Oolite of Yorkshire consists largely of shales and sandstones, +which assume much the aspect of a true coal-field, thin seams of coal having +actually been worked in them for more than a century. A rich harvest of fossil +ferns has been obtained from them, as at Gristhorpe, near Scarborough (Figure +350). They contain also Cycadeae, of which family a magnificent specimen has +been described by Mr. Williamson under the name Zamia gigas, and a fossil called +Equisetum Columnare (see Figure 397), which maintains an upright position in +sandstone strata over a wide area. Shells of Estheria and Unio, collected by Mr. +Bean from these Yorkshire coal-bearing beds, point to the estuary or fluviatile +origin of the deposit. + +At Brora, in Sutherlandshire, a coal formation, probably coeval with the above, +or at least belonging to some of the lower divisions of the Oolitic period, has +been mined extensively for a century or more. It affords the thickest stratum of +pure vegetable matter hitherto detected in any secondary rock in England. One +seam of coal of good quality has been worked three and a half feet thick, and +there are several feet more of pyritous coal resting upon it. + +(FIGURE 351. Terebratula fimbria, Sowerby. Inferior Oolite marl. Cotswold +Hills.) + +(FIGURE 352. Rhynchonella spinosa, Schloth. Inferior Oolite.) + +(FIGURE 353. Pholadomya fidicula, Sowerby. One-third natural size. Inferior +Oolite.) + +(FIGURE 354. Pleurotomaria granulata, Sowerby. Ferruginous Oolite, Normandy. +Inferior Oolite, England.) + +(FIGURE 355. Pleurotomaria ornata, Sowerby Sp. Inferior Oolite.) + +(FIGURE 356. Collyrites (Dysaster) ringens, Agassiz. Inferior Oolite, +Somersetshire.) + +(FIGURE 357. Ammonites Humphresianus, Sowerby. Inferior Oolite.) + +(FIGURE 358. Ammonites Braikenridgii, Sowerby. Oolite, Scarborough. Inferior +Oolite, Dundry; Calvados; etc.) + +(FIGURE 359. Ostrea Marshii. One-half natural size. Middle and Lower Oolite.) + +Among the characteristic shells of the Inferior Oolite, I may instance +Terebratula fimbria (Figure 351), Rhynchonella spinosa (Figure 352), and +Pholadomya fidicula (Figure 353). The extinct genus Pleurotomaria is also a form +very common in this division as well as in the Oolitic system generally. It +resembles the Trochus in form, but is marked by a deep cleft (a, Figures 354, +355) on one side of the mouth. The Collyrites (Dysaster) ringens (Figure 356) is +an Echinoderm common to the Inferior Oolite of England and France, as are the +two Ammonites (Figures 357, 358). + +PALAEONTOLOGICAL RELATIONS OF THE OOLITIC STRATA. + +Observations have already been made on the distinctness of the organic remains +of the Oolitic and Cretaceous strata, and the proportion of species common to +the different members of the Oolite. Between the Lower Oolite and the Lias there +is a somewhat greater break, for out of 256 mollusca of the Upper Lias, thirty- +seven species only pass up into the Inferior Oolite. + +In illustration of shells having a great vertical range, it may be stated that +in England some few species pass up from the Lower to the Upper Oolite, as, for +example, Rhynchonella obsoleta, Lithodomus inclusus, Pholadomya ovalis, and +Trigonia costata. + +(FIGURE 360. Ammonites macrocephalus, Schloth. One-third natural size. Great +Oolite and Oxford Clay.) + +Of all the Jurassic Ammonites of Great Britain, A. macrocephalus (Figure 360), +which is common to the Great Oolite and Oxford Clay, has the widest range. + +We have every reason to conclude that the gaps which occur, both between the +larger and smaller sections of the English Oolites, imply intervals of time, +elsewhere represented by fossiliferous strata, although no deposit may have +taken place in the British area. This conclusion is warranted by the partial +extent of many of the minor and some of the larger divisions even in England. + + +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. + +LIAS. + +The English provincial name of Lias has been very generally adopted for a +formation of argillaceous limestone, marl, and clay, which forms the base of the +Oolite, and is classed by many geologists as part of that group. The peculiar +aspect which is most characteristic of the Lias in England, France, and Germany, +is an alternation of thin beds of blue or grey limestone, having a surface which +becomes light-brown when weathered, these beds being separated by dark-coloured, +narrow argillaceous partings, so that the quarries of this rock, at a distance, +assume a striped and ribbon-like appearance. + +The Lias has been divided in England into three groups, the Upper, Middle, and +Lower. The Upper Lias consists first of sands, which were formerly regarded as +the base of the Oolite, but which, according to Dr. Wright, are by their fossils +more properly referable to the Lias; secondly, of clay shale and thin beds of +limestone. The Middle Lias, or marl-stone series, has been divided into three +zones; and the Lower Lias, according to the labours of Quenstedt, Oppel, +Strickland, Wright, and others, into seven zones, each marked by its own group +of fossils. This Lower Lias averages from 600 to 900 feet in thickness. + +From Devon and Dorsetshire to Yorkshire all these divisions, observes Professor +Ramsay, are constant; and from top to bottom we can not assert that anywhere +there is actual unconformity between any two subdivisions, whether of the larger +or smaller kind. + +In the whole of the English Lias there are at present known about 937 species of +mollusca, and of these 267 are Cephalopods, of which class more than two-thirds +are Ammonites, the Nautilus and Belemnite also abounding. The whole series has +been divided by zones characterised by particular Ammonites; for while other +families of shells pass from one division to another in numbers varying from +about 20 to 50 per cent, these cephalopods are almost always limited to single +zones, as Quenstedt and Oppel have shown for Germany, and Dr. Wright and others +for England. + +As no actual unconformity is known from the top of the Upper to the bottom of +the Lower Lias, and as there is a marked uniformity in the mineral character of +almost all the strata, it is somewhat difficult to account even for such partial +breaks as have been alluded to in the succession of species, if we reject the +hypothesis that the old species were in each case destroyed at the close of the +deposition of the rocks containing them, and replaced by the creation of new +forms when the succeeding formation began. I agree with Professor Ramsay in not +accepting this hypothesis. No doubt some of the old species occasionally died +out, and left no representatives in Europe or elsewhere; others were locally +exterminated in the struggle for life by species which invaded their ancient +domain, or by varieties better fitted for a new state of things. Pauses also of +vast duration may have occurred in the deposition of strata, allowing time for +the modification of organic life throughout the globe, slowly brought about by +variation accompanied by extinction of the original forms. + +FOSSILS OF THE LIAS. + +(FIGURE 361. Plagiostoma (Lima) giganteum, Sowerby. Inferior Oolite and Lias.) + +(FIGURE 362. Gryphaea incurva, Sowerby. (G. arcuata, Lam.) Lias.) + +(FIGURE 363. Avicula inaequivalvis, Sowerby. Lower Lias.) + +(FIGURE 364. Avicula cygnipes, Phil. Lower Lias, Gloucestershire and Yorkshire. +a. Lower valve. +b. Upper valve.) + +(FIGURE 365. Hippopodium ponderosum, Sowerby. 1/4 diameter. Lias, Cheltenham) + +(FIGURE 366. Spiriferina (Spirifera) Walcotti, Sowerby. Lower Lias.) + +(FIGURE 367. Leptaena Moorei, Davidson. Upper Lias, Ilminster.) + +The name of Gryphite limestone has sometimes been applied to the Lias, in +consequence of the great number of shells which it contains of a species of +oyster, or Gryphaea (Figure 362). A large heavy shell called Hippopodium (Figure +365), allied to Cypricardia, is also characteristic of the upper part of the +Lower Lias. In this formation occur also the Aviculas, Figures 363 and 364. The +Lias formation is also remarkable for being the newest of the secondary rocks in +which brachiopoda of the genera Spirifer and Leptaena (Figures 366, 367) occur, +although the former is slightly modified in structure so as to constitute the +subgenus Spiriferina, Davidson, and the Leptaena has dwindled to a shell smaller +in size than a pea. No less than eight or nine species of Spiriferina are +enumerated by Mr. Davidson as belonging to the Lias. Palliobranchiate mollusca +predominate greatly in strata older than the Trias; but, so far as we yet know, +they did not survive the Liassic epoch. + +(FIGURE 368. Ammonites Bucklandi, Sowerby. Ammonites bisulcatus, Brug. One- +eighth diameter of original. +a. Side view. +b. Front view, showing mouth and bisulcated keel. Characteristic of the lower +part of the Lias of England and the Continent.) + +(FIGURE 369. Ammonites planorbis, Sowerby. One-half diameter of original. From +the base of the Lower Lias of England and the Continent.) + +(FIGURE 370. Nautilus truncatus, Sowerby. Lias.) + +(FIGURE 371. Ammonites bifrons, Brug. Ammonites Walcotti, Sowerby. Upper Lias +shales.) + +(FIGURE 372. Ammonites margaritatus, Montf. Syn. Ammonites Stokesi, Sowerby. +Middle Lias.) + +Allusion has already been made to numerous zones in the Lias having each their +peculiar Ammonites. Two of these occur near the base of the Lower Lias, having a +united thickness, varying from 40 to 80 feet. The upper of these is +characterised by Ammonites Bucklandi, and the lower by Ammonites planorbis (see +Figures 368, 369). (Quarterly Journal volume 16 page 376.) Sometimes, however, +there is a third intermediate zone, that of Ammonites angulatus, which is the +equivalent of the zone called the infra-lias on the Continent, the species of +which are for the most part common to the superior group marked by Ammonites +Bucklandi. + +(FIGURE 373. Extracrinus (Pentacrinus) Briareus. Miller. 1/2 natural size. +(Body, arms, and part of stem.) Lower Lias, Lyme Regis.) + +(FIGURE 374. Palaeocoma (Ophioderma) tenuibrachiata. E. Forbes. Middle Lias, +Seatown, Dorset.) + +Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites are +conspicuous. (See Figure 373.) Of Palaeocoma (Ophioderma) Egertoni (Figure 374), +referable to the Ophiuridae of Muller, perfect specimens have been met with in +the Middle Lias beds of Dorset and Yorkshire. + +The Extracrinus Briareus (removed by Major Austin from Pentacrinus on account of +generic differences) occurs in tangled masses, forming thin beds of considerable +extent, in the Lower Lias of Dorset, Gloucestershire, and Yorkshire. The remains +are often highly charged with pyrites. This Crinoid, with its innumerable +tentacular arms, appears to have been frequently attached to the driftwood of +the liassic sea, in the same manner as Barnacles float about on wood at the +present day. There is another species of Extracrinus and several of Pentacrinus +in the Lias; and the latter genus is found in nearly all the formations from the +Lias to the London Clay inclusive. It is represented in the present seas by the +delicate and rare Pentacrinus caput-medusae of the Antilles, which, with +Comatula, is one of the few surviving members of the ancient family of the +Crinoids, represented by so many extinct genera in the older formations. + +FISHES OF THE LIAS. + +(FIGURE 375. Scales of Lepidotus gigas. Agass. +a. Two of the scales detached.) + +(FIGURE 376. Aechmodus Leachii and Dapedius monilifer. +a. Aechmodus. Restored outline. +b. Scales of Aechmodus Leachii. +c. Scales of Dapedius monilifer.) + +(FIGURE 377. Acrodus nobilis, Agassiz (tooth); commonly called "fossil leech." +Lias, Lyme Regis, and Germany.) + +The fossil fish, of which there are no less than 117 species known as British, +resemble generically those of the Oolite, but differ, according to M. Agassiz, +from those of the Cretaceous period. Among them is a species of Lepidotus (L. +gigas, Agassiz), Figure 375, which is found in the Lias of England, France, and +Germany. (Agassiz Poissons Fossiles volume 2 tab. 28, 29.) This genus was before +mentioned (Chapter 18) as occurring in the Wealden, and is supposed to have +frequented both rivers and sea-coasts. Another genus of Ganoids (or fish with +hard, shining, and enamelled scales), called Aechmodus (Figure 376), is almost +exclusively Liassic. The teeth of a species of Acrodus, also, are very abundant +in the Lias (Figure 377). + +(FIGURE 378. Hybodus reticulatus, Agassiz. Lias, Lyme Regis. +a. Part of fin, commonly called Ichthyodorulite. +b. Tooth.) + +(FIGURE 379. Chimaera monstrosa. (Agassiz Poissons Fossiles volume 3 tab. C +Figure 1.) +a. Spine forming anterior part of the dorsal fin.) + +SBut the remains of fish which have excited more attention than any others are +those large bony spines called ichthyodorulites (a, Figure 378), which were once +supposed by some naturalists to be jaws, and by others weapons, resembling those +of the living Balistes and Silurus; but which M. Agassiz has shown to be neither +the one nor the other. The spines, in the genera last mentioned, articulate with +the backbone, whereas there are no signs of any such articulation in the +ichthyodorulites. These last appear to have been bony spines which formed the +anterior part of the dorsal fin, like that of the living genera Cestracion and +Chimaera (see a, Figure 379). In both of these genera, the posterior concave +face is armed with small spines, as in that of the fossil Hybodus (Figure 378), +a placoid fish of the shark family found fossil at Lyme Regis. Such spines are +simply imbedded in the flesh, and attached to strong muscles. "They serve," says +Dr. Buckland, "as in the Chimaera (Figure 379), to raise and depress the fin, +their action resembling that of a movable mast, raising and lowering backward +the sail of a barge." (Bridgewater Treatise page 290.) + +REPTILES OF THE LIAS. + +(FIGURE 380. Skeleton of Ichthyosaurus communis, restored by Conybeare and +Cuvier. +a. Costal vertebrae.) + +(FIGURE 381. Skeleton of Plesiosaurus dolichodeirus, restored by Reverend W.D. +Conybeare. +a. Cervical vertebra.) + +It is not, however, the fossil fish which form the most striking feature in the +organic remains of the Lias; but the Enaliosaurian reptiles, which are +extraordinary for their number, size, and structure. Among the most singular of +these are several species of Ichthyosaurus and Plesiosaurus (Figures 380, 381). +The genus Ichthyosaurus, or fish-lizard, is not confined to this formation, but +has been found in strata as high as the White Chalk of England, and as low as +the Trias of Germany, a formation which immediately succeeds the Lias in the +descending order. It is evident from their fish-like vertebrae, their paddles, +resembling those of a porpoise or whale, the length of their tail, and other +parts of their structure, that the Ichthyosaurs were aquatic. Their jaws and +teeth show that they were carnivorous; and the half-digested remains of fishes +and reptiles, found within their skeletons, indicate the precise nature of their +food. + +Mr. Conybeare was enabled, in 1824, after examining many skeletons nearly +perfect, to give an ideal restoration of the osteology of this genus, and of +that of the Plesiosaurus (Geological Society Transactions Second Series volume 1 +page 49.). (See Figures 380, 381.) The latter animal had an extremely long neck +and small head, with teeth like those of the crocodile, and paddles analogous to +those of the Ichthyosaurus, but larger. It is supposed to have lived in shallow +seas and estuaries, and to have breathed air like the Ichthyosaur and our modern +cetacea. (Conybeare and De la Beche, Geological Transactions First Series volume +5 page 559; and Buckland Bridgewater Treatise page 203.) Some of the reptiles +above mentioned were of formidable dimensions. One specimen of Ichthyosaurus +platydon, from the Lias at Lyme, now in the British Museum, must have belonged +to an animal more than 24 feet in length; and there are species of Plesiosaurus +which measure from 18 to 20 feet in length. The form of the Ichthyosaurus may +have fitted it to cut through the waves like the porpoise; as it was furnished +besides its paddles with a tail-fin so constructed as to be a powerful organ of +motion; but it is supposed that the Plesiosaurus, at least the long-necked +species (Figure 381), was better suited to fish in shallow creeks and bays +defended from heavy breakers. + +It is now very generally agreed that these extinct saurians must have inhabited +the sea; and it was urged that as there are now chelonians, like the tortoise, +living in fresh water, and others, as the turtle, frequenting the ocean, so +there may have been formerly some saurians proper to salt, others to fresh +water. The common crocodile of the Ganges is well-known to frequent equally that +river and the brackish and salt water near its mouth; and crocodiles are said in +like manner to be abundant both in the rivers of the Isla de Pinos (Isle of +Pines), south of Cuba, and in the open sea round the coast. In 1835 a curious +lizard (Amblyrhynchus cristatus) was discovered by Mr. Darwin in the Galapagos +Islands. (See Darwin Naturalist's Voyage page 385 Murray.) It was found to be +exclusively marine, swimming easily by means of its flattened tail, and +subsisting chiefly on seaweed. One of them was sunk from the ship by a heavy +weight, and on being drawn up after an hour was quite unharmed. + +The families of Dinosauria, crocodiles, and Pterosauria or winged reptiles, are +also represented in the Lias. + +SUDDEN DESTRUCTION OF SAURIANS. + +It has been remarked, and truly, that many of the fish and saurians, found +fossil in the Lias, must have met with sudden death and immediate burial; and +that the destructive operation, whatever may have been its nature, was often +repeated. + +"Sometimes," says Dr. Buckland, "scarcely a single bone or scale has been +removed from the place it occupied during life; which could not have happened +had the uncovered bodies of these saurians been left, even for a few hours, +exposed to putrefaction, and to the attacks of fishes and other smaller animals +at the bottom of the sea." (Bridgewater Treatise page 115.) Not only are the +skeletons of the Ichthyosaurs entire, but sometimes the contents of their +stomachs still remain between their ribs, as before remarked, so that we can +discover the particular species of fish on which they lived, and the form of +their excrements. Not unfrequently there are layers of these coprolites, at +different depths in the Lias, at a distance from any entire skeletons of the +marine lizards from which they were derived; "as if," says Sir H. De la Beche, +"the muddy bottom of the sea received small sudden accessions of matter from +time to time, covering up the coprolites and other exuviae which had accumulated +during the intervals." (Geological Researches page 334.) It is further stated +that, at Lyme Regis, those surfaces only of the coprolites which lay uppermost +at the bottom of the sea have suffered partial decay, from the action of water +before they were covered and protected by the muddy sediment that has afterwards +permanently enveloped them. + +Numerous specimens of the Calamary or pen-and-ink fish, (Geoteuthis bollensis) +have also been met with in the Lias at Lyme, with the ink-bags still distended, +containing the ink in a dried state, chiefly composed of carbon, and but +slightly impregnated with carbonate of lime. These Cephalopoda, therefore, must, +like the saurians, have been soon buried in sediment; for, if long exposed after +death, the membrane containing the ink would have decayed. (Buckland Bridgewater +Treatise page 307.) + +As we know that river-fish are sometimes stifled, even in their own element, by +muddy water during floods, it can not be doubted that the periodical discharge +of large bodies of turbid fresh water in the sea may be still more fatal to +marine tribes. In the "Principles of Geology" I have shown that large quantities +of mud and drowned animals have been swept down into the sea by rivers during +earthquakes, as in Java in 1699; and that indescribable multitudes of dead +fishes have been seen floating on the sea after a discharge of noxious vapours +during similar convulsions. But in the intervals between such catastrophes, +strata may have accumulated slowly in the sea of the Lias, some being formed +chiefly of one description of shell, such as ammonites, others of gryphites. + +FRESH-WATER DEPOSITS.-- INSECT BEDS. + +(FIGURE 382. Wing of a neuropterous insect, from the Lower Lias, +Gloucestershire. (Reverend P.B. Brodie.)) + +From the above remarks the reader will infer that the Lias is for the most part +a marine deposit. Some members, however, of the series have an estuarine +character, and must have been formed within the influence of rivers. At the base +of the Upper and Lower Lias respectively, insect-beds appear to be almost +everywhere present throughout the Midland and South-western districts of +England. These beds are crowded with the remains of insects, small fish, and +crustaceans, with occasional marine shells. One band in Gloucestershire, rarely +exceeding a foot in thickness, has been named the "insect limestone." It passes +upward, says the Reverend P.B. Brodie, into a shale containing Cypris and +Estheria, and is full of the wing-cases of several genera of Coleoptera, with +some nearly entire beetles, of which the eyes are preserved. (A History of +Fossil Insects etc 1846 London.) The nervures of the wings of neuropterous +insects (Figure 382) are beautifully perfect in this bed. Ferns, with Cycads and +leaves of monocotyledonous plants, and some apparently brackish and fresh-water +shells, accompany the insects in several places, while in others marine shells +predominate, the fossils varying apparently as we examine the bed nearer or +farther from the ancient land, or the source whence the fresh water was derived. +After studying 300 specimens of these insects from the Lias, Mr. Westwood +declares that they comprise both wood-eating and herb-devouring beetles, of the +Linnean genera Elater, Carabus, etc., besides grasshoppers (Gryllus), and +detached wings of dragon-flies and may-flies, or insects referable to the +Linnean genera Libellula, Ephemera, Hemerobius, and Panorpa, in all belonging to +no less than twenty-four families. The size of the species is usually small, and +such as taken alone would imply a temperate climate; but many of the associated +organic remains of other classes must lead to a different conclusion. + +FOSSIL PLANTS. + +Among the vegetable remains of the Lias, several species of Zamia have been +found at Lyme Regis, and the remains of coniferous plants at Whitby. M. Ad. +Brongniart enumerates forty-seven liassic acrogens, most of them ferns; and +fifty gymnosperms, of which thirty-nine are cycads, and eleven conifers. Among +the cycads the predominance of Zamites, and among the ferns the numerous genera +with leaves having reticulated veins (as in Figure 349), are mentioned as +botanical characteristics of this era. (Tableau des Veg. Foss. 1849 page 105.) +The absence as yet from the Lias and Oolite of all signs of dicotyledonous +angiosperms is worthy of notice. The leaves of such plants are frequent in +tertiary strata, and occur in the Cretaceous, though less plentifully (see +Chapter 17). The angiosperms seem, therefore, to have been at the least +comparatively rare in these older secondary periods, when more space was +occupied by the Cycads and Conifers. + +ORIGIN OF THE OOLITE AND LIAS. + +The entire group of Oolite and Lias consists of repeated alternations of clay, +sandstone, and limestone, following each other in the same order. Thus the clays +of the Lias are followed by the sands now considered (see Chapter 20) as +belonging to the same formation, though formerly referred to the Inferior +Oolite, and these sands again by the shelly and coralline limestone called the +Great or Bath Oolite. So, in the Middle Oolite, the Oxford Clay is followed by +calcareous grit and coral rag; lastly, in the Upper Oolite, the Kimmeridge Clay +is followed by the Portland Sand and limestone (see Figure 298). (Conybeare and +Philips's Outlines etc. page 166.) The clay beds, however, as Sir H. de la Beche +remarks, can be followed over larger areas than the sand or sandstones. +(Geological Researches page 337.) It should also be remembered that while the +Oolite system becomes arenaceous and resembles a coal-field in Yorkshire, it +assumes in the Alps an almost purely calcareous form, the sands and clays being +omitted; and even in the intervening tracts it is more complicated and variable +than appears in ordinary descriptions. Nevertheless, some of the clays and +intervening limestones do retain, in reality, a pretty uniform character for +distances of from 400 to 600 miles from east to west and north to south. + +In order to account for such a succession of events, we may imagine, first, the +bed of the ocean to be the receptacle for ages of fine argillaceous sediment, +brought by oceanic currents, which may have communicated with rivers, or with +part of the sea near a wasting coast. This mud ceases, at length, to be conveyed +to the same region, either because the land which had previously suffered +denudation is depressed and submerged, or because the current is deflected in +another direction by the altered shape of the bed of the ocean and neighbouring +dry land. By such changes the water becomes once more clear and fit for the +growth of stony zoophytes. Calcareous sand is then formed from comminuted shell +and coral, or, in some cases, arenaceous matter replaces the clay; because it +commonly happens that the finer sediment, being first drifted farthest from +coasts, is subsequently overspread by coarse sand, after the sea has grown +shallower, or when the land, increasing in extent, whether by upheaval or by +sediment filling up parts of the sea, has approached nearer to the spots first +occupied by fine mud. + +The increased thickness of the limestones in those regions, as in the Alps and +Jura, where the clays are comparatively thin, arises from the calcareous matter +having been derived from species of corals and other organic beings which live +in clear water, far from land, to the growth of which the influx of mud would be +unfavourable. Portions therefore of these clays and limestones have probably +been formed contemporaneously to a greater extent than we can generally prove, +for the distinctness of the species of organic beings would be caused by the +difference of conditions between the more littoral and the more pelagic areas +and the different depths and nature of the sea-bottom. Independently of those +ascending and descending movements which have given rise to the superposition of +the limestones and clays, and by which the position of land and sea are made in +the course of ages to vary, the geologist has the difficult task of allowing for +the contemporaneous thinning out in one direction and thickening in another, of +the successive organic and inorganic deposits of the same era. + + +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. + +BEDS OF PASSAGE BETWEEN THE LIAS AND TRIAS-- RHAETIC BEDS. + +We have mentioned in the last chapter that the base of the Lower Lias is +characterised, both in England and Germany, by beds containing distinct species +of Ammonites, the lowest subdivision having been called the zone of Ammonites +planorbis. Below this zone, on the boundary line between the Lias and the strata +of which we are about to treat, called "Trias," certain cream-coloured +limestones devoid of fossils are usually found. These white beds were called by +William Smith the White Lias, and they have been shown by Mr. Charles Moore to +belong to a formation similar to one in the Rhaetian Alps of Bavaria, to which +Mr. Gumbel has applied the name of Rhaetic. They have also long been known as +the Koessen beds in Germany, and may be regarded as beds of passage between the +Lias and Trias. They are named the Penarth beds by the Government surveyors of +Great Britain, from Penarth, near Cardiff, in Glamorganshire, where they +sometimes attain a thickness of fifty feet. + +(FIGURE 383. Cardium rhaeticum, Merrian. Natural size. Rhaetic Beds.) + +(FIGURE 384. Pecten Valoniensis. Dfr. 1/2 natural size. Portrush, Ireland, etc. +Rhaetic Beds.) + +(FIGURE 385. Avicula contorta. Portlock. Portrush, Ireland, etc. Natural size. +Rhaetic Beds.) + +The principal member of this group has been called by Dr. Wright the Avicula +contorta bed, as this shell is very abundant, and has a wide range in Europe. +(Dr. Wright, on Lias and Bone Bed, Quarterly Geological Journal 1860 volume 16.) +General Portlock first described the formation as it occurs at Portrush, in +Antrim, where the Avicula contorta is accompanied by Pecten Valoniensis, as in +Germany. + +The best known member of the group, a thin band or bone-breccia, is conspicuous +among the black shales in the neighbourhood of Axmouth in Devonshire, and in the +cliffs of Westbury-on-Severn, as well as at Aust and other places on the borders +of the Bristol Channel. It abounds in the remains of saurians and fish, and was +formerly classed as the lowest bed of the Lias; but Sir P. Egerton first pointed +out, in 1841, that it should be referred to the Upper New Red Sandstone, because +it contained an assemblage of fossil fish which are either peculiar to this +stratum, or belong to species well-known in the Muschelkalk of Germany. These +fish belong to the genera Acrodus, Hybodus, Gyrolepis, and Saurichthys. + +(FIGURE 386. Hybodus plicatilis, Agassiz. Teeth. Bone-bed, Aust and Axmouth.) + +(FIGURE 387. Saurichthys apicalis, Agassiz. Tooth; natural size and magnified. +Axmouth.) + +(FIGURE 388. Gyrolepis tenuistriatus, Agassiz. Scale; natural size and +magnified. Axmouth.) + +Among those common to the English bone-bed and the Muschelkalk of Germany are +Hybodus plicatilis (Figure 386), Saurychthys apicalis (Figure 387), Gyrolepis +tenuistriatus (Figure 388), and G. Albertii. Remains of saurians, Plesiosaurus +among others, have also been found in the bone-bed, and plates of an Encrinus. +It may be questioned whether some of those fossils which have the most Triassic +character may not have been derived from the destruction of older strata, since +in bone-beds, in general, many of the organic remains are undoubtedly +derivative. + +TRIASSIC MAMMIFER. + +(FIGURE 389. Microlestes antiquus, Plieninger. Molar tooth, magnified. Upper +Trias. Diegerloch, near Stuttgart, Wurtemberg. +a. View of inner side? +b. Same, outer side? +c. Same in profile. +d. Crown of same.) + +In North-western Germany, as in England, there occurs beneath the Lias a +remarkable bone breccia. It is filled with shells and with the remains of fishes +and reptiles, almost all the genera of which, and some even of the species, +agree with those of the subjacent Trias. This breccia has accordingly been +considered by Professor Quenstedt, and other German geologists of high +authority, as the newest or uppermost part of the Trias. Professor Plieninger +found in it, in 1847, the molar tooth of a small Triassic mammifer, called by +him Microlestes antiquus. He inferred its true nature from its double fangs, and +from the form and number of the protuberances or cusps on the flat crown; and +considering it as predaceous, probably insectivorous, he called it Microlestes +from micros, little, and lestes, a beast of prey. Soon afterwards he found a +second tooth, also at the same locality, Diegerloch, about two miles to the +south-east of Stuttgart. + +No anatomist had been able to give any feasible conjecture as to the affinities +of this minute quadruped until Dr. Falconer, in 1857, recognised an unmistakable +resemblance between its teeth and the two back molars of his new genus +Plagiaulax (Figure 306), from the Purbeck strata. This would lead us to the +conclusion that Microlestes was marsupial and plant-eating. + +In Wurtemberg there are two bone-beds, namely, that containing the Microlestes, +which has just been described, which constitutes, as we have seen, the uppermost +member of the Trias, and another of still greater extent, and still more rich in +the remains of fish and reptiles, which is of older date, intervening between +the Keuper and Muschelkalk. + +The genera Saurichthys, Hybodus, and Gyrolepis are found in both these breccias, +and one of the species, Saurichthys Mongeoti, is common to both bone-beds, as is +also a remarkable reptile called Nothosaurus mirabilis. The saurian called +Belodon by H. von Meyer, of the Thecodont family, is another Triassic form, +associated at Diegerloch with Microlestes. + +TRIAS OF ENGLAND. + +Between the Lias and the Coal (or Carboniferous group) there is interposed, in +the midland and western counties of England, a great series of red loams, +shales, and sandstones, to which the name of the "New Red Sandstone formation" +was first given, to distinguish it from other shales and sandstones called the +"Old Red," often identical in mineral character, which lie immediately beneath +the coal. The name of "Red Marl" has been incorrectly applied to the red clays +of this formation, as before explained (Chapter 2), for they are remarkably free +from calcareous matter. The absence, indeed, of carbonate of lime, as well as +the scarcity of organic remains, together with the bright red colour of most of +the rocks of this group, causes a strong contrast between it and the Jurassic +formations before described. + +The group in question is more fully developed in Germany than in England or +France. It has been called the Trias by German writers, or the Triple Group, +because it is separable into three distinct formations, called the "Keuper," the +"Muschelkalk," and the "Bunter-sandstein." Of these the middle division, or the +Muschelkalk, is wholly wanting in England, and the uppermost (Keuper) and lowest +(Bunter) members of the series are not rich in fossils. + +UPPER TRIAS OR KEUPER. + +In certain grey indurated marls below the bone-bed Mr. Boyd Dawkins has found at +Watchet, on the coast of Somersetshire, a molar tooth of Microlestes, enabling +him to refer to the Trias strata formerly supposed to be Liassic. Mr. Charles +Moore had previously discovered many teeth of mammalia of the same family near +Frome, in Somersetshire, in the contents of a vertical fissure traversing a mass +of carboniferous limestone. The top of this fissure must have communicated with +the bed of the Triassic sea, and probably at a point not far from the ancient +shore on which the small marsupials of that era abounded. + +This upper division of the Trias called the Keuper is of great thickness in the +central counties of England, attaining, according to Mr. Hull's estimate, no +less than 3450 feet in Cheshire, and it covers a large extent of country between +Lancashire and Devonshire. + +(FIGURE 390. Estheria minuta, Bronn.) + +In Worcestershire and Warwickshire in sandstone belonging to the uppermost part +of the Keuper the bivalve crustacean Estheria minuta occurs. The member of the +English "New Red" containing this shell, in those parts of England, is, +according to Sir Roderick Murchison and Mr. Strickland, 600 feet thick, and +consists chiefly of red marl or slate, with a band of sandstone. +Ichthyodorulites, or spines of Hybodus, teeth of fishes, and footprints of +reptiles were observed by the same geologists in these strata. + +(FIGURE 391. Hyperodapedon Gordoni. Left palate, maxillary. (Showing the two +rows of palatal teeth on opposite sides of the jaw.) +a. Under surface. +b. Exterior right side.) + +In the Upper Trias or Keuper the remains of two saurians of the order Lacertilia +have been found. The one called Rhynchosaurus occurred at Grinsell near +Shrewsbury, and is characterised by having a small bird-like skull and jaws +without teeth. The other Hyperodapedon (Figure 391) was first noticed in 1858, +near Elgin, in strata now recognised as Upper Triassic, and afterwards in beds +of about the same age in the neighbourhood of Warwick. Remains of the same genus +have been found both in Central India and Southern Africa in rocks believed to +be of Triassic age. The Hyperodapedon has been shown by Professor Huxley to be a +terrestrial reptile having numerous palatal teeth, and closely allied to the +living Sphenodon of New Zealand. + +The recent discoveries of a living saurian in New Zealand so closely allied to +this supposed extinct division of the Lacertilia seems to afford an illustration +of a principle pointed out by Mr. Darwin of the survival in insulated tracts, +after many changes in physical geography, of orders of which the congeners have +become extinct on continents where they have been exposed to the severer +competition of a larger progressive fauna. + +(FIGURE 392. Tooth of Labyrinthodon; natural size. Warwick sandstone.) + +(FIGURE 393. Transverse section of upper part of tooth of Labyrinthodon Jaegeri, +Owen (Mastodonsaurus Jaegeri, Meyer); natural size, and a segment magnified. +a. Pulp cavity, from which the processes of pulp and dentine radiate.) + +Teeth of Labyrinthodon (Figure 392) found in the Keuper in Warwickshire were +examined microscopically by Professor Owen, and compared with other teeth from +the German Keuper. He found after careful investigation that neither of them +could be referred to true saurians, although they had been named Mastodonsaurus +and Phytosaurus by Jager. It appeared that they were of the Batrachian order, +and of gigantic dimensions in comparison with any representatives of that order +now living. Both the Continental and English fossil teeth exhibited a most +complicated texture, differing from that previously observed in any reptile, +whether recent or extinct, but most nearly analogous to the Ichthyosaurus. A +section of one of these teeth exhibits a series of irregular folds, resembling +the labyrinthic windings of the surface of the brain; and from this character +Professor Owen has proposed the name Labyrinthodon for the new genus. Figure 393 +of part of one is given from his "Odontography," plate 64, a. The entire length +of this tooth is supposed to have been about three inches and a half, and the +breadth at the base one inch and a half. + +ROCK-SALT. + +In Cheshire and Lancashire there are red clays containing gypsum and salt of the +age of the Trias which are between 1000 and 1500 feet thick. In some places +lenticular masses of pure rock-salt nearly 100 feet thick are interpolated +between the argillaceous beds. At the base of the formation beneath the rock- +salt occur the Lower Sandstones and Marl, called provincially in Cheshire +"water-stones," which are largely quarried for building. They are often ripple- +marked, and are impressed with numerous footprints of reptiles. + +The basement beds of the Keuper rest with a slight unconformability upon an +eroded surface of the "Bunter" next to be described. + +LOWER TRIAS OR BUNTER. + +(FIGURE 394. Single footstep of Cheirotherium. Bunter-sandstein, Saxony, one- +eighth of natural size.) + +(FIGURE 395. Line of footsteps on slab of sandstone. Hildburghausen, in Saxony.) + +The lower division or English representative of the "Bunter" attains a thickness +of 1500 feet in the counties last mentioned, according to Professor Ramsay. +Besides red and green shales and red sandstones, it comprises much soft white +quartzose sandstone, in which the trunks of silicified trees have been met with +at Allesley Hill, near Coventry. Several of them were a foot and a half in +diameter, and some yards in length, decidedly of coniferous wood, and showing +rings of annual growth. (Buckland Proceedings of the Geological Society volume 2 +page 439 and Murchison and Strickland Geological Transactions Second Series +volume 5 page 347.) Impressions, also, of the footsteps of animals have been +detected in Lancashire and Cheshire in this formation. Some of the most +remarkable occur a few miles from Liverpool, in the whitish quartzose sandstone +of Storton Hill, on the west side of the Mersey. They bear a close resemblance +to tracks first observed in this member of the Upper New Red Sandstone, at the +village of Hesseberg, near Hildburghausen, in Saxony. For many years these +footprints have been referred to a large unknown quadruped, provisionally named +Cheirotherium by Professor Kaup, because the marks both of the fore and hind +feet resembled impressions made by a human hand. (See Figure 394.) The foot- +marks at Hesseberg are partly concave, and partly in relief, the former, or the +depressions, are seen upon the upper surface of the sandstone slabs, but those +in relief are only upon the lower surfaces, being, in fact, natural casts, +formed in the subjacent footprints as in moulds. The larger impressions, which +seem to be those of the hind foot, are generally eight inches in length, and +five in width, and one was twelve inches long. Near each large footstep, and at +a regular distance (about an inch and a half) before it, a smaller print of a +fore foot, four inches long and three inches wide, occurs. The footsteps follow +each other in pairs, each pair in the same line, at intervals of fourteen inches +from pair to pair. The large as well as the small steps show the great toes +alternately on the right and left side; each step makes the print of five toes, +the first, or great toe, being bent inward like a thumb. Though the fore and +hind foot differ so much in size, they are nearly similar in form. + +As neither in Germany nor in England had any bones or teeth been met with in the +same identical strata as the footsteps, anatomists indulged, for several years, +in various conjectures respecting the mysterious animals from which they might +have been derived. Professor Kaup suggested that the unknown quadruped might +have been allied to the Marsupialia; for in the kangaroo the first toe of the +fore foot is in a similar manner set obliquely to the others, like a thumb, and +the disproportion between the fore and hind feet is also very great. But M. Link +conceived that some of the four species of animals of which the tracks had been +found in Saxony might have been gigantic Batrachians, and when it was afterwards +inferred that the Labyrinthodon was an air-breathing reptile, it was conjectured +by Professor Owen that it might be one and the same as the Cheirotherium. + +DOLOMITIC CONGLOMERATE OF BRISTOL. + +(FIGURE 396. Tooth of Thecodontosaurus; three times magnified. Riley and +Stutchbury. Dolomitic conglomerate. Redland, near Bristol.) + +Near Bristol, in Somersetshire, and in other counties bordering the Severn, the +lowest strata belonging to the Triassic series consist of a conglomerate or +breccia resting unconformably upon the Old Red Sandstone, and on different +members of the Carboniferous rocks, such as the Coal Measures, Millstone Grit, +and Mountain Limestone. This mode of superposition will be understood by +reference to the section below Dundry Hill (Figure 85), where No. 4 is the +dolomitic conglomerate. Such breccias may have been partly the result of the +subaerial waste of an old land-surface which gradually sank down and suffered +littoral denudation in proportion as it became submerged. The pebbles and +fragments of older rocks which constitute the conglomerate are cemented together +by a red or yellow base of dolomite, and in some places the encrinites and other +fossils derived from the Mountain Limestone are so detached from the parent +rocks that they have the deceptive appearance of belonging to a fauna +contemporaneous with the dolomitic beds in which they occur. The imbedded +fragments are both rounded and angular, some consisting of sandstone from the +coal-measures, being of vast size, and weighing nearly a ton. Fractured bones +and teeth of saurians which are truly of contemporaneous origin are dispersed +through some parts of the breccia, and two of these reptiles called Thecodont +saurians, named from the manner in which the teeth were implanted in the +jawbone, obtained great celebrity because the patches of red conglomerate in +which they were found, near Bristol, were originally supposed to be of Permian +or Palaeozoic age, and therefore the only representatives in England of +vertebrate animals of so high a grade in rocks of such antiquity. The teeth of +these saurians are conical, compressed, and with finely serrated edges (see +Figure 396); they are referred by Professor Huxley to the Dinosaurian order. + +ORIGIN OF RED SANDSTONE AND ROCK-SALT. + +In various parts of the world, red and mottled clays and sandstones, of several +distinct geological epochs, are found associated with salt, gypsum, and +magnesian limestone, or with one or all of these substances. There is, +therefore, in all likelihood, a general cause for such a coincidence. +Nevertheless, we must not forget that there are dense masses of red and +variegated sandstones and clays, thousands of feet in thickness, and of vast +horizontal extent, wholly devoid of saliferous or gypseous matter. There are +also deposits of gypsum and of common salt, as in the blue-clay formation of +Sicily, without any accompanying red sandstone or red clay. + +These red deposits may be accounted for by the decomposition of gneiss and mica +schist, which in the eastern Grampians of Scotland has produced a mass of +detritus of precisely the same colour as the Old Red Sandstone. + +It is a general fact, and one not yet accounted for, that scarcely any fossil +remains are ever preserved in stratified rocks in which this oxide of iron +abounds; and when we find fossils in the New or Old Red Sandstone in England, it +is in the grey, and usually calcareous beds, that they occur. The saline or +gypseous interstratified beds may have been produced by submarine gaseous +emanations, or hot mineral springs, which often continue to flow in the same +spots for ages. Beds of rock-salt are, however, more generally attributed to the +evaporation of lakes or lagoons communicating at intervals with the ocean. In +Cheshire two beds of salt occur of the extraordinary thickness of 90 or even 100 +feet, and extending over an area supposed to be 150 miles in diameter. The +adjacent beds present ripple-marked sandstones and footprints of animals at so +many levels as to imply that the whole area underwent a slow and gradual +depression during the formation of the red sandstone. + +Major Harris, in his "Highlands of Ethiopia," describes a salt lake, called the +Bahr Assal, near the Abyssinian frontier, which once formed the prolongation of +the Gulf of Tadjara, but was afterwards cut off from the gulf by a broad bar of +lava or of land upraised by an earthquake. "Fed by no rivers, and exposed in a +burning climate to the unmitigated rays of the sun, it has shrunk into an +elliptical basin, seven miles in its transverse axis, half filled with smooth +water of the deepest caerulean hue, and half with a solid sheet of glittering +snow-white salt, the offspring of evaporation." "If," says Mr. Hugh Miller, "we +suppose, instead of a barrier of lava, that sand-bars were raised by the surf on +a flat arenaceous coast during a slow and equable sinking of the surface, the +waters of the outer gulf might occasionally topple over the bar, and supply +fresh brine when the first stock had been exhausted by evaporation." + +The Runn of Cutch, as I have shown elsewhere (Principles of Geology chapter +27.), is a low region near the delta of the Indus, equal in extent to about a +quarter of Ireland, which is neither land nor sea, being dry during part of +every year, and covered by salt water during the monsoons. Here and there its +surface is incrusted over with a layer of salt caused by the evaporation of sea- +water. A subsiding movement has been witnessed in this country during +earthquakes, so that a great thickness of pure salt might result from a +continuation of such sinking. + +TRIAS OF GERMANY. + +In Germany, as before hinted, chapter 21, the Trias first received its name as a +Triple Group, consisting of two sandstones with an intermediate marine +calcareous formation, which last is wanting in England. + +NOMENCLATURE OF TRIAS. + +COLUMN 1: GERMAN. + +COLUMN 2: FRENCH. + +COLUMN 3: ENGLISH. + +Keuper: Marnes irisees: Saliferous and gypseous shales and sandstone. + +Muschelkalk: Muschelkalk, ou calcaire coquillier: Wanting in England. + +Bunter-sandstein: Gres bigarre: Sandstone and quartzose conglomerate. + +KEUPER. + +(FIGURE 397. Equisetites columnaris. (Syn. Equisetum columnare.) Fragment of +stem, and a small portion of same magnified. Keuper.) + +The first of these, or the Keuper, underlying the beds before described as +Rhaetic, attains in Wurtemberg a thickness of about 1000 feet. It is divided by +Alberti into sandstone, gypsum, and carbonaceous clay-slate. (Monog. des Bunter- +Sandsteins.) Remains of reptiles called Nothosaurus and Phytosaurus, have been +found in it with Labyrinthodon; the detached teeth, also, of placoid fish and of +Rays, and of the genera Saurichthys and Gyrolepis (Figures 387, 388). The plants +of the Keuper are generically very analogous to those of the oolite and lias, +consisting of ferns, equisetaceous plants, cycads, and conifers, with a few +doubtful monocotyledons. A few species such as Equisetites columnaris, are +common to this group and the oolite. + +ST. CASSIAN AND HALLSTADT BEDS (SEE MAP, FIGURE 398). + +(FIGURE 398. Map of Tyrol and Styria showing St. Cassian and Hallstadt Beds.) + +The sandstones and clay of the Keuper resemble the deposits of estuaries and a +shallow sea near the land, and afford, in the north-west of Germany, as in +France and England, but a scanty representation of the marine life of that +period. We might, however, have anticipated, from its rich reptilian fauna, that +the contemporaneous inhabitants of the sea of the Keuper period would be very +numerous, should we ever have an opportunity of bringing their remains to light. +This, it is believed, has at length been accomplished, by the position now +assigned to certain Alpine rocks called the "St. Cassian beds," the true place +of which in the series was until lately a subject of much doubt and discussion. +It has been proved that the Hallstadt beds on the northern flanks of the +Austrian Alps correspond in age with the St. Cassian beds on their southern +declivity, and the Austrian geologists, M. Suess of Vienna and others, have +satisfied themselves that the Hallstadt formation is referable to the period of +the Upper Trias. Assuming this conclusion to be correct, we become acquainted +suddenly and unexpectedly with a rich marine fauna belonging to a period +previously believed to be very barren of organic remains, because in England, +France, and Northern Germany the upper Trias is chiefly represented by beds of +fresh or brackish water origin. + +(FIGURE 399. Scoliostoma, St. Cassian.) + +(FIGURE 400. Platystoma Suessii, Hornes. From Hallstadt.) + +(FIGURE 401. Koninckia Leonhardi, Wissmann. +a. Ventral view. Part of ventral valve removed to show the vascular impressions +of dorsal valve. +b. Interior of dorsal valve, showing spiral processes restored. +c. Vertical section of both valves. Part shaded black showing place occupied by +the animal, and the dorsal valve following the curve of the ventral.) + +About 600 species of invertebrate fossils occur in the Hallstadt and St. Cassian +beds, many of which are still undescribed; some of the Mollusca are of new and +peculiar genera, as Scoliostoma, Figure 399, and Platystoma, Figure 400, among +the Gasteropoda; and Koninckia, Figure 401, among the Brachiopoda. + +TABLE 21.1 GENERA OF FOSSIL MOLLUSCA IN THE ST. CASSIAN AND HALLSTADT BEDS. + +COLUMN 1: COMMON TO OLDER ROCKS. +Orthoceras. +Bactrites. +Macrocheilus. +Loxonema. +Holopella. +Murchisonia. +Porcellia. +Athyris. +Retzia. +Cyrtina. +Euomphalus. + +COLUMN 2: CHARACTERISTIC TRIASSIC GENERA. +Ceratites. +Cochloceras. +Choristoceras. +Rhabdoceras. +Aulacoceras. +Scoliostoma (reaches its maximum in the Trias, but passes down to older rocks). +Naticella. +Platystoma. +Ptychostoma. +Euchrysalis. +Halobia. +Hornesia. +Amphiclina. +Koninckia. +Cassianella. (Reach their maximum in the Trias, but pass up to newer rocks.) +Myophoria. (Reach their maximum in the Trias, but pass up to newer rocks.) + +COLUMN 3: COMMON TO NEWER ROCKS. +Ammonites. +Chemnitzia. +Cerithium. +Monodonta. +Opis. +Sphoera. +Cardita. +Myoconcha. +Hinnites. +Monotis. +Plicatula. +Pachyrisma. +Thecidium. + +Table 21.1 of genera of marine shells from the Hallstadt and St. Cassian beds, +drawn up first on the joint authority of M. Suess and the late Dr. Woodward, and +since corrected by Messrs. Etheridge and Tate, shows how many connecting links +between the fauna of primary and secondary Palaeozoic and Mesozoic rocks are +supplied by the St. Cassian and Hallstadt beds. + +The first column marks the last appearance of several genera which are +characteristic of Palaeozoic strata. The second shows those genera which are +characteristic of the Upper Trias, either as peculiar to it, or, as in the three +cases marked by asterisks, reaching their maximum of development at this era. +The third column marks the first appearance in Triassic rocks of genera destined +to become more abundant in later ages. + +It is only, however, when we contemplate the number of species by which each of +the above-mentioned genera are represented that we comprehend the peculiarities +of what is commonly called the St. Cassian fauna. Thus, for example, the +Ammonite, which is not common to older rocks, is represented by no less than +seventy-three species; whereas Loxonema, which is only known as common to older +rocks, furnishes fifteen Triassic species. Cerithium, so abundant in tertiary +strata, and which still lives, is represented by no less than fourteen species. +As the Orthoceras had never been met with in the marine Muschelkalk, much +surprise was naturally felt that seven or eight species of the genus should +appear in the Hallstadt beds, assuming these last to belong to the Upper Trias. +Among these species are some of large dimensions, associated with large +Ammonites with foliated lobes, a form never seen before so low in the series, +while the Orthoceras had never been seen so high. + +On the whole, the rich marine fauna of Hallstadt and St. Cassian, now generally +assigned to the lowest members of the Upper Trias or Keuper, leads us to suspect +that when the strata of the Triassic age are better known, especially those +belonging to the period of the Bunter sandstone, the break between the +Palaeozoic and Mesozoic Periods may be almost effaced. Indeed some geologists +are not yet satisfied that the true position of the St. Cassian beds (containing +so great an admixture of types, having at once both Mesozoic and Palaeozoic +affinities) is made out, and doubt whether they have yet been clearly proved to +be newer than the Muschelkalk. + +MUSCHELKALK. + +(FIGURE 402. Ceratites nodosus, Schloth. Muschelkalk, Germany. Side and front +view, showing the denticulated outline of the septa dividing the chambers.) + +(FIGURE 403. Gervilia (Avicula) socialis, Schloth. Characteristic shell of the +Muschelkalk.) + +The next member of the Trias in Germany, the Muschelkalk, which underlies the +Keuper before described, consists chiefly of a compact greyish limestone, but +includes beds of dolomite in many places, together with gypsum and rock-salt. +This limestone, a formation wholly unrepresented in England, abounds in fossil +shells, as the name implies. Among the Cephalopoda there are no belemnites, and +no ammonites with foliated sutures, as in the Lias, and Oolite, and the +Hallstadt beds; but we find instead a genus allied to the Ammonite, called +Ceratites by de Haan, in which the descending lobes (Figure 402) terminate in a +few small denticulations pointing inward. Among the bivalve crustacea, the +Estheria minuta, Bronn (see Figure 390), is abundant, ranging through the +Keuper, Muschelkalk, and Bunter-sandstein; and Gervillia socialis (Figure 403), +having a similar range, is found in great numbers in the Muschelkalk of Germany, +France, and Poland. + +(FIGURE 404. Encrinus liliiformis, Schlott. Syn. E. moniliformis. Body, arms, +and part of stem. +a. Section of stem. Muschelkalk.) + +(FIGURE 405. Aspidura loricata, Agassiz. +a. Upper side. +b. Lower side. Muschelkalk.) + +(FIGURE 406. Palatal teeth of Placodus gigas. Muschelkalk.) + +The abundance of the heads and stems of lily encrinites, Encrinus liliiformis +(Figure 404), (or Encrinites moniliformis), shows the slow manner in which some +beds of this limestone have been formed in clear sea-water. The star-fish called +Aspidura loricata (Figure 405) is as yet peculiar to the Muschelkalk. In the +same formation are found the skull and teeth of a reptile of the genus Placodus +(see Figure 406), which was referred originally by Munster, and afterwards by +Agassiz, to the class of fishes. But more perfect specimens enabled Professor +Owen, in 1858, to show that this fossil animal was a Saurian reptile, which +probably fed on shell-bearing mollusks, and used its short and flat teeth, so +thickly coated with enamel, for pounding and crushing the shells. + +BUNTER-SANDSTEIN. + +(FIGURE 407. Voltzia heterophylla. (Syn. Voltzia brevifolia.) +b. Portion of same magnified to show fructification. Sulzbad. Bunter-sandstein.) + +The Bunter-sandstein consists of various-coloured sandstones, dolomites, and red +clays, with some beds, especially in the Hartz, of calcareous pisolite or roe- +stone, the whole sometimes attaining a thickness of more than 1000 feet. The +sandstone of the Vosges is proved, by its fossils, to belong to this lowest +member of the Triassic group. At Sulzbad (or Soultz-les-bains), near Strasburg, +on the flanks of the Vosges, many plants have been obtained from the "bunter," +especially conifers of the extinct genus Voltzia, of which the fructification +has been preserved. (See Figure 407.) Out of thirty species of ferns, cycads, +conifers, and other plants, enumerated by M. Ad. Brongniart, in 1849, as coming +from the "Gres bigarre," or Bunter, not one is common to the Keuper. + +The footprints of Labyrinthodon observed in the clays of this formation at +Hildburghausen, in Saxony, have already been mentioned. Some idea of the variety +and importance of the terrestrial vertebrate fauna of the three members of the +Trias in Northern Germany may be derived from the fact that in the great +monograph by the late Hermann von Meyer on the reptiles of the Trias, the +remains of no less than eighty distinct species are described and figured. + +TRIAS OF THE UNITED STATES. + +NEW RED SANDSTONE OF THE VALLEY OF THE CONNECTICUT RIVER. + +(FIGURE 408. Footprints of a bird, Turner's Falls, Valley of the Connecticut.) + +In a depression of the granitic or hypogene rocks in the States of Massachusetts +and Connecticut strata of red sandstone, shale, and conglomerate are found, +occupying an area more than 150 miles in length from north to south, and about +five to ten miles in breadth, the beds dipping to the eastward at angles varying +from 5 to 50 degrees. The extreme inclination of 50 degrees is rare, and only +observed in the neighbourhood of masses of trap which have been intruded into +the red sandstone while it was forming, or before the newer parts of the deposit +had been completed. Having examined this series of rocks in many places, I feel +satisfied that they were formed in shallow water, and for the most part near the +shore, and that some of the beds were from time to time raised above the level +of the water, and laid dry, while a newer series, composed of similar sediment, +was forming. + +According to Professor Hitchcock, the footprints of no less than thirty-two +species of bipeds, and twelve of quadrupeds, have been already detected in these +rocks. Thirty of these are believed to be those of birds, four of lizards, two +of chelonians, and six of batrachians. The tracks have been found in more than +twenty places, scattered through an extent of nearly 80 miles from north to +south, and they are repeated through a succession of beds attaining at some +points a thickness of more than 1000 feet. (Hitchcock Mem. of the American +Academy New Series volume 3 page 129 1848.) + +The bipedal impressions are, for the most part, trifid, and show the same number +of joints as exist in the feet of living tridactylous birds. Now, such birds +have three phalangeal bones for the inner toe, four for the middle, and five for +the outer one (see Figure 408); but the impression of the terminal joint is that +of the nail only. The fossil footprints exhibit regularly, where the joints are +seen, the same number; and we see in each continuous line of tracks the three- +jointed and five-jointed toes placed alternately outward, first on the one side, +and then on the other. In some specimens, besides impressions of the three toes +in front, the rudiment is seen of the fourth toe behind. It is not often that +the matrix has been fine enough to retain impressions of the integument or skin +of the foot; but in one fine specimen found at Turner's Falls, on the +Connecticut, by Dr. Deane, these markings are well preserved, and have been +recognised by Professor Owen as resembling the skin of the ostrich, and not that +of reptiles. + +The casts of the footprints show that some of the fossil bipeds of the red +sandstone of Connecticut had feet four times as large as the living ostrich, but +scarcely, perhaps, larger than the Dinornis of New Zealand, a lost genus of +feathered giants related to the Apteryx, of which there were many species which +have left their bones and almost entire skeletons in the superficial alluvium of +that island. By referring to what was said of the Iguanodon of the Wealden, the +reader will perceive that the Dinosaur was somewhat intermediate between +reptiles and birds, and left a series of tridactylous impressions on the sand. + +To determine the exact age of the red sandstone and shale containing these +ancient footprints, in the United States, is not possible at present. No fossil +shells have yet been found in the deposit, nor plants in a determinable state. +The fossil fish are numerous and very perfect; but they are of a peculiar type, +called Ischypterus, by Sir Philip Egerton, from the great size and strength of +the fulcral rays of the dorsal fin, from ischus, strength, and pteron, a fin. + +The age of the Connecticut beds can not be proved by direct superposition, but +may be presumed from the general structure of the country. That structure proves +them to be newer than the movements to which the Appalachian or Allegheny chain +owes its flexures, and this chain includes the ancient or palaeozoic coal- +formation among its contorted rocks. + +COAL-FIELD OF RICHMOND, VIRGINIA. + +In the State of Virginia, at the distance of about 13 miles eastward of +Richmond, the capital of that State, there is a coal-field occurring in a +depression of the granite rocks, and occupying a geological position analogous +to that of the New Red Sandstone, above-mentioned, of the Connecticut valley. It +extends 26 miles from north to south, and from four to twelve from east to west. + +The plants consist chiefly of zamites, calamites, equiseta, and ferns, and, upon +the whole, are considered by Professor Heer to have the nearest affinity to +those of the European Keuper. + +The equiseta are very commonly met with in a vertical position more or less +compressed perpendicularly. It is clear that they grew in the places where they +are now buried in strata of hardened sand and mud. I found them maintaining +their erect attitude, at points many miles apart, in beds both above and between +the seams of coal. In order to explain this fact, we must suppose such shales +and sandstones to have been gradually accumulated during the slow and repeated +subsidence of the whole region. + +(FIGURE 409. Triassic coal-shale, Richmond, Virginia. +a. Estheria ovata. +b. Young of same. +c. Natural size of a. +d. Natural size of b.) + +The fossil fish are Ganoids, some of them of the genus Catopterus, others +belonging to the liassic genus Tetragonolepis (Aechmodus), see Figure 376. Two +species of Entomostraca called Estheria are in such profusion in some shaly beds +as to divide them like the plates of mica in micaceous shales (see Figure 409). + +These Virginian coal-measures are composed of grits, sandstones, and shales, +exactly resembling those of older or primary date in America and Europe, and +they rival, or even surpass, the latter in the richness and thickness of the +coal-seams. One of these, the main seam, is in some places from 30 to 40 feet +thick, composed of pure bituminous coal. The coal is like the finest kinds +shipped at Newcastle, and when analysed yields the same proportions of carbon +and hydrogen-- a fact worthy of notice, when we consider that this fuel has been +derived from an assemblage of plants very distinct specifically, and in part +generically, from those which have contributed to the formation of the ancient +or palaeozoic coal. + +TRIASSIC MAMMIFER. + +In North Carolina, the late Professor Emmons has described the strata of the +Chatham coal-field, which correspond in age to those near Richmond, in Virginia. +In beds underlying them he has met with three jaws of a small insectivorous +mammal which he has called Dromatherium sylvestre, closely allied to +Spalacotherium. Its nearest living analogue, says Professor Owen, "is found in +Myrmecobius; for each ramus of the lower jaw contained ten small molars in a +continuous series, one canine, and three conical incisors-- the latter being +divided by short intervals." + +LOW GRADE OF EARLY MAMMALS FAVOURABLE TO THE THEORY OF PROGRESSIVE DEVELOPMENT. + +There is every reason to believe that this fossil quadruped is at least as +ancient as the Microlestes of the European Trias described in Chapter 21; and +the fact is highly important, as proving that a certain low grade of marsupials +had not only a wide range in time, from the Trias to the Purbeck, or uppermost +oolitic strata of Europe, but had also a wide range in space, namely, from +Europe to North America, in an east and west direction, and, in regard to +latitude, from Stonesfield, in 52 degrees N., to that of North Carolina, 35 +degrees N. + +If the three localities in Europe where the most ancient mammalia have been +found-- Purbeck, Stonesfield, and Stuttgart-- had belonged all of them to +formations of the same age, we might well have imagined so limited an area to +have been peopled exclusively with pouched quadrupeds, just as Australia now is, +while other parts of the globe were inhabited by placentals; for Australia now +supports one hundred and sixty species of marsupials, while the rest of the +continents and islands are tenanted by about seventeen hundred species of +mammalia, of which only forty-six are marsupial, namely, the opossums of North +and South America. But the great difference of age of the strata in each of +these three localities seems to indicate the predominance throughout a vast +lapse of time (from the era of the Upper Trias to that of the Purbeck beds) of a +low grade of quadrupeds; and this persistency of similar generic and ordinal +types in Europe while the species were changing, and while the fish, reptiles, +and mollusca were undergoing great modifications, would naturally lead us to +suspect that there must also have been a vast extension in space of the same +marsupial forms during that portion of the Secondary or Mesozoic epoch which has +been termed "the age of reptiles." Such an inference as to the wide geographical +range of the ancient marsupials has been confirmed by the discovery in the Trias +of North America of the above-mentioned Dromatherium. The predominance in +earlier ages of these mammalia of a low grade, and the absence, so far as our +investigations have yet gone, of species of higher organisation, whether aquatic +or terrestrial, is certainly in favour of the theory of progressive development. + + +PRIMARY OR PALAEOZOIC SERIES. + +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. + +In pursuing our examination of the strata in descending order, we have next to +pass from the base of the Secondary or Mesozoic to the uppermost or newest of +the Primary or Palaeozoic formations. As this point has been selected as a line +of demarkation for one of the three great divisions of the fossiliferous series, +the student might naturally expect that by aid of lithological and +palaeontological characters he would be able to recognise without difficulty a +distinct break between the newer and older group. But so far is this from being +the case in Great Britain, that nowhere have geologists found more difficulty in +drawing the line of separation than between the Secondary and Primary series. +The obscurity has arisen from the great resemblance in colour and mineral +character of the Triassic and Permian red marls and sandstones, and the scarcity +and often total absence in them of organic remains. The thickness of the strata +belonging to each group amounts in some places to several thousand feet; and by +dint of a careful examination of their geological position, and of those fossil, +animal, and vegetable forms which are occasionally met with in some members of +each series, it has at length been made clear that the older or Permian rocks +are more connected with the Primary or Palaeozoic than with the Secondary or +Mesozoic strata already described. + +The term Permian has been proposed for this group by Sir R. Murchison, from +Perm, a Russian province, where it occupies an area twice the size of France, +and contains a great abundance and variety of fossils, both vertebrate and +invertebrate. Professor Sedgwick in 1832 described what is now recognised as the +central member of this group, the Magnesian limestone, showing that it attained +a thickness of 600 feet along the north-east of England, in the counties of +Durham, Yorkshire, and Nottinghamshire, its lower part often passing into a +fossiliferous marl-slate and resting on an inferior Red Sandstone, the +equivalent of the Rothliegendes of Germany. (Transactions of the Geological +Society London Second Series volume 3 page 37.) It has since been shown that +some of the Red Sandstones of newer date also belong to the Permian group; and +it appears from the observations of Mr. Binney, Sir R. Murchison, Mr. Harkness, +and others, that it is in the region where the limestone is most largely +developed, as, for example, in the county of Durham, that the associated red +sandstones or sedimentary rocks are thinnest, whereas in the country where the +latter are thickest the calcareous member is reduced to thirty, or even +sometimes to ten feet. It is clear, therefore, says Mr. Hull, that the +sedimentary region in the north of England area has been to the westward, and +the calcareous area to the eastward; and that in this group there has been a +development from opposite directions of the two types of strata. + +In illustration of this he has given us the following table: + +TABLE 22.1. THICKNESS OF PERMIAN STRATA IN NORTH OF ENGLAND. + +COLUMN 1: NAME OF STRATA. + +COLUMN 2: THICKNESS IN FEET IN N.W. OF ENGLAND. + +COLUMN 3: THICKNESS IN FEET IN N.E. OF ENGLAND. + +Upper Permian (Sedimentary): 600 : 50-100. +Middle Permian (Calcareous): 10-30 : 600. +Lower Permian (Sedimentary): 3000 : 100-250. (Edward Hull Ternary +Classification Quarterly Journal of Science No. 23 1869.) + +UPPER PERMIAN. + +What is called in this table the Upper Permian will be seen to attain its chief +thickness in the north-west, or on the coast of Cumberland, as at St. Bee's +Head, where it is described by Sir Roderick Murchison as consisting of massive +red sandstones with gypsum resting on a thin course of Magnesian Limestone with +fossils, which again is connected with the Lower Red Sandstone, resembling the +upper one in such a manner that the whole forms a continuous series. No fossil +footprints have been found in this Upper as in the Lower Red Sandstone. + +MIDDLE PERMIAN-- MAGNESIAN LIMESTONE AND MARL-SLATE. + +(FIGURE 410. Schizodus Schlotheimi, Geinitz. Permian crystalline limestone.) + +(FIGURE 411. The hinge of Schizodus truncatus, King. Permian.) + +(FIGURE 412. Mytilus septifer, King. Syn. Modiola acuminata, Sowerby. Permian +crystalline limestone.) + +This formation is seen upon the coast of Durham and Yorkshire, between the Wear +and the Tees. Among its characteristic fossils are Schizodus Schlotheimi (Figure +410) and Mytilus septifer (Figure 412). These shells occur at Hartlepool and +Sunderland, where the rock assumes an oolitic and botryoidal character. Some of +the beds in this division are ripple-marked. In some parts of the coast of +Durham, where the rock is not crystalline, it contains as much as 44 per cent of +carbonate of magnesia, mixed with carbonate of lime. In other places-- for it is +extremely variable in structure-- it consists chiefly of carbonate of lime, and +has concreted into globular and hemispherical masses, varying from the size of a +marble to that of a cannon-ball, and radiating from the centre. Occasionally +earthy and pulverulent beds pass into compact limestone or hard granular +dolomite. Sometimes the limestone appears in a brecciated form, the fragments +which are united together not consisting of foreign rocks but seemingly composed +of the breaking-up of the Permian limestone itself, about the time of its +consolidation. Some of the angular masses in Tynemouth cliff are two feet in +diameter. + +(FIGURE 413. Magnesian Limestone, Humbleton Hill, near Sunderland. (King's +Monograph Plate 2.) +a. Fenestella retiformis, Schlot, sp. Syn. Gorgonia infundibuliformis, Goldf.; +Retepora flustracea, Phillips. +b. Part of the same highly magnified.) + +The magnesian limestone sometimes becomes very fossiliferous and includes in it +delicate bryozoa, one of which, Fenestella retiformis (Figure 413), is a very +variable species, and has received many different names. It sometimes attains a +large size, single specimens measuring eight inches in width. The same bryozoan, +with several other British species, is also found abundantly in the Permian of +Germany. + +(FIGURE 414. Productus horridus, Sowerby. (P. calvus, Sowerby) Sunderland and +Durham, in Magnesian Limestone; Zechstein and Kupferschiefer, Germany.) + +(FIGURE 415. Lingula Crednerii. (Geinitz.) Magnesian Limestone, and +Carboniferous Marl-slate, Durham; Zechstein, Thuringia.) + +The total known fauna of the Permian series of Great Britain at present numbers +147 species, of which 77, or more than half, are mollusca. Not one of these is +common to rocks newer than the Palaeozoic, and the brachiopods are the only +group which have furnished species common to the more ancient or Carboniferous +rocks. Of these Lingula Crednerii (Figure 415) is an example. There are 25 +Gasteropods and only one cephalopod, Nautilus Freieslebeni, which is also found +in the German Zechstein. + +(FIGURE 416. Spirifera alata, Schloth. Syn. Trigonotreta undulata, Sowerby., +King's Monograph. Magnesian Limestone.) + +Shells of the genera Productus (Figure 414) and Strophalosia (the latter of +allied form with hinge teeth), which do not occur in strata newer than the +Permian, are abundant in the ordinary yellow magnesian limestone, as will be +seen in the valuable memoirs of Messrs. King and Howse. They are accompanied by +certain species of Spirifera (Figure 416), Lingula Crednerii (Figure 415), and +other brachiopoda of the true primary or palaeozoic type. Some of this same +tribe of shells, such as Camarophoria, allied to Rhynchonella, Spiriferina, and +two species of Lingula, are specifically the same as fossils of the +carboniferous rocks. Avicula, Arca, and Schizodus (Figure 410), and other +lamellibranchiate bivalves, are abundant, but spiral univalves are very rare. + +(FIGURE 417. Restored outline of a fish of the genus Palaeoniscus, Agassiz. +Palaeothrissum, Blainville.) + +Beneath the limestone lies a formation termed the marl-stone, which consists of +hard calcareous shales, marl-slate, and thin-bedded limestones. At East +Thickley, in Durham, where it is thirty feet thick, this slate has yielded many +fine specimens of fossil fish-- of the genera Palaeoniscus ten species, +Pygopterus two species, Coelacanthus two species, and Platysomus two species, +which as genera are common to the older Carboniferous formation, but the Permian +species are peculiar, and, for the most part, identical with those found in the +marl-slate or copper-slate of Thuringia. + +(FIGURE 418. Shark. Heterocercal.) + +(FIGURE 419. Shad. (Clupea. Herring tribe. Homocercal.) + +The Palaeoniscus above-mentioned belongs to that division of fishes which M. +Agassiz has called "Heterocercal," which have their tails unequally bilobate, +like the recent shark and sturgeon, and the vertebral column running along the +upper caudal lobe. (See Figure 418.) The "Homocercal" fish, which comprise +almost all the 9000 species at present known in the living creation, have the +tail-fin either single or equally divided; and the vertebral column stops short, +and is not prolonged into either lobe. (See Figure 419.) Now it is a singular +fact, first pointed out by Agassiz, that the heterocercal form, which is +confined to a small number of genera in the existing creation, is universal in +the magnesian limestone, and all the more ancient formations. It characterises +the earlier periods of the earth's history, whereas in the secondary strata, or +those newer than the Permian, the homocercal tail predominates. + +A full description has been given by Sir Philip Egerton of the species of fish +characteristic of the marl-slate, in Professor King's monograph before referred +to, where figures of the ichthyolites, which are very entire and well preserved, +will be found. Even a single scale is usually so characteristically marked as to +indicate the genus, and sometimes even the particular species. They are often +scattered through the beds singly, and may be useful to a geologist in +determining the age of the rock. + +(FIGURES 420-425. SCALES OF FISH. MAGNESIAN LIMESTONE. + +(FIGURE 420. Palaeoniscus comptus, Agassiz. Scale, magnified. Marl-slate.) + +(FIGURE 421. Palaeoniscus elegans, Sedgwick. Under surface of scale, magnified. +Marl-slate.) + +(FIGURE 422. Palaeoniscus glaphyrus, Agassiz. Under surface of scale, magnified. +Marl-slate.) + +(FIGURE 423. Coelacanthus granulatus, Agassiz. Granulated surface of scale, +magnified. Marl-slate.) + +(FIGURE 424. Pygopterus mandibularis. Agassiz. Marl-slate. +a. Outside of scale, magnified. +b. Under surface of same.) + +(FIGURE 425. Acrolepis Sedgwickii, Agassiz. Outside of scale, magnified. Marl- +slate.)) + +We are indebted to Messrs. Hancock and Howse for the discovery in this marl- +slate at Midderidge, Durham, of two species of Protosaurus, a genus of reptiles, +one representative of which, P. Speneri, has been celebrated ever since the year +1810 as characteristic of the Kupfer-schiefer or Permian of Thuringia. Professor +Huxley informs us that the agreement of the Durham fossil with Hermann von +Meyer's figure of the German specimen is most striking. Although the head is +wanting in all the examples yet found, they clearly belong to the Lacertian +order, and are therefore of a higher grade than any other vertebrate animal +hitherto found fossil in a Palaeozoic rock. Remains of Labyrinthodont reptiles +have also been met with in the same slate near Durham. + +LOWER PERMIAN. + +The inferior sandstones which lie beneath the marl-slate consist of sandstone +and sand, separating the Magnesian Limestone from the coal, in Yorkshire and +Durham. In some instances, red marl and gypsum have been found associated with +these beds. They have been classed with the Magnesian Limestone by Professor +Sedgwick, as being nearly co-extensive with it in geographical range, though +their relations are very obscure. But the principal development of Lower Permian +is, as we have seen by Mr. Hull's Table 22.1, in the northwest, where the +Penrith sandstone, as it has been called, and the associated breccias and purple +shales are estimated by Professor Harkness to attain a thickness of 3000 feet. +Organic remains are generally wanting, but the leaves and wood of coniferous +plants, and in one case a cone, have been found. Also in the purple marls of +Corncockle Muir near Dumfries, very distinct footprints of reptiles occur, +originally referred to the Trias, but shown by Mr. Binney in 1856 to be Permian. +No bones of the animals which they represent have yet been discovered. + +ANGULAR BRECCIAS IN LOWER PERMIAN. + +A striking feature in these beds is the occasional occurrence, especially at the +base of the formation, of angular and sometimes rounded fragments of +Carboniferous and older rocks of the adjoining districts being included in a +paste of red marl. Some of the angular masses are of huge size. + +In the central and southern counties, where the Middle Permian or Magnesian +Limestone is wanting, it is difficult to separate the upper and lower +sandstones, and Mr. Hull is of opinion that the patches of this formation found +here and there in Worcestershire, Shropshire, and other counties may have been +deposited in a sea separated from the northern basin by a barrier of +Carboniferous rocks running east and west, and now concealed under the Triassic +strata of Cheshire. Similar breccias to those before described are found in the +more southern counties last mentioned, where their appearance is rendered more +striking by the marked contrast they present to the beds of well-rolled and +rounded pebbles of the Trias occupying a large area in the same region. + +Professor Ramsay refers the angular form and large size of the fragments +composing these breccias to the action of floating ice in the sea. These masses +of angular rock, some of them weighing more than half a ton, and lying +confusedly in a red, unstratified marl, like stones in boulder-drift, are in +some cases polished, striated, and furrowed like erratic blocks in the moraine +of a glacier. They can be shown in some cases to have travelled from the parent +rocks, thirty or more miles distant, and yet not to have lost their angular +shape. (Ramsay Quarterly Geological Journal 1855; and Lyell Principles of +Geology volume 1 page 223 10th edition.) + +PERMIAN ROCKS OF THE CONTINENT. + +Germany is the classic ground of the Magnesian Limestone now called Permian. The +formation was well studied by the miners of that country a century ago as +containing a thin band of dark-coloured cupriferous shale, characterised at +Mansfield in Thuringia by numerous fossil fish. Beneath some variegated +sandstones (not belonging to the Trias, though often confounded with it) they +came down first upon a dolomitic limestone corresponding to the upper part of +our Middle Permian, and then upon a marl-slate richly impregnated with copper +pyrites, and containing fish and reptiles (Protosaurus) identical in species +with those of the corresponding marl-slate of Durham. To the limestone they gave +the name of Zechstein, and to the marl-slate that of Mergel-schiefer or Kupfer- +schiefer. Beneath the fossiliferous group lies the Rothliegendes or Rothtodt- +liegendes, meaning the red-lyer or red-dead-lyer, so-called by the German miners +from its colour, and because the copper had DIED OUT when they reached this +underlying non-metalliferous member of the series. This red under-lyer is, in +fact, a great deposit of red sandstone, breccia, and conglomerate with +associated porphyry, basalt, and amygdaloid. + +According to Sir R. Murchison, the Permian rocks are composed, in Russia, of +white limestone, with gypsum and white salt; and of red and green grits, +occasionally with copper ore; also magnesian limestones, marl-stones, and +conglomerates. + +PERMIAN FLORA. + +(FIGURE 426. Walchia piniformis, Schloth. Permian, Saxony. (Gutbier, Die +Versteinerungen des Permischen Systemes in Sachsen volume 2 plate 10.) +a. Branch. +b. Twig of the same. +c. Leaf magnified.) + +About 18 or 20 species of plants are known in the Permian rocks of England. None +of them pass down into the Carboniferous series, but several genera, such as +Alethopteris, Neuropteris, Walchia, and Ullmania, are common to the two groups. +The Permian flora on the Continent appears, from the researches of MM. Murchison +and de Verneuil in Russia, and of MM. Geinitz and von Gutbier in Saxony, to be, +with a few exceptions, distinct from that of the coal. + +In the Permian rocks of Saxony no less than 60 species of fossil plants have +been met with. Two or three of these, as Calamites gigas, Sphenopteris erosa, +and S. lobata, are also met with in the government of Perm in Russia. Seven +others, and among them Neuropteris Loshii, Pecopteris arborescens, and P. +similis, and several species of Walchia (see Figure 426), a genus of Conifers, +called Lycopodites by some authors, are said by Geinitz to be common to the +coal-measures. + +(FIGURE 427. Cardiocarpon Ottonis. Gutbier, Permian, Saxony. 1/2 diameter.) + +(FIGURE 428. Neoggerathia cuneifolia. Brongniart. (Murchison's Russia volume 2 +Plate A figure 3.) + +Among the genera also enumerated by Colonel Gutbier are the fruit called +Cardiocarpon (see Figure 427), Asterophyllites, and Annularia, so characteristic +of the Carboniferous period; also Lepidodendron, which is common to the Permian +of Saxony, Thuringia, and Russia, although not abundant. Neoggerathia (see +Figure 428), the leaves of which have parallel veins without a midrib, and to +which various generic synonyms, such as Cordaites, Flabellaria, and Poacites, +have been given, is another link between the Permian and Carboniferous +vegetation. Coniferae, of the Araucarian division, also occur; but these are +likewise met with both in older and newer rocks. The plants called Sigillaria +and Stigmaria, so marked a feature in the Carboniferous period, are as yet +wanting in the true Permian. + +Among the remarkable fossils of the Rothliegendes, or lowest part of the Permian +in Saxony and Bohemia, are the silicified trunks of tree-ferns called +generically Psaronius. Their bark was surrounded by a dense mass of air-roots, +which often constituted a great addition to the original stem, so as to double +or quadruple its diameter. The same remark holds good in regard to certain +living extra-tropical arborescent ferns, particularly those of New Zealand. + +Upon the whole, it is evident that the Permian plants approach much nearer to +the Carboniferous flora than to the Triassic; and the same may be said of the +Permian fauna. + + +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. + +PRINCIPAL SUBDIVISIONS OF THE CARBONIFEROUS GROUP. + +The next group which we meet with in the descending order is the Carboniferous, +commonly called "The Coal," because it contains many beds of that mineral, in a +more or less pure state, interstratified with sandstones, shales, and +limestones. The coal itself, even in Great Britain and Belgium, where it is most +abundant, constitutes but an insignificant portion of the whole mass. In South +Wales, for example, the thickness of the coal-bearing strata has been estimated +at between 11,000 and 12,000 feet, while the various coal seams, about 80 in +number, do not, according to Professor Phillips, exceed in the aggregate 120 +feet. + +The Carboniferous formation assumes various characters in different parts even +of the British Islands. It usually comprises two very distinct members: first, +the sedimentary beds, usually called the Coal-measures, of mixed fresh-water, +terrestrial, and marine origin, often including seams of coal; secondly, that +named in England the Mountain or Carboniferous Limestone, of purely marine +origin, and made up chiefly of corals, shells, and encrinites, and resting on +shales called the shales of the Mountain Limestone. + +TABLE 23.1. + +In the south-western part of our island, in Somersetshire and South Wales, the +three divisions usually spoken of are: + +1. Coal-measures: strata of shale, sandstone, and grit, from 600 to 12,000 feet +thick, with occasional seams of coal. + +2. Millstone grit: a coarse quartzose sandstone passing into a conglomerate, +sometimes used for millstones, with beds of shale; usually devoid of coal; +occasionally above 600 feet thick. + +3. Mountain or Carboniferous Limestone: a calcareous rock containing marine +shells, corals, and encrinites; devoid of coal; thickness variable, sometimes +more than 1500 feet. + +If the reader will refer to the section in Figure 85, he will see that the Upper +and Lower Coal-measures of the coal-field near Bristol are divided by a +micaceous flaggy sandstone called the Pennant Rock. The Lower Coal-measures of +the same section rest sometimes, especially in the north part of the basin, on a +base of coarse grit called the Millstone Grit (No. 2 of the above Table 23.1.) + +In the South Welsh coal-field Millstone Grit occurs in like manner at the base +of the productive coal. It is called by the miners the "Farewell Rock," as when +they reach it they have no longer any hopes of obtaining coal at a greater depth +in the same district. In the central and northern coal-fields of England this +same grit, including quartz pebbles, with some accompanying sandstones and +shales containing coal plants, acquires a thickness of several thousand feet, +lying beneath the productive coal-measures, which are nearly 10,000 feet thick. + +Below the Millstone Grit is a continuation of similar sandstones and shales +called by Professor Phillips the Yoredale series, from Yoredale, in Yorkshire, +where they attain a thickness of from 800 to 1000 feet. At several intervals +bands of limestone divide this part of the series, one of which, called the Main +Limestone or Upper Scar Limestone, composed in great part of encrinites, is 70 +feet thick. Thin seams of coal also occur in these lower Yoredale beds in +Yorkshire, showing that in the same region there were great alternations in the +state of the surface. For at successive periods in the same area there prevailed +first terrestrial conditions favourable to the growth of pure coal, secondly, a +sea of some depth suited to the formation of Carboniferous Limestone, and, +thirdly, a supply of muddy sediment and sand, furnishing the materials for +sandstone and shale. There is no clear line of demarkation between the Coal- +measures and the Millstone Grit, nor between the Millstone Grit and underlying +Yoredale rocks. + +On comparing a series of vertical sections in a north-westerly direction from +Leicestershire and Warwickshire into North Lancashire, we find, says Mr. Hull, +within a distance of 120 miles an augmentation of the sedimentary materials to +the extent of 16,000 feet. + +Leicestershire and Warwickshire: 2,600 feet. +North Staffordshire: 9,000 feet. +South Lancashire: 12,130 feet. +North Lancashire: 18,700 feet. + +In central England, where the sedimentary beds are reduced to about 3000 feet in +all, the Carboniferous Limestone attains an enormous thickness, as much as 4000 +feet at Ashbourne, near Derby, according to Mr. Hull's estimate. To a certain +extent, therefore, we may consider the calcareous member of the formation as +having originated simultaneously with the accumulation of the materials of grit, +sandstone, and shale, with seams of coal; just as strata of mud, sand, and +pebbles, several thousand feet thick, with layers of vegetable matter, are now +in the process of formation in the cypress swamps and delta of the Mississippi, +while coral reefs are forming on the coast of Florida and in the sea of the +Bermuda islands. For we may safely conclude that in the ancient Carboniferous +ocean those marine animals which were limestone builders were never freely +developed in areas where the rivers poured in fresh water charged with sand or +clay; and the limestone could only become several thousand feet thick in parts +of the ocean which remained perfectly clear for ages. + +The calcareous strata of the Scotch coal-fields, those of Lanarkshire, the +Lothians, and Fife, for example, are very insignificant in thickness when +compared to those of England. They consist of a few beds intercalated between +the sandstones and shales containing coal and ironstone, the combined thickness +of all the limestones amounting to no more than 150 feet. The vegetation of some +of these northern sedimentary beds containing coal may be older than any of the +coal-measures of central and southern England, as being coeval with the Mountain +Limestone of the south. In Ireland the limestone predominates over the coal- +bearing sands and shales. We may infer the former continuity of several of the +coal-fields in northern and central England, not only from the abrupt manner in +which they are cut off at their outcrop, but from their remarkable +correspondence in the succession and character of particular beds. But the +limited extent to which these strata are exposed at the surface is not merely +owing to their former denudation, but even in a still greater degree to their +having been largely covered by the New Red Sandstone, as in Cheshire, and here +and there by the Permian strata, as in Durham. + +It has long been the opinion of the most eminent geologists that the coal-fields +of Yorkshire and Lancashire were once united, the upper Coal-measures and the +overlying Millstone Grit and Yoredale rocks having been subsequently removed; +but what is remarkable, is the ancient date now assigned to this denudation, for +it seems that a thickness of no less than 10,000 feet of the coal-measures had +been carried away before the deposition even of the lower Permian rocks which +were thrown down upon the already disturbed truncated edges of the coal-strata. +(Edward Hull Quarterly Geological Journal volume 24 page 327.) The carboniferous +strata most productive of workable coal have so often a basin-shaped arrangement +that these troughs have sometimes been supposed to be connected with the +original conformation of the surface upon which the beds were deposited. But it +is now admitted that this structure has been owing to movements of the earth's +crust of upheaval and subsidence, and that the flexure and inclination of the +beds has no connection with the original geographical configuration of the +district. + +COAL-MEASURES. + +I shall now treat more particularly of the productive coal-measures, and their +mode of origin and organic remains. + +COAL-FORMED ON LAND. + +In South Wales, already alluded to, where the coal-measures attain a thickness +of 12,000 feet, the beds throughout appear to have been formed in water of +moderate depth, during a slow, but perhaps intermittent, depression of the +ground, in a region to which rivers were bringing a never-failing supply of +muddy sediment and sand. The same area was sometimes covered with vast forests, +such as we see in the deltas of great rivers in warm climates, which are liable +to be submerged beneath fresh or salt water should the ground sink vertically a +few feet. + +In one section near Swansea, in South Wales, where the total thickness of strata +is 3246 feet, we learn from Sir H. De la Beche that there are ten principal +masses of sandstone. One of these is 500 feet thick, and the whole of them make +together a thickness of 2125 feet. They are separated by masses of shale, +varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in +number, are generally from one to five feet thick, one of them, which has two or +three layers of clay interposed, attaining nine feet. At other points in the +same coal-field the shales predominate over the sandstones. Great as is the +diversity in the horizontal extent of individual coal-seams, they all present +one characteristic feature, in having, each of them, what is called its +UNDERCLAY. These underclays, co-extensive with every layer of coal, consist of +arenaceous shale, sometimes called fire-stone, because it can be made into +bricks which stand the fire of a furnace. They vary in thickness from six inches +to more than ten feet; and Sir William Logan first announced to the scientific +world in 1841 that they were regarded by the colliers in South Wales as an +essential accompaniment of each of the eighty or more seams of coal met with in +their coal-field. They are said to form the FLOOR on which the coal rests; and +some of them have a slight admixture of carbonaceous matter, while others are +quite blackened by it. + +All of them, as Sir William Logan pointed out, are characterised by inclosing a +peculiar species of fossil vegetable called Stigmaria, to the exclusion of other +plants. It was also observed that, while in the overlying shales, or "roof" of +the coal, ferns and trunks of trees abound without any Stigmariae, and are +flattened and compressed, those singular plants of the underclay most commonly +retain their natural forms, unflattened and branching freely, and sending out +their slender rootlets, formerly thought to be leaves, through the mud in all +directions. Several species of Stigmaria had long been known to botanists, and +described by them, before their position under each seam of coal was pointed +out, and before their true nature as the roots of trees (some having been +actually found attached to the base of Sigillaria stumps) was recognised. It was +conjectured that they might be aquatic, perhaps floating plants, which sometimes +extended their branches and leaves freely in fluid mud, in which they were +finally enveloped. + +Now that all agree that these underclays are ancient soils, it follows that in +every instance where we find them they attest the terrestrial nature of the +plants which formed the overlying coal, which consists of the trunks, branches, +and leaves of the same plants. The trunks have generally fallen prostrate in the +coal, but some of them still remain at right angles to the ancient soils (see +Figure 440). Professor Goppert, after examining the fossil vegetables of the +coal-fields of Germany, has detected, in beds of pure coal, remains of plants of +every family hitherto known to occur fossil in the carboniferous rocks. Many +seams, he remarks, are rich in Sigillariae, Lepidodendra, and Stigmariae, the +latter in such abundance as to appear to form the bulk of the coal. In some +places, almost all the plants were calamites, in others ferns. (Quarterly +Geological Journal volume 5 Mem. page 17.) + +Between the years 1837 and 1840, six fossil trees were discovered in the coal- +fields of Lancashire, where it is intersected by the Bolton railway. They were +all at right angles to the plane of the bed, which dips about 15 degrees to the +south. The distance between the first and the last was more than 100 feet, and +the roots of all were imbedded in a soft argillaceous shale. In the same plane +with the roots is a bed of coal, eight or ten inches thick, which has been found +to extend across the railway, or to the distance of at least ten yards. Just +above the covering of the roots, yet beneath the coal-seam, so large a quantity +of the Lepidostrobus variabilis was discovered inclosed in nodules of hard clay, +that more than a bushel was collected from the small openings around the base of +some of the trees (see Figure 457 of this genus). The exterior trunk of each was +marked by a coating of friable coal, varying from one-quarter to three-quarters +of an inch in thickness; but it crumbled away on removing the matrix. The +dimensions of one of the trees is 15 1/2 feet in circumference at the base, 7 +1/2 feet at the top, its height being eleven feet. All the trees have large +spreading roots, solid and strong, sometimes branching, and traced to a distance +of several feet, and presumed to extend much farther. + +In a colliery near Newcastle a great number of Sigillariae occur in the rock as +if they had retained the position in which they grew. No less than thirty, some +of them four or five feet in diameter, were visible within an area of 50 yards +square, the interior being sandstone, and the bark having been converted into +coal. Such vertical stems are familiar to our coal-miners, under the name of +coal-pipes. They are much dreaded, for almost every year in the Bristol, +Newcastle, and other coal-fields, they are the cause of fatal accidents. Each +cylindrical cast of a tree, formed of solid sandstone, and increasing gradually +in size towards the base, and being without branches, has its whole weight +thrown downward, and receives no support from the coating of friable coal which +has replaced the bark. As soon, therefore, as the cohesion of this external +layer is overcome, the heavy column falls suddenly in a perpendicular or oblique +direction from the roof of the gallery whence coal has been extracted, wounding +or killing the workman who stands below. It is strange to reflect how many +thousands of these trees fell originally in their native forests in obedience to +the law of gravity; and how the few which continued to stand erect, obeying, +after myriads of ages, the same force, are cast down to immolate their human +victims. + +(FIGURE 429. Ground-plan of a fossil forest, Parkfield Colliery, near +Wolverhampton, showing the position of 73 trees in a quarter of an acre.) + +It has been remarked that if, instead of working in the dark, the miner was +accustomed to remove the upper covering of rock from each seam of coal, and to +expose to the day the soils on which ancient forests grew, the evidence of their +former growth would be obvious. Thus in South Staffordshire a seam of coal was +laid bare in the year 1844, in what is called an open work at Parkfield +colliery, near Wolverhampton. In the space of about a quarter of an acre the +stumps of no less than 73 trees with their roots attached appeared, as shown in +Figure 429, some of them more than eight feet in circumference. The trunks, +broken off close to the root, were lying prostrate in every direction, often +crossing each other. One of them measured 15, another 30 feet in length, and +others less. They were invariably flattened to the thickness of one or two +inches, and converted into coal. Their roots formed part of a stratum of coal +ten inches thick, which rested on a layer of clay two inches thick, below which +was a second forest resting on a two-foot seam of coal. Five feet below this, +again, was a third forest with large stumps of Lepidodendra, Calamites, and +other trees. + +BLENDING OF COAL-SEAMS. + +Both in England and North America seams of coal are occasionally observed to be +parted from each other by layers of clay and sand, and, after they have been +persistent for miles, to come together and blend in one single bed, which is +then found to be equal in the aggregate to the thickness of the several seams. I +was shown by Mr. H.D. Rogers a remarkable example of this in Pennsylvania. In +the Shark Mountain, near Pottsville, in that State, there are thirteen seams of +anthracite coal, some of them more than six feet thick, separated by beds of +white quartzose grit and a conglomerate of quartz pebbles, often of the size of +a hen's egg. Between Pottsville and the Lehigh Summit Mine, seven of these seams +of coal, at first widely separated, are, in the course of several miles, brought +nearer and nearer together by the gradual thinning out of the intervening +coarse-grained strata and their accompanying shales, until at length they +successively unite and form one mass of coal between forty and fifty feet thick, +very pure on the whole, though with a few thin partings of clay. This mass of +coal I saw quarried in the open air at Mauch Chunk, on the Bear Mountain. The +origin of such a vast thickness of vegetable remains, so unmixed, on the whole, +with earthy ingredients, can be accounted for in no other way than by the +growth, during thousands of years, of trees and ferns in the manner of peat-- a +theory which the presence of the Stigmaria in situ under each of the seven +layers of anthracite fully bears out. The rival hypothesis, of the drifting of +plants into a sea or estuary, leaves the non-intermixture of sediment, or of +clay, sand, and pebbles, with the pure coal wholly unexplained. + +(FIGURE 430. Uniting of distinct coal-seams.) + +The late Mr. Bowman was the first who gave a satisfactory explanation of the +manner in which distinct coal-seams, after maintaining their independence for +miles, may at length unite, and then persist throughout another wide area with a +thickness equal to that which the separate seams had previously maintained. + +Let A-C (Figure 430) be a three-foot seam of coal originally laid down as a mass +of vegetable matter on the level area of an extensive swamp, having an under- +clay, f-g, through which the Stigmariae or roots of the trees penetrate as +usual. One portion, B-C, of this seam of coal is now inclined; the area of the +swamp having subsided as much as 25 feet at E-C, and become for a time submerged +under salt, fresh, or brackish water. Some of the trees of the original forest +A-B-C fell down, others continued to stand erect in the new lagoon, their stumps +and part of their trunks becoming gradually enveloped in layers of sand and mud, +which at length filled up the new piece of water C-E. + +When this lagoon has been entirely silted up and converted into land, the +forest-covered surface A-B will extend once more over the whole area A-B-E, and +a second mass of vegetable matter, D-E, forming three feet more of coal, will +accumulate. We then find in the region E-C two seams of coals, each three feet +thick, with their respective under-clays, with erect buried trees based upon the +surface of the lower coal, the two seams being separated by 25 feet of +intervening shale and sandstone. Whereas in the region A-B, where the growth of +the forest has never been interrupted by submergence, there will simply be one +seam, two yards thick, corresponding to the united thickness of the beds B-E and +B-C. It may be objected that the uninterrupted growth of plants during the +interval of time required for the filling up of the lagoon will have caused the +vegetable matter in the region D-A-B to be thicker than the two distinct seams E +and C, and no doubt there would actually be a slight excess representing one or +more generation of trees and plants forming the undergrowth; but this excess of +vegetable matter, when compressed into coal, would be so insignificant in +thickness that the miner might still affirm that the seam D-A throughout the +area D-A-B was equal to the two seams C and E. + +CAUSE OF THE PURITY OF COAL. + +The purity of the coal itself, or the absence in it of earthy particles and +sand, throughout areas of vast extent, is a fact which appears very difficult to +explain when we attribute each coal-seam to a vegetation growing in swamps. It +has been asked how, during river inundations capable of sweeping away the leaves +of ferns and the stems and roots of Sigillariae and other trees, could the +waters fail to transport some fine mud into the swamps? One generation after +another of tall trees grew with their roots in mud, and their leaves and +prostrate trunks formed layers of vegetable matter, which was afterwards covered +with mud since turned to shale. Yet the coal itself, or altered vegetable +matter, remained all the while unsoiled by earthy particles. This enigma, +however perplexing at first sight, may, I think, be solved by attending to what +is now taking place in deltas. The dense growth of reeds and herbage which +encompasses the margins of forest-covered swamps in the valley and delta of the +Mississippi is such that the fluviatile waters, in passing through them, are +filtered and made to clear themselves entirely before they reach the areas in +which vegetable matter may accumulate for centuries, forming coal if the climate +be favourable. There is no possibility of the least intermixture of earthy +matter in such cases. Thus in the large submerged tract called the "Sunk +Country," near New Madrid, forming part of the western side of the valley of the +Mississippi, erect trees have been standing ever since the year 1811-12, killed +by the great earthquake of that date; lacustrine and swamp plants have been +growing there in the shallows, and several rivers have annually inundated the +whole space, and yet have been unable to carry in any sediment within the outer +boundaries of the morass, so dense is the marginal belt of reeds and brush-wood. +It may be affirmed that generally, in the "cypress swamps" of the Mississippi, +no sediment mingles with the vegetable matter accumulated there from the decay +of trees and semi-aquatic plants. As a singular proof of this fact, I may +mention that whenever any part of a swamp in Louisiana is dried up, during an +unusually hot season, and the wood set on fire, pits are burnt into the ground +many feet deep, or as far down as the fire can descend without meeting with +water, and it is then found that scarcely any residuum or earthy matter is left. +At the bottom of all these "cypress swamps" a bed of clay is found, with roots +of the tall cypress (Taxodium distichum), just as the under-clays of the coal +are filled with Stigmaria. + +CONVERSION OF COAL INTO ANTHRACITE. + +It appears from the researches of Liebig and other eminent chemists, that when +wood and vegetable matter are buried in the earth exposed to moisture, and +partially or entirely excluded from the air, they decompose slowly and evolve +carbonic acid gas, thus parting with a portion of their original oxygen. By this +means they become gradually converted into lignite or wood-coal, which contains +a larger proportion of hydrogen than wood does. A continuance of decomposition +changes this lignite into common or bituminous coal, chiefly by the discharge of +carbureted hydrogen, or the gas by which we illuminate our streets and houses. +According to Bischoff, the inflammable gases which are always escaping from +mineral coal, and are so often the cause of fatal accidents in mines, always +contain carbonic acid, carbureted hydrogen, nitrogen, and olefiant gas. The +disengagement of all these gradually transforms ordinary or bituminous coal into +anthracite, to which the various names of glance-coal, coke, hard-coal, culm, +and many others, have been given. + +There is an intimate connection between the extent to which the coal has in +different regions parted with its gaseous contents, and the amount of +disturbance which the strata have undergone. The coincidence of these phenomena +may be attributed partly to the greater facility afforded for the escape of +volatile matter, when the fracturing of the rocks has produced an infinite +number of cracks and crevices. The gases and water which are made to penetrate +these cracks are probably rendered the more effective as metamorphic agents by +increased temperature derived from the interior. It is well known that, at the +present period, thermal waters and hot vapours burst out from the earth during +earthquakes, and these would not fail to promote the disengagement of volatile +matter from the Carboniferous rocks. + +In Pennsylvania the strata of coal are horizontal to the westward of the +Alleghany Mountains, where the late Professor H.D. Rogers pointed out that they +were most bituminous; but as we travel south-eastward, where they no longer +remain level and unbroken, the same seams become progressively debitumenized in +proportion as the rocks become more bent and distorted. At first, on the Ohio +River, the proportion of hydrogen, oxygen, and other volatile matters ranges +from forty to fifty per cent. Eastward of this line, on the Monongahela, it +still approaches forty per cent, where the strata begin to experience some +gentle flexures. On entering the Alleghany Mountains, where the distinct +anticlinal axes begin to show themselves, but before the dislocations are +considerable, the volatile matter is generally in the proportion of eighteen or +twenty per cent. At length, when we arrive at some insulated coal-fields +associated with the boldest flexures of the Appalachian chain, where the strata +have been actually turned over, as near Pottsville, we find the coal to contain +only from six per cent of volatile matter, thus becoming a genuine anthracite. + +CLAY-IRONSTONE. + +Bands and nodules of clay-ironstone are common in coal-measures, and are formed, +says Sir H. De la Beche, of carbonate of iron mingled mechanically with earthy +matter, like that constituting the shales. Mr. Hunt, of the Museum of Practical +Geology, instituted a series of experiments to illustrate the production of this +substance, and found that decomposing vegetable matter, such as would be +distributed through all coal strata, prevented the further oxidation of the +proto-salts of iron, and converted the peroxide into protoxide by taking a +portion of its oxygen to form carbonic acid. Such carbonic acid, meeting with +the protoxide of iron in solution, would unite with it and form a carbonate of +iron; and this mingling with fine mud, when the excess of carbonic acid was +removed, might form beds or nodules of argillaceous ironstone. (Memoirs of the +Geological Survey pages 51, 255, etc.) + +INTERCALATED MARINE BEDS IN COAL. + +(FIGURE 431. Microconchus (Spirorbis) carbonarius, Murchison. Natural size and +magnified. +b. Variety of same.) + +(FIGURE 432. Cythere (Leperditia) inflata. Natural size and magnified. +Murchison.) + +(FIGURE 433. Goniatites Listeri, Martin. Coal-measures, Yorkshire and +Lancashire.) + +(FIGURE 434. Aviculopecten papyraceus, Goldf. (Pecten papyraceus, Sowerby.)) + +Both in the coal-fields of Europe and America the association of fresh, +brackish-water, and marine strata with coal-seams of terrestrial origin is +frequently recognised. Thus, for example, a deposit near Shrewsbury, probably +formed in brackish water, has been described by Sir R. Murchison as the youngest +member of the coal-measures of that district, at the point where they are in +contact with the overlying Permian group. It consists of shales and sandstones +about 150 feet thick, with coal and traces of plants; including a bed of +limestone varying from two to nine feet in thickness, which is cellular, and +resembles some lacustrine limestones of France and Germany. It has been traced +for 30 miles in a straight line, and can be recognised at still more distant +points. The characteristic fossils are a small bivalve, having the form of a +Cyclas or Cyrena, also a small entomostracan, Cythere inflata (Figure 432), and +the microscopic shell of an annelid of an extinct genus called Microconchus +(Figure 431), allied to Spirorbis. In the coal-field of Yorkshire there are +fresh-water strata, some of which contain shells referred to the family +Unionidae; but in the midst of the series there is one thin but very widely- +spread stratum, abounding in fishes and marine shells, such as Goniatites +Listeri (Figure 433), Orthoceras, and Aviculopecten papyraceus, Goldf. (Figure +434). + +INSECTS IN EUROPEAN COAL. + +Articulate animals of the genus Scorpion were found by Count Sternberg in 1835 +in the coal-measures of Bohemia, and about the same time in those of Coalbrook +Dale by Mr. Prestwich, were also true insects, such as beetles of the family +Curculionidae, a neuropterous insect of the genus Corydalis, and another related +to the Phasmidae, have been found. + +(FIGURE 435. Wing of a Grasshopper. Gryllacris lithanthraca, Goldenberg. Coal, +Saarbruck, near Treves.) + +From the coal of Wetting, in Westphalia, several specimens of the cockroach or +Blatta family, and the wing of a cricket (Acridites) have been described by +Germar. Professor Goldenberg published, in 1854, descriptions of no less than +twelve species of insects from the nodular clay-ironstone of Saarbruck, near +Treves. (Dunker and V. Meyer Palaeontology volume 4 page 17.) Among them are +several Blattinae, three species of Neuroptera, one beetle of the Scarabaeus +family, a grasshopper or locust, Gryllacris (see Figure 435), and several white +ants or Termites. Professor Goldenberg showed me, in 1864, the wing of a white +ant, found low down in the productive coal-measures of Saarbruck, in the +interior of a flattened Lepidodendron. It is much larger than that of any known +living species of the same genus. + +BATRACHIAN REPTILES IN COAL. + +(FIGURE 436. Archegosaurus minor, Goldfuss. Fossil reptile from the coal- +measures, Saarbruck.) + +(FIGURE 437. Imbricated covering of skin of Archegosaurus medius, Goldf. +Magnified.) + +No vertebrated animals more highly organised than fish were known in rocks of +higher antiquity than the Permian until the year 1844, when the Apateon +pedestris, Meyer, was discovered in the coal-measures of Munster-Appel in +Rhenish Bavaria, and three years later, in 1847, Professor von Dechen found +three other distinct species of the same family of Amphibia in the Saarbruck +coal-field above alluded to. These were described by the late Professor Goldfuss +under the generic name of Archegosaurus. The skulls, teeth, and the greater +portions of the skeleton, nay, even a large part of the skin, of two of these +reptiles have been faithfully preserved in the centre of spheroidal concretions +of clay-ironstone. The largest of these, Archegosaurus Decheni, must have been +three feet six inches long. Figure 436 represents the skull and neck bones of +the smallest of the three, of the natural size. They were considered by Goldfuss +as saurians, but by Herman von Meyer as most nearly allied to the Labyrinthodon +before mentioned (Chapter 21), and the remains of the extremities leave no doubt +they were quadrupeds, "provided," says Von Meyer, "with hands and feet +terminating in distinct toes; but these limbs were weak, serving only for +swimming or creeping." The same anatomist has pointed out certain points of +analogy between their bones and those of the Proteus anguinus; and Professor +Owen has observed that they make an approach to the Proteus in the shortness of +their ribs. Two specimens of these ancient reptiles retain a large part of the +outer skin, which consisted of long, narrow, wedge-shaped, tile-like, and horny +scales, arranged in rows (see Figure 437). + +In 1865, several species belonging to three different genera of the same family +of perennibranchiate Batrachians were found in the coal-field of Kilkenny in +bituminous shale at the junction of the coal with the underlying Stigmaria- +bearing clay. They were, probably, inhabitants of a marsh, and the large +processes projecting from the vertebrae of their tail imply, according to +Professor Huxley, great powers of swimming. They were of the Labyrinthodont +family, and their association with the fish of the coal, of which so large a +proportion are ganoids, reminds us that the living perennibranchiate amphibia of +America frequent the same rivers as the ganoid Lepidostei or bony pikes. + +LABYRINTHODONT FOOTPRINTS IN COAL-MEASURES. + +(FIGURE 438. Slab of sandstone from the coal-measures of Pennsylvania, with +footprints of air-breathing reptile and casts of cracks. Scale one-sixth the +original.) + +In 1844, the very year when the Apateon, before mentioned, of the coal was first +met with in the country between the Moselle and the Rhine, Dr. King published an +account of the footprints of a large reptile discovered by him in North America. +These occur in the coal-strata of Greensburg, in Westmoreland County, +Pennsylvania; and I had an opportunity of examining them when in that country in +1846. The footmarks were first observed standing out in relief from the lower +surface of slabs of sandstone, resting on thin layers of fine unctuous clay. I +brought away one of these masses, which is represented in Figure 438. It +displays, together with footprints, the casts of cracks (a, a') of various +sizes. The origin of such cracks in clay, and casts of the same, has before been +explained, and referred to the drying and shrinking of mud, and the subsequent +pouring of sand into open crevices. It will be seen that some of the cracks, as +at b, c, traverse the footprints, and produce distortion in them, as might have +been expected, for the mud must have been soft when the animal walked over it +and left the impressions; whereas, when it afterwards dried up and shrank, it +would be too hard to receive such indentations. + +We may assume that the reptile which left these prints on the ancient sands of +the coal-measures was an air-breather, because its weight would not have been +sufficient under water to have made impressions so deep and distinct. The same +conclusion is also borne out by the casts of the cracks above described, for +they show that the clay had been exposed to the air and sun, so as to have dried +and shrunk. + +NOVA SCOTIA COAL-MEASURES. + +The sedimentary strata in which thin seams of coal occur attain a thickness, as +we have seen, of 18,000 feet in the north of England exclusive of the Mountain +Limestone, and are estimated by Von Dechen at over 20,000 feet in Rhenish +Prussia. But the finest example in the world of a natural exposure in a +continuous section ten miles long, occurs in the sea-cliffs bordering a branch +of the Bay of Fundy, in Nova Scotia. These cliffs, called the "South Joggins," +which I first examined in 1842, and afterwards with Dr. Dawson in 1845, have +lately been admirably described by the last-mentioned geologist in detail, and +his evidence is most valuable as showing how large a portion of this dense mass +was formed on land, or in swamps where terrestrial vegetation flourished, or in +fresh-water lagoons. (Acadian Geology second edition 1868.) His computation of +the thickness of the whole series of carboniferous strata as exceeding three +miles, agrees with the measurement made independently by Sir William Logan in +his survey of this coast. + +There is no reason to believe that in this vast succession of strata, comprising +some marine as well as many fresh-water and terrestrial formations, there is any +repetition of the same beds. There are no faults to mislead the geologist, and +cause him to count the same beds over more than once, while some of the same +plants have been traced from the top to the bottom of the whole series, and are +distinct from the flora of the antecedent Devonian formation of Canada. Eighty- +one seams of coal, varying in thickness from an inch to about five feet, have +been discovered, and no less than seventy-one of these have been actually +exposed in the sea-cliffs. + +(FIGURE 439. Section of the cliffs of the South Joggins, near Minudie, Nova +Scotia (from north to south through coal with upright trees and sandstone and +shale). +c. Grindstone. +d, g. Alternations of sandstone, shale, and coal containing upright trees. +e, f. Portion of cliff, given on a larger scale in Figure 440. +f. Four-foot coal, main seam. +h, i. Shale with fresh-water mussels, see below.) + +In the section in Figure 439, which I examined in 1842, the beds from c to i are +seen all dipping the same way, their average inclination being at an angle of 24 +degrees S.S.W. The vertical height of the cliffs is from 150 to 200 feet; and +between d and g-- in which space I observed seventeen trees in an upright +position, or, to speak more correctly, at right angles to the planes of +stratification-- I counted nineteen seams of coal, varying in thickness from two +inches to four feet. At low tide a fine horizontal section of the same beds is +exposed to view on the beach, which at low tide extends sometimes 200 yards from +the base of the cliff. The thickness of the beds alluded to, between d and g, is +about 2500 feet, the erect trees consisting chiefly of large Sigillariae, +occurring at ten distinct levels, one above the other. The usual height of the +buried trees seen by me was from six to eight feet; but one trunk was about 25 +feet high and four feet in diameter, with a considerable bulge at the base. In +no instance could I detect any trunk intersecting a layer of coal, however thin; +and most of the trees terminated downward in seams of coal. Some few only were +based on clay and shale; none of them, except Calamites, on sandstone. The erect +trees, therefore, appeared in general to have grown on beds of vegetable matter. +In the underclays Stigmaria abounds. + +These root-bearing beds have been found under all the coal-seams, and such old +soils are at present the most destructible masses in the whole cliff, the +sandstones and laminated shales being harder and more capable of resisting the +action of the waves and the weather. Originally the reverse was doubtless true, +for in the existing delta of the Mississippi those clays in which the +innumerable roots of the deciduous cypress and other swamp trees ramify in all +directions are seen to withstand far more effectually the undermining power of +the river, or of the sea at the base of the delta, than do beds of loose sand or +layers of mud not supporting trees. It is obvious that if this sand or mud be +afterwards consolidated and turned to sandstone and hard shale, it would be the +least destructible. + +(FIGURE 440. Erect fossil trees. Coal-measures, Nova Scotia.) + +In regard to the plants, they belonged to the same genera, and most of them to +the same species, as those met with in the distant coal-fields of Europe. Dr. +Dawson has enumerated more than 150 species, two-thirds of which are European, a +greater agreement than can be said to exist between the same Nova Scotia flora +and that of the coal-fields of the United States. By referring to the section in +Figure 439, the position of the four-foot coal will be perceived, and in Figure +440 (a section made by me in 1842 of a small portion) that from e to f of the +same cliff is exhibited, in order to show the manner of occurrence of erect +fossil trees at right angles to the planes of the inclined strata. + +In the sandstone which filled their interiors, I frequently observed fern- +leaves, and sometimes fragments of Stigmaria, which had evidently entered +together with sediment after the trunk had decayed and become hollow, and while +it was still standing under water. Thus the tree, a, Figure 440, represented in +the bed e in the section, Figure 439, is a hollow trunk five feet eight inches +in length, traversing various strata, and cut off at the top by a layer of clay +two feet thick, on which rests a seam of coal (b, Figure 440) one foot thick. On +this coal again stood two large trees (c and d), while at a greater height the +trees f and g rest upon a thin seam of coal (e), and above them is an underclay, +supporting the four-foot coal. + +Occasionally the layers of matter in the inside of the tree are more numerous +than those without; but it is more common in the coal-measures of all countries +to find a cylinder of pure sandstone-- the cast of the interior of a tree-- +intersecting a great many alternating beds of shale and sandstone, which +originally enveloped the trunk as it stood erect in the water. Such a want of +correspondence in the materials outside and inside, is just what we might expect +if we reflect on the difference of time at which the deposition of sediment will +take place in the two cases; the imbedding of the tree having gone on for many +years before its decay had made much progress. In many places distinct proof is +seen that the enveloping strata took years to accumulate, for some of the +sandstones surrounding erect sigillarian trunks support at different levels +roots and stems of Calamites; the Calamites having begun to grow after the older +Sigillariae had been partially buried. + +The general absence of structure in the interior of the large fossil trees of +the Coal implies the very durable nature of their bark, as compared with their +woody portion. The same difference of durability of bark and wood exists in +modern trees, and was first pointed out to me by Dr. Dawson, in the forests of +Nova Scotia, where the Canoe Birch (Betula papyracea) has such tough bark that +it may sometimes be seen in the swamps looking externally sound and fresh, +although consisting simply of a hollow cylinder with all the wood decayed and +gone. When portions of such trunks have become submerged in the swamps they are +sometimes found filled with mud. One of the erect fossil trees of the South +Joggins fifteen feet in height, occurring at a higher level than the main coal, +has been shown by Dr. Dawson to have a coniferous structure, so that some +Coniferae of the Coal period grew in the same swamps as Sigillariae, just as now +the deciduous Cypress (Taxodium distichum) abounds in the marshes of Louisiana +even to the edge of the sea. + +When the carboniferous forests sank below high-water mark, a species of +Spirorbis or Serpula (Figure 431), attached itself to the outside of the stumps +and stems of the erect trees, adhering occasionally even to the interior of the +bark-- another proof that the process of envelopment was very gradual. These +hollow upright trees, covered with innumerable marine annelids, reminded me of a +"cane-brake," as it is commonly called, consisting of tall reeds, Arundinaria +macrosperma, which I saw in 1846, at the Balize, or extremity of the delta of +the Mississippi. Although these reeds are fresh-water plants, they were covered +with barnacles, having been killed by an incursion of salt-water over an extent +of many acres, where the sea had for a season usurped a space previously gained +from it by the river. Yet the dead reeds, in spite of this change, remained +standing in the soft mud, enabling us to conceive how easily the larger +Sigillariae, hollow as they were but supported by strong roots, may have +resisted an incursion of the sea. + +The high tides of the Bay of Fundy, rising more than 60 feet, are so destructive +as to undermine and sweep away continually the whole face of the cliffs, and +thus a new crop of erect fossil trees is brought into view every three or four +years. They are known to extend over a space between two and three miles from +north to south, and more than twice that distance from east to west, being seen +in the banks of streams intersecting the coal-field. + +STRUCTURE OF COAL. + +The bituminous coal of Nova Scotia is similar in composition and structure to +that of Great Britain, being chiefly derived from sigillarioid trees mixed with +leaves of ferns and of a Lycopodiaceous tree called Cordaites (Noeggerathia, +etc., for genus, see Figure 428), supposed by Dawson to have been deciduous, and +which had broad parallel veined leaves without a mid-rib. On the surface of the +seams of coal are large quantities of mineral charcoal, which doubtless consist, +as Dr. Dawson suggests, of fragments of wood which decayed in the open air, as +would naturally be expected in swamps where so many erect trees were preserved. +Beds of cannel-coal display, says Dr. Dawson, such a microscopical structure and +chemical composition as shows them to have been of the nature of fine vegetable +mud such as accumulates in the shallow ponds of modern swamps. The underclays +are loamy soils, which must have been sufficiently above water to admit of +drainage, and the absence of sulphurets, and the occurrence of carbonate of iron +in them, prove that when they existed as soils, rain-water, and not sea-water, +percolated them. With the exception, perhaps, of Asterophyllites (see Figure +461), there is a remarkable absence from the coal-measures of any form of +vegetation properly aquatic, the true coal being a sub-aerial accumulation in +soil that was wet and swampy but not permanently submerged. + +AIR-BREATHERS OF THE COAL. + +If we have rightly interpreted the evidence of the former existence at more than +eighty different levels of forests of trees, some of them of vast extent, and +which lasted for ages, giving rise to a great accumulation of vegetable matter, +it is natural to ask whether there were not many air-breathing inhabitants of +these same regions. As yet no remains of mammalia or birds have been found, a +negative character common at present to all the Palaeozoic formations; but in +1852 the osseous remains of a reptile, the first ever met with in the +carboniferous strata of the American continent, were found by Dr. Dawson and +myself. We detected them in the interior of one of the erect Sigillariae before +alluded to as of such frequent occurrence in Nova Scotia. The tree was about two +feet in diameter, and consisted of an external cylinder of bark, converted into +coal, and an internal stony axis of black sandstone, or rather mud and sand +stained black by carbonaceous matter, and cemented together with fragments of +wood into a rock. These fragments were in the state of charcoal, and seem to +have fallen to the bottom of the hollow tree while it was rotting away. The +skull, jaws, and vertebrae of a reptile, probably about 2 1/2 feet in length +(Dendrerpeton Acadianum, Owen), were scattered through this stony matrix. The +shell, also, of a Pupa (see Figure 442), the first land-shell ever met with in +the coal or in beds older than the tertiary, was observed in the same stony +mass. Dr. Wyman of Boston pronounced the reptile to be allied in structure to +Menobranchus and Menopoma, species of batrachians, now inhabiting the North +American rivers. The same view was afterwards confirmed by Professor Owen, who +also pointed out the resemblance of the cranial plates to those seen in the +skull of Archegosaurus and Labyrinthodon. (Quarterly Geological Journal volume 9 +page 58.) Whether the creature had crept into the hollow tree while its top was +still open to the air, or whether it was washed in with mud during a flood, or +in whatever other manner it entered, must be matter of conjecture. + +Footprints of two reptiles of different sizes had previously been observed by +Dr. Harding and Dr. Gesner on ripple-marked flags of the lower coal-measures in +Nova Scotia (No. 2, Figure 447), evidently made by quadrupeds walking on the +ancient beach, or out of the water, just as the recent Menopoma is sometimes +observed to do. + +The remains of a second and smaller species of Dendrerpeton, D. Oweni, were also +found accompanying the larger one, and still retaining some of its dermal +appendages; and in the same tree were the bones of a third small lizard-like +reptile, Hylonomus Lyelli, seven inches long, with stout hind limbs, and fore +limbs comparatively slender, supposed by Dr. Dawson to be capable of walking and +running on land. (Dawson Air-Breathers of the Coal in Nova Scotia Montreal +1863.) + +(FIGURE 441. Xylobius Sigillariae, Dawson. Coal, Nova Scotia. +a. Natural size. +b. Anterior part, magnified. +c. Caudal extremity, magnified.) + +(FIGURE 442. Pupa vetusta, Dawson. +a. Natural size. +b. Magnified.) + +In a second specimen of an erect stump of a hollow tree 15 inches in diameter, +the ribbed bark of which showed that it was a Sigillaria, and which belonged to +the same forest as the specimen examined by us in 1852, Dr. Dawson obtained not +only fifty specimens of Pupa vetusta (Figure 442), and nine skeletons of +reptiles belonging to four species, but also several examples of an articulated +animal resembling the recent centipede or gally-worm, a creature which feeds on +decayed vegetable matter (see Figure 441). Under the microscope, the head, with +the eyes, mandible, and labrum, are well seen. It is interesting, as being the +earliest known representative of the myriapods, none of which had previously +been met with in rocks older than the oolite or lithographic slate of Germany. + +Some years after the discovery of the first Pupa, Dr. Dawson, carefully +examining the same great section containing so many buried forests in the cliffs +of Nova Scotia, discovered another bed, separated from the tree containing +Dendrerpeton by a mass of strata more than 1200 feet thick. As there were 21 +seams of coal in this intervening mass, the length of time comprised in the +interval is not to be measured by the mere thickness of the sandstones and +shales. This lower bed is an underclay seven feet thick, with stigmarian +rootlets, and the small land-shells occurring in it are in all stages of growth. +They are chiefly confined to a layer about two inches thick, and are unmixed +with any aquatic shells. They were all originally entire when imbedded, but are +most of them now crushed, flattened, and distorted by pressure; they must have +been accumulated, says Dr. Dawson, in mud deposited in a pond or creek. + +(FIGURE 443. Zonites (Conulus) priscus, Carpenter. +a. Natural size. +b. Magnified.) + +The surface striae of Pupa vetusta, when magnified 50 diameters, present exactly +the same appearance as a portion corresponding in size of the common English +Pupa juniperi, and the internal hexagonal cells, magnified 500 diameters, show +the internal structure of the fossil and recent Pupa to be identical. In 1866 +Dr. Dawson discovered in this lower bed, so full of the Pupa, another land-shell +of the genus Helix (sub-genus Zonites), see Figure 443. (Dawson Acadian Geology +1868 page 385.) + +None of the reptiles obtained from the coal-measures of the South Joggins are of +a higher grade than the Labyrinthodonts, but some of these were of very great +size, two caudal vertebrae found by Mr. Marsh in 1862 measuring two and a half +inches in diameter, and implying a gigantic aquatic reptile with a powerful +swimming tail. + +Except some obscure traces of an insect found by Dr. Dawson in a coprolite of a +terrestrial reptile occurring in a fossil tree, no specimen of this class has +been brought to light in the Joggins. But Mr. James Barnes found in a bed of +shale at Little Grace Bay, Cape Breton, the wing of an Ephemera, which must have +measured seven inches from tip to tip of the expanded wings-- larger than any +known living insect of the Neuropterous family. + +That we should have made so little progress in obtaining a knowledge of the +terrestrial fauna of the Coal is certainly a mystery, but we have no reason to +wonder at the extreme rarity of insects, seeing how few are known in the +carboniferous rocks of Europe, worked for centuries before America was +discovered, and now quarried on so enormous a scale. These European rocks have +not yet produced a single land-shell, in spite of the millions of tons of coal +annually extracted, and the many hundreds of soils replete with the fossil roots +of trees, and the erect trunks and stumps preserved in the position in which +they grew. In many large coal-fields we continue as much in the dark respecting +the invertebrate air-breathers then living, as if the coal had been thrown down +in mid-ocean. The early date of the carboniferous strata can not explain the +enigma, because we know that while the land supported a luxuriant vegetation, +the contemporaneous seas swarmed with life-- with Articulata, Mollusca, Radiata, +and Fishes. The perplexity in which we are involved when we attempt to solve +this problem may be owing partly to our want of diligence as collectors, but +still more perhaps to ignorance of the laws which govern the fossilisation of +land-animals, whether of high or low degree. + +CARBONIFEROUS RAIN-PRINTS. + +(FIGURES 444 and 445. On green shale, from Cape Breton, Nova Scotia. + +(FIGURE 444. Carboniferous rain-prints with worm-tracks (a, b) on green shale, +from Cape Breton, Nova Scotia. Natural size.) + +(FIGURE 445. Casts of rain-prints on a portion of the same slab (Figure 444), +seen to project on the under side of an incumbent layer of arenaceous shale. +Natural size. The arrow represents the supposed direction of the shower.)) + +At various levels in the coal measures of Nova Scotia, ripple-marked sandstones, +and shales with rain-prints, were seen by Dr. Dawson and myself, but still more +perfect impressions of rain were discovered by Mr. Brown, near Sydney, in the +adjoining island of cape Breton. They consist of very delicate markings on +greenish slates, accompanied by worm-tracks (a, b, Figure 444), such as are +often seen between high and low water mark on the recent mud of the Bay of +Fundy. + +The great humidity of the climate of the Coal period had been previously +inferred from the number of its ferns and the continuity of its forests for +hundreds of miles; but it is satisfactory to have at length obtained such +positive proofs of showers of rain, the drops of which resembled in their +average size those which now fall from the clouds. From such data we may presume +that the atmosphere of the Carboniferous period corresponded in density with +that now investing the globe, and that different currents of air varied then as +now in temperature, so as to give rise, by their mixture, to the condensation of +aqueous vapour. + +FOLDING AND DENUDATION OF THE BEDS INDICATED BY THE NOVA SCOTIA COAL-STRATA. + +(FIGURE 446. Cone and branch of Lepidodendron corrugatum. Lower Carboniferous, +New Brunswick.) + +The series of events which are indicated by the great section of the coal-strata +in Nova Scotia consist of a gradual and long-continued subsidence of a tract +which throughout most of the period was in the state of a delta, though +occasionally submerged beneath a sea of moderate depth. Deposits of mud and sand +were first carried down into a shallow sea on the low shores of which the +footprints of reptiles were sometimes impressed (see above). Though no regular +seams of coal were formed, the characteristic imbedded coal-plants are of the +genera Cyclopteris and Alethopteris, agreeing with species occurring at much +higher levels, and distinct from those of the antecedent Devonian group. The +Lepidodendron corrugatum (see Figure 446), a plant predominating in the Lower +Carboniferous group of Europe, is also conspicuous in these shallow-water beds, +together with many fishes and entomostracans. A more rapid rate of subsidence +sometimes converted part of the sea into deep clear water, in which there was a +growth of coral which was afterwards turned into crystalline limestone, and +parts of it, apparently by the action of sulphuric acid, into gypsum. In spite +of continued sinking, amounting to several thousand feet, the sea might in time +have been rendered shallow by the growth of coral, had not its conversion into +land or swampy ground been accelerated by the pouring in of sand and the advance +of the delta accompanied with such fluviatile and brackish-water formations as +are common in lagoons. + +(FIGURE 447. Diagram section from north, through Minudie, S. Joggins, Shoulie R. +and Cobequid Mountains, south, showing the curvature and supposed denudation of +the Carboniferous strata in Nova Scotia. +A. Anticlinal axis of Minudie. +B. Synclinal of Shoulie River. +1. Coal-measures. +2. Lower Carboniferous.) + +The amount to which the bed of the sea sank down in order to allow of the +formation of so vast a thickness of rock of sedimentary and organic origin is +expressed by the total thickness of the Carboniferous strata, including the +coal-measures, No. 1, and the rocks which underlie them, No. 2, Figure 447. + +After the strata No. 2 had been elaborated, the conditions proper to a great +delta exclusively prevailed, the subsidence still continuing so that one forest +after another grew and was submerged until their under-clays with roots, and +usually seams of coal, were left at more than eighty distinct levels. Here and +there, also, deposits bearing testimony to the existence of fresh or brackish- +water lagoons, filled with calcareo-bituminous mud, were formed. In these beds +(h and i, Figure 439) are found fresh-water bivalves or mussels allied to +Anodon, though not identical with that or any living genus, and called Naiadites +carbonarius by Dawson. They are associated with small entomostracous crustaceans +of the genus Cythere, and scales of small fishes. Occasionally some of the +calamite brakes and forests of Sigillariae and Coniferae were exposed in the +flood season, or sometimes, perhaps, by slight elevatory movements to the +denuding action of the river or the sea. + +In order to interpret the great coast section exposed to view on the shores of +the Bay of Fundy, the student must, in the first place, understand that the +newest or last-mentioned coal formations would have been the only ones known to +us (for they would have covered all the others), had there not been two great +movements in opposite directions, the first consisting of a general sinking of +three miles, which took place during the Carboniferous Period, and the second an +upheaval of more limited horizontal extent, by which the anticlinal axis A was +formed. That the first great change of level was one of subsidence is proved by +the fact that there are shallow-water deposits at the base of the Carboniferous +series, or in the lowest beds of No. 2. + +Subsequent movements produced in the Nova Scotia and the adjoining New Brunswick +coal-fields the usual anticlinal and synclinal flexures. In order to follow +these, we must survey the country for about thirty miles round the South +Joggins, or the region where the erect trees described in the foregoing pages +are seen. As we pass along the cliffs for miles in a southerly direction, the +beds containing these fossil trees, which were mentioned as dipping about 18 +degrees south, are less and less inclined, until they become nearly horizontal +in the valley of a small river called the Shoulie, as ascertained by Dr. Dawson. +After passing this synclinal line the beds begin to dip in an opposite or north- +easterly direction, acquiring a steep dip where they rest unconformably on the +edges of the Upper Silurian strata of the Cobequid Hills, as shown in Figure +447. But if we travel northward towards Minudie from the region of the coal- +seams and buried forests, we find the dip of the coal-strata increasing from an +angle of 18 degrees to one of more than 40 degrees, lower beds being continually +exposed to view until we reach the anticlinal axis A and see the lower +Carboniferous formation, No. 2, at the surface. The missing rocks removed by +denudation are expressed by the faint lines at A, and thus the student will see +that, according to the principles laid down in the seventh chapter, we are +enabled, by the joint operations of upheaval and denudation, to look, as it +were, about three miles into the interior of the earth without passing beyond +the limits of a single formation. + + +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. + +VEGETATION OF THE COAL PERIOD. + +In the last chapter we have seen that the seams of coal, whether bituminous or +anthracitic, are derived from the same species of plants, and Goppert has +ascertained that the remains of every family of plants scattered through the +shales and sandstones of the coal-measures are sometimes met with in the pure +coal itself-- a fact which adds greatly to the geological interest of this +flora. + +The coal-period was called by Adolphe Brongniart the age of Acrogens, so great +appears to have been the numerical preponderance of flowerless or cryptogamic +plants of the families of ferns, club-mosses, and horse-tails. (For botanical +nomenclature see Chapter 17.) He reckoned the known species in 1849 at 500, and +the number has been largely increased by recent research in spite of reductions +owing to the discovery that different parts of even the same plants had been +taken for distinct species. Notwithstanding these changes, Brongniart's +generalisation concerning this flora still holds true, namely, that the state of +the vegetable world was then extremely different from that now prevailing, not +only because the cryptogamous plants constituted nearly the whole flora, but +also because they were, on the whole, more highly developed than any belonging +to the same class now existing, and united some forms of structure now only +found separately and in distinct orders. The only phaenogamous plants were +constitute any feature in the coal are the coniferae; monocotyledonous +angiosperms appear to have been very rare, and the dicotyledonous, with one or +two doubtful exceptions, were wanting. For this we are in some measure prepared +by what we have seen of the Secondary or Mesozoic floras if, consistently with +the belief in the theory of evolution, we expect to find the prevalence of +simpler and less specialised organisms in older rocks. + +FERNS. + +(FIGURE 448. Pecopteris elliptica, Bunbury. (Sir C. Bunbury Quarterly Geological +Journal volume 2 1845.) Frostburg.) + +We are struck at the first glance with the similarity of the ferns to those now +living. In the fossil genus Pecopteris, for example (Figure 448), it is not easy +to decide whether the fossils might not be referred to the same genera as those +established for living ferns; whereas, in regard to some of the other +contemporary families of plants, with the exception of the fir tribe, it is not +easy to guess even the class to which they belong. The ferns of the +Carboniferous period are generally without organs of fructification, but in the +few instances in which these do occur in a fit state for microscopical +investigations they agree with those of the living ferns. + +(FIGURE 449. Caulopteris primaeva, Lindley.) + +When collecting fossil specimens from the coal-measures of Frostburg, in +Maryland, I found in the iron-shales several species with well-preserved rounded +spots or marks of the sori (see Figure 448). In the general absence of such +characters they have been divided into genera distinguished chiefly by the +branching of the fronds and the way in which the veins of the leaves are +disposed. The larger portion are supposed to have been of the size of ordinary +European ferns, but some were decidedly arborescent, especially the group called +Caulopteris (see Figure 449) by Lindley, and the Psaronius of the upper or +newest coal-measures, before alluded to (Chapter 22). All the recent tree-ferns +belong to one tribe (Polypodiaceae), and to a small number only of genera in +that tribe, in which the surface of the trunk is marked with scars, or +cicatrices, left after the fall of the fronds. These scars resemble those of +Caulopteris. + +(FIGURES 450, 451 and 452. Living tree-ferns of different genera. (Ad. Brong.) + +(FIGURE 450. Tree-fern from Isle of Bourbon.) + +(FIGURE 451. Cyathea glauca, Mauritius.) + +(FIGURE 452. Tree-fern from Brazil.)) + +No less than 130 species of ferns are enumerated as having been obtained from +the British coal-strata, and this number is more than doubled if we include the +Continental and American species. Even if we make some reduction on the ground +of varieties which have been mistaken, in the absence of their fructification, +for species, still the result is singular, because the whole of Europe affords +at present no more than sixty-seven indigenous species. + +LYCOPODIACEAE-- LEPIDODENDRON. + +(FIGURES 453, 454 and 455. Lepidodendron Sternbergii. Coal-measures, near +Newcastle. + +(FIGURE 453. Branching trunk, 49 feet long, supposed to have belonged to L. +Sternbergii. (Foss. Flo. 203.)) + +(FIGURE 454. Branching stem with bark and leaves of L. Sternbergii. (Foss. Flo. +4.) + +(FIGURE 455. Portion of same nearer the root. Natural size. (Ibid.))) + +(FIGURE 456. Lycopodium densum. +a. Living species. New Zealand. +b. Branch; natural size. +c. Part of same, magnified.) + +About forty species of fossil plants of the Coal have been referred to this +genus, more than half of which are found in the British coal-measures. They +consist of cylindrical stems or trunks, covered with leaf-scars. In their mode +of branching, they are always dichotomous (see Figure 454). They belong to the +Lycopodiaceae, bearing sporangia and spores similar to those of the living +representatives of this family (Figure 457); and although most of the +Carboniferous species grew to the size of large trees, Mr. Carruthers has found +by careful measurement that the volume of the fossil spores did not exceed that +of the recent club-moss, a fact of some geological importance, as it may help to +explain the facility with which these seeds may have been transported by the +wind, causing the same wide distribution of the species of the fossil forests in +Europe and America which we now observe in the geographical distribution of so +many living families of cryptogamous plants. The Figures 453-455 represent a +fossil Lepidodendron, 49 feet long, found in Jarrow Colliery, near Newcastle, +lying in shale parallel to the planes of stratification. Fragments of others, +found in the same shale, indicate, by the size of the rhomboidal scars which +cover them, a still greater magnitude. The living club-mosses, of which there +are about 200 species, are most abundant in tropical climates. They usually +creep on the ground, but some stand erect, as the Lycopodium densum from New +Zealand (see Figure 456), which attains a height of three feet. + +(FIGURE 457. Lepidostrobus ornatus, Brong. Shropshire. +a. (Body) half natural size. +b. Portion of a section, showing the large sporangia in their natural position, +and each supported by its bract or scale. +c. Spores in these sporangia, highly magnified. (Hooker Mem. Geological Survey +volume 2 part 2 page 440.) + +In the Carboniferous strata of Coalbrook Dale, and in many other coal-fields, +elongated cylindrical bodies, called fossil cones, named Lepidostrobus by M. +Adolphe Brongniart, are met with. (See Figure 457.) They often form the nucleus +of concretionary balls of clay-ironstone, and are well preserved, exhibiting a +conical axis, around which a great quantity of scales were compactly imbricated. +The opinion of M. Brongniart that the Lepidostrobus is the fruit of +Lepidodendron has been confirmed, for these strobili or fruits have been found +terminating the tip of a branch of a well-characterised Lepidodendron in +Coalbrook Dale and elsewhere. + +EQUISETACEAE. + +(FIGURE 458. Calamites Sucowii, Brong.; natural size. Common in coal throughout +Europe.) + +(FIGURE 459. Stem of Figure 458, as restored by Dr. Dawson.) + +(FIGURE 460. Radical termination of a Calamite. Nova Scotia.) + +To this family belong two fossil genera of the coal, Equisetites and Calamites. +The Calamites were evidently closely related to the modern horse-tails +(Equiseta) differing principally in their great size, the want of sheaths at the +joints, and some details of fructification. They grew in dense brakes on sandy +and muddy flats in the manner of modern Equisetaceae, and their remains are +frequent in the coal. Seven species of this plant occur in the great Nova Scotia +section before described, where the stems of some of them five inches in +diameter, and sometimes eight feet high, may be seen terminating downward in a +tapering root (see Figure 460). + +(FIGURE 461. Asterophillites foliosus. (Foss. Flo. 25.) Coal-measures, +Newcastle.) + +(FIGURE 462. Annularia sphenophylloides, Dawson.) + +(FIGURE 463. Sphenophyllum erosum, Dawson.) + +Botanists are not yet agreed whether the Asterophyllites, a species of which is +represented in Figure 461, can form a separate genus from the Calamite, from +which, however, according to Dr. Dawson, its foliage is distinguished by a true +mid-rib, which is wanting in the leaves known to belong to some Calamites. +Figures 462 and 463 represent leaves of Annularia and Sphenophyllum, common in +the coal, and believed by Mr. Carruthers to be leaves of Calamites. Dr. +Williamson, who has carefully studied the Calamites, thinks that they had a +fistular pith, exogenous woody stem, and thick smooth bark, which last having +always disappeared, leaves a fluted stem, as represented in Figure 459. + +SIGILLARIA. + +(FIGURE 464. Sigillaria laevigata, Brong.) + +A large portion of the trees of the Carboniferous period belonged to this genus, +of which as many as 28 species are enumerated as British. The structure, both +internal and external, was very peculiar, and, with reference to existing types, +very anomalous. They were formerly referred, by M. Ad. Brongniart, to ferns, +which they resemble in the scalariform texture of their vessels and, in some +degree, in the form of the cicatrices left by the base of the leaf-stalks which +have fallen off (see Figure 464). But some of them are ascertained to have had +long linear leaves, quite unlike those of ferns. They grew to a great height, +from 30 to 60, or even 70 feet, with regular cylindrical stems, and without +branches, although some species were dichotomous towards the top. Their fluted +trunks, from one to five feet in diameter, appear to have decayed more rapidly +in the interior than externally, so that they became hollow when standing; and +when thrown prostrate, they were squeezed down and flattened. Hence, we find the +bark of the two opposite sides (now converted into bright shining coal) +constitute two horizontal layers, one upon the other, half an inch, or an inch, +in their united thickness. These same trunks, when they are placed obliquely or +vertically to the planes of stratification, retain their original rounded form, +and are uncompressed, the cylinder of bark having been filled with sand, which +now affords a cast of the interior. + +Dr. Hooker inclined to the belief that the Sigillariae may have been +cryptogamous, though more highly developed than any flowerless plants now +living. Dr. Dawson having found in some species what he regards as medullary +rays, thinks with Brongniart that they have some relation to gymnogens, while +Mr. Carruthers leans to the opinion that they belong to the Lycopodiaceae. + +STIGMARIA. + +(FIGURE 465. Stigmaria attached to a trunk of Sigillaria.) + +This fossil, the importance of which has already been pointed out in Chapter 23, +was originally conjectured to be an aquatic plant. It is now ascertained to be +the root of Sigillaria. The connection of the roots with the stem, previously +suspected, on botanical grounds, by Brongniart, was first proved, by actual +contact, in the Lancashire coal-field, by Mr. Binney. The fact has lately been +shown, even more distinctly, by Mr. Richard Brown, in his description of the +Stigmariae occurring in the under-clays of the coal-seams of the Island of Cape +Breton, in Nova Scotia. In a specimen of one of these, represented in Figure +465, the spread of the roots was sixteen feet, and some of them sent out +rootlets, in all directions, into the surrounding clay. + +(FIGURE 466. Stigmaria ficoides, Brong. 1/4 natural size. (Foss. Flo. 32.)) + +(FIGURE 467. Stigmaria ficoides, Brong. Surface of another individual of same +species, showing form of tubercles. (Foss. Flo. 34.)) + +In the sea-cliffs of the South Joggins in Nova Scotia, I examined several erect +Sigillariae, in company with Dr. Dawson, and we found that from the lower +extremities of the trunk they sent out Stigmariae as roots. All the stools of +the fossil trees dug out by us divided into four parts, and these again +bifurcated, forming eight roots, which were also dichotomous when traceable far +enough. The cylindrical rootlets formerly regarded as leaves are now shown by +more perfect specimens to have been attached to the root by fitting into deep +cylindrical pits. In the fossil there is rarely any trace of the form of these +cavities, in consequence of the shrinkage of the surrounding tissues. Where the +rootlets are removed, nothing remains on the surface of the Stigmaria but rows +of mammillated tubercles (see Figures 466, 467), which have formed the base of +each rootlet. These protuberances may possibly indicate the place of a joint at +the lower extremity of the rootlet. Rows of these tubercles are arranged +spirally round each root, which have always a medullary axis and woody system +much resembling that of Sigillaria, the structure of the vessels being, like it, +scalariform. + +CONIFERAE. + +(FIGURE 468. Fragment of coniferous wood, Dadoxylon, of Endlicher, fractured +longitudinally; from Coalbrook Dale. W.C. Williamson. (Manchester Philosophical +Mem. volume 9 1851.) +a. Bark. +b. Woody zone or fibre (pleurenchyma). +c. Medulla or pith. +d. Cast of hollow pith or "Sternbergia.") + +(FIGURE 469. Fragment of coniferous wood, Dadoxylon, of Endlicher. Magnified +portion of Figure 468; transverse section. +b-b. Woody fibre. +c. Pith. +e, e, e. Medullary rays.) + +The coniferous trees of this period are referred to five genera; the woody +structure of some of them showing that they were allied to the Araucarian +division of pines, more than to any of our common European firs. Some of their +trunks exceeded forty-four feet in height. Many, if not all of them, seem to +have differed from living Coniferae in having large piths; for Professor +Williamson has demonstrated the fossil of the coal-measures called Sternbergia +to be the pith of these trees, or rather the cast of cavities formed by the +shrinking or partial absorption of the original medullary axis (see Figures 468, +469). This peculiar type of pith is observed in living plants of very different +families, such as the common Walnut and the White Jasmine, in which the pith +becomes so reduced as simply to form a thin lining of the medullary cavity, +across which transverse plates of pith extend horizontally, so as to divide the +cylindrical hollow into discoid interspaces. When these interspaces have been +filled up with inorganic matter, they constitute an axis to which, before their +true nature was known, the provisional name of Sternbergia (d, d, Figure 468) +was given. In the above specimen the structure of the wood (b, Figures 468 and +469) is coniferous, and the fossil is referable to Endlicher's fossil genus +Dadoxylon. + +(FIGURE 470. Trigonocarpum ovatum, Lindley and Hutton. Peel Quarry, Lancashire.) + +(FIGURE 471. Trigonocarpum olivaeforme, Lindley, with its fleshy envelope. +Felling Colliery, Newcastle.) + +The fossil named Trigonocarpon (Figures 470 and 471), formerly supposed to be +the fruit of a palm, may now, according to Dr. Hooker, be referred, like the +Sternbergia, to the Coniferae. Its geological importance is great, for so +abundant is it in the coal-measures, that in certain localities the fruit of +some species may be procured by the bushel; nor is there any part of the +formation where they do not occur, except the under-clays and limestone. The +sandstone, ironstone, shales, and coal itself, all contain them. Mr. Binney has +at length found in the clay-ironstone of Lancashire several specimens displaying +structure, and from these, says Dr. Hooker, we learn that the Trigonocarpon +belonged to that large section of existing coniferous plants which bear fleshy +solitary fruits, and not cones. It resembled very closely the fruit of the +Chinese genus Salisburia, one of the Yew tribe, or Taxoid conifers. + +ANGIOSPERMS. + +(FIGURE 472. Antholithes. Felling Colliery, Newcastle.) + +The curious fossils called Antholithes by Lindley have usually been considered +to be flower spikes, having what seems a calyx and linear petals (see Figure +472). Dr. Hooker, after seeing very perfect specimens, also thought that they +resembled the spike of a highly-organised plant in full flower, such as one of +the Bromeliaceae, to which Professor Lindley had at first compared them. Mr. +Carruthers, who has lately examined a large series in different museums, +considers it to be a dicotyledonous angiosperm allied to Orobanche (broom-rape), +which grew, not on the soil, but parasitically on the trees of the coal forests. + +(FIGURE 473. Pothocites Grantonii, Pat. Coal-measures, Edinburgh. +c. Stem and spike; 1/2 natural size. +b. Remains of the spathe magnified. +c. Portion of spike magnified. +d. One of the calyces magnified.) + +In the coal-measures of Granton, near Edinburgh, a remarkable fossil (Figure +473) was found and described in 1840, by Dr. Robert Paterson. (Transactions of +the Botanical Society of Edinburgh volume 1 1844.) It was compressed between +layers of bituminous shale, and consists of a stem bearing a cylindrical spike, +a, which in the portion preserved in the slate exhibits two subdivisions and +part of a third. The spike is covered on the exposed surface with the four-cleft +calyces of the flowers arranged in parallel rows. The stem shows, at b, a little +below the spike, remains of a lateral appendage, which is supposed to indicate +the beginning of the spathe. The fossil has been referred to the Aroidiae, and +there is every probability that it is a true member of this order. There can at +least be no doubt as to the high grade of its organisation, and that it belongs +to the monocotyledonous angiosperms. Mr. Carruthers has carefully examined the +original specimen in the Botanical Museum, Edinburgh, and thinks it may have +been an epiphyte. + +CLIMATE OF THE COAL PERIOD. + +As to the climate of the Coal, the Ferns and the Coniferae are perhaps the two +classes of plants which may be most relied upon as leading us to safe +conclusions, as the genera are nearly allied to living types. All botanists +admit that the abundance of ferns implies a moist atmosphere. But the coniferae, +says Hooker, are of more doubtful import, as they are found in hot and dry, and +in cold and dry climates; in hot and moist, and in cold and moist regions. In +New Zealand the coniferae attain their maximum in numbers, constituting 1/62 +part of all the flowering plants; whereas in a wide district around the Cape of +Good Hope they do not form 1/1600 of the phenogamic flora. Besides the conifers, +many species of ferns flourish in New Zealand, some of them arborescent, +together with many lycopodiums; so that a forest in that country may make a +nearer approach to the carboniferous vegetation than any other now existing on +the globe. + +MARINE FAUNA OF THE CARBONIFEROUS PERIOD. + +It has already been stated that the Carboniferous or Mountain Limestone +underlies the coal-measures in the South of England and Wales, whereas in the +North, and in Scotland, marine calcareous rocks partly of the age of the +Mountain Limestone alternate with shales and sandstones, containing seams of +coal. In its most calcareous form the Mountain Limestone is destitute of land- +plants, and is loaded with marine remains-- the greater part, indeed, of the +rock being made up bodily of crinoids, corals, and bryozoa with interspersed +mollusca. + +CORALS. + +(FIGURE 474. Palaeozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA +RUGOSA, Milne Edwards and Jules Haime. +a. Vertical section of Campophyllum flexuosum, (Cyathophyllum, Goldfuss); 1/2 +natural size: from the Devonian of the Eifel. The lamellae are seen around the +inside of the cup; the walls consist of cellular tissue; and large transverse +plates, called tubulae, divide the interior into chambers. +b. Arrangement of the lamellae in Polycoelia profunda, Germar, sp.; natural +size: from the Magnesian Limestone, Durham. +This diagram shows the quadripartite arrangement of the primary septa, +characteristic of palaeozoic corals, there being four principal and eight +intermediate lamellae, the whole number in this type being always a multiple of +four. +c. Stauria astraeiformis, Milne Edwards. Young group, natural size. Upper +Silurian, Gothland. The lamellae or septal system in each cup are divided by +four prominent ridges into four groups.) + +(FIGURE 475. Neozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA +APOROSA, M. Edwards and J. Haime. +a. Parasmilia centralis, Mantell, sp. Vertical section; natural size. Upper +Chalk, Gravesend. In this type the lamellae are massive, and extend to the axis +or columella composed of loose cellular tissue, without any transverse plates +like those in Figure 474, a. +b. Cyathina Bowerbankii, Ed. and H. Transverse section, enlarged. Gault, +Folkestone. In this coral the primary septa are a multiple of six. The twelve +principal plates reach the columella, and between each pair there are three +secondaries, in all forty-eight. The short intermediate plates which proceed +from the columella are not counted. They are called pali. +c. Fungia patellaris, Lamarck. Recent; very young state. Diagram of its six +primary and six secondary septa, magnified. The sextuple arrangement is always +more manifest in the young than in the adult state.) + +The corals deserve especial notice, as the cup-and-star corals, which have the +most massive and stony skeletons, display peculiarities of structure by which +they may be distinguished generally, as MM. Milne Edwards and Haime first +pointed out, from all species found in strata newer than the Permian. There is, +in short, an ancient or PALAEOZOIC, and a modern or NEOZOIC type, if, by the +latter term, we designate (as proposed by Professor E. Forbes) all strata from +the triassic to the most modern, inclusive. The accompanying diagrams (Figures +474, 475) may illustrate these types. + +It will be seen that the more ancient corals have what is called a quadripartite +arrangement of the chief plates or LAMELLAE-- parts of the skeleton which +support the organs of reproduction. The number of these lamellae in the +Palaeozoic type is 4, 8, 16, etc.; while in the Neozoic type the number is 6, +12, 24, or some other multiple of six; and this holds good, whether they be +simple forms, as in Figures 474, a, and 475, a, or aggregate clusters of +corallites, as in 474, c. But further investigations have shown in this, as in +all similar grand generalisations in natural history, that there are exceptions +to the rule. Thus in the Lower Greensand Holocystis elegans (Ed. and H.) and +other forms have the Palaeozoic type, and Dr. Duncan has shown to what extent +the Neozoic forms penetrate downward into the Carboniferous and Devonian rocks. + +(FIGURE 476. Lithostrotion basaltiforme, Phil. sp. (Lithostrotion striatum, +Fleming; Astraea basaltiformis, Conyb. and Phill.). England, Ireland, Russia, +Iowa, and westward of the Mississippi, United States. (D.D. Owen.) + +(FIGURE 477. Lonsdaleia floriformis, Martin, sp., M. Edwards. (Lithostrotion +floriforme, Fleming. Strombodes.) +a. Young specimen, with buds or corallites on the disk, illustrating calicular +gemmation. +b. Part of a full-grown compound mass. Bristol, etc.; Russia.) + +From a great number of lamelliferous corals met with in the Mountain Limestone, +two species (Figures 476, 477) have been selected, as having a very wide range, +extending from the eastern borders of Russia to the British Isles, and being +found almost everywhere in each country. These fossils, together with numerous +species of Zaphrentis, Amplexus, Cyathophyllum, Clisiophyllum, Syringopora, and +Michelinia, form a group of rugose corals widely different from any that +followed them. (For figures of these corals, see Palaeontographical Society's +Monographs 1852.) + +BRYOZOA AND CRINOIDEA. + +(FIGURE 478. Cyathocrinus planus, Miller. Body and arms. Mountain Limestone.) + +(FIGURE 479. Cyathocrinus caryocrinoides, M'Coy. +a. Surface of one of the joints of the stem. +b. Pelvis or body; called also calyx or cup. +c. One of the pelvic plates.) + +Of the Bryozoa, the prevailing forms are Fenestella, Hemitrypa, and Polypora, +and these often form considerable beds. Their net-like fronds are easily +recognised. Crinoidea are also numerous in the Mountain Limestone (see Figures +478, 479), two genera, Pentremites and Codonaster, being peculiar to this +formation in Europe and North America. + +(FIGURE 480. Palaechinus gigas, M'Coy. Reduced one-third. Mountain Limestone. +Ireland.) + +In the greater part of them, the cup or pelvis, Figure 479, b, is greatly +developed in size in proportion to the arms, although this is not the case in +Figure 478. The genera Poteriocrinus, Cyathocrinus, Pentremites, Actinocrinus, +and Platycrinus, are all of them characteristic of this formation. Other +Echinoderms are rare, a few Sea-Urchins only being known: these have a complex +structure, with many more plates on their surface than are seen in the modern +genera of the same group. One genus, the Palaechinus (Figure 480), is the +analogue of the modern Echinus, but has four, five, or six rows of plates in the +interambulacral region or area, whereas the modern genera have only two. The +other, Archaeocidaris, represents, in like manner, the Cidaris of the present +seas. + +MOLLUSCA. + +(FIGURE 481. Productus semireticulatus, Martin, sp. (P. antiquatus, Sowerby.) +Mountain Limestone. England, Russia, the Andes, etc.) + +(FIGURE 482. Spirifera trigonalis, Martin, sp. Mountain Limestone. Derbyshire, +etc.) + +(FIGURE 483. Spirifera glabra, Martin, sp. Mountain Limestone.) + +The British Carboniferous mollusca enumerated by Mr. Etheridge comprise 653 +species referable to 86 genera, occurring chiefly in the Mountain Limestone. +(Quarterly Geological Journal volume 23 page 674 1867.) Of this large number +only 40 species are common to the underlying Devonian rocks, 9 of them being +Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda (or +Palliobranchiates). This latter group constitutes the larger part of the +Carboniferous Mollusca, 157 species being known in Great Britain alone, and it +will be found to increase in importance in the fauna of the primary rocks the +lower we descend in the series. Perhaps the most characteristic shells of the +formation are large species of Productus, such as P. giganteus, p. +hemisphericus, P. semireticulatus (Figure 481), and P. scabriculus. Large +plaited spirifers, as Spirifera striata, S. rotundata, and S. trigonalis (Figure +482), also abound; and smooth species, such as Spirifera glabra (Figure 483), +with its numerous varieties. + +(FIGURE 484. Terebratula hastata, Sowerby, with radiating bands of colour. +Mountain Limestone. Derbyshire, Ireland, Russia, etc.) + +(FIGURE 485. Aviculopecten sublobatus, Phill. Mountain Limestone. Derbyshire, +Yorkshire.) + +(FIGURE 486. Pleurotomaria carinata, Sowerby. (P. flammigera, Phillips). +Mountain Limestone. Derbyshire, etc.) + +Among the brachiopoda, Terebratula hastata (Figure 484) deserves mention, not +only for its wide range, but because it often retains the pattern of the +original coloured stripes which ornamented the living shell. These coloured +bands are also preserved in several lamellibranchiate bivalves, as in +Aviculopecten (Figure 485), in which dark stripes alternate with a light ground. +In some also of the spiral univalves the pattern of the original painting is +distinctly retained, as in Pleurotomaria (Figure 486), which displays wavy +blotches, resembling the colouring in many recent trochidae. + +(FIGURE 487. Euomphalus pentangulatus, Sowerby. Mountain Limestone. +a. Upper side. +b. Lower or umbilical side. +c. View showing mouth, which is less pentagonal in older individuals. +d. View of polished section, showing internal chambers.) + +Some few of the carboniferous mollusca, such as Avicula, Nucula (sub-genus +Ctenodonta), Solemya, and Lithodomus, belong no doubt to existing genera; but +the majority, though often referred to as living types, such as Isocardia, +Turritella, and Buccinum, belong really to forms which appear to have become +extinct at the close of the Palaeozoic epoch. Euomphalus is a characteristic +univalve shell of this period. In the interior it is divided into chambers +(Figure 487, d), the septa or partitions not being perforated as in +foraminiferous shells, or in those having siphuncles, like the Nautilus. The +animal appears to have retreated at different periods of its growth from the +internal cavity previously formed, and to have closed all communication with it +by a septum. The number of chambers is irregular, and they are generally wanting +in the innermost whorl. The animal of the recent Turritella communis partitions +off in like manner as it advances in age a part of its spire, forming a shelly +septum. + +(FIGURE 488. Bellerophon costatus, Sowerby. Mountain Limestone.) + +More than twenty species of the genus Bellerophon (see Figure 488), a shell like +the living Argonaut without chambers, occur in the Mountain Limestone. The genus +is not met with in strata of later date. It is most generally regarded as +belonging to the pelagic Nucleobranchiata and the family Atlantidae, partly +allied to the Glass-Shell, Carinaria; but by some few it is thought to be a +simple form of Cephalopod. + +(FIGURE 489. Portion of Orthoceras laterale. Phill. Mountain Limestone.) + +(FIGURE 490. Goniatites crenistra, Phillips. Mountain Limestone. North America, +Britain, Germany, etc. +a. Lateral view. +b. Front view, showing the mouth.) + +The carboniferous Cephalopoda do not depart so widely from the living type (the +Nautilus) as do the more ancient Silurian representatives of the same order; yet +they offer some remarkable forms. Among these is Orthoceras, a siphuncled and +chambered shell, like a Nautilus uncoiled and straightened (Figure 489). Some +species of this genus are several feet long. The Goniatite is another genus, +nearly allied to the Ammonite, from which it differs in having the lobes of the +septa free from lateral denticulations, or crenatures; so that the outline of +these is angular, continuous, and uninterrupted. The species represented in +Figure 490 is found in most localities, and presents the zigzag character of the +septal lobes in perfection. The dorsal position of the siphuncle, however, +clearly distinguishes the Goniatite from the Nautilus, and proves it to have +belonged to the family of the Ammonites, from which, indeed, some authors do not +believe it to be generically distinct. + +FOSSIL FISH. + +(FIGURE 491. Psammodus porosus, Agassiz. Bone-bed, Mountain Limestone. Bristol, +Armagh.) + +(FIGURE 492. Cochliodus contortus, Agassiz. Bone-bed, Mountain Limestone. +Bristol, Armagh.) + +The distribution of these is singularly partial; so much so, that M. De Koninck +of Liege, the eminent palaeontologist, once stated to me that, in making his +extensive collection of the fossils of the Mountain Limestone of Belgium, he had +found no more than four or five examples of the bones or teeth of fishes. +Judging from Belgian data, he might have concluded that this class of vertebrata +was of extreme rarity in the Carboniferous seas; whereas the investigation of +other countries has led to quite a different result. Thus, near Clifton, on the +Avon, as well as at numerous places around the Bristol basin from the Mendip +Hills to Tortworth, there is a celebrated "bone-bed," almost entirely made up of +ichthyolites. It occurs at the base of the Lower Limestone shales immediately +resting upon the passage beds of the Old Red Sandstone. Similar bone-beds occur +in the Carboniferous Limestone of Armagh, in Ireland, where they are made up +chiefly of the teeth of fishes of the Placoid order, nearly all of them rolled +as if drifted from a distance. Some teeth are sharp and pointed, as in ordinary +sharks, of which the genus Cladodus afford an illustration; but the majority, as +in Psammodus and Cochliodus, are, like the teeth of the Cestracion of Port +Jackson (see Figure 261), massive palatal teeth fitted for grinding. (See +Figures 491, 492.) + +There are upward of seventy other species of fossil fish known in the Mountain +Limestone of the British Islands. The defensive fin-bones of these creatures are +not infrequent at Armagh and Bristol; those known as Oracanthus, Ctenocanthus, +and Onchus are often of a very large size. Ganoid fish, such as Holoptychius, +also occur; but these are far less numerous. The great Megalichthys Hibberti +appears to range from the Upper Coal-measures to the lowest Carboniferous +strata. + +FORAMINIFERA. + +(FIGURE 493. Fusulina cylindrica, d'Orbigny. Magnified 3 diameters. Mountain +Limestone.) + +In the upper part of the Mountain Limestone group in the south-west of England, +near Bristol, limestones having a distinct oolitic structure alternate with +shales. In these rocks the nucleus of every minute spherule is seen, under the +microscope, to consist of a small rhizopod or foraminifer. This division of the +lower animals, which is represented so fully at later epochs by the Nummulites +and their numerous minute allies, appears in the Mountain Limestone to be +restricted to a very few species, among which Textularia, Nodosaria, Endothyra, +and Fusulina (Figure 493), have been recognised. The first two genera are common +to this and all the after periods; the third has been found in the Upper +Silurian, but is not known above the Carboniferous strata; the fourth (Figure +493) is characteristic of the Mountain Limestone in the United States, Arctic +America, Russia, and Asia Minor, but is also known in the Permian. + + +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. + +CLASSIFICATION OF THE TWO TYPES OF OLD RED SANDSTONE. + +We have seen that the Carboniferous strata are surmounted by the Permian and +Trias, both originally included in England under the name "New Red Sandstone," +from the prevailing red colour of the strata. Under the coal came other red +sandstones and shales which were distinguished by the title of "Old Red +Sandstone." Afterwards the name of "Devonian" was given by Sir R. Murchison and +Professor Sedgwick to marine fossiliferous strata which, in the south of +England, occupy a similar position between the overlying coal and the underlying +Silurian formations. + +It may be truly said that in the British Isles the rocks of this age present +themselves in their mineral aspect, and even to some extent in their fossil +contents, under two very different forms; the one as distinct from the other as +are often lacustrine or fluviatile from marine strata. It has indeed been +suggested that by far the greater part of the deposits belonging to what may be +termed the Old Red Sandstone type are of fresh-water origin. The number of land- +plants, the character of the fishes, and the fact that the only shell yet +discovered belongs to the genus Anodonta, must be allowed to lend no small +countenance to this opinion. In this case the difficulty of classification when +the strata of this type are compared in different regions, even where they are +contiguous, may arise partly from their having been formed in distinct +hydrographical basins, or in the neighbourhood of the land in shallow parts of +the sea into which large bodies of fresh-water entered, and where no marine +mollusca or corals could flourish. Under such geographical conditions the +limited extent of some kinds of sediment, as well as the absence of those marine +forms by which we are able to identify or contrast marine formations, may be +explained, while the great thickness of the rocks, which might seem at first +sight to require a corresponding depth of water, can often be shown to have been +due to the gradual sinking down of the bottom of the estuary or sea where the +sediment was accumulated. + +Another active cause of local variation in Scotland was the frequency of +contemporaneous volcanic eruptions; some of the rocks derived from this source, +as between the Grampians and the Tay, having formed islands in the sea, and +having been converted into shingle and conglomerate, before the upper portions +of the red shales and sandstones were superimposed. + +The dearth of calcareous matter over wide areas is characteristic of the Old Red +Sandstone. This is, no doubt, in great part due to the absence of shells and +corals; but why should these be so generally wanting in all sedimentary rocks +the colour of which is determined by the red oxide of iron? Some geologists are +of opinion that the waters impregnated with this oxide were prejudicial to +living beings, others that strata permeated with this oxide would not preserve +such fossil remains. + +In regard to the two types, the Old Red Sandstone and the Devonian, I shall +first treat of them separately, and then allude to the proofs of their having +been to a great extent contemporaneous. That they constitute a series of rocks +intermediate in date between the lowest Carboniferous and the uppermost Silurian +is not disputed by the ablest geologists; and it can no longer be contended that +the Upper, Middle, and Lower Old Red Sandstone preceded in date the three +divisions to which, by aid of the marine shells, the Devonian rocks have been +referred, while, on the other hand, we have not yet data for enabling us to +affirm to what extent the subdivisions of the one series may be the equivalents +in time of those of the other. + +UPPER OLD RED SANDSTONE. + +(FIGURE 494. Anodonta Jukesii, Forbes. Upper Devonian, Kiltorkan, Ireland.) + +(FIGURE 495. Bifurcating branch of Lepidodendron Griffithsii, Brongn. Upper +Devonian, Kilkenny.) + +(FIGURE 496. Palaeopteris Hibernica, Schimp. (Cyclopteris Hibernica), Edward +Forbes (Adiantites, Gop.). Upper Devonian, Kilkenny.) + +The highest beds of the series in Scotland, lying immediately below the coal in +Fife, are composed of yellow sandstone well seen at Dura Den, near Coupar, in +Fife, where, although the strata contain no mollusca, fish have been found +abundantly, and have been referred to the genera Holoptychius, Pamphractus, +Glyptopomus, and many others. In the county of Cork, in Ireland, a similar +yellow sandstone occurs containing fish of genera characteristic of the Scotch +Old Red Sandstone, as for example Coccosteus (a form represented by many species +in the Old Red Sandstone and by one only in the Carboniferous group), and +Glytolepis and Asterolepis, both exclusively confined to the "Old Red." In the +same Irish sandstone at Kiltorkan has been found an Anodonta or fresh-water +mussel, the only shell hitherto discovered in the Old Red Sandstone of the +British Isles (see Figure 494). In the same formation are found the fern (Figure +496) and the Lepidodendron (Figure 495), and other species of plants, some of +which, Professor Heer remarks, agree specifically with species from the lower +carboniferous beds. This induces him to lean to the opinion long ago advocated +by Sir Richard Griffiths, that the yellow sandstone, in spite of its fish +remains, should be classed as Lower Carboniferous, an opinion which I am not yet +prepared to adopt. Between the Mountain Limestone and the yellow sandstone in +the south-west of Ireland there intervenes a formation no less than 5000 feet +thick, called the "Carboniferous slate," and at the base of this, in some +places, are local deposits, such as the Glengariff Grits, which appear to be +beds of passage between the Carboniferous and Old Red Sandstone groups. + +It is a remarkable result of the recent examination of the fossil flora of Bear +Island, latitude 74 degrees 30' N., that Professor Heer has described as +occurring in that part of the Arctic region (nearly twenty-six degrees to the +north of the Irish locality) a flora agreeing in several of its species with +that of the yellow sandstones of Ireland. This Bear Island flora is believed by +Professor Heer to comprise species of plants some of which ascend even to the +higher stages of the European Carboniferous formation, or as high as the +Mountain Limestone and Millstone Grit. Palaeontologists have long maintained +that the same species which have a wide range in space are also the most +persistent in time, which may prepare us to find that some plants having a vast +geographical range may also have endured from the period of the Upper Devonian +to that of the Millstone Grit. + +(FIGURE 497. Scale of Holoptychius nobilissimus, Agassiz. Clashbinnie. 1/2 +natural size.) + +(FIGURE 498. Holoptychius, as restored by Professor Huxley. +a. The fringed pectoral fins. +b. The fringed ventral fins. +c. Anal fin. +d, e. Dorsal fins.) + +Outliers of the Upper "Old Red" occur unconformably on older members of the +group, and the formation represented at Whiteness, near Arbroath, a, Figure 55, +may probably be one of these outliers, though the want of organic remains +renders this uncertain. It is not improbable that the beds given in this section +as Nos. 1, 2, and 3, may all belong to the early part of the period of the Upper +Old Red, as some scales of Holoptychius nobilissimus have been found scattered +through these beds, No. 2, in Strathmore. Another nearly allied Holoptychius +occurs in Dura Den, see Figure 498 of this fish and also Figure 497 of one of +its scales, as these last are often the only parts met with; being scattered in +Forfarshire through red-coloured shales and sandstones, as are scales of a large +species of the same genus in a corresponding matrix in Herefordshire. (Siluria +4th edition page 265.) The number of fish obtained from the British Upper Old +Red Sandstone amounts to fifteen species referred to eleven genera. + +Sir R. Murchison groups with this upper division of the Old Red of Scotland +certain light-red and yellow sandstones and grits which occur in the +northernmost part of the mainland, and extend also into the Orkney and Shetland +Islands. They contain Calamites and other plants which agree generically with +Carboniferous forms. + +MIDDLE OLD RED SANDSTONE. + +In the northern part of Scotland there occur a great series of bituminous +schists and flagstones, to the fossil fish of which attention was first called +by the late Hugh Miller. They were afterwards described by Agassiz, and the +rocks containing them were examined by Sir R. Murchison and Professor Sedgwick, +in Caithness, Cromarty, Moray, Nairn, Gamrie in Banff, and the Orkneys and +Shetlands, in which great numbers of fossil fish have been found. These were at +first supposed to be the oldest known vertebrate animals, as in Cromarty the +beds in which they occur seem to form the base of the Old Red system resting +almost immediately on the crystalline or metamorphic rocks. But in fact these +fish-bearing beds, when they are traced from north to south, or to the central +parts of Scotland, thin out, so that their relative age to the Lower Old Red +Sandstone, presently to be mentioned, was not at first detected, the two +formations not appearing in superposition in the same district. In Caithness, +however, many hundred feet below the fish-zone of the middle division, remains +of Pteraspis were found by Mr. Peach in 1861. This genus has never yet been +found in either of the two higher divisions of the Old Red Sandstone, and +confirms Sir R. Murchison's previous suspicion that the rocks in which it occurs +belong to the Lower "Old Red," or agree in age with the Arbroath paving-stone. +(Siluria 4th edition page 258.) + +FOSSIL FISH OF THE MIDDLE OLD RED SANDSTONE. + +The Devonian fish were referred by Agassiz to two of his great orders, namely, +the Placoids and Ganoids. Of the first of these, which in the Recent period +comprise the shark, the dog-fish, and the ray, no entire skeletons are +preserved, but fin-spines, called ichthyodorulites, and teeth occur. On such +remains the genera Onchus, Odontacanthus, and Ctenodus, a supposed cestraciont, +and some others, have been established. + +(FIGURE 499. Polypterus. See Agassiz, "Recherces sur les Poissons Fossiles." +Living in the Nile and other African rivers. +a. One of the fringed pectoral fins. +b. One of the ventral fins. +c. Anal fin. +d. Dorsal fin, or row of finlets.) + +(FIGURE 500. Restoration of Osteolepis. Pander. Old Red Sandstone, or Devonian. +a. One of the fringed pectoral fins. +b. One of the ventral fins. +c. Anal fin. +d, e. Dorsal fins.) + +By far the greater number of the Old Red Sandstone fishes belong to a sub-order +of Ganoids instituted by Huxley in 1861, and for which he has proposed the name +of Crossopterygidae (Abridged from crossotos, a fringe, and pteryx, a fin.), or +the fringe-finned, in consideration of the peculiar manner in which the fin-rays +of the paired fins are arranged so as to form a fringe round a central lobe, as +in the Polypterus (see a, Figure 499), a genus of which there are several +species now inhabiting the Nile and other African rivers. The reader will at +once recognise in Osteolepis (Figure 500), one of the common fishes of the Old +Red Sandstone, many points of analogy with Polypterus. They not only agree in +the structure of the fin, at first pointed out by Huxley, but also in the +position of the pectoral, ventral, and anal fins, and in having an elongated +body and rhomboidal scales. On the other hand, the tail is more symmetrical in +the recent fish, which has also an apparatus of dorsal finlets of a very +abnormal character, both as to number and structure. As to the dorsals of +Osteolepis, they are regular in structure and position, having nothing +remarkable about them, except that there are two of them, which is comparatively +unusual in living fish. + +Among the "fringe-finned" Ganoids we find some with rhomboidal scales, such as +Osteolepis, Figure 500; others with cycloidal scales, as Holoptychius, before +mentioned (see Figure 498). In the genera Dipterus and Diplopterus, as Hugh +Miller pointed out, and in several other of the fringe-finned genera, as in +Gyroptychius and Glyptolepis, the two dorsals are placed far backward, or +directly over the ventral and anal fins. The Asterolepis was a ganoid fish of +gigantic dimensions. A. Asmusii, Eichwald, a species characteristic of the Old +Red Sandstone of Russia, as well as that of Scotland, attained the length of +between twenty and thirty feet. It was clothed with strong bony armour, embossed +with star-like tubercles, but it had only a cartilaginous skeleton. The mouth +was furnished with two rows of teeth, the outer ones small and fish-like, the +inner larger and with a reptilian character. The Asterolepis occurs also in the +Devonian rocks of North America. + + If we except the Placoids already alluded to, and a few other families of +doubtful affinities, all the Old Red Sandstone fishes are Ganoids, an order so +named by Agassiz from the shining outer surface of their scales; but Professor +Huxley has also called our attention to the fact that, while a few of the +primary and the great majority of the secondary Ganoids resemble the living bony +pike, Lepidosteus, or the Amia, genera now found in North American rivers, and +one of them, Lepidosteus, extending as far south as Guatemala, the +Crossopterygii, or fringe-finned Ichthyolites, of the Old Red are closely +related to the African Polypterus, which is represented by five or six species +now inhabiting the Nile and the rivers of Senegal. These North American and +African Ganoids are quite exceptional in the living creation; they are entirely +confined to the northern hemisphere, unless some species of Polypterus range to +the south of the line in Africa; and, out of about 9000 living species of fish +known to M. Gunther, and of which more than 6000 are now preserved in the +British Museum, they probably constitute no more than nine. + +If many circumstances favour the theory of the fresh-water origin of the Old Red +Sandstone, this view of its nature is not a little confirmed by our finding that +it is in Llake Superior and the other inland Canadian seas of fresh water, and +in the Mississippi and African rivers, that we at present find those fish which +have the nearest affinity to the fossil forms of this ancient formation. + +(FIGURE 501. Pterichthys, Agassiz; Upper side, showing mouth; as restored by H. +Miller.) + +Among the anomalous forms of Old Red fishes not referable to Huxley's +Crossopterygii is the Pterichthys, of which five species have been found in the +middle division of the Old Red of Scotland. Some writers have compared their +shelly covering to that of Crustaceans, with which, however, they have no real +affinity. The wing-like appendages, whence the genus is named, were first +supposed by Hugh Miller to be paddles, like those of the turtle; and there can +now be no doubt that they do really correspond with the pectoral fins. + +The number of species of fish already obtained from the middle division of the +Old Red Sandstone in Great Britain is about 70, and the principal genera, +besides Osteolepis and Pterichthys, already mentioned, are Glyptolepis, +Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus, and Acanthoides. + +LOWER OLD RED SANDSTONE. + +(FIGURE 502. Cephalaspis Lyellii, Agassiz. Length 6 3/4 inches. From a specimen +in my collection found at Glammiss, in Forfarshire. (See other figures, Agassiz, +volume 2 table 1 a and 1 b. +a. One of the peculiar scales with which the head is covered when perfect. These +scales are generally removed, as in the specimen above figured. +b, c. Scales from different parts of the body and tail.) + +The third or lowest division south of the Grampians consists of grey paving- +stone and roofing-slate, with associated red and grey shales; these strata +underlie a dense mass of conglomerate. In these grey beds several remarkable +fish have been found of the genus named by Agassiz Cephalaspis, or "buckler- +headed," from the extraordinary shield which covers the head (see Figure 502), +and which has often been mistaken for that of a trilobite, such as Asaphus. A +species of Pteraspis, of the same family, has also been found by the Reverend +Hugh Mitchell in beds of corresponding age in Perthshire; and Mr. Powrie +enumerates no less than five genera of the family Acanthodidae, the spines, +scales, and other remains of which have been detected in the grey flaggy +sandstones. (Powrie Geological Quarterly Journal volume 20 page 417.) + +(FIGURE 503. Pterygotus anglicus, Agassiz. Middle portion of the back of the +head called the seraphim.) + +(FIGURE 504. Pterygotus anglicus, Agassiz. Forfarshire. Ventral aspect. Restored +by H. Wodward, F.G.S. +a. Carapace, showing the large sessile eyes at the anterior angles. +b. The metastoma or post-oral plate (serving the office of a lower lip). +c, c. Chelate appendages (antennules). +d. First pair of simple palpi (antennae). +e. Second pair of simple palpi (mandibles). +f. Third pair of simple palpi (first maxillae). +g. Pair of swimming feet with their broad basal joints, whose serrated edges +serve the office of maxillae. +h. Thoracic plate covering the first two thoracic segments, which are indicated +by the figures 1, 2, and a dotted line. +1-6. Thoracic segments. +7-12. Abdominal segments. +13. Telson, or tail-plate.) + +In the same formation at Carmylie, in Forfarshire, commonly known as the +Arbroath paving-stone, fragments of a huge crustacean have been met with from +time to time. They are called by the Scotch quarrymen the "Seraphim," from the +wing-like form and feather-like ornament of the thoracic appendage, the part +most usually met with. Agassiz, having previously referred some of these +fragments to the class of fishes, was the first to recognise their crustacean +character, and, although at the time unable correctly to determine the true +relation of the several parts, he figured the portions on which he founded his +opinion, in the first plate of his "Poissons Fossiles du Vieux Gres Rouge." + +A restoration in correct proportion to the size of the fragments of P. anglicus +(Figure 504), from the Lower Old Red Sandstone of Perthshire and Forfarshire, +would give us a creature measuring from five to six feet in length, and more +than one foot across. + +The largest crustaceans living at the present day are the Inachus Kaempferi, of +De Haan, from Japan (a brachyurous or short-tailed crab), chiefly remarkable for +the extraordinary length of its limbs; the fore-arm measuring four feet in +length, and the others in proportion, so that it covers about 25 square feet of +ground; and the Limulus Moluccanus, the great King Crab of China and the Eastern +seas, which, when adult, measures 1 1/2 foot across its carapace, and is three +feet in length. + +(FIGURE 505. Parka decipiens, Fleming. In sandstone of lower beds of Old Red, +Ley's Mill, Forfarshire.) + +(FIGURE 506. Parka decipiens, Fleming. In shale of Lower Old Red, Park Hill, +Fife.) + +(FIGURE 507. Shale of Old Red Sandstone. Forfarshire. With impression of plants +and eggs of Crustaceans. +a. Two pair of ova? resembling those of large Salamanders or Tritons-- on the +same leaf. +b, b. Detached ova.) + +Besides some species of Pterygotus, several of the allied genus Eurypterus occur +in the Lower Old Red Sandstone, and with them the remains of grass-like plants +so abundant in Forfarshire and Kincardineshire as to be useful to the geologist +by enabling him to identify the inferior strata at distant points. Some +botanists have suggested that these plants may be of the family Fluviales, and +of fresh-water genera. They are accompanied by fossils, called "berries" by the +quarrymen, which they compared to a compressed blackberry (see Figures 505, +506), and which were called "Parka" by Dr. Fleming. They are now considered by +Mr. Powrie to be the eggs of crustaceans, which is highly probable, for they +have not only been found with Pterygotus anglicus in Forfarshire and Perthshire, +but also in the Upper Silurian strata of England, in which species of the same +genus, Pterygotus, occur. + +The grandest exhibitions, says Sir R. Murchison, of the Old Red Sandstone in +England and Wales appear in the escarpments of the Black Mountains and in the +Fans of Brecon and Carmarthen, the one 2862, and the other 2590 feet above the +sea. The mass of red and brown sandstone in these mountains is estimated at not +less than 10,000 feet, clearly intercalated between the Carboniferous and +Silurian strata. No shells or corals have ever been found in the whole series, +not even where the beds are calcareous, forming irregular courses of +concretionary lumps called "corn-stones," which may be described as mottled red +and green earthy limestones. The fishes of this lowest English Old Red are +Cephalaspis and Pteraspis, specifically different from species of the same +genera which occur in the uppermost Ludlow or Silurian tilestones. Crustaceans +also of the genus Eurypterus are met with. + +MARINE OR DEVONIAN TYPE. + +We may now speak of the marine type of the British strata intermediate between +the Carboniferous and Silurian, in treating of which we shall find it much more +easy to identify the Upper, Middle, and Lower divisions with strata of the same +age in other countries. It was not until the year 1836 that Sir R. Murchison and +Professor Sedgwick discovered that the culmiferous or anthracitic shales and +sandstones of North Devon, several thousand feet thick, belonged to the coal, +and that the beds below them, which are of still greater thickness, and which, +like the carboniferous strata, had been confounded under the general name +"graywacke," occupied a geological position corresponding to that of the Old Red +Sandstone already described. In this reform they were aided by a suggestion of +Mr. Lonsdale, who, after studying the Devonshire fossils, perceived that they +belonged to a peculiar palaeontological type of intermediate character between +the Carboniferous and Silurian. + +It is in the north of Devon that these formations may best be studied, where +they have been divided into an Upper, Middle, and Lower Group, and where, +although much contorted and folded, they have for the most part escaped being +altered by intrusive trap-rocks and by granite, which in Dartmoor and the more +southern parts of the same county have often reduced them to a crystalline or +metamorphic state. + +TABLE 25.1 DEVONIAN SERIES IN NORTH DEVON. + +UPPER DEVONIAN OR PILTON GROUP. + +a. Sandy slates and schists with fossils, 36 species out of 110 common to the +Carboniferous group (Pilton, Barnstaple, etc.), resting on soft schists in which +fossils are very abundant (Croyde, etc.), and which pass down into + +b. Yellow, brown, and red sandstone, with land plants (Cyclopteris, etc.) and +marine shells. One zone, characterised by the abundance of cucullaea (Baggy +Point, Marwood, Sloly, etc.) resting on hard grey and reddish sandstone and +micaceous flags, no fossils yet found (Dulverton, Pickwell, Down, etc.) + +MIDDLE DEVONIAN OR ILFRACOMBE GROUP. + +a. Green glossy slates of considerable thickness, no fossils yet recorded from +these beds (Mortenoe, Lee Bay, etc.). + +b. Slates and schists, with several irregular courses of limestone containing +shells and corals like those of the Plymouth Limestone (Combe Martin, +Ilfracombe, etc.). + +LOWER DEVONIAN OR LYNTON GROUP. + +a. Hard, greenish, red, and purple sandstone-- no fossils yet found (Hangman +Hill, etc.). + +b. Soft slates with subordinate sandstones-- fossils numerous at various +horizons-- Orthis, Corals, Encrinites, etc. (Valley of Rocks, Lynmouth, etc.). + +Table 25.1 exhibits the sequence of the strata or subdivisions as seen both on +the sea-coast of the British Channel and in the interior of Devon. It will be +seen that in all main points it agrees with the table drawn up in 1864 for the +sixth edition of my "Elements." Mr. Etheridge has since published an excellent +account of the different subdivisions of the rocks and their fossils, and has +also pointed out their relation to the corresponding marine strata of the +Continent. (Quarterly Geological Journal volume 23 1867.) The slight +modifications introduced in my table since 1864 are the result of a tour made in +1870 in company with Mr. T. Mck. Hughes, when we had the advantage of Mr. +Etheridge's memoir as our guide. + +The place of the sandstones of the Foreland is not yet clearly made out, as they +are cut off by a great fault and disturbance. + +UPPER DEVONIAN ROCKS. + +(FIGURE 508. Spirifera disjuncta, Sowerby. Syn. Sp. Verneuilii, Murch. Upper +Devonian, Boulogne.) + +(FIGURE 509. Phacops latifrons, Bronn. Characteristic of the Devonian in Europe, +Asia, and N. and S. America.) + +(FIGURE 510. Clymenia linearis, Munster. Petherwyn, Cornwall; Elbersreuth, +Bavaria.) + +(FIGURE 511. Cypridina serrato-striata, Sandberger, Weilburg, etc.; Cornwall, +Nassau, Saxony, Belgium.) + +The slates and sandstones of Barnstaple (a and b of the preceding section) +contain the shell Spirifera disjuncta, Sowerby (S. Verneuilii, Murch.), (see +Figure 508), which has a very wide range in Europe, Asia Minor, and even China; +also Strophalosia caperata, together with the large trilobite Phacops latifrons, +Bronn. (See Figure 509), which is all but world-wide in its distribution. The +fossils are numerous, and comprise about 150 species of mollusca, a fifth of +which pass up into the overlying Carboniferous rocks. To this Upper Devonian +belong a series of limestones and slates well developed at Petherwyn, in +Cornwall, where they have yielded 75 species of fossils. The genus of +Cephalopoda called Clymenia (Figure 510) is represented by no less than eleven +species, and strata occupying the same position in Germany are called Clymenien- +Kalk, or sometimes Cypridinen-Schiefer, on account of the number of minute +bivalve shells of the crustacean called Cypridina serrato-striata (Figure 511), +which is found in these beds, in the Rhenish provinces, the Harz, Saxony, and +Silesia, as well as in Cornwall and Belgium. + +MIDDLE DEVONIAN ROCKS. + +(FIGURE 512. Heliolites porosa, Goldf. sp. (Porites pyriformis, Lonsd.) +a. Portion of the same magnified. Middle Devonian, Torquay, Plymouth; Eifel.) + +(FIGURE 513. Favosites cervicornis, Blainv. S. Devon, from a polished specimen. +a. Portion of the same magnified, to show the pores.) + +(FIGURE 514. Cyathophyllum caespitosum, Goldf.; Plymouth and Ilfracombe. +b. A terminal star. +c. Vertical section, exhibiting transverse plates, and part of another branch.) + +We come next to the most typical portion of the Devonian system, including the +great limestones of Plymouth and Torbay, replete with shells, trilobites, and +corals. Of the corals 51 species are enumerated by Mr. Etheridge, none of which +pass into the Carboniferous formation. Among the genera we find Favosites, +Heliolites, and Cyathophyllum. The two former genera are very frequent in +Silurian rocks: some few even of the species are said to be common to the +Devonian and Silurian groups, as, for example, Favosites cervicornis (Figure +513), one of the commonest of all the Devonshire fossils. The Cyathophyllum +caespitosum (Figure 514) and Heliolites pyriformis (Figure 512) are species +peculiar to this formation. + +(FIGURE 515. Stringocephalus Burtini, Def. +a. Valves united. +b. Interior of ventral or large valve, showing thick partition and portion of a +large process which projects from the dorsal valve across the shell.) + +(FIGURE 516. Uncites Gryphus, Def. Middle Devonian. S. Devon and the Continent.) + +With the above are found no less than eleven genera of stone-lilies or crinoids, +some of them, such as Cupressocrinites, distinct from any Carboniferous forms. +The mollusks, also, are no less characteristic; of 68 species of Brachiopoda, +ten only are common to the Carboniferous Limestone. The Stringocephalus Burtini +(Figure 515) and Uncites Gryphus (Figure 516) may be mentioned as exclusively +Middle Devonian genera, and extremely characteristic of the same division in +Belgium. The Stringocephalus is also so abundant in the Middle Devonian of the +banks of the Rhine as to have suggested the name of Stringocephalus Limestone. +The only two species of Brachiopoda common to the Silurian and Devonian +formations are Atrypa reticularis (Figure 532), which seems to have been a +cosmopolite species, and Strophomena rhomboidalis. + +(FIGURE 517. Megalodon cucullatus, Sowerby. Eifel; also Bradley, S. Devon. +a. The valves united. +b. Interior of valve, showing the large cardinal tooth.) + +(FIGURE 518. Conularia ornata, D'Arch. and De Vern. (Geological Transactions +Sec. Ser. volume 6. Plate 29.) Refrath, near Cologne.) + +(FIGURE 519. Bronteus flabellifer, Goldf. Mid. Devon; S. Devon; and the Eifel.) + +Among the peculiar lamellibranchiate bivalves common to the Plymouth limestone +of Devonshire and the Continent, we find the Megalodon (Figure 517). There are +also twelve genera of Gasteropods which have yielded 36 species, four of which +pass to the Carboniferous group, namely Macrocheilus, Acroculia, Euomphalus, and +Murchisonia. Pteropods occur, such as Conularia (Figure 518), and Cephalopods, +such as Cyrtoceras, Gyroceras, Orthoceras, and others, nearly all of genera +distinct from those prevailing in the Upper Devonian Limestone, or Clymenien- +kalk of the Germans already mentioned. Although but few species of Trilobites +occur, the characteristic Bronteus flabellifer (Figure 519) is far from rare, +and all collectors are familiar with its fan-like tail. In this same group, +called, as before stated, the Stringocephalus, or Eifel Limestone, in Germany, +several fish remains have been detected, and among others the remarkable genus +Coccosteus, covered with its tuberculated bony armour; and these ichthyolites +serve, as Sir R. Murchison observes (Siluria page 362), to identify this middle +marine Devonian with the Old Red Sandstone of Britain and Russia. + +(FIGURE 520. Calceola sandalina, Lam. Eifel; also South Devon. +a. Ventral valve. +b. Inner side of dorsal valve.) + +Beneath the Eifel Limestone (the great central and typical member of "the +Devonian" on the Continent) lie certain schists called by German writers +"Calceola-schiefer," because they contain in abundance a fossil body of very +curious structure, Calceola sandalina (Figure 520), which has been usually +considered a brachiopod, but which some naturalists have lately referred to a +Goniophyllum, supposing it to be an abnormal form of the order Zoantharia rugosa +(see Figure 474), differing from all other corals in being furnished with a +strong operculum. This is by no means a rare fossil in the slaty limestone of +South Devon, and, like the Eifel form, is confined to the middle group of this +country. + +LOWER DEVONIAN ROCKS. + +(FIGURE 521. Spirifera mucronata, Hall. Devonian of Pennsylvania.) + +A great series of sandstones and glossy slates, with Crinoids, Brachiopods, and +some corals, occurring on the coast at Lynmouth and the neighbourhood, and +called the Lynton Group (see Table 25.1), form the lowest member of the Devonian +in North Devon. Among the 18 species of all classes enumerated by Mr. Etheridge, +two-thirds are common to the Middle Devonian, but only one, the ubiquitous +Atrypa reticularis, can with certainty be identified with Silurian species. +Among the characteristic forms are Alveolites suborbicularis, also common to +this formation in the Rhine, and Orthis arcuata, very widely spread in the North +Devon localities. But we may expect a large addition to the number of fossils +whenever these strata shall have been carefully searched. The Spirifer Sandstone +of Sandberger, as exhibited in the rocks bordering the Rhine between Coblentz +and Caub, belong to this Lower division, and the same broad-winged Spirifers +distinguish the Devonian strata of North America. + +(FIGURE 522. Homalonotus armatus, Burmeister. Lower Devonian; Daun, in the +Eifel; and S. Devon. +Obs. The two rows of spines down the body give an appearance of more distinct +trilobation than really occurs in this or most other species of the genus.) + +Among the Trilobites of this era several large species of Homalonotus (Figure +522) are conspicuous. The genus is still better known as a Silurian form, but +the spinose species appear to belong exclusively to the "Lower Devonian," and +are found in Britain, Europe, and the Cape of Good Hope. + +DEVONIAN OF RUSSIA. + +The Devonian strata of Russia extend, according to Sir R. Murchison, over a +region more spacious than the British Isles; and it is remarkable that, where +they consist of sandstone like the "Old Red" of Scotland and Central England, +they are tenanted by fossil fishes often of the same species and still oftener +of the same genera as the British, whereas when they consist of limestone they +contain shells similar to those of Devonshire, thus confirming, as Sir Roderick +has pointed out, the contemporaneous origin which had been previously assigned +to formations exhibiting two very distinct mineral types in different parts of +Britain. (Murchison's Siluria page 329.) The calcareous and the arenaceous rocks +of Russia above alluded to alternate in such a manner as to leave no doubt of +their having been deposited in different parts of the same great period. + +DEVONIAN STRATA IN THE UNITED STATES AND CANADA. + +(FIGURE 523. Psilophyton princeps, Dawson, Quarterly Geological Journal volume +15 1863; and Canada Survey 1863. Species characteristic of the whole Devonian +series in North America. +a. Fruit; natural size. +b. Stem; natural size. +c. Scalariform tissue of the axis highly magnified.) + +Between the Carboniferous and Silurian strata there intervenes, in the United +States and Canada, a great series of formations referable to the Devonian group, +comprising some strata of marine origin abounding in shells and corals, and +others of shallow-water and littoral origin in which terrestrial plants abound. +The fossils, both of the deep and shallow water strata, are very analogous to +those of Europe, the species being in some cases the same. In Eastern Canada Sir +W. Logan has pointed out that in the peninsula of Gaspe, south of the estuary of +St. Lawrence, a mass of sandstone, conglomerate, and shale referable to this +period occurs, rich in vegetable remains, together with some fish-spines. Far +down in the sandstones of Gaspe, Dr. Dawson found, in 1869, an entire specimen +of the genus Cephalaspis, a form so characteristic, as we have already seen, of +the Scotch Lower Old Red Sandstone. Some of the sandstones are ripple-marked, +and towards the upper part of the whole series a thin seam of coal has been +observed, measuring, together with some associated carbonaceous shale, about +three inches in thickness. It rests on an under-clay in which are the roots of +Psilophyton (see Figure 523). At many other levels rootlets of this same plant +have been shown by Principal Dawson to penetrate the clays, and to play the same +part as do the rootlets of Stigmaria in the coal formation. + +We had already learnt from the works of Goppert, Unger, and Bronn that the +European plants of the Devonian epoch resemble generically, with few exceptions, +those already known as Carboniferous; and Dr. Dawson, in 1859, enumerated 32 +genera and 69 species which he had then obtained from the State of New York and +Canada. A perusal of his catalogue (Quarterly Geological Journal volume 15 page +477 1859; also volume 18 page 296 1862.), comprising Coniferae, Sigillariae, +Calamites, Asterophyllites, Lepidodendra, and ferns of the genera Cyclopteris, +Neuropteris, Sphenopteris, and others, together with fruits, such as +Cardiocarpum and Trigonocarpum, might dispose geologists to believe that they +were presented with a list of Carboniferous fossils, the difference of the +species from those of the coal-measures, and even a slight admixture of genera +unknown in Europe, being naturally ascribed to geographical distribution and the +distance of the New from the Old World. But fortunately the coal formation is +fully developed on the other side of the Atlantic, and is singularly like that +of Europe, both lithologically and in the species of its fossil plants. There is +also the most unequivocal evidence of relative age afforded by superposition, +for the Devonian strata in the United States are seen to crop out from beneath +the Carboniferous on the borders of Pennsylvania and New York, where both +formations are of great thickness. + +The number of American Devonian plants has now been raised by Dr. Dawson to 120, +to which we may add about 80 from the European flora of the same age, so that +already the vegetation of this period is beginning to be nearly half as rich as +that of the coal-measures which have been studied for so much longer a time and +over so much wider an area. The Psilophyton above alluded to is believed by Dr. +Dawson to be a lycopodiaceous plant, branching dichotomously (see P. princeps, +Figure 523), with stems springing from a rhizome, which last has circular +areoles, much resembling those of Stigmaria, and like it sending forth +cylindrical rootlets. The extreme points of some of the branchlets are rolled up +so as to resemble the croziers or circinate vernation of ferns; the leaves or +bracts, a, supposed to belong to the same plant, are described by Dawson as +having inclosed the fructification. The remains of Psilophyton princeps have +been traced through all the members of the Devonian series in America, and Dr. +Dawson has lately recognised it in specimens of Old Red Sandstone from the north +of Scotland. + +The monotonous character of the Carboniferous flora might be explained by +imagining that we have only the vegetation handed down to us of one set of +stations, consisting of wide swampy flats. But Dr. Dawson supposes that the +geographical conditions under which the Devonian plants grew were more varied, +and had more of an upland character. If so, the limitation of this more ancient +flora, represented by so many genera and species, to the gymnospermous and +cryptogamous orders, and the absence or extreme rarity of plants of higher +grade, lead us naturally to speculate on the theory of progressive development, +however difficult it may be to avail ourselves of this explanation, so long as +we meet with even a few exceptional cases of what may seem to be +monocotyledonous or dicotyledonous exogens. + +DEVONIAN INSECTS OF CANADA. + +The earliest known insects were brought to light in 1865 in the Devonian strata +of St. John's, New Brunswick, and are referred by Mr. Scudder to four species of +Neuroptera. One of them is a gigantic Ephemera, and measured five inches in +expanse of wing. + +Like many other ancient animals, says Dr. Dawson, they show a remarkable union +of characters now found in distinct orders of insects, or constitute what have +been named "synthetic types." Of this kind is a stridulating or musical +apparatus like that of the cricket in an insect otherwise allied to the +Neuroptera. This structure, as Dr. Dawson observes, if rightly interpreted by +Mr. Scudder, introduces us to the sounds of the Devonian woods, bringing before +our imagination the trill and hum of insect life that enlivened the solitudes of +these strange old forests. + + +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. + +CLASSIFICATION OF THE SILURIAN ROCKS. + +We come next in descending order to that division of Primary or Palaeozoic rocks +which immediately underlie the Devonian group or Old Red Sandstone. For these +strata Sir Roderick Murchison first proposed the name of Silurian when he had +studied and classified them in that part of Wales and some of the contiguous +counties of England which once constituted the kingdom of the Silures, a tribe +of ancient Britons. Table 26.1 will explain the two principal divisions, Upper +and Lower, of the Silurian rocks, and the minor subdivisions usually adopted, +comprehending all the strata originally embraced in the Silurian system by Sir +Roderick Murchison. The formations below the Arenig or Stiper-stones group are +treated of in the next chapter, when the "Primordial" or Cambrian group is +described. + +TABLE 26.1. SILURIAN ROCKS (THICKNESS GIVEN IN FEET). + +UPPER SILURIAN ROCKS. + +1. LUDLOW FORMATION: + +a. Upper Ludlow beds: 780. +b. Lower Ludlow beds: 1,050. + +2. WENLOCK FORMATION: + +a. Wenlock limestone and shale and +b. Woolhope limestone and shale, and Denbighshire grits: above 4,000. + +3. LLANDOVERY FORMATION (Beds of passage between Upper and Lower Silurian): + +a. Upper Llandovery (May-Hill beds): 800. +b. Lower Llandovery: 600-1,000. + +LOWER SILURIAN ROCKS. + +1. BALA AND CARADOC BEDS, including volcanic rocks: 12,000. + +2. LLANDEILO FLAGS, including volcanic rocks: 4,500. + +3. ARENIG OR STIPER-STONES GROUP, including volcanic rocks: above 10,000. + +UPPER SILURIAN ROCKS. + +1. LUDLOW FORMATION. + +This member of the Upper Silurian group, as will be seen by Table 26.1, is of +great thickness, and subdivided into two parts-- the Upper Ludlow and the Lower +Ludlow. Each of these may be distinguished near the town of Ludlow, and at other +places in Shropshire and Herefordshire, by peculiar organic remains; but out of +more than 500 species found in the Ludlow formation as a whole, not more than +five species per hundred are common to the overlying Devonian. The student may +refer to the excellent tables given in the last edition of Sir R. Murchison's +Siluria for a list of the organic remains of all classes distributed through the +different subdivisions of the Upper and Lower Silurian. + +A. UPPER LUDLOW: DOWNTON SANDSTONE. + +At the top of this subdivision there occur beds of fine-grained yellowish +sandstone and hard reddish grits which were formerly referred by Sir R. +Murchison to the Old Red Sandstone, under the name of "Tilestones." In mineral +character this group forms a transition from the Silurian to the Old Red +Sandstone, the strata of both being conformable; but it is now ascertained that +the fossils agree in great part specifically, and in general character entirely, +with those of the underlying Upper Ludlow rocks. Among these are Orthoceras +bullatum, Platyschisma helicites, Bellerophon trilobatus, Chonetes lata, etc., +with numerous defenses of fishes. + +These beds, therefore, now generally called the "Downton Sandstone," are classed +as the newest member of the Upper Silurian. They are well seen at Downton +Castle, near Ludlow, where they are quarried for building, and at Kington, in +Herefordshire. In the latter place, as well as at Ludlow, crustaceans of the +genera Pterygotus (for genus see Figure 504) and Eurypterus are met with. + +BONE-BED OF THE UPPER LUDLOW. + +At the base of the Downton sandstones there occurs a bone-bed which deserves +especial notice as affording the most ancient example of fossil fish occurring +in any considerable quantity. It usually consists of one or two thin layers of +brown bony fragments near the junction of the Old Red Sandstone and the Ludlow +rocks, and was first observed by Sir R. Murchison near the town of Ludlow, where +it is three or four inches thick. It has since been traced to a distance of 45 +miles from that point into Gloucestershire and other counties, and is commonly +not more than an inch thick, but varies to nearly a foot. Near Ludlow two bone- +beds are observable, with 14 feet of intervening strata full of Upper Ludlow +fossils. (Murchison's Siluria page 140.) At that point immediately above the +upper fish-bed numerous small globular bodies have been found, which were +determined by Dr. Hooker to be the sporangia of a cryptogamic land-plant, +probably lycopodiaceous. + +(FIGURE 524. Onchus tenuistriatus, Agassiz. Bone-bed. Upper Silurian. Ludlow.) + +(FIGURE 525. Shagreen-scales of a placoid fish, Thelodus parvidens, Agassiz. +Bone-bed, Upper Ludlow.) + +(FIGURE 526. Plectrodus mirabilis, Agassiz. Bone-bed, Upper Ludlow.) + +Most of the fish have been referred by Agassiz to his placoid order, some of +them to the genus Onchus, to which the spine (Figure 524) and the minute scales +(Figure 525) are supposed to belong. It has been suggested, however, that Onchus +may be one of those Acanthodian fish referred by Agassiz to his Ganoid order, +which are so characteristic of the base of the Old Red Sandstone in Forfarshire, +although the species of the Old Red are all different from these of the Silurian +beds now under consideration. The jaw and teeth of another predaceous genus +(Figure 526) have also been detected, together with some specimens of Pteraspis +Ludensis. As usual in bone-beds, the teeth and bones are, for the most part, +fragmentary and rolled. + +GREY SANDSTONE AND MUDSTONE, ETC. + +(FIGURE 527. Orthis elegantula, Dalm. Var. Orbicularis, Sowerby. Upper Ludlow.) + +(FIGURE 528. Rhynchonella navicula, Sowerby. Ludlow Beds.) + +The next subdivision of the Upper Ludlow consists of grey calcareous sandstone, +or very commonly a micaceous stone, decomposing into soft mud, and contains, +besides the shells mentioned above, Lingula cornea, Orthis orbicularis, a round +variety of O. elegantula, Modiolopsis platyphylla, Grammysia cingulata, all +characteristic of the Upper Ludlow. The lowest or mud-stone beds contain +Rhynchonella navicula (Figure 528), which is common to this bed and the Lower +Ludlow. As usual in Palaeozoic strata older than the coal, the brachiopodous +mollusca greatly outnumber the lamellibranchiate (see below); but the latter are +by no means unrepresented. Among other genera, for example, we observe Avicula +and Pterinea, Cardiola, Ctenodonta (sub-genus of Nucula), Orthonota, +Modiolopsis, and Palaearca. + +Some of the Upper Ludlow sandstones are ripple-marked, thus affording evidence +of gradual deposition; and the same may be said of the accompanying fine +argillaceous shales, which are of great thickness, and have been provincially +named "mud-stones." In some of these shales stems of crinoidea are found in an +erect position, having evidently become fossil on the spots where they grew at +the bottom of the sea. The facility with which these rocks, when exposed to the +weather, are resolved into mud, proves that, notwithstanding their antiquity, +they are nearly in the state in which they were first thrown down. + +b. LOWER LUDLOW BEDS. + +(FIGURE 529. Pentamerus Knightii, Sowerby. Aymestry. One-half natural size. +a. View of both valves united. +b. Longitudinal section through both valves, showing the central plates or +septa.) + +The chief mass of this formation consists of a dark grey argillaceous shale with +calcareous concretions, having a maximum thickness of 1000 feet. In some places, +and especially at Aymestry, in Herefordshire, a subcrystalline and argillaceous +limestone, sometimes 50 feet thick, overlies the shale. Sir R. Murchison +therefore classes this Aymestry limestone as holding an intermediate position +between the Upper and Lower Ludlow, but Mr. Lightbody remarks that at Mocktrie, +near Leintwardine, the Lower Ludlow shales, with their characteristic fossils, +occur both above and below a similar limestone. This limestone around Aymestry +and Sedgeley is distinguished by the abundance of Pentamerus Knightii, Sowerby +(Figure 529), also found in the Lower Ludlow and Wenlock shale. This genus of +brachiopoda was first found in Silurian strata, and is exclusively a palaeozoic +form. The name was derived from pente, five, and meros, a part, because both +valves are divided by a central septum, making four chambers, and in one valve +the septum itself contains a small chamber, making five. The size of these septa +is enormous compared with those of any other brachiopod shell; and they must +nearly have divided the animal into two equal halves; but they are, +nevertheless, of the same nature as the septa or plates which are found in the +interior of Spirifera, Terebratula, and many other shells of this order. Messrs. +Murchison and De Verneuil discovered this species dispersed in myriads through a +white limestone of Upper Silurian age, on the banks of the Is, on the eastern +flank of the Urals in Russia, and a similar species is frequent in Sweden. + +(FIGURE 530. Lingula Lewisii, J. Sowb. Abberley Hills.) + +(FIGURE 531. Rhynchonella (Terebratula) Wilsoni, Sowerby. Aymestry.) + +(FIGURE 532. Atrypa reticularis, Linn. (Terebratula affinis, Min. Con.) +Aymestry. +a. Upper valve. +b. Lower valve. +c. Anterior margin of the valves.) + +Three other abundant shells in the Aymestry limestone are, 1st, Lingula Lewisii +(Figure 530); second, Rhynchonella Wilsoni, Sowerby. (Figure 531), which is also +common to the Lower Ludlow and Wenlock limestone; third, Atrypa reticularis, +Linn. (Figure 532), which has a very wide range, being found in every part of +the Upper Silurian system, and even ranging up into the Middle Devonian series. + +The Aymestry Limestone contains many shells, especially brachiopoda, corals, +trilobites, and other fossils, amounting on the whole to 74 species, all except +three or four being common to the beds either above or below. + +(FIGURE 533. Phragmoceras ventricosum, J. Sowerby. (Orthoceras ventricosum, +Stein.) Aymestry; one-quarter natural size.) + +(FIGURE 534. Lituites (Trochoceras) giganteus, J. Sowerby. Near Ludlow; also in +the Aymestry and Wenlock Limestones; 1/4 natural size.) + +(FIGURE 535. Fragment of Orthoceras Ludense, J. Sowerby. Leintwardine, +Shropshire.) + +The Lower Ludlow Shale contains, among other fossils, many large cephalopoda not +known in newer rocks, as the Phragmoceras of Broderip, and the Lituites of +Breynius (see Figures 533, 534). The latter is partly straight and partly +convoluted in a very flat spire. The Orthoceras Ludense (Figure 535), as well as +the cephalopod last mentioned, occurs in this member of the species. + +A species of Graptolite, G. priodon, Bronn (Figure 545), occurs plentifully in +the Lower Ludlow. This fossil, referred, though somewhat doubtfully, to a form +of hydrozoid or sertularian polyp, has not yet been met with in strata above the +Silurian. + +Star-fish, as Sir R. Murchison points out, are by no means rare in the Lower +Ludlow rock. These fossils, of which six extinct genera are now known in the +Ludlow series, represented by 18 species, remind us of various living forms now +found in our British seas, both of the families Asteriadae and Ophiuridae. + +OLDEST KNOWN FOSSIL FISH. + +Until 1859 there was no example of a fossil fish older than the bone-bed of the +Upper Ludlow, but in that year a specimen of Pteraspis was found at Church Hill, +near Leintwardine, in Shropshire, by Mr. J.E. Lee of Caerleon, F.G.S., in shale +below the Aymestry limestone, associated with fossil shells of the Lower Ludlow +formation-- shells which differ considerably from those characterising the Upper +Ludlow already described. This discovery is of no small interest as bearing on +the theory of progressive development, because, according to Professor Huxley, +the genus Pteraspis is allied to the sturgeon, and therefore by no means of low +grade in the piscine class. + +It is a fact well worthy of notice that no remains of vertebrata have yet been +met with in any strata older than the Lower Ludlow. + +When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals, +and other fossils already obtained from more ancient Silurian formations, Upper, +Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer +in the series were ever studied with equal diligence, and over so vast an area, +without yielding a single ichthyolite. Yet we must hesitate before we accept, +even on such evidence, so sweeping a conclusion, as that the globe, for ages +after it was inhabited by all the great classes of invertebrata, remained wholly +untenanted by vertebrate animals. + +TABLE 26.2. DATES OF THE DISCOVERY OF DIFFERENT CLASSES OF FOSSIL VERTEBRATA; +SHOWING THE GRADUAL PROGRESS MADE IN TRACING THEM TO ROCKS OF HIGHER ANTIQUITY. + +COLUMN 1: YEAR. + +COLUMN 2: FORMATIONS. + +COLUMN 3: GEOGRAPHICAL LOCALITIES. + +MAMMALIA: + +1798: Upper Eocene: Paris (Gypsum of Montmartre). (George Cuvier, Bulletin Soc. +Philom. 20.) + +1818: Lower Oolite: Stonesfield. (In 1818, Cuvier, visiting the Museum of +Oxford, decided on the mammalian character of a jaw from Stonesfield. See also +above Chapter 19.) + +1847: Upper Trias: Stuttgart. (Professor Plieninger. See above Chapter 21.) + +AVES: + +1782: Upper Eocene: Paris (Gypsum of Montmartre). (Cuvier, Ossemens Foss. Art. +"Oiseaux.") + +1839: Lower Eocene: Isle of Sheppey (London Clay). (Professor Owen Geological +Transactions second series volume 6 page 203 1839.) + +1854: Lower Eocene: Woolwich Beds. (Upper part of the Woolwich beds. Prestwich +Quarterly Geological Journal volume 10 page 157.) + +1855: Lower Eocene: Meudon (Plastic Clay). (Gastornis Parisiensis. Owen +Quarterly Geological Journal volume 12 page 204 1856.) + +1858: Chloritic Series, or Upper Greensand: Cambridge. (Coprolitic bed, in the +Upper Greensand. See above Chapter 17.) + +1863: Upper Oolite: Solenhofen. (The Archaeopteryx macrura, Owen. See above +Chapter 19.) + +REPTILIA: + +1710: Permian (or Zechstein): Thuringia. (The fossil monitor of Thuringia +(Protosaurus Speneri, V. Meyer) was figured by Spener of Berlin in 1810. +(Miscel. Berlin.)) + +1844: Carboniferous: Saarbruck, near Treves. (See Chapter 23.) + +PISCES: + +1709: Permian (or Kupferschiefer): Thuringia. (Memorabilia Saxoniae Subterr. +Leipsic 1709.) + +1793: Carboniferous (Mountain Limestone): Glasgow. (History of Rutherglen by +David Ure, 1793.) + +1828: Devonian: Caithness. (Sedgwick and Murchison Geological Transactions +second series volume 3 page 141 1828.) + +1840: Upper Ludlow: Ludlow. (Sir R. Murchison. See Chapter 26.) + +1859: Lower Ludlow: Leintwardine. (See Chapter 26.) + +Obs.-- The evidence derived from foot-prints, though often to be relied on, is +omitted in the above table, as being less exact than that founded on bones and +teeth. + +In Table 26.2 a few dates are set before the reader of the discovery of +different classes of animals in ancient rocks, to enable him to perceive at a +glance how gradual has been our progress in tracing back the signs of vertebrata +to formations of high antiquity. Such facts may be useful in warning us not to +assume too hastily that the point which our retrospect may have reached at the +present moment can be regarded as fixing the date of the first introduction of +any one class of beings upon the earth. + +2. WENLOCK FORMATION. + +We next come to the Wenlock formation, which has been divided into Wenlock +limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits. + +a. WENLOCK LIMESTONE. + +This limestone, otherwise well known to collectors by the name of the Dudley +Limestone, forms a continuous ridge in Shropshire, ranging for about 20 miles +from S.W. to N.E., about a mile distant from the nearly parallel escarpment of +the Aymestry limestone. This ridgy prominence is due to the solidity of the +rock, and to the softness of the shales above and below it. Near Wenlock it +consists of thick masses of grey subcrystalline limestone, replete with corals, +encrinites, and trilobites. It is essentially of a concretionary nature; and the +concretions, termed "ball-stones" in Shropshire, are often enormous, even 80 +feet in diameter. They are of pure carbonate of lime, the surrounding rock being +more or less argillaceous (Murchison's Siluria chapter 6.) Sometimes in the +Malvern Hills this limestone, according to Professor Phillips, is oolitic. + +(FIGURE 536. Halysites catenularius, Linn. sp. Upper and Lower Silurian.) + +(FIGURE 537. Favosites Gothlandica, Lam. Dudley. +a. Portion of a large mass; less than the natural size. +b. Magnified portion, to show the pores and the partitions in the tubes.) + +(FIGURE 538. Omphyma turbinatum, Linn. Sp. (Cyathophyllum, Goldfuss) Wenlock +Limestone, Shropshire.) + +Among the corals, in which this formation is so rich, 53 species being known, +the "chain-coral," Halysites catenularius (Figure 536), may be pointed out as +one very easily recognised, and widely spread in Europe, ranging through all +parts of the Silurian group, from the Aymestry limestone to near the bottom of +the Llandeilo rocks. Another coral, the Favosites Gothlandica (Figure 537), is +also met with in profusion in large hemispherical masses, which break up into +columnar and prismatic fragments, like that here figured (Figure 537, b). +Another common form in the Wenlock limestone is the Omphyma turbinatum (Figure +538), which, like many of its modern companions, reminds us of some cup-corals; +but all the Silurian genera belong to the palaeozoic type before mentioned +(Chapter 24), exhibiting the quadripartite arrangement of the septalamellae +within the cup. + +(FIGURE 539. Pseudocrintes bifasciatus, Pearce. Wenlock Limestone, Dudley.) + +Among the numerous Crinoids, several peculiar species of Cyathocrinus (for genus +see Figures 478, 479) contribute their calcareous stems, arms, and cups towards +the composition of the Wenlock limestone. Of Cystideans there are a few very +remarkable forms, most of them peculiar to the Upper Silurian formation, as, for +example, the Pseudocrinites, which was furnished with pinnated fixed arms, as +represented in Figure 539. (E. Forbes Mem. Geological Survey volume 2 page 496.) + +(FIGURE 540. Strophomena (Leptaena) depressa, Sowerby. Wenlock and Ludlow +Rocks.) + +The Brachiopoda are, many of them, of the same species as those of the Aymestry +limestone; as, for example, Atrypa reticularis (Figure 532), and Strophomena +depressa (Figure 540); but the latter species ranges also from the Ludlow rocks, +through the Wenlock shale, to the Caradoc Sandstone. + +(FIGURE 541. Calymene Blumenbachii, Brong. Ludlow, Wenlock, and Bala beds.) + +(FIGURE 542. Phacops (Asaphus) caudatus, Brong. Wenlock and Ludlow Rocks.) + +(FIGURE 543. Sphaerexochus mirus, Beyrich; coiled up. Wenlock Limestone, Dudley; +also found in Ohio, North America.) + +(FIGURE 544. Homalonotus delphinocephalus, Konig. Wenlock Limestone, Dudley +Castle.) + +The crustaceans are represented almost exclusively by Trilobites, which are very +conspicuous, 22 being peculiar. The Calymene Blumenbachii (Figure 541), called +the "Dudley Trilobite," was known to collectors long before its true place in +the animal kingdom was ascertained. It is often found coiled up like the common +Oniscus or wood-louse, and this is so usual a circumstance among certain genera +of trilobites as to lead us to conclude that they must have habitually resorted +to this mode of protecting themselves when alarmed. The other common species is +the Phacops caudatus (Asaphus caudatus), Brong. (see Figure 542), and this is +conspicuous for its large size and flattened form. Sphaerexochus mirus (Figure +543) is almost a globe when rolled up, the forehead or glabellum of this species +being extremely inflated. The Homalonotus, a form of Trilobite in which the +tripartite division of the dorsal crust is almost lost (see Figure 544), is very +characteristic of this division of the Silurian series. + +WENLOCK SHALE. + +(FIGURE 545. Graptolithus priodon, Bronn. Ludlow and Wenlock shales.) + +The Wenlock Shale, observes Sir R. Murchison, is infinitely the largest and most +persistent member of the Wenlock formation, for the limestone often thins out +and disappears. The shale, like the Lower Ludlow, often contains elliptical +concretions of impure earthy limestone. In the Malvern district it is a mass of +finely levigated argillaceous matter, attaining, according to Professor +Phillips, a thickness of 640 feet, but it is sometimes more than 1000 feet thick +in Wales, and is worked for flag-stones and slates. The prevailing fossils, +besides corals and trilobites, and some crinoids, are several small species of +Orthis, Cardiola, and numerous thin-shelled species of Orthoceratites. + +About six species of Graptolite, a peculiar group of sertularian fossils before +alluded to as being confined to Silurian rocks, occur in this shale. Of fossils +of this genus, which is very characteristic of the Lower Silurian, I shall again +speak in the sequel. + +b. WOOLHOPE BEDS. + +Though not always recognised as a separate subdivision of the Wenlock, the +Woolhope beds, which underlie the Wenlock shale, are of great importance. +Usually they occur as massive or nodular limestones, underlaid by a fine shale +or flag-stone; and in other cases, as in the noted Denbighshire sandstones, as a +coarse grit of very great thickness. This grit forms mountain ranges through +North and South Wales, and is generally marked by the great sterility of the +soil where it occurs. It contains the usual Wenlock fossils, but with the +addition of some common in the uppermost Ludlow rock, such as Chonetes lata and +Bellerophon trilobatus. The chief fossils of the Woolhope limestone are Illaenus +Barriensis, Homalonotus delphinocephalus (Figure 544), Strophomena imbrex, and +Rhynchonella Wilsoni (Figure 531). The latter attains in the Woolhope beds an +unusual size for the species, the specimens being sometimes twice as large as +those found in the Wenlock limestone. + +In some places below the Wenlock formation there are shales of a pale or purple +colour, which near Tarannon attain a thickness of about 1000 feet; they can be +traced through Radnor and Montgomery to North Wales, according to Messrs. Jukes +and Aveline. By the latter geologist they have been identified with certain +shales above the May-Hill Sandstone, near Llandovery, but, owing to the extreme +scarcity of fossils, their exact position remains doubtful. + +3. LLANDOVERY GROUP-- BEDS OF PASSAGE. + +We now come to beds respecting the classification of which there has been much +difference of opinion, and which in fact must be considered as beds of passage +between Upper and Lower Silurian. I formerly adopted the plan of those who class +them as Middle Silurian, but they are scarcely entitled to this distinction, +since after about 1400 Silurian species have been compared the number peculiar +to the group in question only gives them an importance equal to such minor +subdivisions as the Ludlow or Bala groups. I therefore prefer to regard them as +the base of the Upper Silurian, to which group they are linked by more than +twice as many species as to the Lower Silurian. By this arrangement the line of +demarkation between the two great divisions, though confessedly arbitrary, is +less so than by any other. They are called Llandovery Rocks, from a town in +South Wales, in the neighbourhood of which they are well developed, and where, +especially at a hill called Noeth Grug, in spite of several faults, their +relations to one another can be clearly seen. + +a. UPPER LLANDOVERY OR MAY-HILL SANDSTONE. + +(FIGURE 546. Pentamerus oblongus, Sowerby. Upper and Lower Llandovery beds. +a, b. Views of the shell itself, from figures in Murchison's Sil. Syst. +c. Cast with portion of shell remaining, and with the hollow of the central +septum filled with spar. +d. Internal cast of a valve, the space once occupied by the septum being +represented by a hollow in which is seen a cast of the chamber within the +septum.) + +(FIGURE 547. Stricklandinia (Pentamerus) lirata, Sowerby.) + +The May-Hill group, which has also been named "Upper Llandovery," by Sir R. +Murchison, ranges from the west of the Longmynd to Builth, Llandovery, and +Llandeilo, and to the sea in Marlow's Bay, where it is seen in the cliffs. It +consists of brownish and yellow sandstones with calcareous nodules, having +sometimes a conglomerate at the base derived from the waste of the Lower +Silurian rocks. These May-Hill beds were formerly supposed to be part of the +Caradoc formation, but their true position was determined by Professor Sedgwick +to be at the base of the Upper Silurian proper. (Quarterly Geological Journal +volume 4 page 215 1853.) The more calcareous portions of the rock have been +called the Pentamerus limestone, because Pentamerus oblongus (Figure 546) is +very abundant in them. It is usually accompanied by P. (Stricklandinia) lirata +(Figure 547); both forms have a wide geographical range, being also met with in +the same part of the Silurian series in Russia and the United States. + +About 228 species of fossils are known in the May-Hill division, more than half +of which are Wenlock species. They consist of trilobites of the genera Illaenus +and Calymene; Brachiopods of the genera Orthis, Atrypa, Leptaena, Pentamerus, +Strophomena, and others; Gasteropods of the genera Turbo, Murchisonia (for +genus, see Figure 567), and Bellerophon; and Pteropods of the genus Conularia. +The Brachiopods, of which there are 66 species, are almost all Upper Silurian. + +(FIGURE 548. Tentaculites annulatus, Schlot. Interior casts in sandstone. Upper +Llandovery, Eastnor Park, near Malvern. Natural size and magnified.) + +Among the fossils of the May-Hill shelly sandstone at Malvern, Tentaculites +annulatus (Figure 548), an annelid, probably allied to Serpula, is found. + +LOWER LLANDOVERY ROCKS. + +Below the May-Hill Group are the Lower Llandovery Rocks, which consist chiefly +of hard slaty rocks, and beds of conglomerate from 600 to 1000 feet in +thickness. The fossils, which are somewhat rare in the lower beds, consist of +128 known species, only eleven of which are peculiar, 83 being common to the +May-Hill group above, and 93 common to the rocks below. Stricklandinia +(Pentamerus) levis, which is common in the Lower Llandovery, becomes rare in the +Upper, while Pentamerus oblongus (Figure 546), which is the characteristic shell +of the Upper Llandovery, occurs but seldom in the Lower. + +LOWER SILURIAN ROCKS. + +The Lower Silurian has been divided into, first, the Bala Group; secondly, the +Llandeilo flags; and, thirdly, the Arenig or Lower Llandeilo formation. + +BALA AND CARADOC BEDS. + +(FIGURE 549. Orthis tricenaria, Conrad. New York; Canada. 1/2 natural size.) + +(FIGURE 550. Orthis vespertilio, Sowerby. Shropshire, N. and S. Wales. One-half +natural size.) + +(FIGURE 551. Orthis (Strophomena) grandis, Sowerby. Two-thirds natural size. +Caradoc Beds, Horderley, Shropshire, and Coniston, Lancashire.) + +The Caradoc sandstone was originally so named by Sir R.I. Murchison from the +mountain called Caer Caradoc, in Shropshire; it consists of shelly sandstones of +great thickness, and sometimes containing much calcareous matter. The rock is +frequently laden with the beautiful trilobite called by Murchison Trinucleus +Caractaci (see Figure 553), which ranges from the base to the summit of the +formation, usually accompanied by Strophomena grandis (see Figure 551), and +Orthis vespertilio (Figure 550), with many other fossils. + +BRACHIOPODA. + +Nothing is more remarkable in these beds, and in the Silurian strata generally +of all countries, than the preponderance of brachiopoda over other forms of +mollusca. Their proportional numbers can by no means be explained by supposing +them to have inhabited seas of great depth, for the contrast between the +palaeozoic and the present state of things has not been essentially altered by +the late discoveries made in our deep-sea dredgings. We find the living +brachiopoda so rare as to form about one forty-fourth of the whole bivalve +fauna, whereas in the Lower Silurian rocks of which we are now about to treat, +and where the brachiopoda reach their maximum, they are represented by more than +twice as many species as the Lamellibranchiate bivalves. + +There may, indeed, be said to be a continued decrease of the proportional number +of this lower tribe of mollusca as we proceed from older to newer rocks. In the +British Devonian, for example, the Brachiopoda number 99, the Lamellibranchiata +58; while in the Carboniferous their proportions are more than reversed, the +Lamellibranchiata numbering 334 species, and the Brachiopoda only 157. In the +Secondary or Cainozoic formations the preponderance of the higher grade of +bivalves becomes more and more marked, till in the tertiary strata it approaches +that observed in the living creation. + +While on this subject, it may be useful to the student to know that a Brachiopod +differs from ordinary bivalves, mussels, cockles, etc., in being always equal- +sided and never quite equi-valved; the form of each valve being symmetrical, it +may be divided into two equal parts by a line drawn from the apex to the centre +of the margin. + +TRILOBITES. + +In the Bala and Caradoc beds the trilobites reach their maximum, being +represented by 111 species referred to 23 genera. + +(FIGURE 552. Young individuals of Trinucleus concentricus (T. ornatus, Barr.). +a. Youngest state. Natural size and magnified; the body rings not at all +developed. +b. A little older. One thorax joint. +c. Still more advanced. Three thorax joints. The fourth, fifth, and sixth +segments are successively produced, probably each time the animal moulted its +crust.) + +(FIGURE 553. Trinucleus concentricus, Eaton. Syn. T. Caractaci, Murch. Ireland; +Wales; Shropshire; North America; Bohemia.) + +Burmeister, in his work on the organisation of trilobites, supposes that they +swam at the surface of the water in the open sea and near coasts, feeding on +smaller marine animals, and to have had the power of rolling themselves into a +ball as a defence against injury. He was also of opinion that they underwent +various transformations analogous to those of living crustaceans. M. Barrande, +author of an admirable work on the Silurian rocks of Bohemia, confirms the +doctrine of their metamorphosis, having traced more than twenty species through +different stages of growth from the young state just after its escape from the +egg to the adult form. He has followed some of them from a point in which they +show no eyes, no joints, or body rings, and no distinct tail, up to the complete +form with the full number of segments. This change is brought about before the +animal has attained a tenth part of its full dimensions, and hence such minute +and delicate specimens are rarely met with. Some of his figures of the +metamorphoses of the common Trinucleus are copied in Figures 552 and 553. It was +not till 1870 that Mr. Billings was enabled, by means of a specimen found in +Canada, to prove that the trilobite was provided with eight legs. + +(FIGURE 554. Palaeaster asperimus, Salt. Caradoc, Welshpool.) + +(FIGURE 555. Echinosphaeronites balticus, Eichwald. (Of the family Cystideae.) +a. Mouth. +b. Point of attachment of stem. Lower Silurian S. and N. Wales.) + +It has been ascertained that a great thickness of slaty and crystalline rocks of +South Wales, as well as those of Snowdon and Bala, in North Wales, which were +first supposed to be of older date than the Silurian sandstones and mudstones of +Shropshire, are in fact identical in age, and contain the same organic remains. +At Bala, in Merionethshire, a limestone rich in fossils occurs, in which two +genera of star-fish, Protaster and Palaeaster, are found; the fossil specimen of +the latter (Figure 554) being almost as uncompressed as if found just washed up +on the sea-beach. Besides the star-fish there occur abundance of those peculiar +bodies called Cystideae. They are the Sphaeronites of old authors, and were +considered by Professor E. Forbes as intermediate between the crinoids and +echinoderms. The Echinosphaeronite here represented (Figure 555) is +characteristic of the Caradoc beds in Wales, and of their equivalents in Sweden +and Russia. + +With it have been found several other genera of the same family, such as +Sphaeronites, Hemicosmites, etc. Among the mollusca are Pteropods of the genus +Conularia of large size (for genus, see Figure 518). About eleven species of +Graptolite are reckoned as belonging to this formation; they are chiefly found +in peculiar localities where black mud abounded. The formation, when traced into +South Wales and Ireland, assumes a greatly altered mineral aspect, but still +retains its characteristic fossils. The known fauna of the Bala group comprises +565 species, 352 of which are peculiar, and 93, as before stated, are common to +the overlying Llandovery rocks. It is worthy of remark that, when it occurs +under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest +of Snowdon, the peculiar species which distinguish it from the Llandeilo beds +are still observable. The formation generally appears to be of shallow-water +origin, and in that respect is contrasted with the group next to be described. +Professor Ramsay estimates the thickness of the Bala Beds, including the +contemporaneous volcanic rocks, stratified and unstratified, as being from +10,000 to 12,000 feet. + +LLANDEILO FLAGS. + +(FIGURE 556. Didymograpsus (Graptolites) Murchisonii, Beck. Llandeilo flags, +Wales.) + +The Lower Silurian strata were originally divided by Sir R. Murchison into the +upper group already described, under the name of Caradoc Sandstone, and a lower +one, called, from a town in Carmarthenshire, the Llandeilo flags. The last +mentioned strata consist of dark-coloured micaceous flags, frequently +calcareous, with a great thickness of shales, generally black, below them. The +same beds are also seen at Builth, in Radnorshire, where they are +interstratified with volcanic matter. + +(FIGURE 557. Diplograpsus pristis, Hisinger. Llandeilo beds, Waterford.) + +(FIGURE 558. Rastrites peregrinus, Barrande. Scotland; Bohemia; Saxony. +Llandeilo flags.) + +(FIGURE 559. Diplograpsus folium, Hisinger. Dumfriesshire; Sweden. Llandeilo +flags.) + +A still lower part of the Llandeilo rocks consists of a black carbonaceous slate +of great thickness, frequently containing sulphate of alumina, and sometimes, as +in Dumfriesshire, beds of anthracite. It has been conjectured that this +carbonaceous matter may be due in great measure to large quantities of imbedded +animal remains, for the number of Graptolites included in these slates was +certainly very great. In Great Britain eleven genera and about 40 species of +Graptolites occur in the Llandeilo flags and underlying Arenig beds. The double +Graptolites, or those with two rows of cells, such as Diplograpsus (Figure 557), +are conspicuous. + +The brachiopoda of the Llandeilo flags, which number 47 species, are in the main +the same as those of the Caradoc Sandstone, but the other mollusca are in great +part of different species. + +(FIGURE 560. Orthoceras duplex, Wahlenberg. Russia and Sweden. (From Murchison's +Siluria.)) + +(FIGURE 561. Asaphus tyrannus, Murchison. Llandeilo; Bishop's Castle; etc.) + +(FIGURE 562. Ogygia Buchii, Burm. Syn. Asaphus Buchii, Brongn. Builth, +Radnorshire; Llandeilo, Carmarthenshire.) + +In Europe generally, as, for example, in Sweden and Russia, no shells are so +characteristic of this formation as Orthoceratites, usually of great size, and +with a wide siphuncle placed on one side instead of being central (see Figure +560). Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the same +beds also are found Bellerophon (see Figure 488) and some Pteropod shells +(Conularia, Theca, etc.), also in spots where sand abounded, lamellibranchiate +bivalves of large size. The Crustaceans were plentifully represented by the +Trilobites, which appear to have swarmed in the Silurian seas just as crabs and +shrimps do in our own; no less than 263 species have been found in the British +Silurian fauna. The genera Asaphus (Figure 561), Ogygia (Figure 562), and +Trinucleus (Figures 552 and 553) form a marked feature of the rich and varied +Trilobitic fauna of this age. + +Beneath the black slates above described of the Llandeilo formation, Graptolites +are still found in great variety and abundance, and the characteristic genera of +shells and trilobites of the Lower Silurian rocks are still traceable downward, +in Shropshire, Cumberland, and North and South Wales, through a vast depth of +shaly beds, in some districts interstratified with trappean formations of +contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed +of such materials as are ejected from craters and deposited immediately on the +bed of the ocean, or washed into it from the land. According to Professor +Ramsay, their thickness is about 3300 feet in North Wales, including those of +the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure, +and, according to the same authority, of an aggregate thickness of 2500 feet. + +ARENIG OR STIPER-STONES GROUP (LOWER LLANDEILO OF MURCHISON). + +(FIGURE 563. Arenicolites linearis, Hall. Arenig beds, Stiper-Stones. +a. Parting between the beds, or planes of bedding.) + +(FIGURE 564. Didymograpsus geminus, Hisinger, sp. Sweden.) + +Next in the descending order are the shales and sandstones in which the +quartzose rocks called Stiper-Stones in Shropshire occur. Originally these +Stiper-Stones were only known as arenaceous quartzose strata in which no organic +remains were conspicuous, except the tubular burrows of annelids (see Figure +563, Arenicolites linearis), which are remarkably common in the Lowest Silurian +in Shropshire, and in the State of New York, in America. They have already been +alluded to as occurring by thousands in the Silurian strata unconformably +overlying the Cambrian, in the mountain of Queenaig, in Sutherlandshire (Figure +82). I have seen similar burrows now made on the retiring of the tides in the +sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by +fishermen and used as bait. When the term Silurian was given by Sir R. +Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the +base of the Silurian system, but no fossil fauna had then been obtained, such as +could alone enable the geologist to draw a line between this member of the +series and the Llandeilo flags above, or a vast thickness of rock below, which +was seen to form the Longmynd hills, and was called "unfossiliferous graywacke." +Professor Sedgwick had described, in 1843, strata now ascertained to be of the +same age as largely developed in the Arenig mountain, in Merionethshire; and the +Skiddaw slates in the Lake-District of Cumberland, studied by the same author, +were of corresponding date, though the number of fossils was, in both cases, too +few for the determination of their true chronological relations. The subsequent +researches of Messrs. Sedgwick and Harkness, in Cumberland, and of Sir R.I. +Murchison and the Government surveyors in Shropshire, have increased the species +to more than sixty. These were examined by Mr. Salter, and shown in the third +edition of "Siluria" (page 52, 1859) to be quite distinct from the fossils of +the overlying Llandeilo flags. Among these the Obolella plumbea, Aeglina +binodosa, Ogygia Selwynii, and Didymograpsus geminus (Figure 564), and D. +Hirundo, are characteristic. + +But, although the species are distinct, the genera are the same as those which +characterise the Silurian rocks above, and none of the characteristic primordial +or Cambrian forms, presently to be mentioned, are intermixed. The same may be +said of a set of beds underlying the Arenig rocks at Ramsay Island and other +places in the neighbourhood of St. David's. These beds, which have only lately +become known to us through the labours of Dr. Hicks (Transactions of the British +Association 1866. Proceedings of the Liverpool Geological Society 1869.), +present already twenty new species, the greater part of them allied generically +to the Arenig rocks. This Arenig group may therefore be conveniently regarded as +the base of the great Silurian system, a system which, by the thickness of its +strata and the changes in animal life of which it contains the record, is more +than equal in value to the Devonian, or Carboniferous, or other principal +divisions, whether of primary or secondary date. + +It would be unsafe to rely on the mere thickness of the strata, considered apart +from the great fluctuations in organic life which took place between the era of +the Llandeilo and that of the Ludlow formation, especially as the enormous pile +of Silurian rocks observed in Great Britain (in Wales more particularly) is +derived in great part from igneous action, and is not confined to the ordinary +deposition of sediment from rivers or the waste of cliffs. + +In volcanic archipelagoes, such as the Canaries, we see the most active of all +known causes, aqueous and igneous, simultaneously at work to produce great +results in a comparatively moderate lapse of time. The outpouring of repeated +streams of lava-- the showering down upon land and sea of volcanic ashes-- the +sweeping seaward of loose sand and cinders, or of rocks ground down to pebbles +and sand, by rivers and torrents descending steeply inclined channels-- the +undermining and eating away of long lines of sea-cliff exposed to the swell of a +deep and open ocean-- these operations combine to produce a considerable volume +of superimposed matter, without there being time for any extensive change of +species. Nevertheless, there would seem to be a limit to the thickness of stony +masses formed even under such favourable circumstances, for the analogy of +tertiary volcanic regions lends no countenance to the notion that sedimentary +and igneous rocks 25,000, much less 45,000 feet thick, like those of Wales, +could originate while one and the same fauna should continue to people the +earth. If, then, we allow that about 25,000 feet of matter may be ascribed to +one system, such as the Silurian, as above described, we may be prepared to +discover in the next series of subjacent rocks a distinct assemblage of species, +or even in great part of genera, of organic remains. Such appears to be the +fact, and I shall therefore conclude with the Arenig beds my enumeration of the +Silurian formations in Great Britain, and proceed to say something of their +foreign equivalents, before treating of rocks older than the Silurian. + +SILURIAN STRATA OF THE CONTINENT OF EUROPE. + +When we turn to the continent of Europe, we discover the same ancient series +occupying a wide area, but in no region as yet has it been observed to attain +great thickness. Thus, in Norway and Sweden, the total thickness of strata of +Silurian age is considerably less than 1000 feet, although the representatives +both of the Upper and Lower Silurian of England are not wanting there. In Russia +the Silurian strata, so far as they are yet known, seem to be even of smaller +vertical dimensions than in Scandinavia, and they appear to consist chiefly of +the Llandovery group, or of a limestone containing Pentamerus oblongus, below +which are strata with fossils corresponding to those of the Llandeilo beds of +England. The lowest rock with organic remains yet discovered is "the Ungulite or +Obolus grit" of St. Petersburg, probably coeval with the Llandeilo flags of +Wales. + +(Figures 565 and 566. Shells of the lowest known Fossiliferous Beds in Russia. + +(FIGURE 565. Siphonotreta unguiculata, Eichwald. From the Lowest Silurian +Sandstone, "Obolus grits," of St. Petersburg. +a. Outside of perforated valve. +b. Interior of same, showing the termination of the foramen within. (Davidson.)) + +(FIGURE 566. Obolus Apollinis, Eichwald. From the same locality. +a. Interior of the larger or ventral valve. +b. Exterior of the upper (dorsal) valve. (Davidson, "Palaeontographic +Monograph."))) + +The shales and grits near St. Petersburg, above alluded to, contain green grains +in their sandy layers, and are in a singularly unaltered state, taking into +account their high antiquity. The prevailing Brachiopods consist of the Obolus +or Ungulite of Pander, and a Siphonotreta (Figures 565, 566). Notwithstanding +the antiquity of this Russian formation, it should be stated that both of these +genera of brachiopods have been also found in the Upper Silurian of England, +i.e. In the Wenlock limestone. + +Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg +announced in 1854 his discovery of remains of foraminifera. These are casts of +the cells; and among five or six forms three are considered by him as referable +to existing genera (e.g., Textularia, Rotalia, and Guttulina). + +SILURIAN STRATA OF THE UNITED STATES. + +Table 26.3. SUBDIVISIONS OF THE SILURIAN STRATA OF NEW YORK. (Strata below the +Oriskany sandstone or base of the Devonian.) + +COLUMN 1: NEW YORK NAMES. + +COLUMN 2: BRITISH EQUIVALENTS. + +1. Upper Pentamerus Limestone: Upper Silurian (or Ludlow and Wenlock +formations). + +2. Encrinal Limestone: Upper Silurian (or Ludlow and Wenlock formations). + +3. Delthyris Shaly Limestone: Upper Silurian (or Ludlow and Wenlock formations). + +4. Pentamerus and Tentaculite Limestones: Upper Silurian (or Ludlow and Wenlock +formations). + +5. Water Lime Group: Upper Silurian (or Ludlow and Wenlock formations). + +6. Onondaga Salt Group: Upper Silurian (or Ludlow and Wenlock formations). + +7. Niagara Group: Upper Silurian (or Ludlow and Wenlock formations). + +8. Clinton Group: Beds of Passage, Llandovery Group. + +9. Medina Sandstone: Beds of Passage, Llandovery Group. + +10. Oneida Conglomerate: Beds of Passage, Llandovery Group. + +11. Gray Sandstone: Beds of Passage, Llandovery Group. + +12. Hudson River Group: Lower Silurian (or Caradoc and Bala, Llandeilo and +Arenig Formations). + +13. Trenton Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig +Formations). + +14. Black-River Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and +Arenig Formations). + +15. Bird's-eye Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and +Arenig Formations). + +16. Chazy Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig +Formations). + +17. Calciferous Sandstone: Lower Silurian (or Caradoc and Bala, Llandeilo and +Arenig Formations). + +The Silurian formations can be advantageously studied in the States of New York, +Ohio, and other regions north and south of the great Canadian lakes. Here they +are often found, as in Russia, nearly in horizontal position, and are more rich +in well-preserved fossils than in almost any spot in Europe. In the State of New +York, where the succession of the beds and their fossils have been most +carefully worked out by the Government surveyors, the subdivisions given in the +first column of Table 26.3 have been adopted. + +In the second column of the same table I have added the supposed British +equivalents. All Palaeontologists, European and American, such as MM. De +Verneuil, D. Sharpe, Professor Hall, E. Billings, and others, who have entered +upon this comparison, admit that there is a marked general correspondence in the +succession of fossil forms, and even species, as we trace the organic remains +downward from the highest to the lowest beds; but it is impossible to parallel +each minor subdivision. + +That the Niagara Limestone, over which the river of that name is precipitated at +the great cataract, together with its underlying shales, corresponds to the +Wenlock limestone and shale of England there can be no doubt. Among the species +common to this formation in America and Europe are Calymene Blumenbachii, +Homalonotus delphinocephalus (Figure 544), with several other trilobites; +Rhynchonella Wilsoni, Figure 531, and Retzia cuneata; Orthis elegantula, +Pentamerus galeatus, with many more brachiopods; Orthoceras annulatum, among the +cephalopodous shells; and Favosites gothlandica, with other large corals. + +The Clinton Group, containing Pentamerus oblongus and Stricklandinia, and +related more nearly by its fossil species with the beds above than with those +below, is the equivalent of the Llandovery Group or beds of passage. + +(FIGURE 567. Murchisonia gracilis, Hall. A fossil characteristic of the Trenton +Limestone. The genus is common in Lower Silurian rocks.) + +The Hudson River Group, and the Trenton Limestone, agree palaeontologically with +the Caradoc or Bala group, containing in common with them several species of +trilobites, such as Asaphus (Isotelus) gigas, Trinucleus concentricus (Figure +553); and various shells, such as Orthis striatula, Orthis biforata (or O. +lynx), O. porcata (O. occidentalis of Hall), and Bellerophon bilobatus. In the +Trenton limestone occurs Murchisonia gracilis, Figure 567, a fossil also common +to the Llandeilo beds in England. + +Mr. D. Sharpe, in his report on the mollusca collected by me from these strata +in North America (Quarterly Geological Journal volume 4.), has concluded that +the number of species common to the Silurian rocks on both sides of the Atlantic +is between 30 and 40 per cent; a result which, although no doubt liable to +future modification, when a larger comparison shall have been made, proves, +nevertheless, that many of the species had a wide geographical range. It seems +that comparatively few of the gasteropods and lamellibranchiate bivalves of +North America can be identified specifically with European fossils, while no +less than two-fifths of the brachiopoda, of which my collection chiefly +consisted, are the same. In explanation of these facts, it is suggested that +most of the recent brachiopoda (especially the orthidiform ones) are inhabitants +of deep water, and that they may have had a wider geographical range than shells +living near shore. The predominance of bivalve mollusca of this peculiar class +has caused the Silurian period to be sometimes styled "the age of brachiopods." + +In Canada, as in the State of New York, the Potsdam Sandstone underlies the +above-mentioned calcareous rocks, but contains a different suite of fossils, as +will be hereafter explained. In parts of the globe still more remote from Europe +the Silurian strata have also been recognised, as in South America, Australia, +and India. In all these regions the facies of the fauna, or the types of organic +life, enable us to recognise the contemporaneous origin of the rocks; but the +fossil species are distinct, showing that the old notion of a universal +diffusion throughout the "primaeval seas" of one uniform specific fauna was +quite unfounded, geographical provinces having evidently existed in the oldest +as in the most modern times. + + +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. + +CAMBRIAN GROUP. + +The characters of the Upper and Lower Silurian rocks were established so fully, +both on stratigraphical and palaeontological data, by Sir Roderick Murchison +after five years' labour, in 1839, when his "Silurian System" was published, +that these formations could from that period be recognised and identified in all +other parts of Europe and in North America, even in countries where most of the +fossils differed specifically from those of the classical region in Britain, +where they were first studied. + +TABLE 27.1. SHOWING THE SUCCESSION OF THE STRATA IN ENGLAND AND WALES WHICH +BELONG TO THE CAMBRIAN GROUP OR THE FOSSILIFEROUS ROCKS OLDER THAN THE ARENIG OR +LOWER LLANDEILO ROCKS: + +UPPER CAMBRIAN. + +TREMADOC SLATES. (Primordial of Barrande in part.) + +LINGULA FLAGS. (Primordial of Barrande.) + +LOWER CAMBRIAN. + +MENEVIAN BEDS. (Primordial of Barrande.) + +LONGMYND GROUP. +a. Harlech Grits. +b. Llanberis slates. + +While Sir R.I. Murchison was exploring in 1833, in Shropshire and the borders of +Wales, the strata which in 1835 he first called Silurian, Professor Sedgwick was +surveying the rocks of North Wales, which both these geologists considered at +that period as of older date, and for which in 1836 Sedgwick proposed the name +of Cambrian. It was afterwards found that a large portion of the slaty rocks of +North Wales, which had been considered as more ancient than the Llandeilo beds +and Stiper-Stones before alluded to, were, in reality, not inferior in position +to those Lower Silurian beds of Murchison, but merely extensive undulations of +the same, bearing fossils identical in species, though these were generally +rarer and less perfectly preserved, owing to the changes which the rocks had +undergone from metamorphic action. To such rocks the term "Cambrian" was no +longer applicable, although it continued to be appropriate to strata inferior to +the Stiper-Stones, and which were older than those of the Lower Silurian group +as originally defined. It was not till 1846 that fossils were found in Wales in +the Lingula flags, the place of which will be seen in Table 27.1. By this time +Barrande had already published an account of a rich collection of fossils which +he had discovered in Bohemia, portions of which he recognised as of +corresponding age with Murchison's Upper and Lower Silurian, while others were +more ancient, to which he gave the name of "Primordial," for the fossils were +sufficiently distinct to entitle the rocks to be referred to a new period. They +consisted chiefly of trilobites of genera distinct from those occurring in the +overlying Silurian formations. These peculiar genera were afterwards found in +rocks holding a corresponding position in Wales, and I shall retain for them the +term Cambrian, as recent discoveries in our own country seem to carry the first +fauna of Barrande, or his primordial type, even into older strata than any which +he found to be fossiliferous in Bohemia. + +The term primordial was intended to express M. Barrande's own belief that the +fossils of the rocks so-called afforded evidence of the first appearance of +vital phenomena on this planet, and that consequently no fossiliferous strata of +older date would or could ever be discovered. The acceptance of such a +nomenclature would seem to imply that we despaired of extending our discoveries +of new and more ancient fossil groups at some future day when vast portions of +the globe, hitherto unexplored, should have been thoroughly surveyed. Already +the discovery of the Laurentian Eozoon in Canada, presently to be mentioned, +discountenances such views. + +UPPER CAMBRIAN. + +TREMADOC SLATES. + +(FIGURE 568. Theca (Cleidotheca) operculata. Lower Tremadoc beds. Tremadoc.) + +The Tremadoc slates of Sedgwick are more than 1000 feet in thickness, and +consist of dark earthy slates occurring near the little town of Tremadoc, +situated on the north side of Cardigan Bay, in Carnarvonshire. These slates were +first examined by Sedgwick in 1831, and were re-examined by him and described in +1846 (Quarterly Geological Journal volume 3 page 156.), after some fossils had +been found in the underlying Lingula flags by Mr. Davis. The inferiority in +position of these Lingula flags to the Tremadoc beds was at the same time +established. The overlying Tremadoc beds were traced by their pisolitic ore from +Tremadoc to Dolgelly. No fossils proper to the Tremadoc slates were then +observed, but subsequently, thirty-six species of all classes have been found in +them, thanks to the researches of Messrs. Salter, Homfray, and Ash. We have +already seen that in the Arenig or Stiper-Stones group, where the species are +distinct, the genera agree with Silurian types; but in these Tremadoc slates, +where the species are also peculiar, there is about an equal admixture of +Silurian types with those which Barrande has termed "primordial." Here, +therefore, it may truly be said that we are entering upon a new domain of life +in our retrospective survey of the past. The trilobites of new species, but of +Lower Silurian genera, belong to Ogygia, Asaphus, and Cheirurus; whereas those +belonging to primordial types, or Barrande's first fauna as well as to the +Lingula flags of Wales, comprise Dikelocephalus, Conocoryphe (for genera see +Figures 577 and 581 (This genus has been substituted for Barrande's +Conocephalus, as the latter term had been preoccupied by the entomologists.)), +Olenus, and Angelina. In the Tremadoc slates are found Bellerophon, Orthoceras, +and Cyrtoceras, all specifically distinct from Lower Silurian fossils of the +same genera: the Pteropods Theca (Figure 568) and Conularia range throughout +these slates; there are no Graptolites. The Lingula (Lingulella) Davisii ranges +from the top to the bottom of the formation, and links it with the zone next to +be described. The Tremadoc slates are very local, and seem to be confined to a +small part of North Wales; and Professor Ramsay supposes them to lie +unconformably on the Lingula flags, and that a long interval of time elapsed +between these formations. Cephalopoda have not yet been found lower than this +group, but it will be observed that they occur here associated with genera of +Trilobites considered by Barrande as characteristically Primordial, some of +which belong to all the divisions of the British Cambrian about to be mentioned. +This renders the absence of cephalopoda of less importance as bearing on the +theory of development. + +LINGULA FLAGS. + +(FIGURES 569 to 571. "Lingula flags" of Dolgelly, and Ffestiniog; N. Wales. + +(FIGURE 569. Hymenocaris vermicauda, Salter. A phyllopod crustacean. One-half +natural size.) + +(FIGURE 570. Lingulella Davisii, M'Coy. +a. One-half natural size. +b. Distorted by cleavage.) + +(FIGURE 571. Olenus micrurus, Salter. One-half natural size.)) + +Next below the Tremadoc slates in North Wales lie micaceous flagstones and +slates, in which, in 1846, Mr. E. Davis discovered the Lingula (Lingulella), +Figure 570, named after him, and from which was derived the name of Lingula +flags. These beds, which are palaeontologically the equivalents of Barrande's +primordial zone, are represented by more than 5000 feet of strata, and have been +studied chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in +North Wales, and at St. David's in South Wales. They have yielded about forty +species of fossils, of which six only are common to the overlying Tremadoc +rocks, but the two formations are closely allied by having several +characteristic "primordial" genera in common. Dikelocephalus, Olenus (Figure +571), and Conocoryphe are prominent forms, as is also Hymenocaris (Figure 569), +a genus of phyllopod crustacean entirely confined to the Lingula Flags. +According to Mr. Belt, who has devoted much attention to these beds, there are +already palaeontological data for subdividing the Lingula Flags into three +sections. (Geological Magazine volume 4.) + +In Merionethshire, according to Professor Ramsay, the Lingula Flags attain their +greatest development; in Carnarvonshire they thin out so as to have lost two- +thirds of their thickness in eleven miles, while in Anglesea and on the Menai +Straits both they and the Tremadoc beds are entirely absent, and the Lower +Silurian rests directly on Lower Cambrian strata. + +LOWER CAMBRIAN. + +MENEVIAN BEDS. + +(FIGURE 572. Paradoxides Davidis, Salter. One-tenth natural size. Menevian beds. +St. David's and Dolgelly.) + +Immediately beneath the Lingula Flags there occurs a series of dark grey and +black flags and slates alternating at the upper part with some beds of +sandstone, the whole reaching a thickness of from 500 to 600 feet. These beds +were formerly classed, on purely lithological grounds, as the base of the +Lingula Flags, but Messrs. Hicks and Salter, to whose exertions we owe almost +all our knowledge of the fossils, have pointed out that the most characteristic +genera found in them are quite unknown in the Lingula Flags, while they possess +many of the strictly Lower Cambrian genera, such as Microdiscus and Paradoxides. +(British Association Report 1865, 1866, 1868 and Quarterly Geological Journal +volumes 21, 25.) They therefore proposed to place them, and it seems to me with +good reason, at the top of the Lower Cambrian under the term "Menevian," Menevia +being the classical name of St. David's. The beds are well exhibited in the +neighbourhood of St. David's in South Wales, and near Dolgelly and Maentwrog in +North Wales. They are the equivalents of the lowest part of Barrande's +Primordial Zone (Etage C). More than forty species have been found in them, and +the group is altogether very rich in fossils for so early a period. The +trilobites are of large size; Paradoxides Davidis (see Figure 572), the largest +trilobite known in England, 22 inches or nearly two feet long, is peculiar to +the Menevian Beds. By referring to the Bohemian trilobite of the same genus +(Figure 576), the reader will at once see how these fossils (though of such +different dimensions) resemble each other in Bohemia and Wales, and other +closely allied species from the two regions might be added, besides some which +are common to both countries. The Swedish fauna, presently to be mentioned, will +be found to be still more nearly connected with the Welsh Menevian. In all these +countries there is an equally marked difference between the Cambrian fossils and +those of the Upper and Lower Silurian rocks. The trilobite with the largest +number of rings, Erinnys venulosa, occurs here in conjunction with Agnostus and +Microdiscus, the genera with the smallest number. Blind trilobites are also +found as well as those which have the largest eyes, such as Microdiscus on the +one hand, and Anoplenus on the other. + +LONGMYND GROUP. + +Older than the Menevian Beds are a thick series of olive green, purple, red and +grey grits and conglomerates found in North and South Wales, Shropshire, and +parts of Ireland and Scotland. They have been called by Professor Sedgwick the +Longmynd or Bangor Group, comprising, first, the Harlech and Barmouth +sandstones; and secondly, the Llanberis slates. + +HARLECH GRITS. + +(FIGURE 573. Histioderma Hibernica, Kinahan. Oldhamia beds. Bray Head, Ireland. +1. Showing opening of burrow, and tube with wrinklings or crossing ridges, +probably produced by a tentacled sea worm or annelid. +2. Lower and curved extremity of tube with five transverse lines.) + +The sandstones of this period attain in the Longmynd hills a thickness of no +less than 6000 feet without any interposition of volcanic matter; in some places +in Merionethshire they are still thicker. Until recently these rocks possessed +but a very scanty fauna. + +With the exception of five species of annelids (see Figure 460) brought to light +by Mr. Salter in Shropshire, and Dr. Kinahan in Wicklow, and an obscure +crustacean form, Palaeopyge Ramsayi, they were supposed to be barren of organic +remains. Now, however, through the labours of Mr. Hicks, they have yielded at +St. David's a rich fauna of trilobites, brachiopods, phyllopods, and pteropods, +showing, together with other fossils, a by no means low state of organisation at +this early period. (British Association Report 1868.) Already the fauna amounts +to 20 species referred to 17 genera. + +A new genus of trilobite called Plutonia Sedgwickii, not yet figured and +described, has been met with in the Harlech grits. It is comparable in size to +the large Paradoxides Davidis before mentioned, has well-developed eyes, and is +covered all over with tubercles. In the same strata occur other genera of +trilobites, namely, Conocoryphe, Paradoxides, Microdiscus, and the Pteropod +Theca (Figure 568), all represented by species peculiar to the Harlech grits. +The sands of this formation are often rippled, and were evidently left dry at +low tides, so that the surface was dried by the sun and made to shrink and +present sun-cracks. There are also distinct impressions of rain-drops on many +surfaces, like those in Figures 444 and 445. + +LANBERIS SLATES. + +(FIGURE 574. Oldhamia radiata, Forbes. Wicklow, Ireland.) + +(FIGURE 575. Oldhamia antiqua, Forbes. Wicklow, Ireland.) + +The slates of Llanberis and Penrhyn in Carnarvonshire, with their associated +sandy strata, attain a great thickness, sometimes about 3000 feet. They are +perhaps not more ancient than the Harlech and Barmouth beds last mentioned, for +they may represent the deposits of fine mud thrown down in the same sea, on the +borders of which the sands above-mentioned were accumulating. In some of these +slaty rocks in Ireland, immediately opposite Anglesea and Carnarvon, two species +of fossils have been found, to which the late Professor E. Forbes gave the name +of Oldhamia. The nature of these organisms is still a matter of discussion among +naturalists. + +CAMBRIAN ROCKS OF BOHEMIA (PRIMORDIAL ZONE OF BARRANDE). + +In the year 1846, as before stated, M. Joachim Barrande, after ten years' +exploration of Bohemia, and after collecting more than a thousand species of +fossils, had ascertained the existence in that country of three distinct faunas +below the Devonian. To his first fauna, which was older than any then known in +this country, he gave the name of Etage C; his two first stages A and B +consisting of crystalline and metamorphic rocks and unfossiliferous schists. +This Etage C or primordial zone proved afterwards to be the equivalent of those +subdivisions of the Cambrian groups which have been above described under the +names of Menevian and Lingula Flags. The second fauna tallies with Murchison's +Lower Silurian, as originally defined by him when no fossils had been discovered +below the Stiper-Stones. The third fauna agrees with the Upper Silurian of the +same author. Barrande, without government assistance, had undertaken single- +handed the geological survey of Bohemia, the fossils previously obtained from +that country having scarcely exceeded 20 in number, whereas he had already +acquired, in 1850, no less than 1100 species, namely, 250 crustaceans (chiefly +Trilobites), 250 Cephalopods, 160 gasteropods and pteropods, 130 acephalous +mollusks, 210 brachiopods, and 110 corals and other fossils. These numbers have +since been almost doubled by subsequent investigations in the same country. + +(Figures 576 to 580. Fossils of the lowest Fossiliferous Beds in Bohemia, or +"Primordial Zone" of Barrande. + +(FIGURE 576. Paradoxides Bohemicus, Barr. About one-half natural size.) + +(FIGURE 577. Conocoryphe striata. Syn. Conocephalus striatus, Emmrich. One-half +natural size. Ginetz and Skrey.) + +(FIGURE 578. Agnostus integer, Beyrich. Natural size and magnified.) + +(FIGURE 579. Agnostus Rex, Barr. Natural size, Skrey.) + +(FIGURE 580. Sao hirsuta, Barrande, in its various stages of growth. The small +lines beneath indicate the true size. In the youngest state, +a, no segments are visible; as the metamorphosis progresses, +b, c, the body segments begin to be developed: in the stage +d the eyes are introduced, but the facial sutures are not completed; at +e the full-grown animal, half its true size, is shown.)) + +In the primordial zone C, he discovered trilobites of the genera Paradoxides, +Conocoryphe, Ellipsocephalus, Sao, Arionellus, Hydrocephalus, and Agnostus. M. +Barrande pointed out that these primordial trilobites have a peculiar facies of +their own dependent on the multiplication of their thoracic segments and the +diminution of their caudal shield or pygidium. + +One of the "primordial" or Upper Cambrian Trilobites of the genus Sao, a form +not found as yet elsewhere in the world, afforded M. Barrande a fine +illustration of the metamorphosis of these creatures, for he traced them through +no less than twenty stages of their development. A few of these changes have +been selected for representation in Figure 580, that the reader may learn the +gradual manner in which different segments of the body and the eyes make their +appearance. + +In Bohemia the primordial fauna of Barrande derived its importance exclusively +from its numerous and peculiar trilobites. Besides these, however, the same +ancient schists have yielded two genera of brachiopods, Orthis and Orbicula, a +Pteropod of the genus Theca, and four echinoderms of the cystidean family. + +CAMBRIAN OF SWEDEN AND NORWAY. + +The Cambrian beds of Wales are represented in Sweden by strata the fossils of +which have been described by a most able naturalist, M. Angelin, in his +"Palaeontologica Suecica" (1852-4). The "alum-schists," as they are called in +Sweden, are horizontal argillaceous rocks which underlie conformably certain +Lower Silurian strata in the mountain called Kinnekulle, south of the great +Wener Lake in Sweden. These schists contain trilobites belonging to the genera +Paradoxides, Olenus, Agnostus, and others, some of which present rudimentary +forms, like the genus last mentioned, without eyes, and with the body segments +scarcely developed, and others, again, have the number of segments excessively +multiplied, as in Paradoxides. Such peculiarities agree with the characters of +the crustaceans met with in the Cambrian strata of Wales; and Dr. Torell has +recently found in Sweden the Paradoxides Hicksii, a well-known Lower Cambrian +fossil. + +At the base of the Cambrian strata in Sweden, which in the neighbourhood of Lake +Wener are perfectly horizontal, lie ripple-marked quartzose sandstones with +worm-tracks and annelid borings, like some of those found in the Harlech grits +of the Longmynd. Among these are some which have been referred doubtfully to +plants. These sandstones have been called in Sweden "fucoid sandstones." The +whole thickness of the Cambrian rocks of Sweden does not exceed 300 feet from +the equivalents of the Tremadoc beds to these sandstones, which last seem to +correspond with the Longmynd, and are regarded by Torell as older than any +fossiliferous primordial rocks in Bohemia. + +CAMBRIAN OF THE UNITED STATES AND CANADA (POTSDAM SANDSTONE). + +(FIGURE 581. Dikelocephalus Minnesotensis. Dale Owen. One-third diameter. A +large crustacean of the Olenoid group. Potsdam sandstone. Falls of St. Croix, on +the Upper Mississippi.) + +This formation, as we learn from Sir W. Logan, is 700 feet thick in Canada; the +upper part consists of sandstone containing fucoids, and perforated by small +vertical holes, which are very characteristic of the rock, and appear to have +been made by annelids (Scolithus linearis). The lower portion is a conglomerate +with quartz pebbles. I have seen the Potsdam sandstone on the banks of the St. +Lawrence, and on the borders of Lake Champlain, where, as at Keesville, it is a +white quartzose fine-grained grit, almost passing into quartzite. It is divided +into horizontal ripple-marked beds, very like those of the Lingula Flags of +Britain, and replete with a small round-shaped Obolella, in such numbers as to +divide the rock into parallel planes, in the same manner as do the scales of +mica in some micaceous sandstones. Among the shells of this formation in +Wisconsin are species of Lingula and Orthis, and several trilobites of the +primordial genus Dikelocephalus (Figure 581). On the banks of the St. Lawrence, +near Beauharnois and elsewhere, many fossil footprints have been observed on the +surface of the rippled layers. They are supposed by Professor Owen to be the +trails of more than one species of articulate animal, probably allied to the +King Crab, or Limulus. + +Recent investigations by the naturalists of the Canadian survey have rendered it +certain that below the level of the Potsdam Sandstone there are slates and +schists extending from New York to Newfoundland, occupied by a series of +trilobitic forms similar in genera, though not in species, to those found in the +European Upper Cambrian strata. + +HURONIAN SERIES. + +Next below the Upper Cambrian occur strata called the Huronian by Sir W. Logan, +which are of vast thickness, consisting chiefly of quartzite, with great masses +of greenish chloritic slate, which sometimes include pebbles of crystalline +rocks derived from the Laurentian formation, next to be described. Limestones +are rare in this series, but one band of 300 feet in thickness has been traced +for considerable distances to the north of Lake Huron. Beds of greenstone are +intercalated conformably with the quartzose and argillaceous members of this +series. No organic remains have yet been found in any of the beds, which are +about 18,000 feet thick, and rest unconformably on the Laurentian rocks. + +LAURENTIAN GROUP. + +In the course of the geological survey carried on under the direction of Sir +W.E. Logan, it has been shown that, northward of the river St. Lawrence, there +is a vast series of crystalline rocks of gneiss, mica-schist, quartzite, and +limestone, more than 30,000 feet in thickness, which have been called +Laurentian, and which are already known to occupy an area of about 200,000 +square miles. They are not only more ancient than the fossiliferous Cambrian +formations above described, but are older than the Huronian last mentioned, and +had undergone great disturbing movements before the Potsdam sandstone and the +other "primordial" or Cambrian rocks were formed. The older half of this +Laurentian series is unconformable to the newer portion of the same. + +UPPER LAURENTIAN OR LABRADOR SERIES. + +The Upper Group, more than 10,000 feet thick, consists of stratified crystalline +rocks in which no organic remains have yet been found. They consist in great +part of feldspars, which vary in composition from anorthite to andesine, or from +those kinds in which there is less than one per cent of potash and soda to those +in which there is more than seven per cent of these alkalies, the soda +preponderating greatly. These feldsparites sometimes form mountain masses almost +without any admixture of other minerals; but at other times they include augite, +which passes into hypersthene. They are often granitoid in structure. One of the +varieties is the same as the apolescent labradorite rock of Labrador. The +Adirondack Mountains in the State of New York are referred to the same series, +and it is conjectured that the hypersthene rocks of Skye, which resemble this +formation in mineral character, may be of the same geological age. + +LOWER LAURENTIAN. + +This series, about 20,000 feet in thickness, is, as before stated, unconformable +to that last mentioned; it consists in great part of gneiss of a reddish tint +with orthoclase feldspar. Beds of nearly pure quartz, from 400 to 600 feet +thick, occur in some places. Hornblendic and micaceous schists are often +interstratified, and beds of limestone, usually crystalline. Beds of plumbago +also occur. That this pure carbon may have been of organic origin before +metamorphism has naturally been conjectured. + +(FIGURES 582 and 583. Eozoon Canadense, Daw. (after Carpenter). Oldest known +organic body. + +(FIGURE 582. Eozoon Canadense, Daw. (after Carpenter). Oldest known organic +body. +a. Chambers of lower tier communicating at +, and separated from adjoining +chambers at o by an intervening septum, traversed by passages. +b. Chambers of an upper tier. +c. Walls of the chambers traversed by fine tubules. (These tubules pass with +uniform parallelism from the inner to the outer surface, opening at regular +distances from each other.) +d. Intermediate skeleton, composed of homogeneous shell substance, traversed by +f. Stoloniferous passages connecting the chambers of the two tiers. +e. Canal system in intermediate skeleton, showing the arborescent saceodic +prolongations. +(Figure 583 shows these bodies in a decalcified state.)) + +(FIGURE 583. Eozoon Canadense, Daw. (after Carpenter). Oldest known organic +body. +Decalcified portion of natural rock, showing CANAL SYSTEM and the several +layers; the acuteness of the planes prevents more than one or two parallel tiers +being observed. Natural size.)) + +There are several of these limestones which have been traced to great distances, +and one of them is from 700 to 1500 feet thick. In the most massive of them Sir +W. Logan observed, in 1859, what he considered to be an organic body much +resembling the Silurian fossil called Stromatopora rugosa. It had been obtained +the year before by Mr. J. MacMullen at the Grand Calumet, on the river Ottawa. +This fossil was examined in 1864 by Dr. Dawson of Montreal, who detected in it, +by aid of the microscope, the distinct structure of a Rhizopod or Foraminifer. +Dr. Carpenter and Professor T. Rupert Jones have since confirmed this opinion, +comparing the structure to that of the well-known nummulite. It appears to have +grown one layer over another, and to have formed reefs of limestone as do the +living coral-building polyp animals. Parts of the original skeleton, consisting +of carbonate of lime, are still preserved; while certain inter-spaces in the +calcareous fossil have been filled up with serpentine and white augite. On this +oldest of known organic remains Dr. Dawson has conferred the name of Eozoon +Canadense (see Figures 582, 583); its antiquity is such that the distance of +time which separated it from the Upper Cambrian period, or that of the Potsdam +sandstone, may, says Sir W. Logan, be equal to the time which elapsed between +the Potsdam sandstone and the nummulitic limestones of the Tertiary period. The +Laurentian and Huronian rocks united are about 50,000 feet in thickness, and the +Lower Laurentian was disturbed before the newer series was deposited. We may +naturally expect the other proofs of unconformability will hereafter be detected +at more than one point in so vast a succession of strata. + +The mineral character of the Upper Laurentian differs, as we have seen, from +that of the Lower, and the pebbles of gneiss in the Huronian conglomerates are +thought to prove that the Laurentian strata were already in a metamorphic state +before they were broken up to supply materials for the Huronian. Even if we had +not discovered the Eozoon, we might fairly have inferred from analogy that as +the quartzites were once beds of sand, and the gneiss and mica-schist derived +from shales and argillaceous sandstones, so the calcareous masses, from 400 to +1000 feet and more in thickness, were originally of organic origin. This is now +generally believed to have been the case with the Silurian, Devonian, +Carboniferous, Oolitic, and Cretaceous limestones and those nummulitic rocks of +tertiary date which bear the closest affinity to the Eozoon reefs of the Lower +Laurentian. The oldest stratified rock in Scotland is that called by Sir R. +Murchison "the fundamental gneiss," which is found in the north-west of Ross- +shire, and in Sutherlandshire (see Figure 82), and forms the whole of the +adjoining island of Lewis, in the Hebrides. It has a strike from north-west to +south-east, nearly at right angles to the metamorphic strata of the Grampians. +On this Laurentian gneiss, in parts of the western Highlands, the Lower Cambrian +and various metamorphic rocks rest unconformably. It seems highly probable that +this ancient gneiss of Scotland may correspond in date with part of the great +Laurentian group of North America. + + +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. + +(FIGURE 584. Section through formations from a, low, to c, high. +a. Hypogene formations, stratified and unstratified. +b. Aqueous formations. +c. Volcanic rocks.) + +The aqueous or fossiliferous rocks having now been described, we have next to +examine those which may be called volcanic, in the most extended sense of that +term. In the diagram (Figure 584) suppose a, a to represent the crystalline +formations, such as the granitic and metamorphic; b, b the fossiliferous strata; +and c, c the volcanic rocks. These last are sometimes found, as was explained in +the first chapter, breaking through a and b, sometimes overlying both, and +occasionally alternating with the strata b, b. + +EXTERNAL FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS. + +The origin of volcanic cones with crater-shaped summits has been explained in +the "Principles of Geology" (Chapters 23 to 27), where Vesuvius, Etna, Santorin, +and Barren Island are described. The more ancient portions of those mountains or +islands, formed long before the times of history, exhibit the same external +features and internal structure which belong to most of the extinct volcanoes of +still higher antiquity; and these last have evidently been due to a complicated +series of operations, varied in kind according to circumstances; as, for +example, whether the accumulation took place above or below the level of the +sea, whether the lava issued from one or several contiguous vents, and, lastly, +whether the rocks reduced to fusion in the subterranean regions happened to have +contained more or less silica, potash, soda, lime, iron, and other ingredients. +We are best acquainted with the effects of eruptions above water, or those +called subaerial or supramarine; yet the products even of these are arranged in +so many ways that their interpretation has given rise to a variety of +contradictory opinions, some of which will have to be considered in this +chapter. + +CONES AND CRATERS. + +(FIGURE 585. Part of the chain of extinct volcanoes called the Monts Dome, +Auvergne. (Scrope.)) + +In regions where the eruption of volcanic matter has taken place in the open +air, and where the surface has never since been subjected to great aqueous +denudation, cones and craters constitute the most striking peculiarity of this +class of formations. Many hundreds of these cones are seen in central France, in +the ancient provinces of Auvergne, Velay, and Vivarais, where they observe, for +the most part, a linear arrangement, and form chains of hills. Although none of +the eruptions have happened within the historical era, the streams of lava may +still be traced distinctly descending from many of the craters, and following +the lowest levels of the existing valleys. The origin of the cone and crater- +shaped hill is well understood, the growth of many having been watched during +volcanic eruptions. A chasm or fissure first opens in the earth, from which +great volumes of steam are evolved. The explosions are so violent as to hurl up +into the air fragments of broken stone, parts of which are shivered into minute +atoms. At the same time melted stone or LAVA usually ascends through the chimney +or vent by which the gases make their escape. Although extremely heavy, this +lava is forced up by the expansive power of entangled gaseous fluids, chiefly +steam or aqueous vapour, exactly in the same manner as water is made to boil +over the edge of a vessel when steam has been generated at the bottom by heat. +Large quantities of the lava are also shot up into the air, where it separates +into fragments, and acquires a spongy texture by the sudden enlargement of the +included gases, and thus forms SCORIAE, other portions being reduced to an +impalpable powder or dust. The showering down of the various ejected materials +round the orifice of eruption gives rise to a conical mound, in which the +successive envelopes of sand and scoriae form layers, dipping on all sides from +a central axis. In the mean time a hollow, called a CRATER, has been kept open +in the middle of the mound by the continued passage upward of steam and other +gaseous fluids. The lava sometimes flows over the edge of the crater, and thus +thickens and strengthens the sides of the cone; but sometimes it breaks down the +cone on one side (see Figure 585), and often it flows out from a fissure at the +base of the hill, or at some distance from its base. + +Some geologists had erroneously supposed, from observations made on recent cones +of eruption, that lava which consolidates on steep slopes is always of a +scoriaceous or vesicular structure, and never of that compact texture which we +find in those rocks which are usually termed "trappean." Misled by this theory, +they have gone so far as to believe that if melted matter has originally +descended a slope at an angle exceeding four or five degrees, it never, on +cooling, acquires a stony compact texture. Consequently, whenever they found in +a volcanic mountain sheets of stony materials inclined at angles of from 5 +degrees to 20 degrees or even more than 30 degrees, they thought themselves +warranted in assuming that such rocks had been originally horizontal, or very +slightly inclined, and had acquired their high inclination by subsequent +upheaval. To such dome-shaped mountains with a cavity in the middle, and with +the inclined beds having what was called a quaquaversal dip or a slope outward +on all sides, they gave the name of "Elevation craters." + +As the late Leopold Von Buch, the author of this theory, had selected the Isle +of Palma, one of the Canaries, as a typical illustration of this form of +volcanic mountain, I visited that island in 1854, in company with my friend Mr. +Hartung, and I satisfied myself that it owes its origin to a series of eruptions +of the same nature as those which formed the minor cones, already alluded to. In +some of the more ancient or Miocene volcanic mountains, such as Mont Dor and +Cantal in central France, the mode of origin by upheaval as above described is +attributed to those dome-shaped masses, whether they possess or not a great +central cavity, as in Palma. Where this cavity is present, it has probably been +due to one or more great explosions similar to that which destroyed a great part +of ancient Vesuvius in the time of Pliny. Similar paroxysmal catastrophes have +caused in historical times the truncation on a grand scale of some large cones +in Java and elsewhere. (Principles volume 2 pages 56 and 145.) + +Among the objections which may be considered as fatal to Von Buch's doctrine of +upheaval in these cases, I may state that a series of volcanic formations +extending over an area six or seven miles in its shortest diameter, as in Palma, +could not be accumulated in the form of lavas, tuffs, and volcanic breccias or +agglomerates without producing a mountain as lofty as that which they now +constitute. But assuming that they were first horizontal, and then lifted up by +a force acting most powerfully in the centre and tilting the beds on all sides, +a central crater having been formed by explosion or by a chasm opening in the +middle, where the continuity of the rocks was interrupted, we should have a +right to expect that the chief ravines or valleys would open towards the central +cavity, instead of which the rim of the great crater in Palma and other similar +ancient volcanoes is entire for more than three parts of the whole +circumference. + +If dikes are seen in the precipices surrounding such craters or central +cavities, they certainly imply rents which were filled up with liquid matter. +But none of the dislocations producing such rents can have belonged to the +supposed period of terminal and paroxysmal upheaval, for had a great central +crater been already formed before they originated, or at the time when they took +place, the melted matter, instead of filling the narrow vents, would have flowed +down into the bottom of the cavity, and would have obliterated it to a certain +extent. Making due allowance for the quantity of matter removed by subaerial +denudation in volcanic mountains of high antiquity, and for the grand explosions +which are known to have caused truncation in active volcanoes, there is no +reason for calling in the violent hypothesis of elevation craters to explain the +structure of such mountains as Teneriffe, the Grand Canary, Palma, or those of +central France, Etna, or Vesuvius, all of which I have examined. With regard to +Etna, I have shown, from observations made by me in 1857, that modern lavas, +several of them of known date, have formed continuous beds of compact stone even +on slopes of 15, 36, and 38 degrees, and, in the case of the lava of 1852, more +than 40 degrees. The thickness of these tabular layers varies from 1 1/2 foot to +26 feet. And their planes of stratification are parallel to those of the +overlying and underlying scoriae which form part of the same currents. (Memoir +on Mount Etna Philosophical Transactions 1858.) + +NOMENCLATURE OF TRAPPEAN ROCKS. + +When geologists first began to examine attentively the structure of the northern +and western parts of Europe, they were almost entirely ignorant of the phenomena +of existing volcanoes. They found certain rocks, for the most part without +stratification, and of a peculiar mineral composition, to which they gave +different names, such as basalt, greenstone, porphyry, trap tuff, and +amygdaloid. All these, which were recognised as belonging to one family, were +called "trap" by Bergmann, from trappa, Swedish for a flight of steps-- a name +since adopted very generally into the nomenclature of the science; for it was +observed that many rocks of this class occurred in great tabular masses of +unequal extent, so as to form a succession of terraces or steps. It was also +felt that some general term was indispensable, because these rocks, although +very diversified in form and composition, evidently belonged to one group, +distinguishable from the Plutonic as well as from the non-volcanic fossiliferous +rocks. + +By degrees familiarity with the products of active volcanoes convinced +geologists more and more that they were identical with the trappean rocks. In +every stream of modern lava there is some variation in character and +composition, and even where no important difference can be recognised in the +proportions of silica, alumina, lime, potash, iron, and other elementary +materials, the resulting materials are often not the same, for reasons which we +are as yet unable to explain. The difference also of the lavas poured out from +the same mountain at two distinct periods, especially in the quantity of silica +which they contain, is often so great as to give rise to rocks which are +regarded as forming distinct families, although there may be every intermediate +gradation between the two extremes, and although some rocks, forming a +transition from the one class to the other, may often be so abundant as to +demand special names. These species might be multiplied indefinitely, and I can +only afford space to name a few of the principal ones, about the composition and +aspect of which there is the least discordance of opinion. + +MINERALS MOST ABUNDANT IN VOLCANIC ROCKS. + +TABLE 28.1. ANALYSIS OF MINERALS MOST ABUNDANT IN THE VOLCANIC AND HYPOGENE +ROCKS. + +COLUMN 1: SILICA. + +COLUMN 2: ALUMINA. + +COLUMN 3: SESQUIOXIDE OF IRON. + +COLUMN 4: PROTOXIDES OF IRON AND MANGANESE. + +COLUMN 5: LIME. + +COLUMN 6: MAGNESIA. + +COLUMN 7: POTASH. + +COLUMN 8: SODA. + +COLUMN 9: OTHER CONSTITUENTS. +In this column the following signs are used: +F. Fluorine; +Li. Lithia; +W. Loss on igniting the mineral, in most instances only Water. + +COLUMN 10: SPECIFIC GRAVITY. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- - + +THE QUARTZ GROUP: + +1 2 3 4 5 6 7 8 9 10. + +Quartz: +100.0 2.6. + +Tridymite: +100.0 2.3. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- - + +THE FELDSPAR GROUP: + +1 2 3 4 5 6 7 8 9 10. + +Orthoclase. Carlsbad, in granite (Bulk): + +65.23 18.26 0.27 .... trace .... 14.66 1.45 .... 2.55. + +Orthoclase. Sanadine, Drachenfels in trachyte (Rammelsberg). + +65.87 18.53 .... .... 0.95 0.30 10.32 3.42 W 0.44 2.55. + +Albite. Arendal, in granite (G. Rose). + +68.46 19.30 .... 0.28 0.68 .... .... 11.27 .... 2.61. + +Oligoclase. Ytterby, in granite (Berzelius). + +61.55 23.80 .... .... 3.18 0.80 0.38 9.67 .... 2.65. + +Oligoclase. Teneriffe, in trachyte (Deville). + +61.55 22.03 .... .... 2.81 0.47 3.44 7.74 .... 2.59. + +Labradorite. Hitteroe, in Labrador-Rock (Waage). + +51.39 29.42 2.90 .... 9.44 0.37 1.10 5.03 W 0.71 2.72. + +Labradorite. Iceland, in volcanic (Damour). + +52.17 29.22 1.90 .... 13.11 .... .... 3.40 .... 2.71. + +Anorthite. Harzburg, in diorite (Streng). + +45.37 34.81 0.59 .... 16.52 0.83 0.40 1.45 W 0.87 2.74. + +Anorthite. Hecla, in volcanic (Waltershausen). + +45.14 32.10 2.03 0.78 18.32 .... 0.22 1.06 .... 2.74. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- - + +Leucite. Vesuvius, 1811, in lava (Rammelsberg). + +56.10 23.22 .... .... .... .... 20.59 0.57 .... 2.48. + +Nepheline. Miask, in Miascite (Scheerer). + +44.30 33.25 0.82 .... 0.32 0.07 5.82 16.02 .... 2.59. + +Nepheline. Vesuvius, in volcanic (Arfvedson). + +44.11 33.73 .... .... .... .... .... 20.46 W 0.62 2.60. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- - + +THE MICA GROUP: + +1 2 3 4 5 6 7 8 9 10. + +Muscovite. Finland, in granite (Rose). + +46.36 36.80 4.53 .... .... .... 9.22 .... F 0.67 2.90. + W 1.84. + +Lepidolite. Cornwall, in granite (Regnault). + +52.40 26.80 .... 1.50 .... .... 9.14 .... F 4.18 2.90. + Li 4.85. + +Biotite. Bodennais (V. Kobell). + +40.86 15.13 13.00 .... .... 22.00 8.83 .... W 0.44 2.70. + +Biotite. Vesuvius, in volcanic (Chodnef). + +40.91 17.71 11.02 .... 0.30 19.04 9.96 .... .... 2.75. + +Phlogopite. New York, in metamorphic limestone (Rammelsberg). + +41.96 13.47 .... 2.67 0.34 27.12 9.37 .... F 2.93 2.81. + W 0.60. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- -- + +Margarite. Nexos (Smith). + +30.02 49.52 1.65 .... 10.82 0.48 1.25 W 5.55 2.99. + +Chlorite. Dauphiny (Marignac). + +26.88 17.52 29.76 .... .... 13.84 .... .... W 11.33 2.87. + +Rapidolite. Pyrenees (Delesse). + +32.10 18.50 .... 0.06 .... 36.70 .... .... W 12.10 2.61. + +Talc. Zillerthal (Delesse). + +63.00 .... .... trace .... 33.60 .... .... W 3.10 2.78. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- -- + +THE AMPHIBOLE AND PYROXENE GROUP. + +1 2 3 4 5 6 7 8 9 10. + +Tremolite. St. Gothard (Rammelsberg) + +58.55 .... .... .... 13.90 26.63 .... .... FW 0.34 2.93. + +Actinolite. Arendal, in granite (Rammelsberg). + +56.77 0.97 .... 5.88 13.56 21.48 .... .... W 2.20 3.02. + +Hornblende. Faymont, in diorite (Deville). + +41.99 11.66 .... 22.22 9.55 12.59 .... 1.02 W 1.47 3.20. + +Hornblende Etna, in volcanic (Waltershausen). + +40.91 13.68 .... 17.49 13.44 13.19 .... .... W 0.85 3.01. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- -- + +Uralite. Ural (Rammelsberg) + +50.75 5.65 .... 17.27 11.59 12.28 .... .... W 1.80 3.14. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- -- + +Augite. Bohemia, in dolerite (Rammelsberg). + +51.12 3.38 0.95 8.08 23.54 12.82 .... .... .... 3.35. + +Augite. Vesuvius, in lava of 1858 (Rammelsberg). + +49.61 4.42 .... 9.08 22.83 14.22 .... .... .... 3.25. + +Diallage. Harz, in Gabbro (Rammelsberg). + +52.00 3.10 .... 9.36 16.29 18.51 .... .... W 1.10 3.23. + +Hypersthene. Labrador, in Labrador-Rock (Damour). + +51.36 0.37 .... 22.59 3.09 21.31 .... .... .... 3.39. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- -- + +THE OLIVINE GROUP. + +1 2 3 4 5 6 7 8 9 10 + +Bronzite. Greenland (V. Kobell). + +58.00 1.33 11.14 .... .... 29.66 .... .... .... 3.20. + +Olivine. Carlsbad, in basalt (Rammelsberg). + +39.34 .... .... 14.85 .... 45.81 .... .... .... 3.40. + +Olivine. Mount Somma, in volcanic (Walmstedt). + +40.08 0.18 .... 15.74 .... 44.22 .... .... .... 3.33. + +-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- +-- -- -- -- -- -- -- -- -- -- -- + +The minerals which form the chief constituents of these igneous rocks are few in +number. Next to quartz, which is nearly pure silica or silicic acid, the most +important are those silicates commonly classed under the several heads of +feldspar, mica, hornblende or augite, and olivine. In Table 28.1, in drawing up +which I have received the able assistance of Mr. David Forbes, the chemical +analysis of these minerals and their varieties is shown, and he has added the +specific gravity of the different mineral species, the geological application of +which in determining the rocks formed by these minerals will be explained in the +sequel. + +From Table 28.1 it will be observed that many minerals are omitted which, even +if they are of common occurrence, are more to be regarded as accessory than as +essential components of the rocks in which they are found. (For analyses of +these minerals see the Mineralogies of Dana and Bristow.) Such are, for example, +Garnet, Epidote, Tourmaline, Idocrase, Andalusite, Scapolite, the various +Zeolites, and several other silicates of somewhat rarer occurrence. Magnetite, +Titanoferrite, and Iron-pyrites also occur as normal constituents of various +igneous rocks, although in very small amount, as also Apatite, or phosphate of +lime. The other salts of lime, including its carbonate or calcite, although +often met with, are invariably products of secondary chemical action. + +The Zeolites, above mentioned, so named from the manner in which they froth up +under the blow-pipe and melt into a glass, differ in their chemical composition +from all the other mineral constituents of volcanic rocks, since they are +hydrated silicates containing from 10 to 25 per cent of water. They abound in +some trappean rocks and ancient lavas, where they fill up vesicular cavities and +interstices in the substance of the rocks, but are rarely found in any quantity +in recent lavas; in most cases they are to be regarded as secondary products +formed by the action of water on the other constituents of the rocks. Among them +the species Analcime, Stilbite, Natrolite, and Chabazite may be mentioned as of +most common occurrence. + +QUARTZ GROUP. + +The microscope has shown that pure quartz is oftener present in lavas than was +formerly supposed. It had been argued that the quartz in granite having a +specific gravity of 2.6, was not of purely igneous origin, because the silica +resulting from fusion in the laboratory has only a specific gravity of 2.3. But +Mr. David Forbes has ascertained that the free quartz in trachytes, which are +known to have flowed as lava, has the same specific gravity as the ordinary +quartz of granite; and the recent researches of Von Rath and others prove that +the mineral Tridymite, which is crystallised silica of specific gravity 2.3 (see +Table 28.1), is of common occurrence in the volcanic rocks of Mexico, Auvergne, +the Rhine, and elsewhere, although hitherto entirely overlooked. + +FELDSPAR GROUP. + +In the Feldspar group (Table 28.1) the five mineral species most commonly met +with as rock constituents are: 1. Orthoclase, often called common or potash- +feldspar. 2. Albite, or soda-feldspar, a mineral which plays a more subordinate +part than was formerly supposed, this name having been given to much which has +since been proved to be Oligoclase. 3. Oligoclase, or soda-lime feldspar, in +which soda is present in much larger proportion than lime, and of which mineral +andesite are andesine, is considered to be a variety. 4. Labradorite, or lime- +soda-feldspar, in which the proportions of lime and soda are the reverse to what +they are in Oligoclase. 5. Anorthite or lime-feldspar. The two latter feldspars +are rarely if ever found to enter into the composition of rocks containing +quartz. + +In employing such terms as potash-feldspar, etc., it must, however, always be +borne in mind that it is only intended to direct attention to the predominant +alkali or alkaline earth in the mineral, not to assert the absence of the +others, which in most cases will be found to be present in minor quantity. Thus +potash-feldspar (orthoclase) almost always contains a little soda, and often +traces of lime or magnesia; and in like manner with the others. The terms +"glassy" and "compact" feldspars only refer to structure, and not to species or +composition; the student should be prepared to meet with any of the above +feldspars in either of these conditions: the glassy state being apparently due +to quick cooling, and the compact to conditions unfavourable to crystallisation; +the so-called "compact feldspar" is also very commonly found to be an admixture +of more than one feldspar species, and frequently also contains quartz and other +extraneous mineral matter only to be detected by the microscope. + +Feldspars when arranged according to their system of crystallisation are +MONOCLINIC, having one axis obliquely inclined; or TRICLINIC, having the three +axes all obliquely inclined to each other. If arranged with reference to their +cleavage they are ORTHOCLASTIC, the fracture taking place always at a right +angle; or PLAGIOCLASTIC, in which the cleavages are oblique to one another. +Orthoclase is orthoclastic and monoclinic; all the other feldspars are +plagioclastic and triclinic. + +MINERALS IN METEORITES. + +That variety of the Feldspar Group which is called Anorthite has been shown by +Rammelsberg to occur in a meteoric stone, and his analysis proves it to be +almost identical in its chemical proportions to the same mineral in the lavas of +modern volcanoes. So also Bronzite (Enstatite) and Olivine have been met with in +meteorites shown by analysis to come remarkably near to these minerals in +ordinary rocks. + +MICA GROUP. + +With regard to the micas, the four principal species (Table 28.1) all contain +potash in nearly the same proportion, but differ greatly in the proportion and +nature of their other ingredients. Muscovite is often called common or potash +mica; Lepidolite is characterised by containing lithia in addition; Biotite +contains a large amount of magnesia and oxide of iron; whilst Phlogopite +contains still more of the former substance. In rocks containing quartz, +muscovite or lepidolite are most common. The mica in recent volcanic rocks, +gabbros, and diorites is usually Biotite, while that so common in metamorphic +limestones is usually, if not always, Phlogopite. + +AMPHIBOLE AND PYROXENE GROUP. + +The minerals included in Table 28.1 under the Amphibole and Pyroxene Group +differ somewhat in their crystallisation form, though they all belong to the +monoclinic system. Amphibole is a general name for all the different varieties +of Hornblende, Actinolite, Tremolite, etc., while Pyroxene includes Augite, +Diallage, Malacolite, Sahlite, etc. The two divisions are so much allied in +chemical composition and crystallographic characters, and blend so completely +one into the other in Uralite, that it is perhaps best to unite them in one +group. + +THEORY OF ISOMORPHISM. + +The history of the changes of opinion on this point is curious and instructive. +Werner first distinguished augite from hornblende; and his proposal to separate +them obtained afterwards the sanction of Hauy, Mohs, and other celebrated +mineralogists. It was agreed that the form of the crystals of the two species +was different, and also their structure, as shown by CLEAVAGE-- that is to say, +by breaking or cleaving the mineral with a chisel, or a blow of the hammer, in +the direction in which it yields most readily. It was also found by analysis +that augite usually contained more lime, less alumina, and no fluoric acid; +which last, though not always found in hornblende, often enters into its +composition in minute quantity. In addition to these characters, it was remarked +as a geological fact, that augite and hornblende are very rarely associated +together in the same rock. It was also remarked that in the crystalline slags of +furnaces augitic forms were frequent, the hornblendic entirely absent; hence it +was conjectured that hornblende might be the result of slow, and augite of rapid +cooling. This view was confirmed by the fact that Mitscherlich and Berthier were +able to make augite artificially, but could never succeed in forming hornblende. +Lastly, Gustavus Rose fused a mass of hornblende in a porcelain furnace, and +found that it did not, on cooling, assume its previous shape, but invariably +took that of augite. The same mineralogist observed certain crystals called +Uralite (see Table 28.1) in rocks from Siberia, which possessed the cleavage and +chemical composition of hornblende, while they had the external form of augite. + +If, from these data, it is inferred that the same substance may assume the +crystalline forms of hornblende or augite indifferently, according to the more +or less rapid cooling of the melted mass, it is nevertheless certain that the +variety commonly called augite, and recognised by a peculiar crystalline form, +has usually more lime in it, and less alumina, than that called hornblende, +although the quantities of these elements do not seem to be always the same. +Unquestionably the facts and experiments above mentioned show the very near +affinity of hornblende and augite; but even the convertibility of one into the +other, by melting and recrystallising, does not perhaps demonstrate their +absolute identity. For there is often some portion of the materials in a crystal +which are not in perfect chemical combination with the rest. Carbonate of lime, +for example, sometimes carries with it a considerable quantity of silex into its +own form of crystal, the silex being mechanically mixed as sand, and yet not +preventing the carbonate of lime from assuming the form proper to it. This is an +extreme case, but in many others some one or more of the ingredients in a +crystal may be excluded from perfect chemical union; and after fusion, when the +mass recrystallises, the same elements may combine perfectly or in new +proportions, and thus a new mineral may be produced. Or some one of the gaseous +elements of the atmosphere, the oxygen for example, may, when the melted matter +reconsolidates, combine with some one of the component elements. + +The different quantity of the impurities or the refuse above alluded to, which +may occur in all but the most transparent and perfect crystals, may partly +explain the discordant results at which experienced chemists have arrived in +their analysis of the same mineral. For the reader will often find that crystals +of a mineral determined to be the same by physical characters, crystalline form, +and optical properties, have been declared by skilful analysers to be composed +of distinct elements. This disagreement seemed at first subversive of the atomic +theory, or the doctrine that there is a fixed and constant relation between the +crystalline form and structure of a mineral and its chemical composition. The +apparent anomaly, however, which threatened to throw the whole science of +mineralogy into confusion, was reconciled to fixed principles by the discoveries +of Professor Mitscherlich at Berlin, who ascertained that the composition of the +minerals which had appeared so variable was governed by a general law, to which +he gave the name of ISOMORPHISM (from isos, equal, and morphe, form). According +to this law, the ingredients of a given species of mineral are not absolutely +fixed as to their kind and quality; but one ingredient may be replaced by an +equivalent portion of some analogous ingredient. Thus, in augite, the lime may +be in part replaced by portions of protoxide of iron, or of manganese, while the +form of the crystal, and the angle of its cleavage planes, remain the same. +These vicarious substitutions, however, of particular elements can not exceed +certain defined limits. + +BASALTIC ROCKS. + +The two principal families of trappean or volcanic rocks are the basalts and the +trachytes, which differ chiefly from each other in the quantity of silica which +they contain. The basaltic rocks are comparatively poor in silica, containing +less than 50 per cent of that mineral, and none in a pure state or as free +quartz, apart from the rest of the matrix. They contain a larger proportion of +lime and magnesia than the trachytes, so that they are heavier, independently of +the frequent presence of the oxides of iron which in some cases forms more than +a fourth part of the whole mass. Abich has, therefore, proposed that we should +weigh these rocks, in order to appreciate their composition in cases where it is +impossible to separate their component minerals. Thus, basalt from Staffa, +containing 47.80 per cent of silica, has a specific gravity of 2.95; whereas +trachyte, which has 66 per cent of silica, has a specific gravity of only 2.68; +trachytic porphyry, containing 69 per cent of silica, a specific gravity of only +2.58. If we then take a rock of intermediate composition, such as that +prevailing in the Peak of Teneriffe, which Abich calls Trachyte-dolerite, its +proportion of silica being intermediate, or 58 per cent, it weighs 2.78, or more +than trachyte, and less than basalt. (Dr. Daubeny on Volcanoes second edition +pages 14, 15.) + +BASALT. + +The different varieties of this rock are distinguished by the names of basalts, +anamezites, and dolerites, names which, however, only denote differences in +texture without implying any difference in mineral or chemical composition: the +term BASALT being used only when the rock is compact, amorphous, and often semi- +vitreous in texture, and when it breaks with a perfect conchoidal fracture; +when, however, it is uniformly crystalline in appearance, yet very close- +grained, the name ANAMESITE (from anamesos, intermediate) is employed, but if +the rock be so coarsely crystallised that its different mineral constituents can +be easily recognised by the eye, it is called DOLERITE (from doleros, +deceitful), in allusion to the difficulty of distinguishing it from some of the +rocks known as Plutonic. + +MELAPHYRE is often quite undistinguishable in external appearance from basalt, +for although rarely so heavy, dark-coloured, or compact, it may present at times +all these varieties of texture. Both these rocks are composed of triclinic +feldspar and augite with more or less olivine, magnetic or titaniferous oxide of +iron, and usually a little nepheline, leucite, and apatite; basalt usually +contains considerably more olivine than melaphyre, but chemically they are +closely allied, although the melaphyres usually contain more silica and alumina, +with less oxides of iron, lime, and magnesia, than the basalts. The Rowley Hills +in Staffordshire, commonly known as Rowley Ragstone, are melaphyre. + +GREENSTONE. + +This name has usually been extended to all granular mixtures, whether of +hornblende and feldspar, or of augite and feldspar. The term DIORITE has been +applied exclusively to compounds of hornblende and triclinic feldspar. LABRADOR- +ROCK is a term used for a compound of labradorite or labrador-feldspar and +hypersthene; when the hypersthene predominates it is sometimes known under the +name of HYPERSTHENE-ROCK. GABBRO and DIABASE are rocks mainly composed of +triclinic feldspars and diallage. All these rocks become sometimes very +crystalline, and help to connect the volcanic with the Plutonic formations, +which will be treated of in Chapter 31. + +The name trachyte (from trachus, rough) was originally given to a coarse +granular feldspathic rock which was rough and gritty to the touch. The term was +subsequently made to include other rocks, such as clinkstone and obsidian, which +have the same mineral composition, but to which, owing to their different +texture, the word in its original meaning would not apply. The feldspars which +occur in Trachytic rocks are invariably those which contain the largest +proportion of silica, or from 60 to 70 per cent of that mineral. Through the +base are usually disseminated crystals of glassy feldspar, mica, and sometimes +hornblende. Although quartz is not a necessary ingredient in the composition of +this rock, it is very frequently present, and the quartz trachytes are very +largely developed in many volcanic districts. In this respect the trachytes +differ entirely from the members of the Basaltic family, and are more nearly +allied to the granites. + +OBSIDIAN. + +Obsidian, Pitchstone, and Pearlstone are only different forms of a volcanic +glass produced by the fusion of trachytic rocks. The distinction between them is +caused by different rates of cooling from the melted state, as has been proved +by experiment. Obsidian is of a black or ash-grey colour, and though opaque in +mass is transparent in thin edges. + +CLINKSTONE OR PHONOLITE. + +Among the rocks of the trachytic family, or those in which the feldspars are +rich in silica, that termed Clinkstone or Phonolite is conspicuous by its +fissile structure, and its tendency to lamination, which is such as sometimes to +render it useful as roofing-slate. It rings when struck with the hammer, whence +its name; is compact, and usually of a greyish blue or brownish colour; is +variable in composition, but almost entirely composed of feldspar. When it +contains disseminated crystals of feldspar, it is called CLINKSTONE PORPHYRY. + +VOLCANIC ROCKS DISTINGUISHED BY SPECIAL FORMS OF STRUCTURE. + +Many volcanic rocks are commonly spoken of under names denoting structure alone, +which must not be taken to imply that they are distinct rocks, i.e., that they +differ from one another either in mineral or chemical composition. Thus the +terms Trachytic porphyry, Trachytic tuff, etc., merely refer to the same rock +under different conditions of mechanical aggregation or crystalline development +which would be more correctly expressed by the use of the adjective, as +porphyritic trachyte, etc., but as these terms are so commonly employed it is +considered advisable to direct the student's attention to them. + +PORPHYRY. + +(FIGURE 586. Porphyry. White crystals of feldspar in a dark base of hornblende +and feldspar.) + +PORPHYRY is one of this class, and very characteristic of the volcanic +formations. When distinct crystals of one or more minerals are scattered through +an earthy or compact base, the rock is termed a porphyry (see Figure 586). Thus +trachyte is usually porphyritic; for in it, as in many modern lavas, there are +crystals of feldspar; but in some porphyries the crystals are of augite, +olivine, or other minerals. If the base be greenstone, basalt, or pitchstone, +the rock may be denominated greenstone-porphyry, pitchstone-porphyry, and so +forth. The old classical type of this form of rock is the red porphyry of Egypt, +or the well-known "Rosso antico." It consists, according to Delesse, of a red +feldspathic base in which are disseminated rose-coloured crystals of the +feldspar called oligoclase, with some plates of blackish hornblende and grains +of oxide of iron (iron-glance). RED QUARTZIFEROUS PORPHYRY is a much more +siliceous rock, containing about 70 or 80 per cent of silex, while that of Egypt +has only 62 per cent. + +AMYGDALOID. + +This is also another form of igneous rock, admitting of every variety of +composition. It comprehends any rock in which round or almond-shaped nodules of +some mineral, such as agate, chalcedony, calcareous spar, or zeolite, are +scattered through a base of wacke, basalt, greenstone, or other kind of trap. It +derives its name from the Greek word amygdalon, an almond. The origin of this +structure can not be doubted, for we may trace the process of its formation in +modern lavas. Small pores or cells are caused by bubbles of steam and gas +confined in the melted matter. After or during consolidation, these empty spaces +are gradually filled up by matter separating from the mass, or infiltered by +water permeating the rock. As these bubbles have been sometimes lengthened by +the flow of the lava before it finally cooled, the contents of such cavities +have the form of almonds. In some of the amygdaloidal traps of Scotland, where +the nodules have decomposed, the empty cells are seen to have a glazed or +vitreous coating, and in this respect exactly resemble scoriaceous lavas, or the +slags of furnaces. + +(FIGURE 587. Scoriaceous lava in part converted into an amygdaloid. Montagne de +la Veille, Department of Puy de Dome, France.) + +Figure 587 represents a fragment of stone taken from the upper part of a sheet +of basaltic lava in Auvergne. One-half is scoriaceous, the pores being perfectly +empty; the other part is amygdaloidal, the pores or cells being mostly filled up +with carbonate of lime, forming white kernels. + +LAVA. + +This term has a somewhat vague signification, having been applied to all melted +matter observed to flow in streams from volcanic vents. When this matter +consolidates in the open air, the upper part is usually scoriaceous, and the +mass becomes more and more stony as we descend, or in proportion as it has +consolidated more slowly and under greater pressure. At the bottom, however, of +a stream of lava, a small portion of scoriaceous rock very frequently occurs, +formed by the first thin sheet of liquid matter, which often precedes the main +current, and solidifies under slight pressure. + +The more compact lavas are often porphyritic, but even the scoriaceous part +sometimes contains imperfect crystals, which have been derived from some older +rocks, in which the crystals pre-existed, but were not melted, as being more +infusible in their nature. Although melted matter rising in a crater, and even +that which enters a rent on the side of a crater, is called lava, yet this term +belongs more properly to that which has flowed either in the open air or on the +bed of a lake or sea. If the same fluid has not reached the surface, but has +been merely injected into fissures below ground, it is called trap. There is +every variety of composition in lavas; some are trachytic, as in the Peak of +Teneriffe; a great number are basaltic, as in Vesuvius and Auvergne; others are +andesitic, as those of Chili; some of the most modern in Vesuvius consist of +green augite, and many of those of Etna of augite and labrador-feldspar. (G. +Hose, Ann. des Mines tome 8 page 32.) + +SCORIAE and PUMICE may next be mentioned, as porous rocks produced by the action +of gases on materials melted by volcanic heat. SCORIAE are usually of a reddish- +brown and black colour, and are the cinders and slags of basaltic or augitic +lavas. PUMICE is a light, spongy, fibrous substance, produced by the action of +gases on trachytic and other lavas; the relation, however, of its origin to the +composition of lava is not yet well understood. Von Buch says that it never +occurs where only labrador-feldspar is present. + +VOLCANIC ASH OR TUFF, TRAP TUFF. + +Small angular fragments of the scoriae and pumice, above-mentioned, and the dust +of the same, produced by volcanic explosions, form the tuffs which abound in all +regions of active volcanoes, where showers of these materials, together with +small pieces of other rocks ejected from the crater, and more or less burnt, +fall down upon the land or into the sea. Here they often become mingled with +shells, and are stratified. Such tuffs are sometimes bound together by a +calcareous cement, and form a stone susceptible of a beautiful polish. But even +when little or no lime is present, there is a great tendency in the materials of +ordinary tuffs to cohere together. The term VOLCANIC ASH has been much used for +rocks of all ages supposed to have been derived from matter ejected in a melted +state from volcanic orifices. We meet occasionally with extremely compact beds +of volcanic materials, interstratified with fossiliferous rocks. These may +sometimes be tuffs, although their density or compactness is such as the cause +them to resemble many of those kinds of trap which are found in ordinary dikes. + +WACKE is a name given to a decomposed state of various trap rocks of the +basaltic family, or those which are poor in silica. It resembles clay of a +yellowish or brown colour, and passes gradually from the soft state to the hard +dolerite, greenstone, or other trap rock from which it has been derived. + +AGGLOMERATE. + +In the neighbourhood of volcanic vents, we frequently observe accumulations of +angular fragments of rocks formed during eruptions by the explosive action of +steam, which shatters the subjacent stony formations, and hurls them up into the +air. They then fall in showers around the cone or crater, or may be spread for +some distance over the surrounding country. The fragments consist usually of +different varieties of scoriaceous and compact lavas; but other kinds of rock, +such as granite or even fossiliferous limestones, may be intermixed; in short, +any substance through which the expansive gases have forced their way. The +dispersion of such materials may be aided by the wind, as it varies in direction +or intensity, and by the slope of the cone down which they roll, or by floods of +rain, which often accompany eruptions. But if the power of running water, or of +the waves and currents of the sea, be sufficient to carry the fragments to a +distance, it can scarcely fail to wear off their angles, and the formation then +becomes a CONGLOMERATE. If occasionally globular pieces of scoriae abound in an +agglomerate, they may not owe their round form to attrition. When all the +angular fragments are of volcanic rocks the mass is usually termed a volcanic +breccia. + +Laterite is a red or brick-like rock composed of silicate of alumina and oxide +of iron. The red layers called "ochre beds," dividing the lavas of the Giant's +Causeway, are laterites. These were found by Delesse to be trap impregnated with +the red oxide of iron, and in part reduced to kaolin. When still more +decomposed, they were found to be clay coloured by red ochre. As two of the +lavas of the Giant's Causeway are parted by a bed of lignite, it is not +improbable that the layers of laterite seen in the Antrim cliffs resulted from +atmospheric decomposition. In Madeira and the Canary Islands streams of lava of +subaerial origin are often divided by red bands of laterite, probably ancient +soils formed by the decomposition of the surfaces of lava-currents, many of +these soils having been coloured red in the atmosphere by oxide of iron, others +burnt into a red brick by the overflowing of heated lavas. These red bands are +sometimes prismatic, the small prisms being at right angles to the sheets of +lava. Red clay or red marl, formed as above stated by the disintegration of +lava, scoriae, or tuff, has often accumulated to a great thickness in the +valleys of Madeira, being washed into them by alluvial action; and some of the +thick beds of laterite in India may have had a similar origin. In India, +however, especially in the Deccan, the term "laterite" seems to have been used +too vaguely to answer the above definition. The vegetable soil in the gardens of +the suburbs of Catania which was overflowed by the lava of 1669 was turned or +burnt into a layer of red brick-coloured stone, or in other words, into +laterite, which may now be seen supporting the old lava-current. + +COLUMNAR AND GLOBULAR STRUCTURE. + +One of the characteristic forms of volcanic rocks, especially of basalt, is the +columnar, where large masses are divided into regular prisms, sometimes easily +separable, but in other cases adhering firmly together. The columns vary, in the +number of angles, from three to twelve; but they have most commonly from five to +seven sides. They are often divided transversely, at nearly equal distances, +like the joints in a vertebral column, as in the Giant's Causeway, in Ireland. +They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions +some in Skye which are about 400 feet long; others, in Morven, not exceeding an +inch. In regard to diameter, those of Ailsa measure nine feet, and those of +Morven an inch or less. (MacCulloch System of Geology volume 2 page 137.) They +are usually straight, but sometimes curved; and examples of both these occur in +the island of Staffa. In a horizontal bed or sheet of trap the columns are +vertical; in a vertical dike they are horizontal. + +(FIGURE 588. Lava of La Coupe d'Ayzac, near Antraigue, in the Department of +Ardeche.) + +It being assumed that columnar trap has consolidated from a fluid state, the +prisms are said to be always at right angles to the COOLING SURFACES. If these +surfaces, therefore, instead of being either perpendicular or horizontal, are +curved, the columns ought to be inclined at every angle to the horizon; and +there is a beautiful exemplification of this phenomenon in one of the valleys of +the Vivarais, a mountainous district in the South of France, where, in the midst +of a region of gneiss, a geologist encounters unexpectedly several volcanic +cones of loose sand and scoriae. From the crater of one of these cones, called +La Coupe d'Ayzac, a stream of lava has descended and occupied the bottom of a +narrow valley, except at those points where the river Volant, or the torrents +which join it, have cut away portions of the solid lava. Figure 588 represents +the remnant of the lava at one of these points. It is clear that the lava once +filled the whole valley up to the dotted line d-a; but the river has gradually +swept away all below that line, while the tributary torrent has laid open a +transverse section; by which we perceive, in the first place, that the lava is +composed, as usual in this country, of three parts: the uppermost, at a, being +scoriaceous, the second b, presenting irregular prisms; and the third, c, with +regular columns, which are vertical on the banks of the Volant, where they rest +on a horizontal base of gneiss, but which are inclined at an angle of 45 +degrees, at g, and are nearly horizontal at f, their position having been +everywhere determined, according to the law before mentioned, by the form of the +original valley. + +(FIGURE 589. Columnar basalt in the Vincentin. (Fortis.) + +In Figure 589, a view is given of some of the inclined and curved columns which +present themselves on the sides of the valleys in the hilly region north of +Vicenza, in Italy, and at the foot of the higher Alps. (Fortis Mem. sur l'Hist. +Nat. de l'Italie tome 1 page 233 plate 7.) Unlike those of the Vivarais, last +mentioned, the basalt of this country was evidently submarine, and the present +valleys have since been hollowed out by denudation. + +(FIGURE 590. Basaltic pillars of the Kasegrotte, Bertrich-Baden, half-way +between Treves and Coblentz. Height of grotto, from 7 to 8 feet.) + +The columnar structure is by no means peculiar to the trap rocks in which augite +abounds; it is also observed in trachyte, and other feldspathic rocks of the +igneous class, although in these it is rarely exhibited in such regular +polygonal forms. It has been already stated that basaltic columns are often +divided by cross-joints. Sometimes each segment, instead of an angular, assumes +a spheroidal form, so that a pillar is made up of a pile of balls, usually +flattened, as in the Cheese-grotto at Bertrich-Baden, in the Eifel, near the +Moselle (Figure 590). The basalt there is part of a small stream of lava, from +30 to 40 feet thick, which has proceeded from one of several volcanic craters, +still extant, on the neighbouring heights. + +In some masses of decomposing greenstone, basalt, and other trap rocks, the +globular structure is so conspicuous that the rock has the appearance of a heap +of large cannon balls. According to M. Delesse, the centre of each spheroid has +been a centre of crystallisation, around which the different minerals of the +rock arranged themselves symmetrically during the process of cooling. But it was +also, he says, a centre of contraction, produced by the same cooling, the +globular form, therefore, of such spheroids being the combined result of +crystallisation and contraction. (Delesse sur les Roches Globuleuses Mem. de la +Soc. Geol. de France 2 ser. tome 4.) + +(FIGURE 591. Globiform pitchstone. Chiaja di Luna, Isle of Ponza. (Scrope.)) + +Mr. Scrope gives as an illustration of this structure a resinous trachyte or +pitchstone-porphyry in one of the Ponza islands, which rise from the +Mediterranean, off the coast of Terracina and Gaeta. The globes vary from a few +inches to three feet in diameter, and are of an ellipsoidal form (see Figure +591). The whole rock is in a state of decomposition, "and when the balls," says +Mr. Scrope, "have been exposed a short time to the weather, they scale off at a +touch into numerous concentric coats, like those of a bulbous root, inclosing a +compact nucleus. The laminae of this nucleus have not been so much loosened by +decomposition; but the application of a ruder blow will produce a still further +exfoliation." (Scrope Geological Transactions second series volume 2 page 205.) + +VOLCANIC OR TRAP DIKES. + +(FIGURE 592. Dike in valley, near Brazen Head, Madeira. (From a drawing of +Captain Basil Hall, R.N.)) + +The leading varieties of the trappean rocks-- basalt, greenstone, trachyte, and +the rest-- are found sometimes in dikes penetrating stratified and unstratified +formations, sometimes in shapeless masses protruding through or overlying them, +or in horizontal sheets intercalated between strata. Fissures have already been +spoken of as occurring in all kinds of rocks, some a few feet, others many yards +in width, and often filled up with earth or angular pieces of stone, or with +sand and pebbles. Instead of such materials, suppose a quantity of melted stone +to be driven or injected into an open rent, and there consolidated, we have then +a tabular mass resembling a wall, and called a trap dike. It is not uncommon to +find such dikes passing through strata of soft materials, such as tuff, scoriae, +or shale, which, being more perishable than the trap, are often washed away by +the sea, rivers, or rain, in which case the dike stands prominently out in the +face of precipices, or on the level surface of a country (see Figure 592). + +(FIGURE 593. Ground-plan of greenstone dikes traversing sandstone. Arran.) + +In the islands of Arran and Skye, and in other parts of Scotland, where +sandstone, conglomerate, and other hard rocks are traversed by dikes of trap, +the converse of the above phenomenon is seen. The dike, having decomposed more +rapidly than the containing rock, has once more left open the original fissure, +often for a distance of many yards inland from the sea-coast. There is yet +another case, by no means uncommon in Arran and other parts of Scotland, where +the strata in contact with the dike, and for a certain distance from it, have +been hardened, so as to resist the action of the weather more than the dike +itself, or the surrounding rocks. When this happens, two parallel walls of +indurated strata are seen protruding above the general level of the country and +following the course of the dike. In Figure 593 a ground plan is given of a +ramifying dike of greenstone, which I observed cutting through sandstone on the +beach near Kildonan Castle, in Arran. The larger branch varies from five to +seven feet in width, which will afford a scale of measurement for the whole. + +(FIGURE 594. Trap dividing and covering sandstone near Suishnish, in Skye. +(MacCulloch.)) + +In the Hebrides and other countries, the same masses of trap which occupy the +surface of the country far and wide, concealing the subjacent stratified rocks, +are seen also in the sea-cliffs, prolonged downward in veins or dikes, which +probably unite with other masses of igneous rock at a greater depth. The largest +of the dikes represented in Figure 594, and which are seen in part of the coast +of Skye, is no less than 100 feet in width. + +Every variety of trap-rock is sometimes found in dikes, as basalt, greenstone, +feldspar-porphyry, and trachyte. The amygdaloidal traps also occur, though more +rarely, and even tuff and breccia, for the materials of these last may be washed +down into open fissures at the bottom of the sea, or during eruption on the land +may be showered into them from the air. Some dikes of trap may be followed for +leagues uninterruptedly in nearly a straight direction, as in the north of +England, showing that the fissures which they fill must have been of +extraordinary length. + +ROCKS ALTERED BY VOLCANIC DIKES. + +After these remarks on the form and composition of dikes themselves, I shall +describe the alterations which they sometimes produce in the rocks in contact +with them. The changes are usually such as the heat of melted matter and of the +entangled steam and gases might be expected to cause. + +PLAS-NEWYDD: DIKE CUTTING THROUGH SHALE. + +A striking example, near Plas-Newydd, in Anglesea, has been described by +Professor Henslow. (Cambridge Transactions volume 1 page 402.) The dike is 134 +feet wide, and consists of a rock which is a compound of feldspar and augite +(dolerite of some authors). Strata of shale and argillaceous limestone, through +which it cuts perpendicularly, are altered to a distance of 30, or even, in some +places, of 35 feet from the edge of the dike. The shale, as it approaches the +trap, becomes gradually more compact, and is most indurated where nearest the +junction. Here it loses part of its schistose structure, but the separation into +parallel layers is still discernible. In several places the shale is converted +into hard porcelanous jasper. In the most hardened part of the mass the fossil +shells, principally Producti, are nearly obliterated; yet even here their +impressions may frequently be traced. The argillaceous limestone undergoes +analogous mutations, losing its earthy texture as it approaches the dike, and +becoming granular and crystalline. But the most extraordinary phenomenon is the +appearance in the shale of numerous crystals of analcime and garnet, which are +distinctly confined to those portions of the rock affected by the dike. (Ibid. +volume 1 page 410.) Some garnets contain as much as 20 per cent of lime, which +they may have derived from the decomposition of the fossil shells or Producti. +The same mineral has been observed, under very analogous circumstances, in High +Teesdale, by Professor Sedgwick, where it also occurs in shale and limestone, +altered by basalt. (Ibid. volume 2 page 175.) + +ANTRIM: DIKE CUTTING THROUGH CHALK. + +In several parts of the county of Antrim, in the north of Ireland, chalk with +flints is traversed by basaltic dikes. The chalk is there converted into +granular marble near the basalt, the change sometimes extending eight or ten +feet from the wall of the dike, being greatest near the point of contact, and +thence gradually decreasing till it becomes evanescent. "The extreme effect," +says Dr. Berger, "presents a dark brown crystalline limestone, the crystals +running in flakes as large as those of coarse primitive (METAMORPHIC) limestone; +the next state is saccharine, then fine grained and arenaceous; a compact +variety, having a porcelanous aspect and a bluish-grey colour, succeeds: this, +towards the outer edge, becomes yellowish-white, and insensibly graduates into +the unaltered chalk. The flints in the altered chalk usually assume a grey +yellowish colour." (Dr. Berger Geological Transactions 1st series volume 3 page +172.) All traces of organic remains are effaced in that part of the limestone +which is most crystalline. + +(FIGURE 595. Basaltic dikes in chalk in Island of Rathlin, Antrim. Ground-plan +as seen on the beach. (Conybeare and Buckland. (Geological Transactions 1st +series volume 3 page 210 and plate 10. +From left to right: chalk: dike 35 ft.: dike 1 ft.: dike 20 ft.: chalk.) + +Figure 595 represents three basaltic dikes traversing the chalk, all within the +distance of 90 feet. The chalk contiguous to the two outer dikes is converted +into a finely granular marble, m, m, as are the whole of the masses between the +outer dikes and the central one. The entire contrast in the composition and +colour of the intrusive and invaded rocks, in these cases, renders the phenomena +peculiarly clear and interesting. Another of the dikes of the north-east of +Ireland has converted a mass of red sandstone into hornstone. By another, the +shale of the coal-measures has been indurated, assuming the character of flinty +slate; and in another place the slate-clay of the lias has been changed into +flinty slate, which still retains numerous impressions of ammonites. (Ibid. +volume 3 page 213; and Playfair Illustration of Huttonian Theory s. 253.) + +It might have been anticipated that beds of coal would, from their combustible +nature, be affected in an extraordinary degree by the contact of melted rock. +Accordingly, one of the greenstone dikes of Antrim, on passing through a bed of +coal, reduces it to a cinder for the space of nine feet on each side. At +Cockfield Fell, in the north of England, a similar change is observed. Specimens +taken at the distance of about thirty yards from the trap are not +distinguishable from ordinary pit-coal; those nearer the dike are like cinders, +and have all the character of coke; while those close to it are converted into a +substance resembling soot. (Sedgwick Cambridge Transactions volume 2 page 37.) + +It is by no means uncommon to meet with the same rocks, even in the same +districts, absolutely unchanged in the proximity of volcanic dikes. This great +inequality in the effects of the igneous rocks may often arise from an original +difference in their temperature, and in that of the entangled gases, such as is +ascertained to prevail in different lavas, or in the same lava near its source +and at a distance from it. The power also of the invaded rocks to conduct heat +may vary, according to their composition, structure, and the fractures which +they may have experienced, and perhaps, also, according to the quantity of water +(so capable of being heated) which they contain. It must happen in some cases +that the component materials are mixed in such proportions as to prepare them +readily to enter into chemical union, and form new minerals; while in other +cases the mass may be more homogeneous, or the proportions less adapted for such +union. + +We must also take into consideration, that one fissure may be simply filled with +lava, which may begin to cool from the first; whereas in other cases the fissure +may give passage to a current of melted matter, which may ascend for days or +months, feeding streams which are overflowing the country above, or being +ejected in the shape of scoriae from some crater. If the walls of a rent, +moreover, are heated by hot vapour before the lava rises, as we know may happen +on the flanks of a volcano, the additional heat supplied by the dike and its +gases will act more powerfully. + +INTRUSION OF TRAP BETWEEN STRATA. + +Masses of trap are not unfrequently met with intercalated between strata, and +maintaining their parallelism to the planes of stratification throughout large +areas. They must in some places have forced their way laterally between the +divisions of the strata, a direction in which there would be the least +resistance to an advancing fluid, if no vertical rents communicated with the +surface, and a powerful hydrostatic pressure were caused by gases propelling the +lava upward. + +RELATION OF TRAPPEAN ROCKS TO THE PRODUCTS OF ACTIVE VOLCANOES. + +When we reflect on the changes above described in the strata near their contact +with trap dikes, and consider how complete is the analogy or often identity in +composition and structure of the rocks called trappean and the lavas of active +volcanoes, it seems difficult at first to understand how so much doubt could +have prevailed for half a century as to whether trap was of igneous or aqueous +origin. To a certain extent, however, there was a real distinction between the +trappean formations and those to which the term volcanic was almost exclusively +confined. A large portion of the trappean rocks first studied in the north of +Germany, and in Norway, France, Scotland, and other countries, were such as had +been formed entirely under water, or had been injected into fissures and +intruded between strata, and which had never flowed out in the air, or over the +bottom of a shallow sea. When these products, therefore, of submarine or +subterranean igneous action were contrasted with loose cones of scoriae, tuff, +and lava, or with narrow streams of lava in great part scoriaceous and porous, +such as were observed to have proceeded from Vesuvius and Etna, the resemblance +seemed remote and equivocal. It was, in truth, like comparing the roots of a +tree with its leaves and branches, which, although the belong to the same plant, +differ in form, texture, colour, mode of growth, and position. The external +cone, with its loose ashes and porous lava, may be likened to the light foliage +and branches, and the rocks concealed far below, to the roots. But it is not +enough to say of the volcano, + +"Quantum vertice in auras +Aetherias, tantum radice in Tartara tendit," + +for its roots do literally reach downward to Tartarus, or to the regions of +subterranean fire; and what is concealed far below is probably always more +important in volume and extent than what is visible above ground. + +(FIGURE 596. Strata intercepted by a trap dike, and covered with alluvium.) + +We have already stated how frequently dense masses of strata have been removed +by denudation from wide areas (see Chapter 6); and this fact prepares us to +expect a similar destruction of whatever may once have formed the uppermost part +of ancient submarine or subaerial volcanoes, more especially as those +superficial parts are always of the lightest and most perishable materials. The +abrupt manner in which dikes of trap usually terminate at the surface (see +Figure 596), and the water-worn pebbles of trap in the alluvium which covers the +dike, prove incontestably that whatever was uppermost in these formations has +been swept away. It is easy, therefore, to conceive that what is gone in regions +of trap may have corresponded to what is now visible in active volcanoes. + +As to the absence of porosity in the trappean formations, the appearances are in +a great degree deceptive, for all amygdaloids are, as already explained, porous +rocks, into the cells of which mineral matter such as silex, carbonate of lime, +and other ingredients, have been subsequently introduced (see above); sometimes, +perhaps, by secretion during the cooling and consolidation of lavas. In the +Little Cumbray, one of the Western Islands, near Arran, the amygdaloid sometimes +contains elongated cavities filled with brown spar; and when the nodules have +been washed out, the interior of the cavities is glazed with the vitreous +varnish so characteristic of the pores of slaggy lavas. Even in some parts of +this rock which are excluded from air and water, the cells are empty, and seem +to have always remained in this state, and are therefore undistinguishable from +some modern lavas. (MacCulloch Western Islands volume 2 page 487.) + +Dr. MacCulloch, after examining with great attention these and the other igneous +rocks of Scotland, observes, "that it is a mere dispute about terms, to refuse +to the ancient eruptions of trap the name of submarine volcanoes; for they are +such in every essential point, although they no longer eject fire and smoke." +The same author also considers it not improbable that some of the volcanic rocks +of the same country may have been poured out in the open air. (System of Geology +volume 2 page 114.) + +It will be seen in the following chapters that in the earth's crust there are +volcanic tuffs of all ages, containing marine shells, which bear witness to +eruptions at many successive geological periods. These tuffs, and the associated +trappean rocks, must not be compared to lava and scoriae which had cooled in the +open air. Their counterparts must be sought in the products of modern submarine +volcanic eruptions. If it be objected that we have no opportunity of studying +these last, it may be answered, that subterranean movements have caused, almost +everywhere in regions of active volcanoes, great changes in the relative level +of land and sea, in times comparatively modern, so as to expose to view the +effects of volcanic operations at the bottom of the sea. + + +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. + +Having in the former part of this work referred the sedimentary strata to a long +succession of geological periods, we have now to consider how far the volcanic +formations can be classed in a similar chronological order. The tests of +relative age in this class of rocks are four: first, superposition and +intrusion, with or without alteration of the rocks in contact; second, organic +remains; third, mineral characters; fourth, included fragments of older rocks. + +Besides these four tests it may be said, in a general way, that volcanic rocks +of Primary or Palaeozoic antiquity differ from those of the Secondary or +Mesozoic age, and these again from the Tertiary and Recent. Not, perhaps, that +they differed originally in a greater degree than the modern volcanic rocks of +one region, such as that of the Andes, differ from those of another, such as +Iceland, but because all rocks permeated by water, especially if its temperature +be high, are liable to undergo a slow transmutation, even when they do not +assume a new crystalline form like that of the hypogene rocks. + +Although subaerial and submarine denudation, as before stated, remove, in the +course of ages, large portions of the upper or more superficial products of +volcanoes, yet these are sometimes preserved by subsidence, becoming covered by +the sea or by superimposed marine deposits. In this way they may be protected +for ages from the waves of the sea, or the destroying action of rivers, while, +at the same time, they may not sink so deep as to be exposed to that Plutonic +action (to be spoken of in Chapter 31) which would convert them into crystalline +rocks. But even in this case they will not remain unaltered, because they will +be percolated by water often of high temperature, and charged with carbonate of +lime, silex, iron, and other mineral ingredients, whereby gradual changes in the +constitution of the rocks may be superinduced. Every geologist is aware how +often silicified trees occur in volcanic tuffs, the perfect preservation of +their internal structure showing that they have not decayed before the +petrifying material was supplied. + +The porous and vesicular nature of a large part, both of the basaltic and +trachytic lavas, affords cavities in which silex and carbonate of lime are +readily deposited. Minerals of the zeolite family, the composition of which has +already been alluded to in Chapter 28, occur in amygdaloids and other trap-rocks +in great abundance, and Daubree's observations have proved that they are not +always simple deposits of substances held in solution by the percolating waters, +being occasionally products of the chemical action of that water on the rock +through which they are filtered, and portions of which are decomposed. From +these considerations it follows that the perfect identity of very ancient and +very modern volcanic formations is scarcely possible. + +TESTS BY SUPERPOSITION. + +(FIGURE 597. Section through sedimentary mass with melted matter.) + +If a volcanic rock rest upon an aqueous deposit, the volcanic must be the newest +of the two; but the like rule does not hold good where the aqueous formation +rests upon the volcanic, for melted matter, rising from below, may penetrate a +sedimentary mass without reaching the surface, or may be forced in conformably +between two strata, as b below D in Figure 597, after which it may cool down and +consolidate. Superposition, therefore, is not of the same value as a test of age +in the unstratified volcanic rocks as in fossiliferous formations. We can only +rely implicitly on this test where the volcanic rocks are contemporaneous, not +where they are intrusive. Now, they are said to be contemporaneous if produced +by volcanic action which was going on simultaneously with the deposition of the +strata with which they are associated. Thus in the section at D (Figure 597), we +may perhaps ascertain that the trap b flowed over the fossiliferous bed c, and +that, after its consolidation, a was deposited upon it, a and c both belonging +to the same geological period. But, on the other hand, we must conclude the trap +to be intrusive, if the stratum a be altered by b at the point of contact, or +if, in pursuing b for some distance, we find at length that it cuts through the +stratum a, and then overlies it as at E. + +(FIGURE 598. Section through sedimentary mass with melted matter.) + +We may, however, be easily deceived in supposing the volcanic rock to be +intrusive, when in reality it is contemporaneous; for a sheet of lava, as it +spreads over the bottom of the sea, can not rest everywhere upon the same +stratum, either because these have been denuded, or because, if newly thrown +down, they thin out in certain places, thus allowing the lava to cross their +edges. Besides, the heavy igneous fluid will often, as it moves along, cut a +channel into beds of soft mud and sand. Suppose the submarine lava F (Figure +598) to have come in contact in this manner with the strata a, b, c, and that +after its consolidation the strata d, e are thrown down in a nearly horizontal +position, yet so as to lie unconformably to F, the appearance of subsequent +intrusion will here be complete, although the trap is in fact contemporaneous. +We must not, therefore, hastily infer that the rock F is intrusive, unless we +find the overlying strata, d, e, to have been altered at their junction, as if +by heat. + +The test of age by superposition is strictly applicable to all stratified +volcanic tuffs, according to the rules already explained in the case of +sedimentary deposits (see Chapter 8). + +TEST OF AGE BY ORGANIC REMAINS. + +We have seen how, in the vicinity of active volcanoes, scoriae, pumice, fine +sand, and fragments of rock are thrown up into the air, and then showered down +upon the land, or into neighbouring lakes or seas. In the tuffs so formed +shells, corals, or any other durable organic bodies which may happen to be +strewed over the bottom of a lake or sea will be imbedded, and thus continue as +permanent memorials of the geological period when the volcanic eruption +occurred. Tufaceous strata thus formed in the neighbourhood of Vesuvius, Etna, +Stromboli, and other volcanoes now in islands or near the sea, may give +information of the relative age of these tuffs at some remote future period when +the fires of these mountains are extinguished. By evidence of this kind we can +establish a coincidence in age between volcanic rocks and the different primary, +secondary, and tertiary fossiliferous strata. + +The tuffs alluded to may not always be marine, but may include, in some places, +fresh-water shells; in others, the bones of terrestrial quadrupeds. The +diversity of organic remains in formations of this nature is perfectly +intelligible, if we reflect on the wide dispersion of ejected matter during late +eruptions, such as that of the volcano of Coseguina, in the province of +Nicaragua, January 19, 1835. Hot cinders and fine scoriae were then cast up to a +vast height, and covered the ground as they fell to the depth of more than ten +feet, for a distance of eight leagues from the crater, in a southerly direction. +Birds, cattle, and wild animals were scorched to death in great numbers, and +buried in ashes. Some volcanic dust fell at Chiapa, upward of 1200 miles, not to +leeward of the volcano, as might have been anticipated, but to windward, a +striking proof of a counter-current in the upper region of the atmosphere; and +some on Jamaica, about 700 miles distant to the north-east. In the sea, also, at +the distance of 1100 miles from the point of eruption, Captain Eden of the +"Conway" sailed 40 miles through floating pumice, among which were some pieces +of considerable size. (Caldcleugh Philosophical Transactions 1836 page 27.) + +TEST OF AGE BY MINERAL COMPOSITION. + +As sediment of homogeneous composition, when discharged from the mouth of a +large river, is often deposited simultaneously over a wide space, so a +particular kind of lava flowing from a crater during one eruption may spread +over an extensive area; thus in Iceland, in 1783, the melted matter, pouring +from Skaptar Jokul, flowed in streams in opposite directions, and caused a +continuous mass the extreme points of which were 90 miles distant from each +other. This enormous current of lava varied in thickness from 100 feet to 600 +feet, and in breadth from that of a narrow river gorge to 15 miles. (See +Principles Index "Skaptar Jokul.") Now, if such a mass should afterwards be +divided into separate fragments by denudation, we might still, perhaps, identify +the detached portions by their similarity in mineral composition. Nevertheless, +this test will not always avail the geologist; for, although there is usually a +prevailing character in lava emitted during the same eruption, and even in the +successive currents flowing from the same volcano, still, in many cases, the +different parts even of one lava-stream, or, as before stated, of one continuous +mass of trap, vary much in mineral composition and texture. + +In Auvergne, the Eifel, and other countries where trachyte and basalt are both +present, the trachytic rocks are for the most part older than the basaltic. +These rocks do, indeed, sometimes alternate partially, as in the volcano of Mont +Dor, in Auvergne; and in Madeira trachytic rocks overlie an older basaltic +series; but the trachyte occupies more generally an inferior position, and is +cut through and overflowed by basalt. It can by no means be inferred that +trachyte predominated at one period of the earth's history and basalt at +another, for we know that trachytic lavas have been formed at many successive +periods, and are still emitted from many active craters; but it seems that in +each region, where a long series of eruptions have occurred, the lavas +containing feldspar more rich in silica have been first emitted, and the escape +of the more augitic kinds has followed. The hypothesis suggested by Mr. Scrope +may, perhaps, afford a solution of this problem. The minerals, he observes, +which abound in basalt are of greater specific gravity than those composing the +feldspathic lavas; thus, for example, hornblende, augite, and olivine are each +more than three times the weight of water; whereas common feldspar and albite +have each scarcely more than 2 1/2 times the specific gravity of water; and the +difference is increased in consequence of there being much more iron in a +metallic state in basalt and greenstone than in trachyte and other allied +feldspathic lavas. If, therefore, a large quantity of rock be melted up in the +bowels of the earth by volcanic heat, the denser ingredients of the boiling +fluid may sink to the bottom, and the lighter remaining above would in that case +be first propelled upward to the surface by the expansive power of gases. Those +materials, therefore, which occupy the lowest place in the subterranean +reservoir will always be emitted last, and take the uppermost place on the +exterior of the earth's crust. + +TEST BY INCLUDED FRAGMENTS. + +We may sometimes discover the relative age of two trap-rocks, or of an aqueous +deposit and the trap on which it rests, by finding fragments of one included in +the other in cases such as those before alluded to, where the evidence of +superposition alone would be insufficient. It is also not uncommon to find a +conglomerate almost exclusively composed of rolled pebbles of trap, associated +with some fossiliferous stratified formation in the neighbourhood of massive +trap. If the pebbles agree generally in mineral character with the latter, we +are then enabled to determine its relative age by knowing that of the +fossiliferous strata associated with the conglomerate. The origin of such +conglomerates is explained by observing the shingle beaches composed of trap- +pebbles in modern volcanoes, as at the base of Etna. + +RECENT AND POST-PLIOCENE VOLCANIC ROCKS. + +I shall now select examples of contemporaneous volcanic rocks of successive +geological periods, to show that igneous causes have been in activity in all +past ages of the world. They have been perpetually shifting the places where +they have broken out at the earth's surface, and we can sometimes prove that +those areas which are now the great theatres of volcanic action were in a state +of perfect tranquillity at remote geological epochs, and that, on the other +hand, in places where at former periods the most violent eruptions took place at +the surface and continued for a great length of time, there has been an entire +suspension of igneous action in historical times, and even, as in the British +Isles, throughout a large part of the antecedent Tertiary Period. + +In the absence of British examples of volcanic rocks newer than the Upper +Miocene, I may state that in other parts of the world, especially in those where +volcanic eruptions are now taking place from time to time, there are tuffs and +lavas belonging to that part of the Tertiary era the antiquity of which is +proved by the presence of the bones of extinct quadrupeds which co-existed with +terrestrial, fresh-water, and marine mollusca of species still living. One +portion of the lavas, tuffs, and trap-dikes of Etna, Vesuvius, and the island of +Ischia has been produced within the historical era; another and a far more +considerable part originated at times immediately antecedent, when the waters of +the Mediterranean were already inhabited by the existing testacea, but when +certain species of elephant, rhinoceros, and other quadrupeds now extinct, +inhabited Europe. + +VESUVIUS. + +I have traced in the "Principles of Geology" the history of the changes which +the volcanic region of Campania is known to have undergone during the last 2000 +years. The aggregate effect of igneous operations during that period is far from +insignificant, comprising as it does the formation of the modern cone of +Vesuvius since the year 79, and the production of several minor cones in Ischia, +together with that of Monte Nuovo in the year 1538. Lava-currents have also +flowed upon the land and along the bottom of the sea-- volcanic sand, pumice, +and scoriae have been showered down so abundantly that whole cities were buried- +- tracts of the sea have been filled up or converted into shoals-- and tufaceous +sediment has been transported by rivers and land-floods to the sea. There are +also proofs, during the same recent period, of a permanent alteration of the +relative levels of the land and sea in several places, and of the same tract +having, near Puzzuoli, been alternately upheaved and depressed to the amount of +more than twenty feet. In connection with these convulsions, there are found, on +the shores of the Bay of Baiae, recent tufaceous strata, filled with articles +fabricated by the hands of man, and mingled with marine shells. + +It has also been stated (Chapter 13), that when we examine this same region, it +is found to consist largely of tufaceous strata, of a date anterior to human +history or tradition, which are of such thickness as to constitute hills from +500 to more than 2000 feet in height. Some of these strata contain marine shells +which are exclusively of living species, others contain a slight mixture, one or +two per cent of species not known as living. + +The ancient part of Vesuvius is called Somma, and consists of the remains of an +older cone which appears to have been partly destroyed by explosion. In the +great escarpment which this remnant of the ancient mountain presents towards the +modern cone of Vesuvius, there are many dikes which are for the most part +vertical, and traverse the inclined beds of lava and scoriae which were +successively superimposed during those eruptions by which the old cone was +formed. They project in relief several inches, or sometimes feet, from the face +of the cliff, being extremely compact, and less destructible than the +intersected tuffs and porous lavas. In vertical extent they vary from a few +yards to 500 feet, and in breadth from one to twelve feet. Many of them cut all +the inclined beds in the escarpment of Somma from top to bottom, others stop +short before they ascend above halfway. In mineral composition they scarcely +differ from the lavas of Somma, the rock consisting of a base of leucite and +augite, through which large crystals of augite and some of leucite are +scattered. + +Nothing is more remarkable than the usual parallelism of the opposite sides of +the dikes, which correspond almost as regularly as the two opposite faces of a +wall of masonry. This character appears at first the more inexplicable, when we +consider how jagged and uneven are the rents caused by earthquakes in masses of +heterogeneous composition, like those composing the cone of Somma. In +explanation of this phenomenon, M. Necker refers us to Sir W. Hamilton's account +of an eruption of Vesuvius in the year 1779, who records the following fact: +"The lavas, when they either boiled over the crater, or broke out from the +conical parts of the volcano, constantly formed channels as regular as if they +had been cut by art down the steep part of the mountain; and whilst in a state +of perfect fusion, continued their course in those channels, which were +sometimes full to the brim, and at other times more or less so, according to the +quantity of matter in motion. + +"These channels (says the same observer), I have found, upon examination after +an eruption, to be in general from two to five or six feet wide, and seven or +eight feet deep. They were often hid from the sight by a quantity of scoriae +that had formed a crust over them; and the lava, having been conveyed in a +covered way for some yards, came out fresh again into an open channel. After an +eruption, I have walked in some of those subterraneous or covered galleries, +which were exceedingly curious, the sides, top, and bottom BEING WORN PERFECTLY +SMOOTH AND EVEN in most parts by the violence of the currents of the red-hot +lavas which they had conveyed for many weeks successively." I was able to verify +this phenomenon in 1858, when a stream of lava issued from a lateral cone. +(Principles of Geology volume 1 page 626.) Now, the walls of a vertical fissure, +through which lava has ascended in its way to a volcanic vent, must have been +exposed to the same erosion as the sides of the channels before adverted to. The +prolonged and uniform friction of the heavy fluid, as it is forced and made to +flow upward, can not fail to wear and smooth down the surfaces on which it rubs, +and the intense heat must melt all such masses as project and obstruct the +passage of the incandescent fluid. + +The rock composing the dikes both in the modern and ancient part of Vesuvius is +far more compact than that of ordinary lava, for the pressure of a column of +melted matter in a fissure greatly exceeds that in an ordinary stream of lava; +and pressure checks the expansion of those gases which give rise to vesicles in +lava. There is a tendency in almost all the Vesuvian dikes to divide into +horizontal prisms, a phenomenon in accordance with the formation of vertical +columns in horizontal beds of lava; for in both cases the divisions which give +rise to the prismatic structure are at right angles to the cooling surfaces. +(See Chapter 28.) + +AUVERGNE. + +Although the latest eruptions in central France seem to have long preceded the +historical era, they are so modern as to have a very intimate connection with +the present superficial outline of the country and with the existing valleys and +river-courses. Among a great number of cones with perfect craters, one called +the Puy de Tartaret sent forth a lava-current which can be traced up to its +crater, and which flowed for a distance of thirteen miles along the bottom of +the present valley to the village of Nechers, covering the alluvium of the old +valley in which were preserved the bones of an extinct species of horse, and of +a lagomys and other quadrupeds all closely allied to recent animals, while the +associated land-shells were of species now living, such as Cyclostoma elegans, +Helix hortensis, H. nemoralis, H. lapicida, and Clausilia rugosa. That the +current which has issued from the Puy de Tartaret may, nevertheless, be very +ancient in reference to the events of human history, we may conclude, not only +from the divergence of the mammiferous fauna from that of our day, but from the +fact that a Roman bridge of such form and construction as continued in use only +down to the fifth century, but which may be older, is now seen at a place about +a mile and a half from St. Nectaire. This ancient bridge spans the river Couze +with two arches, each about fourteen feet wide. These arches spring from the +lava of Tartaret, on both banks, showing that a ravine precisely like that now +existing had already been excavated by the river through that lava thirteen or +fourteen centuries ago. + +While the river Couze has in most cases, as at the site of this ancient bridge, +been simply able to cut a deep channel through the lava, the lower portion of +which is shown to be columnar, the same torrent has in other places, where the +valley was contracted to a narrow gorge, had power to remove the entire mass of +basaltic rock, causing for a short space a complete breach of continuity in the +volcanic current. The work of erosion has been very slow, as the basalt is tough +and hard, and one column after another must have been undermined and reduced to +pebbles, and then to sand. During the time required for this operation, the +perishable cone of Tartaret, occupying the lowest part of the great valley +descending from Mont Dor (see Chapter 30), and damming up the river so as to +cause the Lake of Chambon, has stood uninjured, proving that no great flood or +deluge can have passed over this region in the interval between the eruption of +Tartaret and our own times. + +PUY DE COME. + +The Puy de Come and its lava-current, near Clermont, may be mentioned as another +minor volcano of about the same age. This conical hill rises from the granitic +platform, at an angle of between 30 and 40 degrees, to the height of more than +900 feet. Its summit presents two distinct craters, one of them with a vertical +depth of 250 feet. A stream of lava takes its rise at the western base of the +hill instead of issuing from either crater, and descends the granitic slope +towards the present site of the town of Pont Gibaud. Thence it pours in a broad +sheet down a steep declivity into the valley of the Sioule, filling the ancient +river-channel for the distance of more than a mile. The Sioule, thus +dispossessed of its bed, has worked out a fresh one between the lava and the +granite of its western bank; and the excavation has disclosed, in one spot, a +wall of columnar basalt about fifty feet high. (Scrope's Central France page 60 +and plate.) + +The excavation of the ravine is still in progress, every winter some columns of +basalt being undermined and carried down the channel of the river, and in the +course of a few miles rolled to sand and pebbles. Meanwhile the cone of Come +remains unimpaired, its loose materials being protected by a dense vegetation, +and the hill standing on a ridge not commanded by any higher ground, so that no +floods of rain-water can descend upon it. There is no end to the waste which the +hard basalt may undergo in future, if the physical geography of the country +continue unchanged-- no limit to the number of years during which the heap of +incoherent and transportable materials called the Puy de Come may remain in an +almost stationary condition. + +PUY DE PARIOU. + +The brim of the crater of the Puy de Pariou, near Clermont, is so sharp, and has +been so little blunted by time, that it scarcely affords room to stand upon. +This and other cones in an equally remarkable state of integrity have stood, I +conceive, uninjured, not IN SPITE of their loose porous nature, as might at +first be naturally supposed, but in consequence of it. No rills can collect +where all the rain is instantly absorbed by the sand and scoriae, as is +remarkably the case on Etna; and nothing but a water-spout breaking directly +upon the Puy de Pariou could carry away a portion of the hill, so long as it is +not rent or ingulfed by earthquakes. + +NEWER PLIOCENE VOLCANIC ROCKS. + +The more ancient portion of Vesuvius and Etna originated at the close of the +Newer Pliocene period, when less than ten, sometimes only one, in a hundred of +the shells differed from those now living. In the case of Etna, it was before +stated (Chapter 13) that Post-pliocene formations occur in the neighbourhood of +Catania, while the oldest lavas of the great volcano are Pliocene. These last +are seen associated with sedimentary deposits at Trezza and other places on the +southern and eastern flanks of the great cone (see Chapter 13). + +CYCLOPEAN ISLANDS. + +The Cyclopean Islands, called by the Sicilians Dei Faraglioni, in the sea-cliffs +of which these beds of clay, tuff, and associated lava are laid open to view, +are situated in the Bay of Trezza, and may be regarded as the extremity of a +promontory severed from the main land. Here numerous proofs are seen of +submarine eruptions, by which the argillaceous and sandy strata were invaded and +cut through, and tufaceous breccias formed. Inclosed in these breccias are many +angular and hardened fragments of laminated clay in different states of +alteration by heat, and intermixed with volcanic sands. + +(FIGURE 599. View of the Isle of Cyclops, in the Bay of Trezza. (Drawn by +Captain Basil Hall, R.N.)) + +The loftiest of the Cyclopean islets, or rather rocks, is about 200 feet in +height, the summit being formed of a mass of stratified clay, the laminae of +which are occasionally subdivided by thin arenaceous layers. These strata dip to +the N.W., and rest on a mass of columnar lava (see Figure 599) in which the tops +of the pillars are weathered, and so rounded as to be often hemispherical. In +some places in the adjoining and largest islet of the group, which lies to the +north-eastward of that represented in Figure 599), the overlying clay has been +greatly altered and hardened by the igneous rock, and occasionally contorted in +the most extraordinary manner; yet the lamination has not been obliterated, but, +on the contrary, rendered much more conspicuous, by the indurating process. + +(FIGURE 600. Contortions of strata in the largest of the Cyclopean Islands.) + +(FIGURE 601. Newer Pliocene strata invaded by lava. Isle of Cyclops (horizontal +section). +a. Lava. +b. Laminated clay and sand. +c. The same altered.) + +In Figure 600 I have represented a portion of the altered rock, a few feet +square, where the alternating thin laminae of sand and clay are contorted in a +manner often observed in ancient metamorphic schists. A great fissure, running +from east to west, nearly divides this larger island into two parts, and lays +open its internal structure. In the section thus exhibited, a dike of lava is +seen, first cutting through an older mass of lava, and then penetrating the +superincumbent tertiary strata. In one place the lava ramifies and terminates in +thin veins, from a few feet to a few inches in thickness (see Figure 601). The +arenaceous laminae are much hardened at the point of contact, and the clays are +converted into siliceous schist. In this island the altered rocks assume a +honey-comb structure on their weathered surface, singularly contrasted with the +smooth and even outline which the same beds present in their usual soft and +yielding state. The pores of the lava are sometimes coated, or entirely filled +with carbonate of lime, and with a zeolite resembling analcime, which has been +called cyclopite. The latter mineral has also been found in small fissures +traversing the altered marl, showing that the same cause which introduced the +minerals into the cavities of the lava, whether we suppose sublimation or +aqueous infiltration, conveyed it also into the open rents of the contiguous +sedimentary strata. + +DIKES OF PALAGONIA. + +(FIGURES 602 and 603. Ground-plan of dikes near Palagonia.) + +(FIGURE 602. Ground-plan of dikes near Palagonia. +a. Lava. +b. Peperino, consisting of volcanic sand, mixed with fragments of lava and +limestone.) + +(FIGURE 603. Ground-plan of dikes near Palagonia. +a. Lava. +b. Peperino, consisting of volcanic sand, mixed with fragments of lava and +limestone.)) + +Dikes of vesicular and amygdaloidal lava are also seen traversing marine tuff or +peperino, west of Palagonia, some of the pores of the lava being empty, while +others are filled with carbonate of lime. In such cases we may suppose the tuff +to have resulted from showers of volcanic sand and scoriae, together with +fragments of limestone, thrown out by a submarine explosion, similar to that +which gave rise to Graham Island in 1831. When the mass was, to a certain +degree, consolidated, it may have been rent open, so that the lava ascended +through fissures, the walls of which were perfectly even and parallel. In one +case, after the melted matter that filled the rent (Figure 602) had cooled down, +it must have been fractured and shifted horizontally by a lateral movement. + +In Figure 603, the lava has more the appearance of a vein, which forced its way +through the peperino. It is highly probable that similar appearances would be +seen, if we could examine the floor of the sea in that part of the Mediterranean +where the waves have recently washed away the new volcanic island; for when a +superincumbent mass of ejected fragments has been removed by denudation, we may +expect to see sections of dikes traversing tuff, or, in other words, sections of +the channels of communication by which the subterranean lavas reached the +surface. + +MADEIRA. + +Although the more ancient portion of the volcanic eruptions by which the island +of Madeira and the neighbouring one of Porto Santo were built up occurred, as we +shall presently see, in the Upper Miocene Period, a still larger part of the +island is of Pliocene date. That the latest outbreaks belonged to the Newer +Pliocene Period, I infer from the close affinity to the present flora of Madeira +of the fossil plants preserved in a leaf-bed in the north-eastern part of the +island. These fossils, associated with some lignite in the ravine of the river +San Jorge, can none of them be proved to be of extinct species, but their +antiquity may be inferred from the following considerations: Firstly-- The leaf- +bed, discovered by Mr. Hartung and myself in 1853, at the height of 1000 feet +above the level of the sea, crops out at the base of a cliff formed by the +erosion of a gorge cut through alternating layers of basalt and scoriae, the +product of a vast succession of eruptions of unknown date, piled up to a +thickness of 1000 feet, and which were all poured out after the plants, of which +about twenty species have been recognised, flourished in Madeira. These lavas +are inclined at an angle of about 15 degrees to the north, and came down from +the great central region of eruption. Their accumulation implies a long period +of intermittent volcanic action, subsequently to which the ravine of San Jorge +was hollowed out. Secondly-- Some few of the plants, though perhaps all of +living species, are supposed to be of genera not now existing in the island. +They have been described by Sir Charles Bunbury and Professor Heer, and the +former first pointed out that many of the leaves are of the laurel type, and +analogous to those now flourishing in the modern forests of Madeira. He also +recognised among them the leaves of Woodwardia radicans, and Davallia +Canariensis, ferns now abundant in Madeira. Thirdly-- the great age of this +leaf-bed of San Jorge, which was perhaps originally formed in the crater of some +ancient volcanic cone afterwards buried under lava, is proved by its belonging +to a part of the eastern extremity of Madeira, which, after the close of the +igneous eruptions, became covered in the adjoining district of Canical with +blown sand in which a vast number of land-shells were buried. These fossil +shells belonged to no less than 36 species, among which are many now extremely +rare in the island, and others, about five per cent, extinct or unknown in any +part of the world. Several of these of the genus Helix are conspicuous from the +peculiarity of their forms, others from their large dimensions. The geographical +configuration of the country shows that this shell-bed is considerably more +modern than the leaf-bed; it must therefore be referred to the Newer Pliocene, +according to the definition of this period given in Chapter 9. + +OLDER PLIOCENE PERIOD.-- ITALY. + +In Tuscany, as at Radicofani, Viterbo, and Aquapendente, and in the Campagna di +Roma, submarine volcanic tuffs are interstratified with the Older Pliocene +strata of the Sub-apennine hills in such a manner as to leave no doubt that they +were the products of eruptions which occurred when the shelly marls and sands of +the Sub-appenine hills were in the course of deposition. This opinion I +expressed after my visit to Italy in 1828 (See 1st edition of Principles of +Geology volume 3 chapters 8 and 14 1833 and former editions of this work chapter +31.), and it has recently (1850) been confirmed by the argument adduced by Sir +R. Murchison in favour of the submarine origin of the tertiary volcanic rocks of +Italy. (Quarterly Geological Journal volume 6 page 281.) These rocks are well- +known to rest conformably on the Sub-apennine marls, even as far south as Monte +Mario, in the suburbs of Rome. On the exact age of the deposits of Monte Mario +new light has recently been thrown by a careful study of their marine fossil +shells, undertaken by MM. Rayneval, Van den Hecke, and Ponzi. They have compared +no less than 160 species with the shells of the Coralline Crag of Suffolk, so +well described by Mr. Searles Wood; and the specific agreement between the +British and Italian fossils is so great, if we make due allowance for +geographical distance and the difference of latitude, that we can have little +hesitation in referring both to the same period, or to the Older Pliocene of +this work. It is highly probable that, between the oldest trachytes of Tuscany +and the newest rocks in the neighbourhood of Naples, a series of volcanic +products might be detected of every age from the Older Pliocene to the +historical epoch. + +PLIOCENE VOLCANOES OF THE EIFEL. + +Some of the most perfect cones and craters in Europe, not even excepting those +of the district round Vesuvius, may be seen on the left or west bank of the +Rhine, near Bonn and Andernach. They exhibit characters distinct from any which +I have observed elsewhere, owing to the large part which the escape of aqueous +vapour has played in the eruptions and the small quantities of lava emitted. The +fundamental rocks of the district are grey and red sandstones and shales, with +some associated limestones, replete with fossils of the Devonian or Old Red +Sandstone group. The volcanoes broke out in the midst of these inclined strata, +and when the present systems of hills and valleys had already been formed. The +eruptions occurred sometimes at the bottom of deep valleys, sometimes on the +summit of hills, and frequently on intervening platforms. In travelling through +this district we often come upon them most unexpectedly, and may find ourselves +on the very edge of a crater before we had been led to suspect that we were +approaching the site of any igneous outburst. Thus, for example, on arriving at +the village of Gemund, immediately south of Daun, we leave the stream, which +flows at the bottom of a deep valley in which strata of sandstone and shale crop +out. We then climb a steep hill, on the surface of which we see the edges of the +same strata dipping inward towards the mountain. When we have ascended to a +considerable height, we see fragments of scoriae sparingly scattered over the +surface; until at length, on reaching the summit, we find ourselves suddenly on +the edge of a tarn, or deep circular lake-basin called the Gemunder Maar. In it +we recognise the ordinary form of a crater, for which we have been prepared by +the occurrence of scoriae scattered over the surface of the soil. But on +examining the walls of the crater we find precipices of sandstone and shale +which exhibit no signs of the action of heat; and we look in vain for those beds +of lava and scoriae, dipping outward on every side, which we have been +accustomed to consider as characteristic of volcanic vents. As we proceed, +however, to the opposite side of the lake, we find a considerable quantity of +scoriae and some lava, and see the whole surface of the soil sparkling with +volcanic sand, and strewed with ejected fragments of half-fused shale, which +preserves its laminated texture in the interior, while it has a vitrified or +scoriform coating. + +Other crater lakes of circular or oval form, and hollowed out of similar ancient +strata, occur in the Upper Eifel, where copious aeriform discharges have taken +place, throwing out vast heaps of pulverized shale into the air. I know of no +other extinct volcanoes where gaseous explosions of such magnitude have been +attended by the emission of so small a quantity of lava. Yet I looked in vain in +the Eifel for any appearances which could lend support to the hypothesis that +the sudden rushing out of such enormous volumes of gas had ever lifted up the +stratified rocks immediately around the vent so as to form conical masses, +having their strata dipping outward on all sides from a central axis, as is +assumed in the theory of elevation craters, alluded to in the last chapter. + +I have already given (Figure 590) an example in the Eifel of a small stream of +lava which issued from one of the craters of that district at Bertrich-Baden. It +shows that when some of these volcanoes were in action the valleys had already +been eroded to their present depth. + +TRASS. + +The tufaceous alluvium called trass, which has covered large areas in the Eifel, +and choked up some valleys now partially re-excavated, is unstratified. Its base +consists almost entirely of pumice, in which are included fragments of basalt +and other lavas, pieces of burnt shale, slate, and sandstone, and numerous +trunks and branches of trees. If, as is probable, this trass was formed during +the period of volcanic eruptions, it may have originated in the manner of the +moya of the Andes. + +We may easily conceive that a similar mass might now be produced, if a copious +evolution of gases should occur in one of the lake-basins. If a breach should be +made in the side of the cone, the flood would sweep away great heaps of ejected +fragments of shale and sandstone, which would be borne down into the adjoining +valleys. Forests might be torn up by such a flood, and thus the occurrence of +the numerous trunks of trees dispersed irregularly through the trass can be +explained. The manner in which this trass conforms to the shape of the present +valleys implies its comparatively modern origin, probably not dating farther +back than the Pliocene Period. + + +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. + +VOLCANIC ROCKS OF THE UPPER MIOCENE PERIOD. + +MADEIRA. + +The greater part of the volcanic eruptions of Madeira, as we have already seen +(Chapter 29), belong to the Pliocene Period, but the most ancient of them are of +Upper Miocene date, as shown by the fossil shells included in the marine tuffs +which have been upraised at San Vicente, in the northern part of the island, to +the height of 1300 feet above the level of the sea. A similar marine and +volcanic formation constitutes the fundamental portion of the neighbouring +island of Porto Santo, forty miles distant from Madeira, and is there elevated +to an equal height, and covered, as in Madeira, with lavas of supra-marine +origin. + +The largest number of fossils have been collected from the tuffs and +conglomerates and some beds of limestone in the island of Baixo, off the +southern extremity of Porto Santo. They amount in this single locality to more +than sixty in number, of which about fifty are mollusca, but many of these are +only casts. Some of the shells probably lived on the spot during the intervals +between eruptions, and some may have been cast up into the water or air together +with muddy ejections, and, falling down again, have been deposited on the bottom +of the sea. The hollows in some of the fragments of vesicular lava of which the +breccias and conglomerates are composed are partially filled with calc-sinter, +being thus half converted into amygdaloids. Among the fossil shells common to +Madeira and Porto Santo, large cones, strombs, and cowries are conspicuous among +the univalves, and Cardium, Spondylus, and Lithodomus among the +lamellibranchiate bivalves, and among the Echinoderms the large Clypeaster +called C. altus, an extinct European Miocene fossil. + +The largest list of fossils has been published by Mr. Karl Meyer, in Hartung's +"Madeira;" but in the collection made by myself, and in a still larger one +formed by Mr. J. Yate Johnson, several remarkable forms not in Meyer's list +occur, as, for example, Pholadomya, and a large Terebra. Mr. Johnson also found +a fine specimen of Nautilus (Atruria) ziczac (Figure 211), a well-known Falunian +fossil of Europe; and in the same volcanic tuff of Baixo, the Echinoderm Brisus +Scillae, a living Mediterranean species, found fossil in the Miocene strata of +Malta. Mr. Meyer identifies one-third of the Madeira shells with known European +Miocene (or Falunian) forms. The huge Strombus of San Vicente and Porto Santo, +S. Italicus, is an extinct shell of the Sub-apennine or Older Pliocene +formations. The mollusca already obtained from various localities of Madeira and +Porto Santo are not less than one hundred in number, and, according to the late +Dr. S.P. Woodward, rather more than a third are of species still living, but +many of these are not now inhabitants of the neighbouring sea. + +It has been remarked (Chapter 16), that in the Older Pliocene and Upper Miocene +deposits of Europe many forms occur of a more southern aspect than those now +inhabiting the nearest sea. In like manner the fossil corals, or Zoantharia, six +in number, which I obtained from Madeira, of the genera Astraea, Sarcinula, +Hydnophora, were pronounced by Mr. Lonsdale to be forms foreign to the adjacent +coasts, and agreeing with the fauna of a sea warmer than that now separating +Madeira from the nearest part of the African coast. We learn, indeed, from the +observations made in 1859, by the Reverend R.T. Lowe, that more than one-half, +or fifty-three in ninety, of the marine mollusks collected by him from the sandy +beach of Mogador are common British species, although Mogador is 18 1/2 degrees +south of the nearest shores of England. The living shells of Madeira and Porto +Santo are in like manner those of a temperate climate, although in great part +differing specifically from those of Mogador. (Linnean Proceedings Zoology +1860.) + +GRAND CANARY. + +In the Canaries, especially in the Grand Canary, the same marine Upper Miocene +formation is found. Stratified tuffs, with intercalated conglomerates and lavas, +are there seen in nearly horizontal layers in sea-cliffs about 300 feet high, +near Las Palmas. Mr. Hartung and I were unable to find marine shells in these +tuffs at a greater elevation than 400 feet above the sea; but as the deposit to +which they belong reaches to the height of 1100 feet or more in the interior, we +conceive that an upheaval of at least that amount has taken place. The +Clypeaster altus, Spondylus gaederopus, Pectunculus pilosus, Cardita calyculata, +and several other shells, serve to identify this formation with that of the +Madeiras, and Ancillaria glandiformis, which is not rare, and some other +fossils, remind us of the faluns of Touraine. + +The sixty-two Miocene species which I collected in the Grand Canary were +referred by the late Dr. S.P. Woodward to forty-seven genera, ten of which are +no longer represented in the neighbouring sea, namely Corbis, an African form, +Hinnites, now living in Oregon, Thecidium (T. Mediterranean, identical with the +Miocene fossil of St. Juvat, in Brittany), Calyptraea, Hipponyx, Nerita, Erato, +Oliva, Ancillaria, and Fasciolaria. + +These tuffs of the southern shores of the Grand Canary, containing the Upper +Miocene shells, appear to be about the same age as the most ancient volcanic +rocks of the island, composed of slaty diabase, phonolite, and trachyte. Over +the marine lavas and tuffs trachytic and basaltic products of subaerial volcanic +origin, between 4000 and 5000 feet in thickness, have been piled, the central +parts of the Grand Canary reaching the height of about 6000 feet above the level +of the sea. A large portion of this mass is of Pliocene date, and some of the +latest lavas have been poured out since the time when the valleys were already +excavated to within a few feet of their present depth. + +On the whole, the rocks of the Grand Canary, an island of a nearly circular +shape, and 6 1/2 geographical miles diameter, exhibit proofs of a long series of +eruptions beginning like those of Madeira, Porto Santo, and the Azores, in the +Upper Miocene period, and continued to the Post-Pliocene. The building up of the +Grand Canary by subaerial eruptions, several thousand feet thick, went on +simultaneously with the gradual upheaval of the earliest products of submarine +eruptions, in the same manner as the Pliocene marine strata of the oldest parts +of Vesuvius and Etna have been upraised during eruptions of Post-tertiary date. + +In proof that movements of elevation have actually continued down to Post- +tertiary times, I may remark that I found raised beaches containing shells of +the Recent Period in the Grand Canary, Teneriffe, and Porto Santo. The most +remarkable raised beach which I observed in the Grand Canary, in the study of +which I was assisted by Don Pedro Maffiotte, is situated in the north-eastern +part of the island at San Catalina, about a quarter of a mile north of Las +Palmas. It intervenes between the base of the high cliff formed of the tuffs +with Miocene shells and the sea-shore. From this beach, at an elevation of +twenty-five feet above high-water mark, and at a distance of about 150 feet from +the present shore, I obtained more than fifty species of living marine shells. +Many of them, according to Dr. S.P. Woodward, are no longer inhabitants of the +contiguous sea, as, for example, Strombus bubonius, which is still living on the +West Coast of Africa, and Cerithium procerum, found at Mozambique; others are +Mediterranean species, as Pecten Jacobaeus and P. polymorphus. Some of these +testacea, such as Cardita squamosa, are inhabitants of deep water, and the +deposit on the whole seems to indicate a depth of water exceeding a hundred +feet. + +AZORES. + +In the island of St. Mary's, one of the Azores, marine fossil shells have long +been known. They are found on the north-east coast on a small projecting +promontory called Ponta do Papagaio (or Point-Parrot), chiefly in a limestone +about twenty feet thick, which rests upon, and is again covered by, basaltic +lavas, scoriae, and conglomerates. The pebbles in the conglomerate are cemented +together with carbonate of lime. + +Mr. Hartung, in his account of the Azores, published in 1860, describes twenty- +three shells from St. Mary's (Hartung Die Azoren 1860 also Insel Gran Canaria, +Madeira und Porto Santo 1864 Leipsig.), of which eight perhaps are identical +with living species, and twelve are with more or less certainty referred to +European Tertiary forms, chiefly Upper Miocene. One of the most characteristic +and abundant of the new species, Cardium Hartungi, not known as fossil in +Europe, is very common in Porto Santo and Baixo, and serves to connect the +Miocene fauna of the Azores and the Madeiras. In some of the Azores, as well as +in the Canary islands, the volcanic fires are not yet extinct, as the recorded +eruptions of Lanzerote, Teneriffe, Palma, St. Michael's, and others, attest. + +LOWER MIOCENE VOLCANIC ROCKS. + +ISLE OF MULL AND ANTRIM. + +I may refer the reader to the account already given (Chapter 15) of leaf-beds at +Ardtun, in the Isle of Mull in the Hebrides, which bear a relation to the +associated volcanic rocks of Lower Miocene date analogous to that which the +Madeira leaf-bed, above described (Chapter 29), bears to the Pliocene lavas of +that island. Mr. Geikie has shown that the volcanic rocks in Mull are above 3000 +feet in thickness. There seems little doubt that the well-known columnar basalt +of Staffa, as well as that of Antrim in Ireland, are of the same age, and not of +higher antiquity, as once suspected. + +THE EIFEL. + +A large portion of the volcanic rocks of the Lower Rhine and the Eifel are +coeval with the Lower Miocene deposits to which most of the "Brown-Coal" of +Germany belongs. The Tertiary strata of that age are seen on both sides of the +Rhine, in the neighbourhood of Bonn, resting unconformably on highly inclined +and vertical strata of Silurian and Devonian rocks. The Brown-Coal formation of +that region consists of beds of loose sand, sandstone, and conglomerate, clay +with nodules of clay-iron-stone, and occasionally silex. Layers of light brown +and sometimes black lignite are interstratified with the clays and sands, and +often irregularly diffused through them. They contain numerous impressions of +leaves and stems of trees, and are extensively worked for fuel, whence the name +of the formation. In several places layers of trachytic tuff are +interstratified, and in these tuffs are leaves of plants identical with those +found in the brown-coal, showing that, during the period of the accumulation of +the latter, some volcanic products were ejected. The igneous rocks of the +Westerwald, and of the mountains called the Siebengebirge, consist partly of +basaltic and partly of trachytic lavas, the latter being in general the more +ancient of the two. There are many varieties of trachyte, some of which are +highly crystalline, resembling a coarse-grained granite, with large separate +crystals of feldspar. Trachytic tuff is also very abundant. + +M. Von Dechen, in his work on the Siebengebirge, has given a copious list of the +animal and vegetable remains of the fresh-water strata associated with the +brown-coal of that part of Germany. (Geognost. Beschreib. des Siebengebirges am +Rhein Bonn 1852.) Plants of the genera Flabellaria, Ceanothus, and Daphnogene, +including D. cinnamomifolia (Figure 155), occur in these beds, with nearly 150 +other plants. The fishes of the brown-coal near Bonn are found in a bituminous +shale, called paper-coal, from being divisible into extremely thin leaves. The +individuals are very numerous; but they appear to belong to a small number of +species, some of which were referred by Agassiz to the genera Leuciscus, Aspius, +and Perca. The remains of frogs also, of extinct species, have been discovered +in the paper-coal; and a complete series may be seen in the museum at Bonn, from +the most imperfect state of the tadpole to that of the full-grown animal. With +these a salamander, scarcely distinguishable from the recent species, has been +found, and the remains of many insects. + +UPPER AND LOWER MIOCENE VOLCANIC ROCKS OF AUVERGNE. + +The extinct volcanoes of Auvergne and Cantal, in central France, seem to have +commenced their eruptions in the Lower Miocene period, but to have been most +active during the Upper Miocene and Pliocene eras. I have already alluded to the +grand succession of events of which there is evidence in Auvergne since the last +retreat of the sea (see Chapter 29). + +The earliest monuments of the Tertiary Period in that region are lacustrine +deposits of great thickness, in the lowest conglomerates of which are rounded +pebbles of quartz, mica-schist, granite, and other non-volcanic rocks, without +the slightest intermixture of igneous products. To these conglomerates succeed +argillaceous and calcareous marls and limestones, containing Lower Miocene +shells and bones of mammalia, the higher beds of which sometimes alternate with +volcanic tuff of contemporaneous origin. After the filling up or drainage of the +ancient lakes, huge piles of trachytic and basaltic rocks, with volcanic +breccias, accumulated to a thickness of several thousand feet, and were +superimposed upon granite, or the contiguous lacustrine strata. The greater +portion of these igneous rocks appear to have originated during the Upper +Miocene and Pliocene periods; and extinct quadrupeds of those eras, belonging to +the genera Mastodon, Rhinoceros, and others, were buried in ashes and beds of +alluvial sand and gravel, which owe their preservation to overspreading sheets +of lava. + +In Auvergne, the most ancient and conspicuous of the volcanic masses is Mont +Dor, which rests immediately on the granitic rocks standing apart from the +fresh-water strata. This great mountain rises suddenly to the height of several +thousand feet above the surrounding platform, and retains the shape of a +flattened and somewhat irregular cone, the slope of which is gradually lost in +the high plain around. This cone is composed of layers of scoriae, pumice- +stones, and their fine detritus, with interposed beds of trachyte and basalt, +which descend often in uninterrupted sheets until they reach and spread +themselves round the base of the mountain. (Scrope Central France page 98.) +Conglomerates, also, composed of angular and rounded fragments of igneous rocks, +are observed to alternate with the above; and the various masses are seen to dip +off from the central axis, and to lie parallel to the sloping flanks of the +mountain. The summit of Mont Dor terminates in seven or eight rocky peaks, where +no regular crater can now be traced, but where we may easily imagine one to have +existed, which may have been shattered by earthquakes, and have suffered +degradation by aqueous agents. Originally, perhaps, like the highest crater of +Etna, it may have formed an insignificant feature in the great pile, and, like +it, may frequently have been destroyed and renovated. + +Respecting the age of the great mass of Mont Dor, we can not come at present to +any positive decision, because no organic remains have yet been found in the +tuffs, except impressions of the leaves of trees of species not yet determined. +It has already been stated (Chapter 15) that the earliest eruptions must have +been posterior in origin to those grits and conglomerates of the fresh-water +formation of the Limagne which contain no pebbles of volcanic rocks. But there +is evidence at a few points, as in the hill of Gergovia, presently to be +mentioned, that some eruptions took place before the great lakes were drained, +while others occurred after the desiccation of those lakes, and when deep +valleys had already been excavated through fresh-water strata. + +The valley in which the cone of Tartaret, above-mentioned (Chapter 29), is +situated affords an impressive monument of the very different dates at which the +igneous eruptions of Auvergne have happened; for while the cone itself is of +Post-Pliocene date, the valley is bounded by lofty precipices composed of sheets +of ancient columnar trachyte and basalt, which once flowed from the summit of +Mont Dor in some part of the Miocene period. These Miocene lavas had accumulated +to a thickness of nearly 1000 feet before the ravine was cut down to the level +of the river Couze, a river which was at length dammed up by the modern cone and +the upper part of its course transformed into a lake. + +GERGOVIA. + +(FIGURE 604. Hill of Gergovia. +Section through (bottom to top) White and green marls: Altered Marl: Dike: +Altered Marl: Limestone and peperino: Tuffs: Blue marls: White and yellow marl: +Basaltic capping.) + +It has been supposed by some observers that there is an alternation of a +contemporaneous sheet of lava with fresh-water strata in the hill of Gergovia, +near Clermont. But this idea has arisen from the intrusion of the dike +represented in Figure 604, which has altered the green and white marls both +above and below. Nevertheless, there is a real alternation of volcanic tuff with +strata containing Lower Miocene fresh-water shells, among others a Melania +allied to M. inquinata (Figure 217), with a Melanopsis and a Unio; there can, +therefore, be no doubt that in Auvergne some volcanic explosions took place +before the drainage of the lakes, and at a time when the Lower Miocene species +of animals and plants still flourished. + +EOCENE VOLCANIC ROCKS. + +MONTE BOLCA. + +The fissile limestone of Monte Bolca, near Verona, has for many centuries been +celebrated in Italy for the number of perfect Ichthyolites which it contains. +Agassiz has described no less than 133 species of fossil fish from this single +deposit, and the multitude of individuals by which many of the species are +represented is attested by the variety of specimens treasured up in the +principal museums of Europe. They have been all obtained from quarries worked +exclusively by lovers of natural history, for the sake of the fossils. Had the +lithographic stone of Solenhofen, now regarded as so rich in fossils, been in +like manner quarried solely for scientific objects, it would have remained +almost a sealed book to palaeontologists, so sparsely are the organic remains +scattered through it. When I visited Monte Bolca, in company with Sir Roderick +Murchison, in 1828, we ascertained that the fish-bearing beds were of Eocene +date, containing well-known species of Nummulites, and that a long series of +submarine volcanic eruptions, evidently contemporaneous, had produced beds of +tuff, which are cut through by dikes of basalt. There is evidence here of a long +series of submarine volcanic eruptions of Eocene date, and during some of them, +as Sir R. Murchison has suggested, shoals of fish were probably destroyed by the +evolution of heat, noxious gases, and tufaceous mud, just as happened when +Graham's Island was thrown up between Sicily and Africa in 1831, at which time +the waters of the Mediterranean were seen to be charged with red mud, and +covered with dead fish over a wide area. (Principles of Geology chapter 26 9th +edition page 432.) + +Associated with the marls and limestones of Monte Bolca are beds containing +lignite and shale with numerous plants, which have been described by Unger and +Massalongo, and referred by them to the Eocene period. I have already cited +(Chapter 16) Professor Heer's remark, that several of the species are common to +Monte Bolca and the white clay of Alum Bay, a Middle Eocene deposit; and the +same botanist dwells on the tropical character of the flora of Monte Bolca and +its distinctness from the sub-tropical flora of the Lower Miocene of Switzerland +and Italy, in which last there is a far more considerable mixture of forms of a +temperate climate, such as the willow, poplar, birch, elm, and others. That +scarcely any one of the Monte Bolca fish should have been found in any other +locality in Europe, is a striking illustration of the extreme imperfection of +the palaeontological record. We are in the habit of imagining that our insight +into the geology of the Eocene period is more than usually perfect, and we are +certainly acquainted with an almost unbroken succession of assemblages of shells +passing one into the other from the era of the Thanet sands to that of the +Bembridge beds or Paris gypsum. The general dearth, therefore, of fish in the +different members of the Eocene series, Upper, Middle, and Lower, might induce a +hasty reasoner to conclude that there was a poverty of ichthyic forms during +this period; but when a local accident, like the volcanic eruptions of Monte +Bolca, occurs, proofs are suddenly revealed to us of the richness and variety of +this great class of vertebrata in the Eocene sea. The number of genera of Monte +Bolca fish is, according to Agassiz, no less than seventy-five, twenty of them +peculiar to that locality, and only eight common to the antecedent Cretaceous +period. No less than forty-seven out of the seventy-five genera make their +appearance for the first time in the Monte Bolca rocks, none of them having been +met with as yet in the antecedent formations. They form a great contrast to the +fish of the secondary strata, as, with the exception of the Placoids, they are +all Teleosteans, only one genus, Pycnodus, belonging to the order of Ganoids, +which form, as before stated, the vast majority of the ichthyolites entombed in +the secondary are Mesozoic rocks. + +CRETACEOUS PERIOD. + +M. Virlet, in his account of the geology of the Morea, page 205, has clearly +shown that certain traps in Greece are of Cretaceous date; as those, for +example, which alternate conformably with cretaceous limestone and greensand +between Kastri and Damala, in the Morea. They consist in great part of diallage +rocks and serpentine, and of an amygdaloid with calcareous kernels, and a base +of serpentine. In certain parts of the Morea, the age of these volcanic rocks is +established by the following proofs: first, the lithographic limestones of the +Cretaceous era are cut through by trap, and then a conglomerate occurs, at +Nauplia and other places, containing in its calcareous cement many well-known +fossils of the chalk and greensand, together with pebbles formed of rolled +pieces of the same serpentinous trap, which appear in the dikes above alluded +to. + +PERIOD OF OOLITE AND LIAS. + +Although the green and serpentinous trap-rocks of the Morea belong chiefly to +the Cretaceous era, as before mentioned, yet it seems that some eruptions of +similar rocks began during the Oolitic period (Boblaye and Virlet Morea page +23.); and it is probable that a large part of the trappean masses, called +ophiolites in the Apennines, and associated with the limestone of that chain, +are of corresponding age. + +TRAP OF THE NEW RED SANDSTONE PERIOD. + +In the southern part of Devonshire, trappean rocks are associated with New Red +Sandstone, and, according to Sir H. De la Beche, have not been intruded +subsequently into the sandstone, but were produced by contemporaneous volcanic +action. Some beds of grit, mingled with ordinary red marl, resemble sands +ejected from a crater; and in the stratified conglomerates occurring near +Tiverton are many angular fragments of trap porphyry, some of them one or two +tons in weight, intermingled with pebbles of other rocks. These angular +fragments were probably thrown out from volcanic vents, and fell upon +sedimentary matter then in the course of deposition. (De la Beche Geological +Proceedings volume 2 page 198.) + +TRAP OF THE PERMIAN PERIOD. + +The recent investigations of Mr. Archibald Geikie in Ayrshire have shown that +some of the volcanic rocks in that county are of Permian age, and it appears +highly probable that the uppermost portion of Arthur's Seat in the suburbs of +Edinburgh marks the site of an eruption of the same era. + +TRAP OF THE CARBONIFEROUS PERIOD. + +Two classes of contemporaneous trap-rocks occur in the coal-field of the Forth, +in Scotland. The newest of these, connected with the higher series of coal- +measures, is well exhibited along the shores of the Forth, in Fifeshire, where +they consist of basalt with olivine, amygdaloid, greenstone, wacke, and tuff. +They appear to have been erupted while the sedimentary strata were in a +horizontal position, and to have suffered the same dislocations which those +strata have subsequently undergone. In the volcanic tuffs of this age are found +not only fragments of limestone, shale, flinty slate, and sandstone, but also +pieces of coal. The other or older class of carboniferous traps are traced along +the south margin of Stratheden, and constitute a ridge parallel with the Ochils, +and extending from Stirling to near St. Andrews. They consist almost exclusively +of greenstone, becoming, in a few instances, earthy and amygdaloidal. They are +regularly interstratified with the sandstone, shale, and iron-stone of the lower +coal-measures, and, on the East Lomond, with Mountain Limestone. I examined +these trap-rocks in 1838, in the cliffs south of St. Andrews, where they consist +in great part of stratified tuffs, which are curved, vertical, and contorted, +like the associated coal-measures. In the tuff I found fragments of +carboniferous shale and limestone, and intersecting veins of greenstone. + +FIFE-- FLISK DIKE. + +A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk, in the +northern part of the county of Fife, which cuts through the grey sandstone and +shale, forming the lowest part of the Old Red Sandstone, but which may probably +be of carboniferous date. It may be traced for many miles, passing through the +amygdaloidal and other traps of the hill called Norman's Law in that parish. In +its course it affords a good exemplification of the passage from the trappean +into the Plutonic, or highly crystalline texture. Professor Gustavus Rose, to +whom I submitted specimens of this dike, found it to be dolerite, and composed +of greenish black augite and Labrador feldspar, the latter being the most +abundant ingredient. A small quantity of magnetic iron, perhaps titaniferous, is +also present. The result of this analysis is interesting, because both the +ancient and modern lavas of Etna consist in like manner of augite, Labradorite, +and titaniferous iron. + +ERECT TREES BURIED IN VOLCANIC ASH AT ARRAN. + +An interesting discovery was made in 1867 by Mr. E.A. Wunsch in the +carboniferous strata of the north-eastern part of the island of Arran. In the +sea-cliff about five miles north of Corrie, near the village of Laggan, strata +of volcanic ash occur, forming a solid rock cemented by carbonate of lime and +enveloping trunks of trees, determined by Mr. Binney to belong to the genera +Sigillaria and Lepidodendron. Some of these trees are at right angles to the +planes of stratification, while others are prostrate and accompanied by leaves +and fruits of the same genera. I visited the spot in company with Mr. Wunsch in +1870, and saw that the trees with their roots, of which about fourteen had been +observed, occur at two distinct levels in volcanic tuffs parallel to each other, +and inclined at an angle of about 40 degrees, having between them beds of shale +and coaly matter seven feet thick. It is evident that the trees were overwhelmed +by a shower of ashes from some neighbouring volcanic vent, as Pompeii was buried +by matter ejected from Vesuvius. The trunks, several of them from three to five +feet in circumference, remained with their Stigmarian roots spreading through +the stratum below, which had served as a soil. The trees must have continued for +years in an upright position after they were killed by the shower of burning +ashes, giving time for a partial decay of the interior, so as to afford hollow +cylinders into which the spores of plants were wafted. These spores germinated +and grew, until finally their stems were petrified by carbonate of lime like +some of the remaining portions of the wood of the containing Sigillaria. Mr. +Carruthers has discovered that sometimes the plants which had thus grown and +become fossil in the inside of a single trunk belonged to several distinct +genera. The fact that the tree-bearing deposits now dip at an angle of 40 +degrees is the more striking, as they must clearly have remained horizontal and +undisturbed during a long period of intermittent and contemporaneous volcanic +action. + +In some of the associated carboniferous shales, ferns and calamites occur, and +all the phenomena of the successive buried forests remind us of the sections in +Figures 439 and 440 of the Nova Scotia coal-measures, with this difference only, +that in the case of the South Joggins the fossilisation of the trees was +effected without the eruption of volcanic matter. + +TRAP OF THE OLD RED SANDSTONE PERIOD. + +By referring to the section explanatory of the structure of Forfarshire, already +given (Chapter 5), the reader will perceive that beds of conglomerate, No. 3, +occur in the middle of the Old Red Sandstone system, 1, 2, 3, 4. The pebbles in +these conglomerates are sometimes composed of granitic and quartzose rocks, +sometimes exclusively of different varieties of trap, which last, although +purposely omitted in the section referred to, is often found either intruding +itself in amorphous masses and dikes into the old fossiliferous tilestones, No. +4, or alternating with them in conformable beds. All the different divisions of +the red sandstone, 1, 2, 3, 4, are occasionally intersected by dikes, but they +are very rare in Nos. 1 and 2, the upper members of the group consisting of red +shale and red sandstone. These phenomena, which occur at the foot of the +Grampians, are repeated in the Sidlaw Hills; and it appears that in this part of +Scotland volcanic eruptions were most frequent in the earlier part of the Old +Red Sandstone period. The trap-rocks alluded to consist chiefly of feldspathic +porphyry and amygdaloid, the kernels of the latter being sometimes calcareous, +often chalcedonic, and forming beautiful agates. We meet also with claystone, +greenstone, compact feldspar, and tuff. Some of these rocks look as if they had +flowed as lavas over the bottom of the sea, and enveloped quartz pebbles which +were lying there, so as to form conglomerates with a base of greenstone, as is +seen in Lumley Den, in the Sidlaw Hills. On either side of the axis of this +chain of hills (see Figure 55), the beds of massive trap, and the tuffs composed +of volcanic sand and ashes, dip regularly to the south-east or north-west, +conformably with the shales and sandstones. + +But the geological structure of the Pentland Hills, near Edinburgh, shows that +igneous rocks were there formed during the newer part of the Devonian or "Old +Red" period. These hills are 1900 feet high above the sea, and consist of +conglomerates and sandstones of Upper Devonian age, resting on the inclined +edges of grits and slates of Lower Devonian and Upper Silurian date. The +contemporaneous volcanic rocks intercalated in this Upper Old Red consist of +feldspathic lavas, or feldstones, with associated tuffs or ashy beds. The lavas +were some of them originally compact, others vesicular, and these last have been +converted into amygdaloids. They consist chiefly of feldstone or compact +feldspar. The Pentland Hills, say Messrs. Maclaren and Geikie, afford evidence +that at the time of the Upper Old Red Sandstone, the district to the south-west +of Edinburgh was for a long while the seat of a powerful volcano, which sent out +massive streams of lava and showers of ash, and continued active until well-nigh +the dawn of the Carboniferous period. (Maclaren Geology of Fife and Lothians. +Geikie Transactions of the Royal Society Edinburgh 1860-1861.) + +SILURIAN VOLCANIC ROCKS. + +It appears from the investigations of Sir R. Murchison in Shropshire, that when +the Lower Silurian strata of that country were accumulating, there were frequent +volcanic eruptions beneath the sea; and the ashes and scoriae then ejected gave +rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to the other +rocks of the Silurian series, and only observable in places where syenitic and +other trap-rocks protrude. These tuffs occur on the flanks of the Wrekin and +Caer Caradoc, and contain Silurian fossils, such as casts of encrinites, +trilobites, and mollusca. Although fossiliferous, the stone resembles a sandy +claystone of the trap family. (Murchison Silurian System etc. page 230.) + +Thin layers of trap, only a few inches thick, alternate in some parts of +Shropshire and Montgomeryshire with sedimentary strata of the Lower Silurian +system. This trap consists of slaty porphyry and granular feldspar rock, the +beds being traversed by joints like those in the associated sandstone, +limestone, and shale, and having the same strike and dip. (Ibid. page 212.) + +In Radnorshire there is an example of twelve bands of stratified trap, +alternating with Silurian schists and flagstones, in a thickness of 350 feet. +The bedded traps consist of feldspar porphyry, and other varieties; and the +interposed Llandeilo flags are of sandstone and shale, with trilobites and +graptolites. (Murchison Silurian System etc. page 325.) + +The Snowdonian hills in Carnarvonshire consist in great part of volcanic tuffs, +the oldest of which are interstratified with the Bala and Llandeilo beds. There +are some contemporaneous feldspathic lavas of this era, which, says Professor +Ramsay, alter the slates on which they repose, having doubtless been poured out +over them, in a melted state, whereas the slates which overlie them having been +subsequently deposited after the lava had cooled and consolidated, have entirely +escaped alteration. But there are greenstones associated with the same +formation, which, although they are often conformable to the slates, are in +reality intrusive rocks. They alter the stratified deposits both above and below +them, and when traced to great distances are sometimes seen to cut through the +slates, and to send off branches. Nevertheless, these greenstones appear to +belong, like the lavas, to the Lower Silurian period. + +CAMBRIAN VOLCANIC ROCKS. + +The Lingula beds in North Wales have been described as 5000 feet in thickness. +In the upper portion of these deposits volcanic tuffs or ashy materials are +interstratified with ordinary muddy sediment, and here and there associated with +thick beds of feldspathic lava. These rocks form the mountains called the Arans +and the Arenigs; numerous greenstones are associated with them, which are +intrusive, although they often run in the lines of bedding for a space. "Much of +the ash," says Professor Ramsay, "seems to have been subaerial. Islands, like +Graham's Island, may have sometimes raised their craters for various periods +above the water, and by the waste of such islands some of the ashy matter became +waterworn, whence the ashy conglomerate. Viscous matter seems also to have been +shot into the air as volcanic bombs, which fell among the dust and broken +crystals (that often form the ashes) before perfect cooling and consolidation +had taken place." (Quarterly Geological Journal volume 9 page 170 1852.) + +LAURENTIAN VOLCANIC ROCKS. + +The Laurentian rocks in Canada, especially in Ottawa and Argenteuil, are the +oldest intrusive masses yet known. They form a set of dikes of a fine-grained +dark greenstone or dolerite, composed of feldspar and pyroxene, with occasional +scales of mica and grains of pyrites. Their width varies from a few feet to a +hundred yards, and they have a columnar structure, the columns being truly at +right angles to the plane of the dike. Some of the dikes send off branches. +These dolerites are cut through by intrusive syenite, and this syenite, in its +turn, is again cut and penetrated by feldspar porphyry, the base of which +consists of petrosilex, or a mixture of orthoclase and quartz. All these trap- +rocks appear to be of Laurentian date, as the Cambrian and Huronian rocks rest +unconformably upon them. (Logan Geology of Canada 1863.) Whether some of the +various conformable crystalline rocks of the Laurentian series, such as the +coarse-grained granitoid and porphyritic varieties of gneiss, exhibiting +scarcely any signs of stratification, and some of the serpentines, may not also +be of volcanic origin, is a point very difficult to determine in a region which +has undergone so much metamorphic action. + + +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. + +The Plutonic rocks may be treated of next in order, as they are most nearly +allied to the volcanic class already considered. I have described, in the first +chapter, these Plutonic rocks as the unstratified division of the crystalline or +hypogene formations, and have stated that 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, whether +thrown up into the air or the sea. They differ also by the absence of pores or +cellular cavities, to which the expansion of the entangled gases gives rise in +ordinary lava, never being scoriaceous or amygdaloidal, and never forming a +porphyry with an uncrystalline base, nor alternating with tuffs. + +From these and other peculiarities it has been inferred that the granites have +been formed at considerable depths in the earth, and have cooled and +crystallised slowly under great pressure, where the contained gases could not +expand. The volcanic rocks, on the contrary, although they also have risen up +from below, have cooled from a melted state more rapidly upon or near the +surface. From this hypothesis of the great depth at which the granites +originated, has been derived the name of "Plutonic rocks." The beginner will +easily conceive that the influence of subterranean heat may extend downward from +the crater of every active volcano to a great depth below, perhaps several miles +or leagues, and the effects which are produced deep in the bowels of the earth +may, or rather must, be distinct; so that volcanic and Plutonic rocks, each +different in texture, and sometimes even in composition, may originate +simultaneously, the one at the surface, the other far beneath it. The Plutonic +formations also agree with the volcanic in having veins or ramifications +proceeding from central masses into the adjoining rocks, and causing alterations +in these last, which will be presently described. They also resemble trap in +containing no organic remains; but they differ in being more uniform in texture, +whole mountain masses of indefinite extent appearing to have originated under +conditions precisely similar. + +The two principal members of the Plutonic family of rocks are Granite and +Syenite, each of which, with their varieties, bear very much the same relation +to each other as the trachytes bear to the basalts. Granite is a compound of +feldspar, quartz, and mica, the feldspars being rich in silica, which forms from +60 to 70 per cent of the whole aggregate. In Syenite quartz is rare or wanting, +hornblende taking the place of mica, and the proportion of silica not exceeding +50 to 60 per cent. + +(FIGURE 605. Mass of granite near the Sharp Tor, Cornwall.) + +(FIGURE 606. Granite having a cuboidal and rude columnar structure, Land's End, +Cornwall.) + +Granite often preserves a very uniform character throughout a wide range of +territory, forming hills of a peculiar rounded form, usually clad with a scanty +vegetation. The surface of the rock is for the most part in a crumbling state, +and the hills are often surmounted by piles of stones like the remains of a +stratified mass, as in Figure 605, and sometimes like heaps of boulders, for +which they have been mistaken. The exterior of these stones, originally +quadrangular, acquires a rounded form by the action of air and water, for the +edges and angles waste away more rapidly than the sides. A similar spherical +structure has already been described as characteristic of basalt and other +volcanic formations, and it must be referred to analogous causes, as yet but +imperfectly understood. Although it is the general peculiarity of granite to +assume no definite shapes, it is nevertheless occasionally subdivided by +fissures, so as to assume a cuboidal, and even a columnar, structure. Examples +of these appearances may be seen near the Land's End, in Cornwall. (See Figure +606.) + +(FIGURES 607 and 608. Graphic granite. + +(FIGURE 607. Graphic granite. Section parallel to the laminae.) + +(FIGURE 608. Graphic granite. Section transverse to the laminae.)) + +Feldspar, quartz, and mica are usually considered as the minerals essential to +granite, the feldspar being most abundant in quantity, and the proportion of +quartz exceeding that of mica. These minerals are united in what is termed a +confused crystallisation; that is to say, there is no regular arrangement of the +crystals in granite, as in gneiss (see Figure 622), except in the variety termed +graphic granite, which occurs mostly in granitic veins. This variety is a +compound of feldspar and quartz, so arranged as to produce an imperfect laminar +structure. The crystals of feldspar appear to have been first formed, leaving +between them the space now occupied by the darker-coloured quartz. This mineral, +when a section is made at right angles to the alternate plates of feldspar and +quartz, presents broken lines, which have been compared to Hebrew characters. +(See Figure 608.) The variety of granite called by the French Pegmatite, which +is a mixture of quartz and common feldspar, usually with some small admixture of +white silvery mica, often passes into graphic granite. + +Ordinary granite, as well as syenite and eurite, usually contains two kinds of +feldspar: First, the common, or orthoclase, in which potash is the prevailing +alkali, and this generally occurs in large crystals of a white or flesh colour; +and secondly, feldspar in smaller crystals, in which soda predominates, usually +of a dead white or spotted, and striated like albite, but not the same in +composition. (Delesse Ann. des Mines 1852 tome 3 page 409 and 1848 tome 13 page +675.) + +As a general rule, quartz, in a compact or amorphous state, forms a vitreous +mass, serving as the base in which feldspar and mica have crystallised; for +although these minerals are much more fusible than silex, they have often +imprinted their shapes upon the quartz. This fact, apparently so paradoxical, +has given rise to much ingenious speculation. We should naturally have +anticipated that, during the cooling of the mass, the flinty portion would be +the first to consolidate; and that the different varieties of feldspar, as well +as garnets and tourmalines, being more easily liquefied by heat, would be the +last. Precisely the reverse has taken place in the passage of most granite +aggregates from a fluid to a solid state, crystals of the more fusible minerals +being found enveloped in hard, transparent, glassy quartz, which has often taken +very faithful casts of each, so as to preserve even the microscopically minute +striations on the surface of prisms of tourmaline. Various explanations of this +phenomenon have been proposed by MM. de Beaumont, Fournet, and Durocher. They +refer to M. Gaudin's experiments on the fusion of quartz, which show that silex, +as it cools, has the property of remaining in a viscous state, whereas alumina +never does. This "gelatinous flint" is supposed to retain a considerable degree +of plasticity long after the granitic mixture has acquired a low temperature. +Occasionally we find the quartz and feldspar mutually imprinting their forms on +each other, affording evidence of the simultaneous crystallisation of both. +(Bulletin 2e serie 4 1304; and d'Archiac Hist. des Progres de la Geol. 1 38.) + +According to the experiments and observations of Gustavus Rose, the quartz of +granite has the specific gravity of 2.6, which characterises silica when it is +precipitated from a liquid solvent, and not that inferior density, namely, 2.3, +which belongs to it when it cools in the laboratory from a state of fusion in +what is called the dry way. By some it had been rashly inferred that the manner +in which the consolidation of granite takes place is exceedingly different from +the cooling of lavas, and that the intense heat supposed to be necessary for the +production of mountain masses of Plutonic rocks might be dispensed with. But Mr. +David Forbes informs me that silica can crystallise in the dry way, and he has +found in quartz forming a constituent part of some trachytes, both from +Guadeloupe and Iceland, glass cavities quite similar to those met with in +genuine volcanic minerals. + +These "glass cavities," which with many other kindred phenomena have been +carefully studied by Mr. Sorby, are those in which a liquid, on cooling, has +become first viscous and then solid without crystallising or undergoing a +definite change in its physical structure. Other cavities which, like those just +mentioned, are frequently discernible under the microscope in the minerals +composing granitic rocks, are filled, some of them with gas or vapour, others +with liquid, and by the movements of the bubbles thus included the distinctness +of such cavities from those filled with a glassy substance can be tested. Mr. +Sorby admits that the frequent occurrence of fluid cavities in the quartz of +granite implies that water was almost always present in the formation of this +rock; but the same may be said of almost all lavas, and it is now more than +forty years since Mr. Scrope insisted on the important part which water plays in +volcanic eruptions, being so intimately mixed up with the materials of the lava +that he supposed it to aid in giving mobility to the fluid mass. It is well +known that steam escapes for months, sometimes for years, from the cavities of +lava when it is cooling and consolidating. As to the result of Mr. Sorby's +experiments and speculations on this difficult subject, they may be stated in a +few words. He concludes that the physical conditions under which the volcanic +and granitic rocks originate are so far similar that in both cases they combine +igneous fusion, aqueous solution, and gaseous sublimation-- the proof, he says, +of the operation of water in the formation of granite being quite as strong as +of that of heat. (See Quarterly Geological Journal volume 14 pages 465, 488.) + +When rocks are melted at great depths water must be present, for two reasons-- +First, because rainwater and seawater are always descending through fissured and +porous rocks, and must at length find their way into the regions of subterranean +heat; and secondly, because in a state of combination water enters largely into +the composition of some of the most common minerals, especially those of the +aluminous class. But the existence of water under great pressure affords no +argument against our attributing an excessively high temperature to the mass +with which it is mixed up. Bunsen, indeed, imagines that in Iceland water +attains a white heat at a very moderate depth. To what extent some of the +metamorphic rocks containing the same minerals as the granites may have been +formed by hydrothermal action without the intervention of intense heat +comparable to that brought into play in a volcanic eruption, will be considered +when we treat of the metamorphic rocks in the thirty-third chapter. + +PORPHYRITIC GRANITE. + +(FIGURE 609. Porphyritic granite. Land's End, Cornwall.) + +This name has been sometimes given to that variety in which large crystals of +common feldspar, sometimes more than three inches in length, are scattered +through an ordinary base of granite. An example of this texture may be seen in +the granite of the Land's End, in Cornwall (Figure 609). The two larger +prismatic crystals in this drawing represent feldspar, smaller crystals of which +are also seen, similar in form, scattered through the base. In this base also +appear black specks of mica, the crystals of which have a more or less perfect +hexagonal outline. The remainder of the mass is quartz, the translucency of +which is strongly contrasted to the opaqueness of the white feldspar and black +mica. But neither the transparency of the quartz nor the silvery lustre of the +mica can be expressed in the engraving. + +The uniform mineral character of large masses of granite seems to indicate that +large quantities of the component elements were thoroughly mixed up together, +and then crystallised under precisely similar conditions. There are, however, +many accidental, or "occasional," minerals, as they are termed, which belong to +granite. Among these black schorl or tourmaline, actinolite, zircon, garnet, and +fluor spar are not uncommon; but they are too sparingly dispersed to modify the +general aspect of the rock. They show, nevertheless, that the ingredients were +not everywhere exactly the same; and a still greater difference may be traced in +the ever-varying proportions of the feldspar, quartz, and mica. + +TALCOSE GRANITE + +Talcose Granite, or Protogine of the French, is a mixture of feldspar, quartz, +and talc. It abounds in the Alps, and in some parts of Cornwall, producing by +its decomposition the kaolin or china clay, more than 12,000 tons of which are +annually exported from that country for the potteries. + +SCHORL-ROCK, AND SCHORLY GRANITE. + +The former of these is an aggregate of schorl, or tourmaline, and quartz. When +feldspar and mica are also present, it may be called schorly granite. This kind +of granite is comparatively rare. + +EURITE, FELDSTONE. + +Eurite is a rock in which the ingredients of granite are blended into a finely +granular mass, mica being usually absent, and, when present, in such minute +flakes as to be invisible to the naked eye. It is sometimes called FELDSTONE, +and when the crystals of feldspar are conspicuous it becomes FELDSPAR PORPHYRY. +All these and other varieties of granite pass into certain kinds of trap-- a +circumstance which affords one of many arguments in favour of what is now the +prevailing opinion, that the granites are also of igneous origin. The contrast +of the most crystalline form of granite to that of the most common and earthy +trap is undoubtedly great; but each member of the volcanic class is capable of +becoming porphyritic, and the base of the porphyry may be more and more +crystalline, until the mass passes to the kind of granite most nearly allied in +mineral composition. + +SYENITIC GRANITE. + +The quadruple compound of quartz, feldspar, mica, and hornblende, may be termed +Syenitic Granite, and forms a passage between the granites and the syenites. +This rock occurs in Scotland and in Guernsey. + +SYENITE. + +We now come to the second division of the Plutonic rocks, or those having less +than 60 per cent of silica, and which, as before stated, are usually called +syenitic. Syenite originally received its name from the celebrated ancient +quarries of Syene, in Egypt. It differs from granite in having hornblende as a +substitute for mica, and being without quartz. Werner at least considered +syenite as a binary compound of feldspar and hornblende, and regarded quartz as +merely one of its occasional minerals. + +MIASCITE. + +Miascite is one of the varieties of syenite most frequently spoken of; it is +composed chiefly of orthoclase and nepheline, with hornblende and quartz as +occasional accessory minerals. It derives its name from Miask, in the Ural +Mountains, where it was first discovered by Gustavus Rose. ZIRCON-SYENITE is +another variety closely allied to Miascite, but containing crystals of Zircon. + +CONNECTION OF THE GRANITES AND SYENITES WITH THE VOLCANIC ROCKS. + +The minerals which constitute alike the Plutonic and volcanic rocks consist, +almost exclusively, of seven elements, namely, silica, alumina, magnesia, lime, +soda, potash, and iron (see Table 28.1); and these may sometimes exist in about +the same proportions in a porous lava, a compact trap, and a crystalline +granite. The same lava, for example, may be glassy, or scoriaceous, or stony, or +porphyritic, according to the more or less rapid rate at which it cools. + +It would be easy to multiply examples and authorities to prove the gradation of +the Plutonic into the trap rocks. On the western side of the Fiord of +Christiania, in Norway, there is a large district of trap, chiefly greenstone- +porphyry and syenitic-greenstone, resting on fossiliferous strata. To this, on +its southern limit, succeeds a region equally extensive of syenite, the passage +from the trappean to the crystalline Plutonic rock being so gradual that it is +impossible to draw a line of demarkation between them. + +"The ordinary granite of Aberdeenshire," says Dr. MacCulloch, "is the usual +ternary compound of quartz, feldspar, and mica; though sometimes hornblende is +substituted for the mica. But in many places a variety occurs which is composed +simply of feldspar and hornblende; and in examining more minutely this duplicate +compound, it is observed in some places to assume a fine grain, and at length to +become undistinguishable from the greenstones of the trap family. It also passes +in the same uninterrupted manner into a basalt, and at length into a soft +claystone, with a schistose tendency on exposure, in no respect differing from +those of the trap islands of the western coast." The same author mentions, that +in Shetland a granite composed of hornblende, mica, feldspar, and quartz +graduates in an equally perfect manner into basalt. (System of Geology volume 1 +pages 157 and 158.) In Hungary there are varieties of trachyte, which, +geologically speaking, are of modern origin, in which crystals, not only of +mica, but of quartz, are common, together with feldspar and hornblende. It is +easy to conceive how such volcanic masses may, at a certain depth from the +surface, pass downward into granite. + +GRANITIC VEINS. + +(Figures 610 and 611. Junction of granite and argillaceous schist in Glen Tilt. +(MacCulloch. (Geological Transactions First Series volume 3 plate 21.)) + +(FIGURE 610. Junction of granite and argillaceous schist in Glen Tilt.) + +(FIGURE 611. Junction of granite and argillaceous schist in Glen Tilt.)) + +I have already hinted at the close analogy in the forms of certain granitic and +trappean veins; and it will be found that strata penetrated by Plutonic rocks +have suffered changes very similar to those exhibited near the contact of +volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating strata of limestone +and argillaceous schist come in contact with a mass of granite. The contact does +not take place as might have been looked for if the granite had been formed +there before the strata were deposited, in which case the section would have +appeared as in Figure 610; but the union is as represented in Figure 611, the +undulating outline of the granite intersecting different strata, and +occasionally intruding itself in torturous veins into the beds of clay-slate and +limestone, from which it differs so remarkably in composition. The limestone is +sometimes changed in character by the proximity of the granitic mass or its +veins, and acquires a more compact texture, like that of hornstone or chert, +with a splintery fracture, and effervescing freely with acids. + +The conversion of the limestone and these and many other instances into a +siliceous rock, effervescing slowly with acids, would be difficult of +explanation, were it not ascertained that such limestones are always impure, +containing grains of quartz, mica, or feldspar disseminated through them. The +elements of these minerals, when the rock has been subjected to great heat, may +have been fused, and so spread more uniformly through the whole mass. + +(FIGURE 612. Granite veins traversing clay slate, Table Mountain, Cape of Good +Hope. (Captain B. Hall Transactions of the Royal Society of Edinburgh volume +7.)) + +(FIGURE 613. Granite veins traversing gneiss, Cape Wrath. (MacCulloch (Western +Islands plate 31.)) + +In the Plutonic, as in the volcanic rocks, there is every gradation from a +torturous vein to the most regular form of a dike, such as intersect the tuffs +and lavas of Vesuvius and Etna. Dikes of granite may be seen, among other +places, on the southern flank of Mount Battock, one of the Grampians, the +opposite walls sometimes preserving an exact parallelism for a considerable +distance. As a general rule, however, granite veins in all quarters of the globe +are more sinuous in their course than those of trap. They present similar shapes +at the most northern point of Scotland, and the southernmost extremity of +Africa, as Figures 612 and 613 will show. + +It is not uncommon for one set of granite veins to intersect another; and +sometimes there are three sets, as in the environs of Heidelberg, where the +granite on the banks of the river Necker is seen to consist of three varieties, +differing in colour, grain, and various peculiarities of mineral composition. +One of these, which is evidently the second in age, is seen to cut through an +older granite; and another, still newer, traverses both the second and the +first. In Shetland there are two kinds of granite. One of them, composed of +hornblende, mica, feldspar, and quartz, is of a dark colour, and is seen +underlying gneiss. The other is a red granite, which penetrates the dark variety +everywhere in veins. (MacCulloch System of Geology volume 2 page 58.) + +(FIGURE 614. Granite veins passing through hornblende slate, Carnsilver Cove, +Cornwall.) + +Figure 614 is a sketch of a group of granite veins in Cornwall, given by Messrs. +Von Oeynhausen and Von Dechen. (Philosophical Magazine and Annals No. 27 New +Series March 1829.) The main body of the granite here is of a porphyritic +appearance, with large crystals of feldspar; but in the veins it is fine- +grained, and without these large crystals. The general height of the veins is +from 16 to 20 feet, but some are much higher. + +Granite, syenite, and those porphyries which have a granitiform structure, in +short all Plutonic rocks, are frequently observed to contain metals, at or near +their junction with stratified formations. On the other hand, the veins which +traverse stratified rocks are, as a general law, more metalliferous near such +junctions than in other positions. Hence it has been inferred that these metals +may have been spread in a gaseous form through the fused mass, and that the +contact of another rock, in a different state of temperature, or sometimes the +existence of rents in other rocks in the vicinity, may have caused the +sublimation of the metals. (Necker Proceedings of the Geological Society No. 26 +page 392.) + +(FIGURE 615. a, b. Quartz vein passing through gneiss and greenstone. Tronstad +Strand, near Christiania.) + +Veins of pure quartz are often found in granite as in many stratified rocks, but +they are not traceable, like veins of granite or trap, to large bodies of rock +of similar composition. They appear to have been cracks, into which siliceous +matter was infiltered. Such segregation, as it is called, can sometimes clearly +be shown to have taken place long subsequently to the original consolidation of +the containing rock. Thus, for example, I observed in the gneiss of Tronstad +Strand, near Drammen, in Norway, the section on the beach shown in Figure 615. +It appears that the alternating strata of whitish granitiform gneiss and black +hornblende-schist were first cut by a greenstone dike, about 2 1/2 feet wide; +then the crack a-b passed through all these rocks, and was filled up with +quartz. The opposite walls of the vein are in some parts incrusted with +transparent crystals of quartz, the middle of the vein being filled up with +common opaque white quartz. + +(FIGURE 616. Euritic porphyry alternating with primary fossiliferous strata, +near Christiania.) + +We have seen that the volcanic formations have been called overlying, because +they not only penetrate others but spread over them. M. Necker has proposed to +call the granites the underlying igneous rocks, and the distinction here +indicated is highly characteristic. It was, indeed, supposed by some of the +earlier observers that the granite of Christiania, in Norway, was intercalated +in mountain masses between the primary or palaeozoic strata of that country, so +as to overlie fossiliferous shale and limestone. But although the granite sends +veins into these fossiliferous rocks, and is decidedly posterior in origin, its +actual superposition in mass has been disproved by Professor Keilhau, whose +observations on this controverted point I had opportunities, in 1837, of +verifying. There are, however, on a smaller scale, certain beds of euritic +porphyry, some a few feet, others many yards in thickness, which pass into +granite, and deserve, perhaps, to be classed as Plutonic rather than trappean +rocks, which may truly be described as interposed conformably between +fossiliferous strata, as the porphyries (a, c, Figure 616) which divide the +bituminous shales and argillaceous limestones, f, f. But some of these same +porphyries are partially unconformable, as b, and may lead us to suspect that +the others also, notwithstanding their appearance of interstratification, have +been forcibly injected. Some of the porphyritic rocks above mentioned are highly +quartzose, others very feldspathic. In proportion as the masses are more +voluminous, they become more granitic in their texture, less conformable, and +even begin to send forth veins into contiguous strata. In a word, we have here a +beautiful illustration of the intermediate gradations between volcanic and +Plutonic rocks, not only in their mineralogical composition and structure, but +also in their relations of position to associated formations. If the term +"overlying" can in this instance be applied to a Plutonic rock, it is only in +proportion as that rock begins to acquire a trappean aspect. + +It has been already hinted that the heat which in every active volcano extends +downward to indefinite depths must produce simultaneously very different effects +near the surface and far below it; and we can not suppose that rocks resulting +from the crystallising of fused matter under a pressure of several thousand +feet, much less several miles, of the earth's crust can exactly resemble those +formed at or near the surface. Hence the production at great depths of a class +of rocks analogous to the volcanic, and yet differing in many particulars, might +have been predicted, even had we no Plutonic formations to account for. How well +these agree, both in their positive and negative characters, with the theory of +their deep subterranean origin, the student will be able to judge by considering +the descriptions already given. + +It has, however, been objected, that if the granitic and volcanic rocks were +simply different parts of one great series, we ought to find in mountain chains +volcanic dikes passing upward into lava and downward into granite. But we may +answer that our vertical sections are usually of small extent; and if we find in +certain places a transition from trap to porous lava, and in others a passage +from granite to trap, it is as much as could be expected of this evidence. + +The prodigious extent of denudation which has been already demonstrated to have +occurred at former periods, will reconcile the student to the belief that +crystalline rocks of high antiquity, although deep in the earth's crust when +originally formed, may have become uncovered and exposed at the surface. Their +actual elevation above the sea may be referred to the same causes to which we +have attributed the upheaval of marine strata, even to the summits of some +mountain chains. + + +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. + +When we adopt the igneous theory of granite, as explained in the last chapter, +and believe that different Plutonic rocks have originated at successive periods +beneath the surface of the planet, we must be prepared to encounter greater +difficulty in ascertaining the precise age of such rocks than in the case of +volcanic and fossiliferous formations. We must bear in mind that the evidence of +the age of each contemporaneous volcanic rock was derived either from lavas +poured out upon the ancient surface, whether in the sea or in the atmosphere, or +from tuffs and conglomerates, also deposited at the surface, and either +containing organic remains themselves or intercalated between strata containing +fossils. But the same tests entirely fail, or are only applicable in a modified +degree, when we endeavour to fix the chronology of a rock which has crystallised +from a state of fusion in the bowels of the earth. In that case we are reduced +to the tests of relative position, intrusion, alteration of the rocks in +contact, included fragments, and mineral character; but all these may yield at +best a somewhat ambiguous result. + +TEST OF AGE BY RELATIVE POSITION. + +Unaltered fossiliferous strata of every age are met with reposing immediately on +Plutonic rocks; as at Christiania, in Norway, where the Post-pliocene deposits +rest on granite; in Auvergne, where the fresh-water Miocene strata, and at +Heidelberg, on the Rhine, where the New Red sandstone occupy a similar place. In +all these, and similar instances, inferiority in position is connected with the +superior antiquity of granite. The crystalline rock was solid before the +sedimentary beds were superimposed, and the latter usually contain in them +rounded pebbles of the subjacent granite. + +TEST BY INTRUSION AND ALTERATION. + +But when Plutonic rocks send veins into strata, and alter them near the point of +contact, in the manner before described (Chapter 31), it is clear that, like +intrusive traps, they are newer than the strata which they invade and alter. +Examples of the application of this test will be given in the sequel. + +TEST BY MINERAL COMPOSITION. + +Notwithstanding a general uniformity in the aspect of Plutonic rocks, we have +seen in the last chapter that there are many varieties, such as syenite, talcose +granite, and others. One of these varieties is sometimes found exclusively +prevailing throughout an extensive region, where it preserves a homogeneous +character; so that, having ascertained its relative age in one place, we can +recognise its identity in others, and thus determine from a single section the +chronological relations of large mountain masses. Having observed, for example, +that the syenitic granite of Norway, in which the mineral called zircon abounds, +has altered the Silurian strata wherever it is in contact, we do not hesitate to +refer other masses of the same zircon-syenite in the south of Norway to a post- +Silurian date. Some have imagined that the age of different granites might, to a +great extent, be determined by their mineral characters alone; syenite, for +instance, or granite with hornblende, being more modern than common or micaceous +granite. But modern investigations have proved these generalisations to have +been premature. + +TEST BY INCLUDED FRAGMENTS. + +This criterion can rarely be of much importance, because the fragments involved +in granite are usually so much altered that they can not be referred with +certainty to the rocks whence they were derived. In the White Mountains, in +North America, according to Professor Hubbard, a granite vein, traversing +granite, contains fragments of slate and trap which must have fallen into the +fissure when the fused materials of the vein were injected from below +(Silliman's Journal No. 69 page 123.), and thus the granite is shown to be newer +than those slaty and trappean formations from which the fragments were derived. + +RECENT AND PLIOCENE PLUTONIC ROCKS, WHY INVISIBLE. + +The explanations already given in the 28th and in the last chapter of the +probable relation of the Plutonic to the volcanic formations, will naturally +lead the reader to infer that rocks of the one class can never be produced at or +near the surface without some members of the other being formed below. It is not +uncommon for lava-streams to require more than ten years to cool in the open +air; and where they are of great depth, a much longer period. The melted matter +poured from Jorullo, in Mexico, in the year 1759, which accumulated in some +places to the height of 550 feet, was found to retain a high temperature half a +century after the eruption. (See Principles Index Jorullo.) We may conceive, +therefore, that great masses of subterranean lava may remain in a red-hot or +incandescent state in the volcanic foci for immense periods, and the process of +refrigeration may be extremely gradual. Sometimes, indeed, this process may be +retarded for an indefinite period by the accession of fresh supplies of heat; +for we find that the lava in the crater of Stromboli, one of the Lipari Islands, +has been in a state of constant ebullition for the last two thousand years; and +we may suppose this fluid mass to communicate with some caldron or reservoir of +fused matter below. In the Isle of Bourbon, also, where there has been an +emission of lava once in every two years for a long period, the lava below can +scarcely fail to have been permanently in a state of liquefaction. If then it be +a reasonable conjecture, that about 2000 volcanic eruptions occur in the course +of every century, either above the waters of the sea or beneath them (Ibid. +Volcanic Eruptions.), it will follow that the quantity of Plutonic rock +generated or in progress during the Recent epoch must already have been +considerable. + +But as the Plutonic rocks originate at some depth in the earth's crust, they can +only be rendered accessible to human observation by subsequent upheaval and +denudation. Between the period when a Plutonic rock crystallises in the +subterranean regions and the era of its protrusion at any single point of the +surface, one or two geological periods must usually intervene. Hence, we must +not expect to find the Recent or even the Pliocene granites laid open to view, +unless we are prepared to assume that sufficient time has elapsed since the +commencement of the Pliocene period for great upheaval and denudation. A +Plutonic rock, therefore, must, in general, be of considerable antiquity +relatively to the fossiliferous and volcanic formations, before it becomes +extensively visible. As we know that the upheaval of land has been sometimes +accompanied in South America by volcanic eruptions and the emission of lava, we +may conceive the more ancient Plutonic rocks to be forced upward to the surface +by the newer rocks of the same class formed successively below-- subterposition +in the Plutonic, like superposition in the sedimentary rocks, being usually +characteristic of a newer origin. + +(FIGURE 617. Diagram showing the relative position which the Plutonic and +sedimentary formations of different ages may occupy. +I. Primary Plutonic rocks. +II. Secondary Plutonic rocks. +III. Tertiary Plutonic rocks. +IV. Post-tertiary Plutonic rocks. +1. Primary fossiliferous or Palaeozoic strata. +2. Secondary or Mesozoic strata. +3. Tertiary or Cainozoic strata. +4. Post-tertiary strata. +The metamorphic rocks are not indicated in this diagram: but the student will +infer, from what is said in Chapters 31 and 33, that some portions of the +stratified formations, Nos. 1 and 2, invaded by granite, will have become +metamorphic.) + +In Figure 617 an attempt is made to show the inverted order in which sedimentary +and Plutonic formations may occur in the earth's crust. The oldest Plutonic +rock, No. I, has been upheaved at successive periods until it has become exposed +to view in a mountain-chain. This protrusion of No. I has been caused by the +igneous agency which produced the newer Plutonic rocks Nos. II, III and IV. Part +of the primary fossiliferous strata, No. I, have also been raised to the surface +by the same gradual process. It will be observed that the Recent STRATA No. 4 +and the Recent GRANITE or Plutonic rock No. IV are the most remote from each +other in position, although of contemporaneous date. According to this +hypothesis, the convulsions of many periods will be required before Recent or +Post-tertiary granite will be upraised so as to form the highest ridges and +central axes of mountain-chains. During that time the RECENT strata No. 4 might +be covered by a great many newer sedimentary formations. + +MIOCENE PLUTONIC ROCKS. + +A considerable mass of syenite, in the Isle of Skye, is described by Dr. +MacCulloch as intersecting limestone and shale, which are of the age of the +lias. The limestone, which at a greater distance from the granite contains +shells, exhibits no traces of them near its junction, where it has been +converted into a pure crystalline marble. (Western Islands volume 1 page 330.) +MacCulloch pointed out that the syenite here, as in Raasay, was newer than the +secondary rocks, and Mr. Geikie has since shown that there is a strong +probability that this Plutonic rock may be of Miocene age, because a similar +Syenite having a true granitic character in its crystallisation has modified the +Tertiary volcanic rocks of Ben More, in Mull, some of which have undergone +considerable metamorphism. + +EOCENE PLUTONIC ROCKS. + +In a former part of this volume (Chapter 16), the great nummulitic formation of +the Alps and Pyrenees was referred to the Eocene period, and it follows that +vast movements which have raised those fossiliferous rocks from the level of the +sea to the height of more than 10,000 feet above its level have taken place +since the commencement of the Tertiary epoch. Here, therefore, if anywhere, we +might expect to find hypogene formations of Eocene date breaking out in the +central axis or most disturbed region of the loftiest chain in Europe. +Accordingly, in the Swiss Alps, even the flysch, or upper portion of the +nummulitic series, has been occasionally invaded by Plutonic rocks, and +converted into crystalline schists of the hypogene class. There can be little +doubt that even the talcose granite or gneiss of Mont Blanc itself has been in a +fused or pasty state since the flysch was deposited at the bottom of the sea; +and the question as to its age is not so much whether it be a secondary or +tertiary granite or gneiss, as whether it should be assigned to the Eocene or +Miocene epoch. + +Great upheaving movements have been experienced in the region of the Andes, +during the Post-tertiary period. In some part, therefore, of this chain, we may +expect to discover tertiary Plutonic rocks laid open to view; and Mr. Darwin's +account of the Chilian Andes, to which the reader may refer, fully realises this +expectation: for he shows that we have strong ground to presume that Plutonic +rocks there exposed on a large scale are of later date than certain Secondary +and Tertiary formations. + +But the theory adopted in this work of the subterranean origin of the hypogene +formations would be untenable, if the supposed fact here alluded to, of the +appearance of tertiary granite at the surface, was not a rare exception to the +general rule. A considerable lapse of time must intervene between the formation +of Plutonic and metamorphic rocks in the nether regions and their emergence at +the surface. For a long series of subterranean movements must occur before such +rocks can be uplifted into the atmosphere or the ocean; and, before they can be +rendered visible to man, some strata which previously covered them must have +been stripped off by denudation. + +We know that in the Bay of Baiae in 1538, in Cutch in 1819, and on several +occasions in Peru and Chili, since the commencement of the present century, the +permanent upheaval or subsidence of land has been accompanied by the +simultaneous emission of lava at one or more points in the same volcanic region. +From these and other examples it may be inferred that the rising or sinking of +the earth's crust, operations by which sea is converted into land, and land into +sea, are a part only of the consequences of subterranean igneous action. It can +scarcely be doubted that this action consists, in a great degree, of the baking, +and occasionally the liquefaction, of rocks, causing them to assume, in some +cases a larger, in others a smaller volume than before the application of heat. +It consists also in the generation of gases, and their expansion by heat, and +the injection of liquid matter into rents formed in superincumbent rocks. The +prodigious scale on which these subterranean causes have operated in Sicily +since the deposition of the Newer Pliocene strata will be appreciated when we +remember that throughout half the surface of that island such strata are met +with, raised to the height of from 50 to that of 2000 and even 3000 feet above +the level of the sea. In the same island also the older rocks which are +contiguous to these marine tertiary strata must have undergone, within the same +period, a similar amount of upheaval. + +The like observations may be extended to nearly the whole of Europe, for, since +the commencement of the Eocene Period, the entire European area, including some +of the central and very lofty portions of the Alps themselves, as I have +elsewhere shown, has, with the exception of a few districts, emerged from the +deep to its present altitude. (See map of Europe, and explanation, in Principles +book 1.) There must, therefore, have been at great depths in the earth's crust, +within the same period, an amount of subterranean change corresponding to this +vast alteration of level affecting a whole continent. + +The principal effect of subterranean movements during the Tertiary Period seems +to have consisted in the upheaval of hypogene formations of an age anterior to +the Carboniferous. The repetition of another series of movements, of equal +violence, might upraise the Plutonic and metamorphic rocks of many secondary +periods; and, if the same force should still continue to act, the next +convulsions might bring up to the day the TERTIARY and RECENT hypogene rocks. In +the course of such changes many of the existing sedimentary strata would suffer +greatly by denudation, others might assume a metamorphic structure, or become +melted down into Plutonic and volcanic rocks. Meanwhile the deposition of a +great thickness of new strata would not fail to take place during the upheaval +and partial destruction of the older rocks. But I must refer the reader to the +last chapter but one of this volume for a fuller explanation of these views. + +PLUTONIC ROCKS OF CRETACEOUS PERIOD. + +(FIGURE 618. Section through three layers (b, c, d) of the Cretaceous series +over granite (A).) + +It will be shown in the next chapter that chalk, as well as lias, has been +altered by granite in the eastern Pyrenees. Whether such granite be cretaceous +or tertiary, can not easily be decided. Suppose b, c, d, Figure 618, to be three +members of the Cretaceous series, the lowest of which, b, has been altered by +the granite A, the modifying influence not having extended so far as c, or +having but slightly affected its lowest beds. Now it can rarely be possible for +the geologist to decide whether the beds d existed at the time of the intrusion +of A, and alteration of b and c, or whether they were subsequently thrown down +upon c. But as some Cretaceous and even Tertiary rocks have been raised to the +height of more than 9000 feet in the Pyrenees, we must not assume that plutonic +formations of the same periods may not have been brought up and exposed by +denudation, at the height of 2000 or 3000 feet on the flanks of that chain. + +PLUTONIC ROCKS OF THE OOLITE AND LIAS. + +(FIGURE 619. Junction of granite with Jurassic or Oolite strata in the Alps, +near Champoleon. (Granite over Altered Rocks over Secondary Schists.)) + +In the Department of the Hautes Alpes, in France, M. Eliede Beaumont traced a +black argillaceous limestone, charged with belemnites, to within a few yards of +a mass of granite. Here the limestone begins to put on a granular texture, but +is extremely fine-grained. When nearer the junction it becomes grey, and has a +saccharoid structure. In another locality, near Champoleon, a granite composed +of quartz, black mica, and rose-coloured feldspar is observed partly to overlie +the secondary rocks, producing an alteration which extends for about 30 feet +downward, diminishing in the beds which lie farthest from the granite. (See +Figure 619.) In the altered mass the argillaceous beds are hardened, the +limestone is saccharoid, the grits quartzose, and in the midst of them is a thin +layer of an imperfect granite. It is also an important circumstance that near +the point of contact, both the granite and the secondary rocks become +metalliferous, and contain nests and small veins of blende, galena, iron, and +copper pyrites. The stratified rocks become harder and more crystalline, but the +granite, on the contrary, softer and less perfectly crystallised near the +junction. (Elie de Beaumont sur les Montagnes de l'Oisans etc. Mem. de la Soc. +d'Hist. Nat. de Paris tome 5.) Although the granite is incumbent in the section +(Figure 619), we can not assume that it overflowed the strata, for the +disturbances of the rocks are so great in this part of the Alps that their +original position is often inverted. + +At Predazzo, in the Tyrol, secondary strata, some of which are limestones of the +Oolitic period, have been traversed and altered by Plutonic rocks, one portion +of which is an augitic porphyry, which passes insensibly into granite. The +limestone is changed into granular marble, with a band of serpentine at the +junction. (Von Buch Annales de Chimie etc.) + +PLUTONIC ROCKS OF CARBONIFEROUS PERIOD. + +The granite of Dartmoor, in Devonshire, was formerly supposed to be one of the +most ancient of the Plutonic rocks, but is now ascertained to be posterior in +date to the culm-measures of that county, which from their position, and, as +containing true coal-plants, are now known to be members of the true +Carboniferous series. This granite, like the syenitic granite of Christiania, +has broken through the stratified formations, on the north-west side of +Dartmoor, the successive members of the culm-measures abutting against the +granite, and becoming metamorphic as they approach. These strata are also +penetrated by granite veins, and Plutonic dikes, called "elvans." (Proceedings +of the Geological Society volume 2 page 562 and Transactions second series +volume 5 page 686.) The granite of Cornwall is probably of the same date, and, +therefore, as modern as the Carboniferous strata, if not newer. + +PLUTONIC ROCKS OF SILURIAN PERIOD. + +(FIGURE 620. Section through Silurian strata and Granite.) + +It has long been known that a very ancient granite near Christiania, in Norway, +is posterior in date to the Lower Silurian strata of that region, although its +exact position in the Palaeozoic series can not be defined. Von Buch first +announced, in 1813, that it was of newer origin than certain limestones +containing orthocerata and trilobites. The proofs consist in the penetration of +granite veins into the shale and limestone, and the alteration of the strata, +for a considerable distance from the point of contact, both of these veins and +the central mass from which they emanate. (See Chapter 31.)Von Buch supposed +that the Plutonic rock alternated with the fossiliferous strata, and that large +masses of granite were sometimes incumbent upon the strata; but this idea was +erroneous, and arose from the fact that the beds of shale and limestone often +dip towards the granite up to the point of contact, appearing as if they would +pass under it in mass, as at a, Figure 620, and then again on the opposite side +of the same mountain, as at b, dip away from the same granite. When the +junctions, however, are carefully examined, it is found that the Plutonic rock +intrudes itself in veins, and nowhere covers the fossiliferous strata in large +overlying masses, as is so commonly the case with trappean formations. (See the +Gaea Norvegica and other works of Keilhau with whom I examined this country.) + +Now this granite, which is more modern than the Silurian strata of Norway, also +sends veins in the same country into an ancient formation of gneiss; and the +relations of the Plutonic rock and the gneiss, at their junction, are full of +interest when we duly consider the wide difference of epoch which must have +separated their origin. + +(FIGURE 621. Granite sending veins into Silurian strata and gneiss. Christiania, +Norway. +a. Inclined gneiss. +b. Silurian strata.) + +The length of this interval of time is attested by the following facts: The +fossiliferous, or Silurian, beds rest unconformably upon the truncated edges of +the gneiss, the inclined strata of which had been denuded before the sedimentary +beds were superimposed (see Figure 621). The signs of denudation are twofold; +first, the surface of the gneiss is seen occasionally, on the removal of the +newer beds containing organic remains, to be worn and smoothed; secondly, +pebbles of gneiss have been found in some of these Silurian strata. Between the +origin, therefore, of the gneiss and the granite there intervened, first, the +period when the strata of gneiss were denuded; secondly, the period of the +deposition of the Silurian deposits upon the denuded and inclined gneiss, a. Yet +the granite produced after this long interval is often so intimately blended +with the ancient gneiss, at the point of junction, that it is impossible to draw +any other than an arbitrary line of separation between them; and where this is +not the case, tortuous veins of granite pass freely through gneiss, ending +sometimes in threads, as if the older rock had offered no resistance to their +passage. These appearances may probably be due to hydrothermal action (see +Chapter 33). I shall merely observe in this place that had such junctions alone +been visible, and had we not learnt, from other sections, how long a period +elapsed between the consolidation of the gneiss and the injection of this +granite, we might have suspected that the gneiss was scarcely solidified, or had +not yet assumed its complete metamorphic character when invaded by the Plutonic +rock. From this example we may learn how impossible it is to conjecture whether +certain granites in Scotland, and other countries, which send veins into gneiss +and other metamorphic rocks, are primary, or whether they may not belong to some +secondary or tertiary period. + +OLDEST GRANITES. + +It is not half a century since the doctrine was very general that all granitic +rocks were PRIMITIVE, that is to say, that they originated before the deposition +of the first sedimentary strata, and before the creation of organic beings (see +above Chapter 1). But so greatly are our views now changed, that we find it no +easy task to point out a single mass of granite demonstrably more ancient than +known fossiliferous deposits. Could we discover some Laurentian strata resting +immediately on granite, there being no alterations at the point of contact, nor +any intersecting granitic veins, we might then affirm the Plutonic rock to have +originated before the oldest known fossiliferous strata. Still it would be +presumptuous, as we have already pointed out (Chapter 26), to suppose that when +a small part only of the globe has been investigated, we are acquainted with the +oldest fossiliferous strata in the crust of our planet. Even when these are +found, we can not assume that there never were any antecedent strata containing +organic remains, which may have become metamorphic. If we find pebbles of +granite in a conglomerate of the Lower Laurentian system, we may then feel +assured that the parent granite was formed before the Laurentian formation. But +if the incumbent strata be merely Cambrian or Silurian, the fundamental granite, +although of high antiquity, may be posterior in date to KNOWN fossiliferous +formations. + +PROTRUSION OF SOLID GRANITE. + +In part of Sutherlandshire, near Brora, common granite, composed of feldspar, +quartz, and mica is in immediate contact with Oolitic strata, and has clearly +been elevated to the surface at a period subsequent to the deposition of those +strata. (Murchison Geological Transactions second series volume 2 page 307.) +Professor Sedgwick and Sir R. Murchison conceive that this granite has been +upheaved in a solid form; and that in breaking through the submarine deposits, +with which it was not perhaps originally in contact, it has fractured them so as +to form a breccia along the line of junction. This breccia consists of fragments +of shale, sandstone, and limestone, with fossils of the oolite, all united +together by a calcareous cement. The secondary strata at some distance from the +granite are but slightly disturbed, but in proportion to their proximity the +amount of dislocation becomes greater. + +Mr. T. McKenney Hughes has suggested to me in explanation of these phenomena +that they may be the effect of the association of more pliant strata with hard +unyielding rocks, the whole of which were subjected simultaneously to great +movements, whether of elevation or subsidence, and of lateral pressure, during +which the more solid granite, being incapable of compression, was forced through +the softer beds of shale, sandstone, and limestone. He remarks that similar +breccias with slickensides are observed on a minor scale where rocks of +different composition and rigidity are contorted together. Such protrusion may +have been brought about by degrees by innumerable shocks of earthquakes repeated +after long intervals of time along the same tract of country. The opening of new +fissures in the hardest rocks is a frequent accompaniment of such convulsions, +and during the consequent vibrations, breccias must often be caused. But these +catastrophes, as we well know, do not imply that the land or sea of the +disturbed region are rendered uninhabitable by living beings, and by no means +indicate a state of things different from that witnessed in the ordinary course +of nature. + + +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. + +We have now considered three distinct classes of rocks: first, the aqueous, or +fossiliferous; secondly, the volcanic; and, thirdly, the Plutonic; and it +remains for us to examine those crystalline (or hypogene) strata to which the +name of METAMORPHIC has been assigned. The last-mentioned term expresses, as +before explained, a theoretical opinion that such strata, after having been +deposited from water, acquired, by the influence of heat and other causes, a +highly crystalline texture. They who still question this opinion may call the +rocks under consideration the stratified hypogene formations or crystalline +schists. + +These rocks, when in their characteristic or normal state, are wholly devoid of +organic remains, and contain no distinct fragments of other rocks, whether +rounded or angular. They sometimes break out in the central parts of mountain +chains, but in other cases extend over areas of vast dimensions, occupying, for +example, nearly the whole of Norway and Sweden, where, as in Brazil, they appear +alike in the lower and higher grounds. However crystalline these rocks may +become in certain regions, they never, like granite or trap, send veins into +contiguous formations. In Great Britain, those members of the series which +approach most nearly to granite in their composition, as gneiss, mica-schist, +and hornblende-schist, are confined to the country north of the rivers Forth and +Clyde. + +Many attempts have been made to trace a general order of succession or +superposition in the members of this family; clay-slate, for example, having +been often supposed to hold invariably a higher geological position than mica- +schist, and mica-schist to overlie gneiss. But although such an order may +prevail throughout limited districts, it is by no means universal. To this +subject, however, I shall again revert, in Chapter 35, where the chronological +relations of the metamorphic rocks are pointed out. + +PRINCIPAL METAMORPHIC ROCKS. + +The following may be enumerated as the principal members of the metamorphic +class:-- gneiss, mica-schist, hornblende-schist, clay-slate, chlorite-schist, +hypogene or metamorphic limestone, and certain kinds of quartz-rock or +quartzite. + +GNEISS. + +(FIGURE 622. Fragment of gneiss, natural size; section made at right angles to +the planes of foliation.) + +The first of these, gneiss, may be called stratified-- or by those who object to +that term, foliated-- granite, being formed of the same materials as granite, +namely, feldspar, quartz, and mica. In the specimen in Figure 622, the white +layers consist almost exclusively of granular feldspar, with here and there a +speck of mica and grain of quartz. The dark layers are composed of grey quartz +and black mica, with occasionally a grain of feldspar intermixed. The rock +splits most easily in the plane of these darker layers, and the surface thus +exposed is almost entirely covered with shining spangles of mica. The +accompanying quartz, however, greatly predominates in quantity, but the most +ready cleavage is determined by the abundance of mica in certain parts of the +dark layer. Instead of consisting of these thin laminae, gneiss is sometimes +simply divided into thick beds, in which the mica has only a slight degree of +parallelism to the planes of stratification. + +Hand specimens may often be obtained from such gneiss which are +undistinguishable from granite, affording an argument to which we shall allude +in the concluding part of this chapter, in favour of those who regard all +granite and syenite not as igneous rocks, but as aqueous formations so altered +as to have lost all signs of their original stratified arrangement. Gneiss in +geology is commonly used to designate not merely stratified and foliated rocks +having the same component materials as granite or syenite, but also in a wider +sense to embrace the formation with which other members of the metamorphic +series, such as hornblende-schist, may alternate, and which are then considered +subordinate to the true gneiss. + +The different varieties of rock allied to gneiss, into which feldspar enters as +an essential ingredient, will be understood by referring to what was said of +granite. Thus, for example, hornblende may be superadded to mica, quartz, and +feldspar, forming a hornblendic or syenitic gneiss; or talc may be substituted +for mica, constituting talcose gneiss (called stratified protogine by the +French), a rock composed of feldspar, quartz, and talc, in distinct crystals or +grains. + +EURITE, which has already been mentioned as a Plutonic rock, occurs also with +precisely the same composition in beds subordinate to gneiss or mica-slate. + +HORNBLENDE-SCHIST is usually black, and composed principally of hornblende, with +a variable quantity of feldspar, and sometimes grains of quartz. When the +hornblende and feldspar are in nearly equal quantities, and the rock is not +slaty, it corresponds in character with the greenstones of the trap family, and +has been called "primitive greenstone." It may be termed hornblende rock, or +amphibolite. Some of these hornblendic masses may really have been volcanic +rocks, which have since assumed a more crystalline or metamorphic texture. + +SERPENTINE is a greenish rock, a silicate of magnesia, in which there is +sometimes from 30 to 40 per cent of magnesia. It enters largely into the +composition of a trap dike cutting through Old Red Sandstone in Forfarshire, and +in that case is probably an altered basaltic dike which had contained much +olivine. The theory of its having been originally a volcanic product +subsequently altered by metamorphism may at first sight seem inconsistent with +its occurrence in large and regularly stratified masses in the metamorphic +series in Scotland, as in Aberdeenshire. But it has been suggested in +explanation that such serpentine may have been originally regularly-bedded trap +tuff, and volcanic breccia, with much olivine, which would still retain a +stratified appearance after their conversion into a metamorphic rock. + +ACTINOLITE SCHIST is a slaty foliated rock, composed chiefly of actinolite, an +emerald-green mineral, allied to hornblende, with some admixture of garnet, +mica, and quartz. + +MICA-SCHIST or MICACEOUS SCHIST is, next to gneiss, one of the most abundant +rocks of the metamorphic series. It is slaty, essentially composed of mica and +quartz, the mica sometimes appearing to constitute the whole mass. Beds of pure +quartz also occur in this formation. In some districts, garnets in regular +twelve-sided crystals form an integrant part of mica-schist. This rock passes by +insensible gradations into clay-slate. + +CLAY-SLATE-- ARGILLACEOUS SCHIST-- ARGILLITE. + +This rock sometimes resembles an indurated clay or shale. It is for the most +part extremely fissile, often affording good roofing-slate. Occasionally it +derives a shining and silky lustre from the minute particles of mica or talc +which it contains. It varies from greenish or bluish-grey to a lead colour; and +it may be said of this, more than of any other schist, that it is common to the +metamorphic and fossiliferous series, for some clay-slates taken from each +division would not be distinguishable by mineral characters alone. It is not +uncommon to meet with an argillaceous rock having the same composition, without +the slaty cleavage, which may be called argillite. + +CHLORITE SCHIST is a green slaty rock, in which chlorite is abundant in +foliated plates, usually blended with minute grains of quartz, or sometimes with +feldspar or mica; often associated with, and graduating into, gneiss and clay- +slate. + +QUARTZITE, or QUARTZ ROCK, is an aggregate of grains of quartz which are either +in minute crystals, or in many cases slightly rounded, occurring in regular +strata, associated with gneiss or other metamorphic rocks. Compact quartz, like +that so frequently found in veins, is also found together with granular +quartzite. Both of these alternate with gneiss or mica-schist, or pass into +those rocks by the addition of mica, or of feldspar and mica. + +CRYSTALLINE, OR METAMORPHIC LIMESTONE. + +This hypogene rock, called by the earlier geologists PRIMARY LIMESTONE, is +sometimes a white crystalline granular marble, which when in thick beds can be +used in sculpture; but more frequently it occurs in thin beds, forming a +foliated schist much resembling in colour and arrangement certain varieties of +gneiss and mica-schist. When it alternates with these rocks, it often contains +some crystals of mica, and occasionally quartz, feldspar, hornblende, talc, +chlorite, garnet, and other minerals. It enters sparingly into the structure of +the hypogene districts of Norway, Sweden, and Scotland, but is largely developed +in the Alps. + +ORIGIN OF THE METAMORPHIC STRATA. + +Having said thus much of the mineral composition of the metamorphic rocks, I may +combine what remains to be said of their structure and history with an account +of the opinions entertained of their probable origin. At the same time, it may +be well to forewarn the reader that we are here entering upon ground of +controversy, and soon reach the limits where positive induction ends, and beyond +which we can only indulge in speculations. It was once a favourite doctrine, and +is still maintained by many, that these rocks owe their crystalline texture, +their want of all signs of a mechanical origin, or of fossil contents, to a +peculiar and nascent condition of the planet at the period of their formation. +The arguments in refutation of this hypothesis will be more fully considered +when I show, in Chapter 35, to how many different ages the metamorphic +formations are referable, and how gneiss, mica-schist, clay-slate, and hypogene +limestone (that of Carrara, for example) have been formed, not only since the +first introduction of organic beings into this planet, but even long after many +distinct races of plants and animals had flourished and passed away in +succession. + +The doctrine respecting the crystalline strata implied in the name metamorphic +may properly be treated of in this place; and we must first inquire whether +these rocks are really entitled to be called stratified in the strict sense of +having been originally deposited as sediment from water. The general adoption by +geologists of the term stratified, as applied to these rocks, sufficiently +attests their division into beds very analogous, at least in form, to ordinary +fossiliferous strata. This resemblance is by no means confined to the existence +in both occasionally of a laminated structure, but extends to every kind of +arrangement which is compatible with the absence of fossils, and of sand, +pebbles, ripple-mark, and other characters which the metamorphic theory supposes +to have been obliterated by Plutonic action. Thus, for example, we behold alike +in the crystalline and fossiliferous formations an alternation of beds varying +greatly in composition, colour, and thickness. We observe, for instance, gneiss +alternating with layers of black hornblende-schist or of green chlorite-schist, +or with granular quartz or limestone; and the interchange of these different +strata may be repeated for an indefinite number of times. In the like manner, +mica-schist alternates with chlorite-schist, and with beds of pure quartz or of +granular limestone. We have already seen that, near the immediate contact of +granitic veins and volcanic dikes, very extraordinary alterations in rocks have +taken place, more especially in the neighbourhood of granite. It will be useful +here to add other illustrations, showing that a texture undistinguishable from +that which characterises the more crystalline metamorphic formations has +actually been superinduced in strata once fossiliferous. + +FOSSILIFEROUS STRATA RENDERED METAMORPHIC BY INTRUSIVE MASSES OF GRANITE. + +(FIGURE 623. Ground-plan of altered slate and limestone near granite. +Christiania. The arrows indicate the dip, and the oblique lines the strike of +the beds.) + +In the southern extremity of Norway there is a large district, on the west side +of the fiord of Christiania, which I visited in 1837 with the late Professor +Keilhau, in which syenitic granite protrudes in mountain masses through +fossiliferous strata, and usually sends veins into them at the point of contact. +The stratified rocks, replete with shells and zoophytes, consist chiefly of +shale, limestone, and some sandstone, and all these are invariably altered near +the granite for a distance of from 50 to 400 yards. The aluminous shales are +hardened, and have become flinty. Sometimes they resemble jasper. Ribboned +jasper is produced by the hardening of alternate layers of green and chocolate- +coloured schist, each stripe faithfully representing the original lines of +stratification. Nearer the granite the schist often contains crystals of +hornblende, which are even met with in some places for a distance of several +hundred yards from the junction; and this black hornblende is so abundant that +eminent geologists, when passing through the country, have confounded it with +the ancient hornblende-schist, subordinate to the great gneiss formation of +Norway. Frequently, between the granite and the hornblende-slate above- +mentioned, grains of mica and crystalline feldspar appear in the schist, so that +rocks resembling gneiss and mica-schist are produced. Fossils can rarely be +detected in these schists, and they are more completely effaced in proportion to +the more crystalline texture of the beds, and their vicinity to the granite. In +some places the siliceous matter of the schist becomes a granular quartz; and +when hornblende and mica are added, the altered rock loses its stratification, +and passes into a kind of granite. The limestone, which at points remote from +the granite is of an earthy texture and blue colour, and often abounds in +corals, becomes a white granular marble near the granite, sometimes siliceous, +the granular structure extending occasionally upward of 400 yards from the +junction; the corals being for the most part obliterated, though sometimes +preserved, even in the white marble. Both the altered limestone and hardened +slate contain garnets in many places, also ores of iron, lead, and copper, with +some silver. These alterations occur equally whether the granite invades the +strata in a line parallel to the general strike of the fossiliferous beds, or in +a line at right angles to their strike, both of which modes of junction will be +seen by the ground-plan in Figure 623. (Keilhau Gaea Norvegica pages 61-63.) + +The granite of Cornwall sends forth veins into a coarse argillaceous-schist, +provincially termed killas. This killas is converted into hornblende-schist near +the contact with the veins. These appearances are well seen at the junction of +the granite and killas, in St. Michael's Mount, a small island nearly 300 feet +high, situated in the bay, at a distance of about three miles from Penzance. The +granite of Dartmoor, in Devonshire, says Sir H. De la Beche, has intruded itself +into the Carboniferous slate and slaty sandstone, twisting and contorting the +strata, and sending veins into them. Hence some of the slate rocks have become +"micaceous; others more indurated, and with the characters of mica-slate and +gneiss; while others again appear converted into a hard zoned rock strongly +impregnated with feldspar." (Geological Manual page 479.) + +We learn from the investigation of M. Dufrenoy that in the eastern Pyrenees +there are mountain masses of granite posterior in date to the formations called +lias and chalk of that district, and that these fossiliferous rocks are greatly +altered in texture, and often charged with iron-ore, in the neighbourhood of the +granite. Thus in the environs of St. Martin, near St. Paul de Fenouillet, the +chalky limestone becomes more crystalline and saccharoid as it approaches the +granite, and loses all trace of the fossils which it previously contained in +abundance. At some points, also, it becomes dolomitic, and filled with small +veins of carbonate of iron, and spots of red iron-ore. At Rancie the lias +nearest the granite is not only filled with iron-ore, but charged with pyrites, +tremolite, garnet, and a new mineral somewhat allied to feldspar, called, from +the place in the Pyrenees where it occurs, "couzeranite." + +"Hornblende-schist," says Dr. MacCulloch, "may at first have been mere clay; for +clay or shale is found altered by trap into Lydian stone, a substance differing +from hornblende-schist almost solely in compactness and uniformity of texture." +(System of Geology volume 1 pages 210, 211.) "In Shetland," remarks the same +author, "argillaceous-schist (or clay-slate), when in contact with granite, is +sometimes converted into hornblende-schist, the schist becoming first siliceous, +and ultimately, at the contact, hornblende-schist." In like manner gneiss and +mica-schist may be nothing more than altered micaceous and argillaceous +sandstones, granular quartz may have been derived from siliceous sandstone, and +compact quartz from the same materials. Clay-slate may be altered shale, and +granular marble may have originated in the form of ordinary limestone, replete +with shells and corals, which have since been obliterated; and, lastly, +calcareous sands and marls may have been changed into impure crystalline +limestones. + +The anthracite and plumbago associated with hypogene rocks may have been coal; +for not only is coal converted into anthracite in the vicinity of some trap +dikes, but we have seen that a like change has taken place generally even far +from the contact of igneous rocks, in the disturbed region of the Appalachians. +At Worcester, in the State of Massachusetts, 45 miles due west of Boston, a bed +of plumbago and impure anthracite occurs, interstratified with mica-schist. It +is about two feet in thickness, and has been made use of both as fuel, and in +the manufacture of lead pencils. At the distance of 30 miles from the plumbago, +there occurs, on the borders of Rhode Island, an impure anthracite in slates +containing impressions of coal-plants of the genera Pecopteris, Neuropteris, +Calamites, etc. This anthracite is intermediate in character between that of +Pennsylvania and the plumbago of Worcester, in which last the gaseous or +volatile matter (hydrogen, oxygen, and nitrogen) is to the carbon only in the +proportion of three per cent. After traversing the country in various +directions, I came to the conclusion that the carboniferous shales or slates +with anthracite and plants, which in Rhode Island often pass into mica-schists, +have at Worcester assumed a perfectly crystalline and metamorphic texture; the +anthracite having been nearly transmuted into that state of pure carbon which is +called plumbago or graphite. (See Lyell Quarterly Geological Journal volume 1 +page 199.) + +Now the alterations above described as superinduced in rocks by volcanic dikes +and granite veins prove incontestably that powers exist in nature capable of +transforming fossiliferous into crystalline strata, a very few simple elements +constituting the component materials common to both classes of rocks. These +elements, which are enumerated in Table 28.1, may be made to form new +combinations by what has been termed Plutonic action, or those chemical changes +which are no doubt connected with the passage of heat, unusually heated steam +and waters, through the strata. + +HYDROTHERMAL ACTION, OR THE INFLUENCE OF STEAM AND GASES IN PRODUCING +METAMORPHISM. + +The experiments of Gregory Watt, in fusing rocks in the laboratory, and allowing +them to consolidate by slow cooling, prove distinctly that a rock need not be +perfectly melted in order that a re-arrangement of its component particles +should take place, and a partial crystallisation ensue. (Philosophical +Transactions 1804.) We may easily suppose, therefore, that all traces of shells +and other organic remains may be destroyed, and that new chemical combinations +may arise, without the mass being so fused as that the lines of stratification +should be wholly obliterated. We must not, however, imagine that heat alone, +such as may be applied to a stone in the open air, can constitute all that is +comprised in Plutonic action. We know that volcanoes in eruption not only emit +fluid lava, but give off steam and other heated gases, which rush out in +enormous volume, for days, weeks, or years continuously, and are even disengaged +from lava during its consolidation. + +We also know that long after volcanoes have spent their force, hot springs +continue for ages to flow out at various points in the same area. In regions, +also, subject to violent earthquakes such springs are frequently observed +issuing from rents, usually along lines of fault or displacement of the rocks. +These thermal waters are most commonly charged with a variety of mineral +ingredients, and they retain a remarkable uniformity of temperature from century +to century. A like uniformity is also persistent in the nature of the earthy, +metallic, and gaseous substances with which they are impregnated. It is well +ascertained that springs, whether hot or cold, charged with carbonic acid, +especially with hydrofluoric acid, which is often present in small quantities, +are powerful causes of decomposition and chemical reaction in rocks through +which they percolate. + +The changes which Daubree has shown to have been produced by the alkaline waters +of Plombieres in the Vosges, are more especially instructive. (Daubree Sur le +Metamorphisme Paris 1860.) These waters have a heat of 160 degrees F., or an +excess of 109 degrees above the average temperature of ordinary springs in that +district. They were conveyed by the Romans to baths through long conduits or +aqueducts. The foundations of some of their works consisted of a bed of concrete +made of lime, fragments of brick, and sandstone. Through this and other masonry +the hot waters have been percolating for centuries, and have given rise to +various zeolites-- apophyllite and chabazite among others; also to calcareous +spar, arragonite, and fluor spar, together with siliceous minerals, such as +opal-- all found in the inter-spaces of the bricks and mortar, or constituting +part of their re-arranged materials. The quantity of heat brought into action in +this instance in the course of 2000 years has, no doubt, been enormous, but the +intensity of it developed at any one moment has been always inconsiderable. + +From these facts and from the experiments and observations of Senarmont, +Daubree, Delesse, Scheerer, Sorby, Sterry Hunt, and others, we are led to infer +that when in the bowels of the earth there are large volumes of matter +containing water and various acids intensely heated under enormous pressure, +these subterranean fluid masses will gradually part with their heat by the +escape of steam and various gases through fissures, producing hot springs; or by +the passage of the same through the pores of the overlying and injected rocks. +Even the most compact rocks may be regarded, before they have been exposed to +the air and dried, in the light of sponges filled with water. According to the +experiments of Henry, water, under a hydrostatic pressure of 96 feet, will +absorb three times as much carbonic acid gas as it can under the ordinary +pressure of the atmosphere. There are other gases, as well as the carbonic acid, +which water absorbs, and more rapidly in proportion to the amount of pressure. +Although the gaseous matter first absorbed would soon be condensed, and part +with its heat, yet the continual arrival of fresh supplies from below might, in +the course of ages, cause the temperature of the water, and with it that of the +containing rock, to be materially raised; the water acts not only as a vehicle +of heat, but also by its affinity for various silicates, which, when some of the +materials of the invaded rocks are decomposed, form quartz, feldspar, mica, and +other minerals. As for quartz, it can be produced under the influence of heat by +water holding alkaline silicates in solution, as in the case of the Plombieres +springs. The quantity of water required, according to Daubree, to produce great +transformations in the mineral structure of rocks, is very small. As to the heat +required, silicates may be produced in the moist way at about incipient red +heat, whereas to form the same in the dry way would require a much higher +temperature. + +M. Fournet, in his description of the metalliferous gneiss near Clermont, in +Auvergne, states that all the minute fissures of the rock are quite saturated +with free carbonic acid gas; which gas rises plentifully from the soil there and +in many parts of the surrounding country. The various elements of the gneiss, +with the exception of the quartz, are all softened; and new combinations of the +acid with lime, iron, and manganese are continually in progress. (See Principles +Index Carbonated Springs etc.) + +The power of subterranean gases is well illustrated by the stufas of St. +Calogero in the Lipari Islands, where the horizontal strata of tuffs, forming +cliffs 200 feet high, have been discoloured in places by the jets of steam often +above the boiling point, called "stufas," issuing from the fissures; and similar +instances are recorded by M. Virlet of corrosion of rocks near Corinth, and by +Dr. Daubeny of decomposition of trachytic rocks by sulphureted hydrogen and +muriatic acid gases in the Solfatara, near Naples. In all these instances it is +clear that the gaseous fluids must have made their way through vast thicknesses +of porous or fissured rocks, and their modifying influence may spread through +the crust for thousands of yards in thickness. + +It has been urged as an argument against the metamorphic theory, that rocks have +a small power of conducting heat, and it is true that when dry, and in the air, +they differ remarkably from metals in this respect. The syenite of Norway, as we +have seen (Chapter 31), has sometimes altered fossiliferous strata both in the +direction of their dip and strike for a distance of a quarter of a mile, but the +theory of gneiss and mica-schist above proposed requires us to imagine that the +same influence has extended through strata miles in thickness. Professor Bischof +has shown what changes may be superinduced, on black marble and other rocks, by +the steam of a hot spring having a temperature of no more than 133 degrees to +167 degrees Fahrenheit, and we are becoming more and more acquainted with the +prominent part which water is playing in distributing the heat of the interior +through mountain masses of incumbent strata, and of introducing into them +various mineral elements in a fluid or gaseous state. Such facts may induce us +to consider whether many granites and other rocks of that class may not +sometimes represent merely the extreme of a similar slow metamorphism. But, on +the other hand, the heat of lava in a volcanic crater when it is white and +glowing like the sun must convince us that the temperature of a column of such a +fluid at the depth of many miles exceeds any heat which can ever be witnessed at +the surface. That large portions of the Plutonic rocks had been formed under the +influence of such intense heat is in perfect accordance with their great volume, +uniform composition, and absence of stratification. The forcing also of veins +into contiguous stratified or schistose rocks is a natural consequence of the +hydrostatic pressure to which columns of molten matter many miles in height must +give rise. + +OBJECTIONS TO THE METAMORPHIC THEORY CONSIDERED. + +It has been objected to the metamorphic theory that the crystalline schists +contain a considerable proportion of potash and soda, whilst the sedimentary +strata out of which they are supposed to have been formed are usually wanting in +alkaline matter. But this reasoning proceeds on mistaken data, for clay, marl, +shale, and slate often contain a considerable proportion of alkali, so much so +as to make them frequently unfit to be burnt into bricks or pottery, and the Old +Red Sandstone in Forfarshire and other parts of Scotland, derived from +disintegration of granite, contains much triturated feldspar rich in potash. In +the common salt by which strata are often largely impregnated, as in Patagonia, +much soda is present, and potash enters largely into the composition of fossil +sea-weeds, and recent analysis has also shown that the carboniferous strata in +England, the Upper and Lower Silurian in East Canada, and the oldest clay-slates +in Norway, all contain as much alkali as is generally present in metamorphic +rocks. + +Another objection has been derived from the alternation of highly crystalline +strata with others less crystalline. The heat, it is said, in its ascent from +below, must have traversed the less altered schists before it reached a higher +and more crystalline bed. In answer to this, it may be observed, that if a +number of strata differing greatly in composition from each other be subjected +to equal quantities of heat, or hydrothermal action, there is every probability +that some will be much more fusible or soluble than others. Some, for example, +will contain soda, potash, lime, or some other ingredient capable of acting as a +flux or solvent; while others may be destitute of the same elements, and so +refractory as to be very slightly affected by the same causes. Nor should it be +forgotten that, as a general rule, the less crystalline rocks do really occur in +the upper, and the more crystalline in the lower part of each metamorphic +series. + + +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. + +We have already seen that chemical forces of great intensity have frequently +acted upon sedimentary and fossiliferous strata long subsequently to their +consolidation, and we may next inquire whether the component minerals of the +altered rocks usually arrange themselves in planes parallel to the original +planes of stratification, or whether, after crystallisation, they more commonly +take up a different position. + +In order to estimate fairly the merits of this question, we must first define +what is meant by the terms cleavage and foliation. There are four distinct forms +of structure exhibited in rocks, namely, stratification, joints, slaty cleavage, +and foliation; and all these must have different names, even though there be +cases where it is impossible, after carefully studying the appearances, to +decide upon the class to which they belong. + +SLATY CLEAVAGE. + +(FIGURE 624. Parallel planes of cleavage intersecting curved strata. +(Sedgwick.)) + +Professor Sedgwick, whose essay "On the Structure of large Mineral Masses" first +cleared the way towards a better understanding of this difficult subject, +observes, that joints are distinguishable from lines of slaty cleavage in this, +that the rock intervening between two joints has no tendency to cleave in a +direction parallel to the planes of the joints, whereas a rock is capable of +indefinite subdivision in the direction of its slaty cleavage. In cases where +the strata are curved, the planes of cleavage are still perfectly parallel. This +has been observed in the slate rocks of part of Wales (see Figure 624), which +consists of a hard greenish slate. The true bedding is there indicated by a +number of parallel stripes, some of a lighter and some of a darker colour than +the general mass. Such stripes are found to be parallel to the true planes of +stratification, wherever these are manifested by ripple-mark or by beds +containing peculiar organic remains. Some of the contorted strata are of a +coarse mechanical structure, alternating with fine-grained crystalline chloritic +slates, in which case the same slaty cleavage extends through the coarser and +finer beds, though it is brought out in greater perfection in proportion as the +materials of the rock are fine and homogeneous. It is only when these are very +coarse that the cleavage planes entirely vanish. In the Welsh hills these planes +are usually inclined at a very considerable angle to the planes of the strata, +the average angle being as much as from 30 to 40 degrees. Sometimes the cleavage +planes dip towards the same point of the compass as those of stratification, but +often to opposite points. (Geological Transactions second series volume 3 page +461.) The cleavage, as represented in Figure 624, is generally constant over the +whole of any area affected by one great set of disturbances, as if the same +lateral pressure which caused the crumpling up of the rock along parallel, +anticlinal, and synclinal axes caused also the cleavage. + +(FIGURE 625. Section in Lower Silurian slates of Cardiganshire, showing the +cleavage planes bent along the junction of the beds. (T. McK. Hughes.)) + +Mr. T. McK. Hughes remarks, that where a rough cleavage cuts flag-stones at a +considerable angle to the planes of stratification, the rock often splits into +large slabs, across which the lines of bedding are frequently seen, but when the +cleavage planes approach within about 15 degrees of stratification, the rock is +apt to split along the lines of bedding. He has also called my attention to the +fact that subsequent movements in a cleaved rock sometimes drag and bend the +cleavage planes along the junction of the beds in the manner indicated in Figure +625. + +JOINTED STRUCTURE. + +In regard to joints, they are natural fissures which often traverse rocks in +straight and well-determined lines. They afford to the quarryman, as Sir R. +Murchison observes, when speaking of the phenomenon, as exhibited in Shropshire +and the neighbouring counties, the greatest aid in the extraction of blocks of +stone; and, if a sufficient number cross each other, the whole mass of rock is +split into symmetrical blocks. The faces of the joints are for the most part +smoother and more regular than the surfaces of true strata. The joints are +straight-cut chinks, sometimes slightly open, and often passing, not only +through layers of successive deposition, but also through balls of limestone or +other matter which have been formed by concretionary action since the original +accumulation of the strata. Such joints, therefore, must often have resulted +from one of the last changes superinduced upon sedimentary deposits. (Silurian +System page 246.) + +(FIGURE 626. Stratification, joints, and cleavage (From Murchison's Silurian +System page 245.)) + +In Figure 626 the flat-surfaces of rock, A, B, C, represent exposed faces of +joints, to which the walls of other joints, J-J, are parallel. S-S are the lines +of stratification; D, D are lines of slaty cleavage, which intersect the rock at +a considerable angle to the planes of stratification. + +In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enormous masses of +limestone are cut through so regularly by nearly vertical partings, and these +joints are often so much more conspicuous than the seams of stratification, that +an inexperienced observer will almost inevitably confound them, and suppose the +strata to be perpendicular in places where in fact they are almost horizontal. +(Introduction to Geology chapter 4.) + +Now such joints are supposed to be analogous to the partings which separate +volcanic and Plutonic rocks into cuboidal and prismatic masses. On a small scale +we see clay and starch when dry split into similar shapes; this is often caused +by simple contraction, whether the shrinking be due to the evaporation of water, +or to a change of temperature. It is well known that many sandstones and other +rocks expand by the application of moderate degrees of heat, and then contract +again on cooling; and there can be no doubt that large portions of the earth's +crust have, in the course of past ages, been subjected again and again to very +different degrees of heat and cold. These alternations of temperature have +probably contributed largely to the production of joints in rocks. + +In many countries where masses of basalt rest on sandstone, the aqueous rock +has, for the distance of several feet from the point of junction, assumed a +columnar structure similar to that of the trap. In like manner some hearth- +stones, after exposure to the heat of a furnace without being melted, have +become prismatic. Certain crystals also acquire by the application of heat a new +internal arrangement, so as to break in a new direction, their external form +remaining unaltered. + +CRYSTALLINE THEORY OF CLEAVAGE. + +Professor Sedgwick, speaking of the planes of slaty cleavage, where they are +decidedly distinct from those of sedimentary deposition, declared, in the essay +before alluded to, his opinion that no retreat of parts, no contraction in the +dimensions of rocks in passing to a solid state, can account for the phenomenon. +He accordingly referred it to crystalline or polar forces acting simultaneously, +and somewhat uniformly, in given directions, on large masses having a +homogeneous composition. + +Sir John Herschel, in allusion to slaty cleavage, has suggested that "if rocks +have been so heated as to allow a commencement of crystallisation-- that is to +say, if they have been heated to a point at which the particles can begin to +move among themselves, or at least on their own axes, some general law must then +determine the position in which these particles will rest on cooling. Probably, +that position will have some relation to the direction in which the heat +escapes. Now, when all, or a majority of particles of the same nature have a +general tendency to one position, that must of course determine a cleavage- +plane. Thus we see the infinitesimal crystals of fresh-precipitated sulphate of +barytes, and some other such bodies, arrange themselves alike in the fluid in +which they float; so as, when stirred, all to glance with one light, and give +the appearance of silky filaments. Some sorts of soap, in which insoluble +margarates exist (Margaric acid is an oleaginous acid, formed from different +animal and vegetable fatty substances. A margarate is a compound of this acid +with soda, potash, or some other base, and is so named from its pearly lustre.), +exhibit the same phenomenon when mixed with water; and what occurs in our +experiments on a minute scale may occur in nature on a great one." (Letter to +the author dated Cape of Good Hope February 20, 1836.) + +MECHANICAL THEORY OF CLEAVAGE. + +Professor Phillips has remarked that in some slaty rocks the form of the outline +of fossil shells and trilobites has been much changed by distortion, which has +taken place in a longitudinal, transverse, or oblique direction. This change, he +adds, seems to be the result of a "creeping movement" of the particles of the +rock along the planes of cleavage, its direction being always uniform over the +same tract of country, and its amount in space being sometimes measurable, and +being as much as a quarter or even half an inch. The hard shells are not +affected, but only those which are thin. (Report British Association Cork 1843 +Section page 60.) Mr. D. Sharpe, following up the same line of inquiry, came to +the conclusion that the present distorted forms of the shells in certain British +slate rocks may be accounted for by supposing that the rocks in which they are +imbedded have undergone compression in a direction perpendicular to the planes +of cleavage, and a corresponding expansion in the direction of the dip of the +cleavage. (Quarterly Geological Journal volume 3 page 87 1847.) + +(FIGURE 627. Vertical section of slate rock in the cliffs near Ilfracombe, North +Devon. Scale one inch to one foot. (Drawn by H.C. Sorby.) +a, b, c, e. Fine-grained slates, the stratification being shown partly by +lighter or darker colours, and partly by different degrees of fineness in the +grain. +d, f. A coarser grained light-coloured sandy slate with less perfect cleavage.) + +Subsequently (1853) Mr. Sorby demonstrated the great extent to which this +mechanical theory is applicable to the slate rocks of North Wales and Devonshire +(On the Origin of Slaty Cleavage by H.C. Sorby Edinburgh New Philosophical +Journal 1853 volume 55 page 137.), districts where the amount of change in +dimensions can be tested and measured by comparing the different effects exerted +by lateral pressure on alternating beds of finer and coarser materials. Thus, +for example, in Figure 627 it will be seen that the sandy bed d-f, which has +offered greater resistance, has been sharply contorted, while the fine-grained +strata, a, b, c, have remained comparatively unbent. The points d and f in the +stratum d-f must have been originally four times as far apart as they are now. +They have been forced so much nearer to each other, partly by bending, and +partly by becoming elongated in the direction of what may be called the longer +axes of their contortions, and lastly, to a certain small amount, by +condensation. The chief result has obviously been due to the bending; but, in +proof of elongation, it will be observed that the thickness of the bed d-f is +now about four times greater in those parts lying in the main direction of the +flexures than in a plane perpendicular to them; and the same bed exhibits +cleavage planes in the direction of the greatest movement, although they are +much fewer than in the slaty strata above and below. + +Above the sandy bed d-f, the stratum c is somewhat disturbed, while the next +bed, b, is much less so, and a not at all; yet all these beds, c, b, and a, must +have undergone an equal amount of pressure with d, the points a and g having +approximated as much towards each other as have d and f. The same phenomena are +also repeated in the beds below d, and might have been shown, had the section +been extended downward. Hence it appears that the finer beds have been squeezed +into a fourth of the space they previously occupied, partly by condensation, or +the closer packing of their ultimate particles (which has given rise to the +great specific gravity of such slates), and partly by elongation in the line of +the dip of the cleavage, of which the general direction is perpendicular to that +of the pressure. "These and numerous other cases in North Devon are analogous," +says Mr. Sorby, "to what would occur if a strip of paper were included in a mass +of some soft plastic material which would readily change its dimensions. If the +whole were then compressed in the direction of the length of the strip of paper, +it would be bent and puckered up into contortions, while the plastic material +would readily change its dimensions without undergoing such contortions; and the +difference in distance of the ends of the paper, as measured in a direct line or +along it, would indicate the change in the dimensions of the plastic material." + +By microscopic examination of minute crystals, and by other observations, Mr. +Sorby has come to the conclusion that the absolute condensation of the slate +rocks amounts upon an average to about one half their original volume. Most of +the scales of mica occurring in certain slates examined by Mr. Sorby lie in the +plane of cleavage; whereas in a similar rock not exhibiting cleavage they lie +with their longer axes in all directions. May not their position in the slates +have been determined by the movement of elongation before alluded to? To +illustrate this theory some scales of oxide of iron were mixed with soft pipe- +clay in such a manner that they inclined in all directions. The dimensions of +the mass were then changed artificially to a similar extent to what has occurred +in slate rocks, and the pipe-clay was then dried and baked. When it was +afterwards rubbed to a flat surface perpendicular to the pressure and in the +line of elongation, or in a plane corresponding to that of the dip of cleavage, +the particles were found to have become arranged in the same manner as in +natural slates, and the mass admitted of easy fracture into thin flat pieces in +the plane alluded to, whereas it would not yield in that perpendicular to the +cleavage. (Sorby as cited above page 741 note.) + +Dr. Tyndall, when commenting in 1856 on Mr. Sorby's experiments, observed that +pressure alone is sufficient to produce cleavage, and that the intervention of +plates of mica or scales of oxide of iron, or any other substances having flat +surfaces, is quite unnecessary. In proof of this he showed experimentally that a +mass of "pure white wax, after having been submitted to great pressure, +exhibited a cleavage more clean than that of any slate-rock, splitting into +laminae of surpassing tenuity." (Tyndall View of the Cleavage of Crystals and +Slate rocks.) He remarks that every mass of clay or mud is divided and +subdivided by surfaces among which the cohesion is comparatively small. On being +subjected to pressure, such masses yield and spread out in the direction of +least resistance, small nodules become converted into laminae separated from +each other by surfaces of weak cohesion, and the result is that the mass cleaves +at right angles to the line in which the pressure is exerted. In further +illustration of this, Mr. Hughes remarks that "concretions which in the +undisturbed beds have their longer axes parallel to the bedding are, where the +rock is much cleaved, frequently found flattened laterally, so as to have their +longer axes parallel to the cleavage planes, and at a considerable angle, even +right angles, to their former position." + +Mr. Darwin attributes the lamination and fissile structure of volcanic rocks of +the trachytic series, including some obsidians in Ascension, Mexico, and +elsewhere, to their having moved when liquid in the direction of the laminae. +The zones consist sometimes of layers of air-cells drawn out and lengthened in +the supposed direction of the moving mass. (Darwin Volcanic Islands pages 69, +70.) + +FOLIATION OF CRYSTALLINE SCHISTS. + +After studying, in 1835, the crystalline rocks of South America, Mr. Darwin +proposed the term FOLIATION for the laminae or plates into which gneiss, mica- +schist, and other crystalline rocks are divided. Cleavage, he observes, may be +applied to those divisional planes which render a rock fissile, although it may +appear to the eye quite or nearly homogeneous. Foliation may be used for those +alternating layers or plates of different mineralogical nature of which gneiss +and other metamorphic schists are composed. + +That the planes of foliation of the crystalline schists in Norway accord very +generally with those of original stratification is a conclusion long since +espoused by Keilhau. (Norske Mag. Naturvidsk. volume 1 page 71.) Numerous +observations made by Mr. David Forbes in the same country (the best probably in +Europe for studying such phenomena on a grand scale) confirm Keilhau's opinion. +In Scotland, also, Mr. D. Forbes has pointed out a striking case where the +foliation is identical with the lines of stratification in rocks well seen near +Crianlorich on the road to Tyndrum, about eight miles from Inverarnon, in +Perthshire. There is in that locality a blue limestone foliated by the +intercalation of small plates of white mica, so that the rock is often scarcely +distinguishable in aspect from gneiss or mica-schist. The stratification is +shown by the large beds and coloured bands of limestone all dipping, like the +folia, at an angle of 32 degrees N.E. (Memoir read before the Geological Society +London January 31, 1855.) In stratified formations of every age we see layers of +siliceous sand with or without mica, alternating with clay, with fragments of +shells or corals, or with seams of vegetable matter, and we should expect the +mutual attraction of like particles to favour the crystallisation of the quartz, +or mica, or feldspar, or carbonate of lime, along the planes of original +deposition, rather than in planes placed at angles of 20 or 40 degrees to those +of stratification. + +We have seen how much the original planes of stratification may be interfered +with or even obliterated by concretionary action in deposits still retaining +their fossils, as in the case of the magnesian limestone (see Chapter 4). Hence +we must expect to be frequently baffled when we attempt to decide whether the +foliation does or does not accord with that arrangement which gravitation, +combined with current-action, imparted to a deposit from water. Moreover, when +we look for stratification in crystalline rocks, we must be on our guard not to +expect too much regularity. The occurrence of wedge-shaped masses, such as +belong to coarse sand and pebbles-- diagonal lamination (Chapter 2)-- ripple- +marked, unconformable stratification,-- the fantastic folds produced by lateral +pressure-- faults of various width-- intrusive dikes of trap-- organic bodies of +diversified shapes, and other causes of unevenness in the planes of deposition, +both on the small and on the large scale, will interfere with parallelism. If +complex and enigmatical appearances did not present themselves, it would be a +serious objection to the metamorphic theory. Mr. Sorby has shown that the +peculiar structure belonging to ripple-marked sands, or that which is generated +when ripples are formed during the deposition of the materials, is distinctly +recognisable in many varieties of mica-schists in Scotland. (H.C. Sorby +Quarterly Geological Journal volume 19 page 401.) + +(FIGURE 628. Lamination of clay-stone. Montagne de Seguinat, near Gavarnie, in +the Pyrenees.) + +In Figure 628 I have represented carefully the lamination of a coarse +argillaceous schist which I examined in 1830 in the Pyrenees. In part it +approaches in character to a green and blue roofing-slate, while part is +extremely quartzose, the whole mass passing downward into micaceous schist. The +vertical section here exhibited is about three feet in height, and the layers +are sometimes so thin that fifty may be counted in the thickness of an inch. +Some of them consist of pure quartz. There is a resemblance in such cases to the +diagonal lamination which we see in sedimentary rocks, even though the layers of +quartz and of mica, or of feldspar and other minerals, may be more distinct in +alternating folia than they were originally. + + +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. + +According to the theory adopted in the last chapter, the metamorphic strata have +been deposited at one period, and have become crystalline at another. We can +rarely hope to define with exactness the date of both these periods, the fossils +having been destroyed by Plutonic action, and the mineral characters being the +same, whatever the age. Superposition itself is an ambiguous test, especially +when we desire to determine the period of crystallisation. Suppose, for example, +we are convinced that certain metamorphic strata in the Alps, which are covered +by cretaceous beds, are altered lias; this lias may have assumed its crystalline +texture in the cretaceous or in some tertiary period, the Eocene for example. + +When discussing the ages of the Plutonic rocks, we have seen that examples occur +of various primary, secondary, and tertiary deposits converted into metamorphic +strata near their contact with granite. There can be no doubt in these cases +that strata once composed of mud, sand, and gravel, or of clay, marl, and shelly +limestone, have for the distance of several yards, and in some instances several +hundred feet, been turned into gneiss, mica-schist, hornblende-schist, chlorite- +schist, quartz rock, statuary marble, and the rest. (See Chapters 33 and 34.) It +may be easy to prove the identity of two different parts of the same stratum; +one, where the rock has been in contact with a volcanic or Plutonic mass, and +has been changed into marble or hornblende-schist, and another not far distant, +where the same bed remains unaltered and fossiliferous; but when hydrothermal +action, as described in Chapter 33, has operated gradually on a more extensive +scale, it may have finally destroyed all monuments of the date of its +development throughout a whole mountain chain, and all the labour and skill of +the most practised observers are required, and may sometimes be at fault. I +shall mention one or two examples of alteration on a grand scale, in order to +explain to the student the kind of reasoning by which we are led to infer that +dense masses of fossiliferous strata have been converted into crystalline rocks. + +EOCENE STRATA RENDERED METAMORPHIC IN THE ALPS. + +In the eastern part of the Alps, some of the Palaeozoic strata, as well as the +older Mesozoic formations, including the oolitic and cretaceous rocks, are +distinctly recognisable. Tertiary deposits also appear in a less elevated +position on the flanks of the Eastern Alps; but in the Central or Swiss Alps, +the Palaeozoic and older Mesozoic formations disappear, and the Cretaceous, +Oolitic, Liassic, and at some points even the Eocene strata, graduate insensibly +into metamorphic rocks, consisting of granular limestone, talc-schist, talcose- +gneiss, micaceous schist, and other varieties. + +As an illustration of the partial conversion into gneiss of portions of a highly +inclined set of beds, I may cite Sir R. Murchison's memoir on the structure of +the Alps. Slates provincially termed "flysch" (see Chapter 16), overlying the +nummulite limestone of Eocene date, and comprising some arenaceous and some +calcareous layers, are seen to alternate several times with bands of granitoid +rock, answering in character to gneiss. In this case heat, vapour, or water at a +high temperature may have traversed the more permeable beds, and altered them so +far as to admit of an internal movement and re-arrangement of the molecules, +while the adjoining strata did not give passage to the same heated gases or +water, or, if so, remained unchanged because they were composed of less fusible +or decomposable materials. Whatever hypothesis we adopt, the phenomena establish +beyond a doubt the possibility of the development of the metamorphic structure +in a tertiary deposit in planes parallel to those of stratification. The strata +appear clearly to have been affected, though in a less intense degree, by that +same Plutonic action which has entirely altered and rendered metamorphic so many +of the subjacent formations; for in the Alps this action has by no means been +confined to the immediate vicinity of granite. Granite, indeed, and other +Plutonic rocks, rarely make their appearance at the surface, notwithstanding the +deep ravines which lay open to view the internal structure of these mountains. +That they exist below at no great depth we can not doubt, for at some points, as +in the Valorsine, near Mont Blanc, granite and granitic veins are observable, +piercing through talcose gneiss, which passes insensibly upward into secondary +strata. + +It is certainly in the Alps of Switzerland and Savoy, more than in any other +district in Europe, that the geologist is prepared to meet with the signs of an +intense development of Plutonic action; for here strata thousands of feet thick +have been bent, folded, and overturned, and marine secondary formations of a +comparatively modern date, such as the Oolitic and Cretaceous, have been +upheaved to the height of 12,000, and some Eocene strata to elevations of 10,000 +feet above the level of the sea; and even deposits of the Miocene era have been +raised 4000 or 5000 feet, so as to rival in height the loftiest mountains in +Great Britain. In one of the sections described by M. Studer in the highest of +the Bernese Alps, namely in the Roththal, a valley bordering the line of +perpetual snow on the northern side of the Jungfrau, there occurs a mass of +gneiss 1000 feet thick, and 15,000 feet long, which I examined, not only resting +upon, but also again covered by strata containing oolitic fossils. These +anomalous appearances may partly be explained by supposing great solid wedges of +intrusive gneiss to have been forced in laterally between strata to which I +found them to be in many sections unconformable. The superposition, also, of the +gneiss to the oolite may, in some cases, be due to a reversal of the original +position of the beds in a region where the convulsions have been on so +stupendous a scale. + +NORTHERN APENNINES.-- CARRARA. + +The celebrated marble of Carrara, used in sculpture, was once regarded as a type +of primitive limestone. It abounds in the mountains of Massa Carrara, or the +"Apuan Alps," as they have been called, the highest peaks of which are nearly +6000 feet high. Its great antiquity was inferred from its mineral texture, from +the absence of fossils, and its passage downward into talc-schist and +garnetiferous mica-schist; these rocks again graduating downward into gneiss, +which is penetrated, at Forno, by granite veins. But the researches of MM. Savi, +Boue, Pareto, Guidoni, De la Beche, Hoffman, and Pilla demonstrated that this +marble, once supposed to be formed before the existence of organic beings, is, +in fact, an altered limestone of the Oolitic period, and the underlying +crystalline schists are secondary sandstones and shales, modified by Plutonic +action. In order to establish these conclusions it was first pointed out that +the calcareous rocks bordering the Gulf of Spezia, and abounding in Oolitic +fossils, assume a texture like that of Carrara marble, in proportion as they are +more and more invaded by certain trappean and Plutonic rocks, such as diorite, +serpentine, and granite, occurring in the same country. + +It was then observed that, in places where the secondary formations are +unaltered, the uppermost consist of common Apennine limestone with nodules of +flint, below which are shales, and at the base of all, argillaceous and +siliceous sandstones. In the limestone fossils are frequent, but very rare in +the underlying shale and sandstone. Then a gradation was traced laterally from +these rocks into another and corresponding series, which is completely +metamorphic; for at the top of this we find a white granular marble, wholly +devoid of fossils, and almost without stratification, in which there are no +nodules of flint, but in its place siliceous matter disseminated through the +mass in the form of prisms of quartz. Below this, and in place of the shales, +are talc-schists, jasper, and hornstone; and at the bottom, instead of the +siliceous and argillaceous sandstones, are quartzite and gneiss. (See notices of +Savi, Hoffman, and others, referred to by Boue, Bull. de la Soc. Geol. de France +tome 5 page 317 and tome 3 page 44; also Pilla, cited by Murchison Quarterly +Geological Journal volume 5 page 266.) Had these secondary strata of the +Apennines undergone universally as great an amount of transmutation, it would +have been impossible to form a conjecture respecting their true age; and then, +according to the method of classification adopted by the earlier geologists, +they would have ranked as primary rocks. In that case the date of their origin +would have been thrown back to an era antecedent to the deposition of the Lower +Silurian or Cambrian strata, although in reality they were formed in the Oolitic +period, and altered at some subsequent and perhaps much later epoch. + +METAMORPHIC STRATA OF OLDER DATE THAN THE SILURIAN AND CAMBRIAN ROCKS. + +It was remarked (Figure 617) that as the hypogene rocks, both stratified and +unstratified, crystallise originally at a certain depth beneath the surface, +they must always, before they are upraised and exposed at the surface, be of +considerable antiquity, relatively to a large portion of the fossiliferous and +volcanic rocks. They may be forming at all periods; but before any of them can +become visible, they must be raised above the level of the sea, and some of the +rocks which previously concealed them must have been removed by denudation. + +In Canada, as we have seen (Chapter 27), the Lower Laurentian gneiss, quartzite, +and limestone may be regarded as metamorphic, because, among other reasons, +organic remains (Eozoon Canadense) have been detected in a part of one of the +calcareous masses. The Upper Laurentian or Labrador series lies unconformably +upon the Lower, and differs from it chiefly in having as yet yielded no fossils. +It consists of gneiss with Labrador-feldspar and feldstones, in all 10,000 feet +thick, and both its composition and structure lead us to suppose that, like the +Lower Laurentian, it was originally of sedimentary origin and owes its +crystalline condition to metamorphic action. The remote date of the period when +some of these old Laurentian strata of Canada were converted into gneiss may be +inferred from the fact that pebbles of that rock are found in the overlying +Huronian formation, which is probably of Cambrian age (Chapter 27). + +The oldest stratified rock of Scotland is the hornblendic gneiss of Lewis, in +the Hebrides, and that of the north-west coast of Ross-shire, represented at the +base of the section given at Figure 82. It is the same as that intersected by +numerous granite veins which forms the cliffs of Cape Wrath, in Sutherlandshire +(see Figure 613), and is conjectured to be of Laurentian age. Above it, as shown +in the section (Figure 82), lie unconformable beds of a reddish or purple +sandstone and conglomerate, nearly horizontal, and between 3000 and 4000 feet +thick. In these ancient grits no fossils have been found, but they are supposed +to be of Cambrian date, for Sir R. Murchison found Lower Silurian strata resting +unconformably upon them. These strata consist of quartzite with annelid burrows +already alluded to (Chapter 7), and limestone in which Mr. Charles Peach was the +first to find, in 1854, three or four species of Orthoceras, also the genera +Cyrtoceras and Lituites, two species of Murchisonia, a Pleurotomaria, a species +of Maclurea, one of Euomphalus, and an Orthis. Several of the species are +believed by Mr. Salter to be identical with Lower Silurian fossils of Canada and +the United States. + +The discovery of the true age of these fossiliferous rocks was one of the most +important steps made of late years in the progress of British Geology, for it +led to the unexpected conclusion that all the Scotch crystalline strata to the +eastward, once called primitive, which overlie the limestone and quartzite in +question, are referable to some part of the Silurian series. + +These Scotch metamorphic strata are of gneiss, mica-schist, and clay-slate of +vast thickness, and having a strike from north-east to south-west almost at +right angles to that of the older Laurentian gneiss before mentioned. The newer +crystalline series, comprising the crystalline rocks of Aberdeenshire, +Perthshire, and Forfarshire, were inferred by Sir R. Murchison to be altered +Silurian strata; and his opinion has been since confirmed by the observations of +three able geologists, Messrs. Ramsay, Harkness, and Geikie. The newest of the +series is a clay-slate, on which, along the southern borders of the Grampians, +the Lower Old Red, containing Cephalaspis Lyelli, Pterygotus Anglicus, and Parka +decipiens, rests unconformably. + +ORDER OF SUCCESSION IN METAMORPHIC ROCKS. + +There is no universal and invariable order of superposition in metamorphic +rocks, although a particular arrangement may prevail throughout countries of +great extent, for the same reason that it is traceable in those sedimentary +formations from which crystalline strata are derived. Thus, for example, we have +seen that in the Apennines, near Carrara, the descending series, where it is +metamorphic, consists of, first, saccharine marble; secondly, talcose-schist; +and thirdly, of quartz-rock and gneiss: where unaltered, of, first, +fossiliferous limestone; secondly, shale; and thirdly, sandstone. + +But if we investigate different mountain chains, we find gneiss, mica-schist, +hornblende-schist, chlorite-schist, hypogene limestone, and other rocks, +succeeding each other, and alternating with each other in every possible order. +It is, indeed, more common to meet with some variety of clay-slate forming the +uppermost member of a metamorphic series than any other rock; but this fact by +no means implies, as some have imagined, that all clay-slates were formed at the +close of an imaginary period when the deposition of the crystalline strata gave +way to that of ordinary sedimentary deposits. Such clay-slates, in fact, are +variable in composition, and sometimes alternate with fossiliferous strata, so +that they may be said to belong almost equally to the sedimentary and +metamorphic order of rocks. It is probable that, had they been subjected to more +intense Plutonic action, they would have been transformed into hornblende- +schist, foliated chlorite-schist, scaly talcose-schist, mica-schist, or other +more perfectly crystalline rocks, such as are usually associated with gneiss. + +UNIFORMITY OF MINERAL CHARACTER IN HYPOGENE ROCKS. + +It is true, as Humboldt has happily remarked, that when we pass to another +hemisphere, we see new forms of animals and plants, and even new constellations +in the heavens; but in the rocks we still recognise our old acquaintances-- the +same granite, the same gneiss, the same micaceous schist, quartz-rock, and the +rest. There is certainly a great and striking general resemblance in the +principal kinds of hypogene rocks in all countries, however different their +ages; but each of them, as we have seen, must be regarded as geological families +of rocks, and not as definite mineral compounds. They are more uniform in aspect +than sedimentary strata, because these last are often composed of fragments +varying greatly in form, size, and colour, and contain fossils of different +shapes and mineral composition, and acquire a variety of tints from the mixture +of various kinds of sediment. The materials of such strata, if they underwent +metamorphism, would be subject to chemical laws, simple and uniform in their +action, the same in every climate, and wholly undisturbed by mechanical and +organic causes. It would, however, be a great error to assume, as some have +done, that the hypogene rocks, considered as aggregates of simple minerals, are +really more homogeneous in their composition than the several members of the +sedimentary series. Not only do the proportional quantities of feldspar, quartz, +mica, hornblende, and other minerals, vary in hypogene rocks bearing the same +name; but what is still more important, the ingredients, as we have seen, of the +same simple mineral are not always constant (Chapter 28 and Table 28.1). + +SUPPOSED AZOIC PERIOD. + +The total absence of any trace of fossils has inclined many geologists to +attribute the origin of the most ancient strata to an azoic period, or one +antecedent to the existence of organic beings. Admitting, they say, the +obliteration, in some cases, of fossils by Plutonic action, we might still +expect that traces of them would oftener be found in certain ancient systems of +slate which can scarcely be said to have assumed a crystalline structure. But in +urging this argument it seems to have been forgotten that there are stratified +formations of enormous thickness, and of various ages, some of them even of +Tertiary date, and which we know were formed after the earth had become the +abode of living creatures, which are, nevertheless, in some districts, entirely +destitute of all vestiges of organic bodies. In some, the traces of fossils may +have been effaced by water and acids, at many successive periods; indeed the +removal of the calcareous matter of fossil shells is proved by the fact of such +organic remains being often replaced by silex or other minerals, and sometimes +by the space once occupied by the fossil being left empty, or only marked by a +faint impression. + +Those who believed the hypogene rocks to have originated antecedently to the +creation of organic beings, imputed the absence of lime, so remarkable in +metamorphic strata, to the non-existence of those mollusca and zoophytes by +which shells and corals are secreted; but when we ascribe the crystalline +formations to Plutonic action, it is natural to inquire whether this action +itself may not tend to expel carbonic acid and lime from the materials which it +reduces to fusion or semi-fusion. Not only carbonate of lime, but also free +carbonic acid gas, is given off plentifully from the soil and crevices of rocks +in regions of active and spent volcanoes, as near Naples and in Auvergne. By +this process, fossil shells or corals may often lose their carbonic acid, and +the residual lime may enter into the composition of augite, hornblende, garnet, +and other hypogene minerals. Although we can not descend into the subterranean +regions where volcanic heat is developed, we can observe in regions of extinct +volcanoes, such as Auvergne and Tuscany, hundreds of springs, both cold and +thermal, flowing out from granite and other rocks, and having their waters +plentifully charged with carbonate of lime. + +If all the calcareous matter transferred in the course of ages by these and +thousands of other springs from the lower part of the earth's crust to the +atmosphere could be presented to us in a solid form, we should find that its +volume was comparable to that of many a chain of hills. Calcareous matter is +poured into lakes and the ocean by a thousand springs and rivers; so that part +of almost every new calcareous rock chemically precipitated, and of many reefs +of shelly and coralline stone, must be derived from mineral matter subtracted by +Plutonic agency, and driven up by gas and steam from fused and heated rocks in +the bowels of the earth. + +The scarcity of limestone in many extensive regions of metamorphic rocks, as in +the Eastern and Southern Grampians of Scotland, may have been the result of some +action of this kind; and if the limestones of the Lower Laurentian in Canada +afford a remarkable exception to the general rule, we must not forget that it is +precisely in this most ancient formation that the Eozoon Canadense has been +found. The fact that some distinct bands of limestone from 700 to 1500 feet +thick occur here, may be connected with the escape from destruction of some few +traces of organic life, even in a rock in which metamorphic action has gone so +far as to produce serpentine, augite, and other minerals found largely +intermixed with the carbonate of lime. + + +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. + +The manner in which metallic substances are distributed through the earth's +crust, and more especially the phenomena of those more or less connected masses +of ore called mineral veins, from which the larger part of the precious metals +used by man are obtained, are subjects of the highest practical importance to +the miner, and of no less theoretical interest to the geologist. + +ON DIFFERENT KINDS OF MINERAL VEINS. + +The mineral veins with which we are most familiarly acquainted are those of +quartz and carbonate of lime, which are often observed to form lenticular masses +of limited extent traversing both hypogene strata and fossiliferous rocks. Such +veins appear to have once been chinks or small cavities, caused, like cracks in +clay, by the shrinking of the mass, during desiccation, or in passing from a +higher to a lower temperature. Siliceous, calcareous, and occasionally metallic +matters have sometimes found their way simultaneously into such empty spaces, by +infiltration from the surrounding rocks. Mixed with hot water and steam, +metallic ores may have permeated the mass until they reached those receptacles +formed by shrinkage, and thus gave rise to that irregular assemblage of veins, +called by the Germans a "stockwerk," in allusion to the different floors on +which the mining operations are in such cases carried on. + +The more ordinary or regular veins are usually worked in vertical shafts, and +have evidently been fissures produced by mechanical violence. They traverse all +kinds of rocks, both hypogene and fossiliferous, and extend downward to +indefinite or unknown depths. We may assume that they correspond with such rents +as we see caused from time to time by the shock of an earthquake. Metalliferous +veins referable to such agency are occasionally a few inches wide, but more +commonly three or four feet. They hold their course continuously in a certain +prevailing direction for miles or leagues, passing through rocks varying in +mineral composition. + +THAT METALLIFEROUS VEINS WERE FISSURES. + +(FIGURES 629, 630 and 631. Vertical sections of the mine of Huel Peever, +Redruth, Cornwall. + +(Figure 629. Vertical section of the mine of Huel Peever, Redruth, Cornwall. +Tin.) + +(FIGURE 630. Vertical section of the mine of Huel Peever, Redruth, Cornwall. +Copper.) + +(FIGURE 631. Vertical section of the mine of Huel Peever, Redruth, Cornwall. +Clay and copper.)) + +As some intelligent miners, after an attentive study of metalliferous veins, +have been unable to reconcile many of their characteristics with the hypothesis +of fissures, I shall begin by stating the evidence in its favour. The most +striking fact, perhaps, which can be adduced in its support is, the coincidence +of a considerable proportion of mineral veins with FAULTS, or those dislocations +of rocks which are indisputably due to mechanical force, as above explained +(Chapter 5). There are even proofs in almost every mining district of a +succession of faults, by which the opposite walls of rents, now the receptacles +of metallic substances, have suffered displacement. Thus, for example, suppose +a-a, Figure 629, to be a tin lode in Cornwall, the term LODE being applied to +veins containing metallic ores. This lode, running east and west, is a yard +wide, and is shifted by a copper lode (b-b) of similar width. The first fissure +(a-a) has been filled with various materials, partly of chemical origin, such as +quartz, fluor-spar, peroxide of tin, sulphuret of copper, arsenical pyrites, +bismuth, and sulphuret of nickel, and partly of mechanical origin, comprising +clay and angular fragments or detritus of the intersected rocks. The plates of +quartz and the ores are, in some places, parallel to the vertical sides or walls +of the vein, being divided from each other by alternating layers of clay or +other earthy matter. Occasionally the metallic ores are disseminated in detached +masses among the vein-stones. + +It is clear that, after the gradual introduction of the tin and other +substances, the second rent (b-b) was produced by another fracture accompanied +by a displacement of the rocks along the plane of b-b. This new opening was then +filled with minerals, some of them resembling those in a-a, as fluor-spar (or +fluate of lime) and quartz; others different, the copper being plentiful and the +tin wanting or very scarce. We must next suppose a third movement to occur, +breaking asunder all the rocks along the line c-c, Figure 630; the fissure, in +this instance, being only six inches wide, and simply filled with clay, derived, +probably, from the friction of the walls of the rent, or partly, perhaps, washed +in from above. This new movement has displaced the rock in such a manner as to +interrupt the continuity of the copper vein (b-b), and, at the same time, to +shift or heave laterally in the same direction a portion of the tin vein which +had not previously been broken. + +Again, in Figure 631 we see evidence of a fourth fissure (d-d), also filled with +clay, which has cut through the tin vein (a-a), and has lifted it slightly +upward towards the south. The various changes here represented are not ideal, +but are exhibited in a section obtained in working an old Cornish mine, long +since abandoned, in the parish of Redruth, called Huel Peever, and described +both by Mr. Williams and Mr. Carne. (Geological Transactions volume 4 page 139; +Transactions of the Royal Geological Society Cornwall volume 2 page 90.) The +principal movement here referred to, or that of c-c, Figure 631, extends through +a space of no less than 84 feet; but in this, as in the case of the other three, +it will be seen that the outline of the country above, d, c, b, a, etc., or the +geographical features of Cornwall, are not affected by any of the dislocations, +a powerful denuding force having clearly been exerted subsequently to all the +faults. (See Chapter 5.) It is commonly said in Cornwall, that there are eight +distinct systems of veins, which can in like manner be referred to as many +successive movements or fractures; and the German miners of the Hartz Mountains +speak also of eight systems of veins, referable to as many periods. + +Besides the proofs of mechanical action already explained, the opposite walls of +veins are often beautifully polished, as if glazed, and are not unfrequently +striated or scored with parallel furrows and ridges, such as would be produced +by the continued rubbing together of surfaces of unequal hardness. These +smoothed surfaces resemble the rocky floor over which a glacier has passed (see +Figure 106). They are common even in cases where there has been no shift, and +occur equally in non-metalliferous fissures. They are called by miners "slicken- +sides," from the German schlichten, to plane, and seite, side. It is supposed +that the lines of the striae indicate the direction in which the rocks were +moved. + +In some of the veins in the mountain limestone of Derbyshire, containing lead, +the vein-stuff, which is nearly compact, is occasionally traversed by what may +be called a vertical crack passing down the middle of the vein. The two faces in +contact are slicken-sides, well polished and fluted, and sometimes covered by a +thin coating of lead-ore. When one side of the vein-stuff is removed, the other +side cracks, especially if small holes be made in it, and fragments fly off with +loud explosions, and continue to do so for some days. The miner, availing +himself of this circumstance, makes with his pick small holes about six inches +apart, and four inches deep, and on his return in a few hours finds every part +ready broken to his hand. (Conybeare and Phil. Geol. page 401 and Farey's +Derbyshire page 243.) + +That a great many veins communicated originally with the surface of the country +above, or with the bed of the sea, is proved by the occurrence in them of well- +rounded pebbles, agreeing with those in superficial alluviums, as in Auvergne +and Saxony. Marine fossil shells, also, have been found at great depths, having +probably been ingulfed during submarine earthquakes. Thus, a gryphaea is stated +by M. Virlet to have been met with in a lead-mine near Semur, in France, and a +madrepore in a compact vein of cinnabar in Hungary. (Fournet Etudes sur les +Depots Metalliferes.) In Bohemia, similar pebbles have been met with at the +depth of 180 fathoms; and in Cornwall, Mr. Carne mentions true pebbles of quartz +and slate in a tin lode of the Relistran Mine, at the depth of 600 feet below +the surface. They were cemented by oxide of tin and bisulphuret of copper, and +were traced over a space more than twelve feet long and as many wide. (carne +Transactions of the Geological Society Cornwall volume 3 page 238.) When +different sets or systems of veins occur in the same country, those which are +supposed to be of contemporaneous origin, and which are filled with the same +kind of metals, often maintain a general parallelism of direction. Thus, for +example, both the tin and copper veins in Cornwall run nearly east and west, +while the lead veins run north and south; but there is no general law of +direction common to different mining districts. The parallelism of the veins is +another reason for regarding them as ordinary fissures, for we observe that +faults and trap dikes, admitted by all to be masses of melted matter which have +filled rents, are often parallel. + +FRACTURE, RE-OPENING AND SUCCESSIVE FORMATION OF VEINS. + +Assuming, then, that veins are simply fissures in which chemical and mechanical +deposits have accumulated, we may next consider the proofs of their having been +filled gradually and often during successive enlargements. + +Werner observed, in a vein near Gersdorff, in Saxony, no less than thirteen beds +of different minerals, arranged with the utmost regularity on each side of the +central layer. This layer was formed of two plates of calcareous spar, which had +evidently lined the opposite walls of a vertical cavity. The thirteen beds +followed each other in corresponding order, consisting of fluor-spar, heavy +spar, galena, etc. In these cases the central mass has been last formed, and the +two plates which coat the walls of the rent on each side are the oldest of all. +If they consist of crystalline precipitates, they may be explained by supposing +the fissure to have remained unaltered in its dimensions, while a series of +changes occurred in the nature of the solutions which rose up from below: but +such a mode of deposition, in the case of many successive and parallel layers, +appears to be exceptional. + +(FIGURE 632. Copper lode, near Redruth, enlarged at six successive periods.) + +If a vein-stone consist of crystalline matter, the points of the crystals are +always turned inward, or towards the centre of the vein; in other words, they +point in the direction where there was space for the development of the +crystals. Thus each new layer receives the impression of the crystals of the +preceding layer, and imprints its crystals on the one which follows, until at +length the whole of the vein is filled: the two layers which meet dovetail the +points of their crystals the one into the other. But in Cornwall, some lodes +occur where the vertical plates, or COMBS, as they are there called, exhibit +crystals so dovetailed as to prove that the same fissure has been often +enlarged. Sir H. De la Beche gives the following curious and instructive example +(Figure 632), from a copper-mine in granite, near Redruth. (Geological Report on +Cornwall page 340.) Each of the plates or combs (a, b, c, d, e, f) is double, +having the points of their crystals turned inward along the axis of the comb. +The sides or walls (2, 3, 4, 5 and 6) are parted by a thin covering of ochreous +clay, so that each comb is readily separable from another by a moderate blow of +the hammer. The breadth of each represents the whole width of the fissure at six +successive periods, and the outer walls of the vein, where the first narrow rent +was formed, consisted of the granitic surfaces 1 and 7. + +A somewhat analogous interpretation is applicable to many other cases, where +clay, sand, or angular detritus, alternate with ores and vein-stones. Thus, we +may imagine the sides of a fissure to be incrusted with siliceous matter, as Von +Buch observed, in Lancerote, the walls of a volcanic crater formed in 1731 to be +traversed by an open rent in which hot vapours had deposited hydrate of silica, +the incrustation nearly extending to the middle. (Principles chapter 27 8th +edition page 422.) Such a vein may then be filled with clay or sand, and +afterwards re-opened, the new rent dividing the argillaceous deposit, and +allowing a quantity of rubbish to fall down. Various metals and spars may then +be precipitated from aqueous solutions among the interstices of this +heterogeneous mass. + +That such changes have repeatedly occurred, is demonstrated by occasional cross- +veins, implying the oblique fracture of previously formed chemical and +mechanical deposits. Thus, for example, M. Fournet, in his description of some +mines in Auvergne worked under his superintendence, observes that the granite of +that country was first penetrated by veins of granite, and then dislocated, so +that open rents crossed both the granite and the granitic veins. Into such +openings, quartz, accompanied by sulphurets of iron and arsenical pyrites, was +introduced. Another convulsion then burst open the rocks along the old line of +fracture, and the first set of deposits were cracked and often shattered, so +that the new rent was filled, not only with angular fragments of the adjoining +rocks, but with pieces of the older vein-stones. Polished and striated surfaces +on the sides or in the contents of the vein also attest the reality of these +movements. A new period of repose then ensued, during which various sulphurets +were introduced, together with hornstone quartz, by which angular fragments of +the older quartz before mentioned were cemented into a breccia. This period was +followed by other dilatations of the same veins, and the introduction of other +sets of mineral deposits, as well as of pebbles of the basaltic lavas of +Auvergne, derived from superficial alluviums, probably of Miocene or even Older +Pliocene date. Such repeated enlargement and re-opening of veins might have been +anticipated, if we adopt the theory of fissures, and reflect how few of them +have ever been sealed up entirely, and that a country with fissures only +partially filled must naturally offer much feebler resistance along the old +lines of fracture than anywhere else. + +CAUSE OF ALTERNATE CONTRACTION AND SWELLING OF VEINS. + +(FIGURES 633 to 635. Irregular fissures. + +(FIGURE 633.) + +(FIGURE 634.) + +(FIGURE 635.)) + +A large proportion of metalliferous veins have their opposite walls nearly +parallel, and sometimes over a wide extent of country. There is a fine example +of this in the celebrated vein of Andreasburg in the Hartz, which has been +worked for a depth of 500 yards perpendicularly, and 200 horizontally, retaining +almost everywhere a width of three feet. But many lodes in Cornwall and +elsewhere are extremely variable in size, being one or two inches in one part, +and then eight or ten feet in another, at the distance of a few fathoms, and +then again narrowing as before. Such alternate swelling and contraction is so +often characteristic as to require explanation. The walls of fissures in +general, observes Sir H. De la Beche, are rarely perfect planes throughout their +entire course, nor could we well expect them to be so, since they commonly pass +through rocks of unequal hardness and different mineral composition. If, +therefore, the opposite sides of such irregular fissures slide upon each other, +that is to say, if there be a fault, as in the case of so many mineral veins, +the parallelism of the opposite walls is at once entirely destroyed, as will be +readily seen by studying Figures 633 to 635. + +Let a-b, Figure 633, be a line of fracture traversing a rock, and let a-b, +Figure 634, represent the same line. Now, if we cut in two a piece of paper +representing this line, and then move the lower portion of this cut paper +sideways from a to a', taking care that the two pieces of paper still touch each +other at the points 1, 2, 3, 4, 5, we obtain an irregular aperture at c, and +isolated cavities at d, d, d, and when we compare such figures with nature we +find that, with certain modifications, they represent the interior of faults and +mineral veins. If, instead of sliding the cut paper to the right hand, we move +the lower part towards the left, about the same distance that it was previously +slid to the right, we obtain considerable variation in the cavities so produced, +two long irregular open spaces, f, f, Figure 635, being then formed. This will +serve to show to what slight circumstances considerable variations in the +character of the openings between unevenly fractured surfaces may be due, such +surfaces being moved upon each other, so as to have numerous points of contact. + +(FIGURE 636. Nipped ores where the course of a vein departs from verticality.) + +Most lodes are perpendicular to the horizon, or nearly so; but some of them have +a considerable inclination or "hade," as it is termed, the angles of dip being +very various. The course of a vein is frequently very straight; but if tortuous, +it is found to be choked up with clay, stones, and pebbles, at points where it +departs most widely from verticality. Hence at places, such as a, Figure 636, +the miner complains that the ores are "nipped," or greatly reduced in quantity, +the space for their free deposition having been interfered with in consequence +of the pre-occupancy of the lode by earthy materials. When lodes are many +fathoms wide, they are usually filled for the most part with earthy matter, and +fragments of rock, through which the ores are disseminated. The metallic +substances frequently coat or encircle detached pieces of rock, which our miners +call "horses" or "riders." That we should find some mineral veins which split +into branches is also natural, for we observe the same in regard to open +fissures. + +CHEMICAL DEPOSITS IN VEINS. + +If we now turn from the mechanical to the chemical agencies which have been +instrumental in the production of mineral veins, it may be remarked that those +parts of fissures which were choked up with the ruins of fractured rocks must +always have been filled with water; and almost every vein has probably been the +channel by which hot springs, so common in countries of volcanoes and +earthquakes, have made their way to the surface. For we know that the rents in +which ores abound extend downward to vast depths, where the temperature of the +interior of the earth is more elevated. We also know that mineral veins are most +metalliferous near the contact of Plutonic and stratified formations, especially +where the former send veins into the latter, a circumstance which indicates an +original proximity of veins at their inferior extremity to igneous and heated +rocks. It is moreover acknowledged that even those mineral and thermal springs +which, in the present state of the globe, are far from volcanoes, are +nevertheless observed to burst out along great lines of upheaval and dislocation +of rocks. (See Dr. Daubeny's Volcanoes.) It is also ascertained that all the +substances with which hot springs are impregnated agree with those discharged in +a gaseous form from volcanoes. Many of these bodies occur as vein-stones; such +as silex, carbonate of lime, sulphur, fluor-spar, sulphate of barytes, magnesia, +oxide of iron, and others. I may add that, if veins have been filled with +gaseous emanations from masses of melted matter, slowly cooling in the +subterranean regions, the contraction of such masses as they pass from a plastic +to a solid state would, according to the experiments of Deville on granite (a +rock which may be taken as a standard), produce a reduction in volume amounting +to 10 per cent. The slow crystallisation, therefore, of such Plutonic rocks +supplies us with a force not only capable of rending open the incumbent rocks by +causing a failure of support, but also of giving rise to faults whenever one +portion of the earth's crust subsides slowly while another contiguous to it +happens to rest on a different foundation, so as to remain unmoved. + +Although we are led to infer, from the foregoing reasoning, that there has often +been an intimate connection between metalliferous veins and hot springs holding +mineral matter in solution, yet we must not on that account expect that the +contents of hot springs and mineral veins would be identical. On the contrary, +M. E. de Beaumont has judiciously observed that we ought to find in veins those +substances which, being least soluble, are not discharged by hot springs-- or +that class of simple and compound bodies which the thermal waters ascending from +below would first precipitate on the walls of a fissure, as soon as their +temperature began slightly to diminish. The higher they mount towards the +surface, the more will they cool, till they acquire the average temperature of +springs, being in that case chiefly charged with the most soluble substances, +such as the alkalies, soda and potash. These are not met with in veins, although +they enter so largely into the composition of granitic rocks. (Bulletin 4 page +1278.) + +To a certain extent, therefore, the arrangement and distribution of metallic +matter in veins may be referred to ordinary chemical action, or to those +variations in temperature which waters holding the ores in solution must +undergo, as they rise upward from great depths in the earth. But there are other +phenomena which do not admit of the same simple explanation. Thus, for example, +in Derbyshire, veins containing ores of lead, zinc, and copper, but chiefly +lead, traverse alternate beds of limestone and greenstone. The ore is plentiful +where the walls of the rent consist of limestone, but is reduced to a mere +string when they are formed of greenstone, or "toad-stone," as it is called +provincially. Not that the original fissure is narrower where the greenstone +occurs, but because more of the space is there filled with vein-stones, and the +waters at such points have not parted so freely with their metallic contents. + +"Lodes in Cornwall," says Mr. Robert W. Fox, "are very much influenced in their +metallic riches by the nature of the rock which they traverse, and they often +change in this respect very suddenly, in passing from one rock to another. Thus +many lodes which yield abundance of ore in granite, are unproductive in clay- +slate, or killas and vice versa. + +SUPPOSED RELATIVE AGE OF THE DIFFERENT METALS. + +After duly reflecting on the facts above described, we can not doubt that +mineral veins, like eruptions of granite or trap, are referable to many distinct +periods of the earth's history, although it may be more difficult to determine +the precise age of veins; because they have often remained open for ages, and +because, as we have seen, the same fissure, after having been once filled, has +frequently been re-opened or enlarged. But besides this diversity of age, it has +been supposed by some geologists that certain metals have been produced +exclusively in earlier, others in more modern times; that tin, for example, is +of higher antiquity than copper, copper than lead or silver, and all of them +more ancient than gold. I shall first point out that the facts once relied upon +in support of some of these views are contradicted by later experience, and then +consider how far any chronological order of arrangement can be recognised in the +position of the precious and other metals in the earth's crust. + +In the first place, it is not true that veins in which tin abounds are the +oldest lodes worked in Great Britain. The government survey of Ireland has +demonstrated that in Wexford veins of copper and lead (the latter as usual being +argentiferous) are much older than the tin of Cornwall. In each of the two +countries a very similar series of geological changes has occurred at two +distinct epochs-- in Wexford, before the Devonian strata were deposited; in +Cornwall, after the Carboniferous epoch. To begin with the Irish mining +district: We have granite in Wexford traversed by granite veins, which veins +also intrude themselves into the Silurian strata, the same Silurian rocks as +well as the veins having been denuded before the Devonian beds were +superimposed. Next we find, in the same county, that elvans, or straight dikes +of porphyritic granite, have cut through the granite and the veins before +mentioned, but have not penetrated the Devonian rocks. Subsequently to these +elvans, veins of copper and lead were produced, being of a date certainly +posterior to the Silurian, and anterior to the Devonian; for they do not enter +the latter, and, what is still more decisive, streaks or layers of derivative +copper have been found near Wexford in the Devonian, not far from points where +mines of copper are worked in the Silurian strata. + +Although the precise age of such copper lodes can not be defined, we may safely +affirm that they were either filled at the close of the Silurian or commencement +of the Devonian period. Besides copper, lead, and silver, there is some gold in +these ancient or primary metalliferous veins. A few fragments also of tin found +in Wicklow in the drift are supposed to have been derived from veins of the same +age. (Sir H. De la Beche MS. Notes on Irish Survey.) + +Next, if we turn to Cornwall, we find there also the monuments of a very +analogous sequence of events. First, the granite was formed; then, about the +same period, veins of fine-grained granite, often tortuous (see Figure 614), +penetrating both the outer crust of granite and the adjoining fossiliferous or +primary rocks, including the coal-measures; thirdly, elvans, holding their +course straight through granite, granitic veins, and fossiliferous slates; +fourthly, veins of tin also containing copper, the first of those eight systems +of fissures of different ages already alluded to. Here, then, the tin lodes are +newer than the elvans. It has, indeed, been stated by some Cornish miners that +the elvans are in some instances posterior to the oldest tin-bearing lodes, but +the observations of Sir H. de la Beche during the survey led him to an opposite +conclusion, and he has shown how the cases referred to in corroboration can be +otherwise interpreted. (Report on the Geology of Cornwall page 310.) We may, +therefore, assert that the most ancient Cornish lodes are younger than the coal- +measures of that part of England, and it follows that they are of a much later +date than the Irish copper and lead of Wexford and some adjoining counties. How +much later, it is not so easy to declare, although probably they are not newer +than the beginning of the Permian period, as no tin lodes have been discovered +in any red sandstone which overlies the coal in the south-west of England. + +There are lead veins in Glamorganshire which enter the lias, and others near +Frome, in Somersetshire, which have been traced into the Inferior Oolite. In +Bohemia, the rich veins of silver of Joachimsthal cut through basalt containing +olivine, which overlies tertiary lignite, in which are leaves of dicotyledonous +trees. This silver, therefore, is decidedly a tertiary formation. In regard to +the age of the gold of the Ural mountains, in Russia, which, like that of +California, is obtained chiefly from auriferous alluvium, it occurs in veins of +quartz in the schistose and granitic rocks of that chain, and is supposed by Sir +R. Murchison, MM. Deverneuil and Keyserling to be newer than the syenitic +granite of the Ural-- perhaps of tertiary date. They observe that no gold has +yet been found in the Permian conglomerates which lie at the base of the Ural +Mountains, although large quantities of iron and copper detritus are mixed with +the pebbles of those Permian strata. Hence it seems that the Uralian quartz +veins, containing gold and platinum, were not formed, or certainly not exposed +to aqueous denudation, during the Permian era. + +In the auriferous alluvium of Russia, California, and Australia, the bones of +extinct land-quadrupeds have been met with, those of the mammoth being common in +the gravel at the foot of the Ural Mountains, while in Australia they consist of +huge marsupials, some of them of the size of the rhinoceros and allied to the +living wombat. They belong to the genera Diprotodon and Nototherium of Professor +Owen. The gold of Northern Chili is associated in the mines of Los Hornos with +copper pyrites, in veins traversing the cretaceo-oolitic formations, so-called +because its fossils have the character partly of the cretaceous and partly of +the oolitic fauna of Europe. (Darwin's South America page 209 etc.) The gold +found in the United States, in the mountainous parts of Virginia, North and +South Carolina, and Georgia, occurs in metamorphic Silurian strata, as well as +in auriferous gravel derived from the same. + +Gold has now been detected in almost every kind of rock, in slate, quartzite, +sandstone, limestone, granite, and serpentine, both in veins and in the rocks +themselves at short distances from the veins. In Australia it has been worked +successfully not only in alluvium, but in vein-stones in the native rock, +generally consisting of Silurian shales and slates. It has been traced on that +continent over more than nine degrees of latitude (between the parallels of 30 +degrees and 39 degrees S.), and over twelve of longitude, and yielded in 1853 an +annual supply equal, if not superior, to that of California; nor is there any +apparent prospect of this supply diminishing, still less of the exhaustion of +the gold-fields. + +ORIGIN OF GOLD IN CALIFORNIA. + +Mr. J. Arthur Phillips, in his treatise "On the Gold Fields of California," has +shown that the ore in the gold workings is derived from drifts, or gravel clay, +and sand, of two distinct geological ages, both comparatively modern, but +belonging to different river-systems, the older of which is so ancient as to be +capped by a thick sheet of lava divided by basaltic columns. (Proceedings of the +Royal Society 1868 page 294.) The auriferous quartz of these drifts is derived +from veins apparently due to hydrothermal agency, proceeding from granite and +penetrating strata supposed to be of Jurassic and Triassic date. The fossil wood +of the drift is sometimes beautifully silicified, and occasionally the trunks of +trees are replaced by iron pyrites, but gold seems not to have been found as in +the pyrites of similarly petrified trees in the drift of Australia. + +The formation of recent metalliferous veins is now going on, according to Mr. +Phillips, in various parts of the Pacific coast. Thus, for example, there are +fissures at the foot of the eastern declivity of the Sierra Nevada in the state +of that name, from which boiling water and steam escape, forming siliceous +incrustations on the sides of the fissures. In one case, where the fissure is +partially filled up with silica inclosing iron and copper pyrites, gold has also +been found in the vein-stone. + +It has been remarked by M. de Beaumont, that lead and some other metals are +found in dikes of basalt and greenstone, as well as in mineral veins connected +with trap-rock, whereas tin is met with in granite and in veins associated with +the Plutonic series. If this rule hold true generally, the geological position +of tin accessible to the miner will belong, for the most part, to rocks older +than those bearing lead. The tin veins will be of higher relative antiquity for +the same reason that the "underlying" igneous formations or granites which are +visible to man are older, on the whole, than the overlying or trappean +formations. + +If different sets of fissures, originating simultaneously at different levels in +the earth's crust, and communicating, some of them with volcanic, others with +heated Plutonic masses, be filled with different metals, it will follow that +those formed farthest from the surface will usually require the longest time +before they can be exposed superficially. In order to bring them into view, or +within reach of the miner, a greater amount of upheaval and denudation must take +place in proportion as they have lain deeper when first formed and filled. A +considerable series of geological revolutions must intervene before any part of +the fissure which has been for ages in the proximity of the Plutonic rock, so as +to receive the gases discharged from it when it was cooling, can emerge into the +atmosphere. But I need not enlarge on this subject, as the reader will remember +what was said in the 30th, 32d, and 35th chapters on the chronology of the +volcanic and hypogene formations. + + +INDEX. + +Abbeville, flint tools of. + +Aberdeenshire, granite of. + +Abich, M., on trachytic rocks. + +Acer trilobatum, Miocene. + +Acrodus nobilis, Lias. + +Acrogens, term explained. + +Acrolepis Sedgwickii, Permian. + +Actaeon acutus, Great Oolite. + +Actinocyclas, in Atlantic mud. + +Actinolite. +-- schist. + +Aechmodus Leachii, Lias. + +Adiantites Hibernica, Old Red. + +Agassiz on fish of Sheppey. +-- on fish of the Brown-Coal. +-- on fish of Monte Bolca. +-- on Old Red fossil fish. +-- on Silurian fish. + +Age of metamorphic rocks. +-- of Plutonic rocks. +-- of strata, tests of. +-- of volcanic rocks. + +Agglomerate described. + +Agnostus integer. A. Rex. + +Air-breathers of the Coal. + +Aix-la-Chapelle, Cretaceous flora of. + +Alabaster defined. + +Alberti on Keuper. + +Albite. + +Aldeby and Chillesford beds. + +Alkali, present in the Palaeozoic strata. + +Alpine blocks on the Jura. + +Alps, age of metamorphic rocks in. +--, nummulitic limestone and flysch of. + +Alum schists of Norway and Sweden. + +Alluvial deposits, Recent and Post-pliocene. + +Alluvium, term explained. +-- in Auvergne. + +Alternations of marine and fresh-water strata. + +Alum Bay beds, plants of the. + +Amblyrhynchus cristatus, a living marine saurian. + +America. See United States, Canada, Nova Scotia. +--, North, Glacial formations of. +--, South, gradual rise of land in. +--, Silurian strata of. + +American character of Lower Miocene flora. +-- forms in Swiss Miocene flora. + +Amiens, flint tools of. + +Ammonites bifrons, Lias. +-- Braikenridgii, Oolite. +-- Bucklandi, Lias. +-- Deshayesii, Neocomian. +-- Humphresianus, Inferior Oolite. +-- Jason, Oxford Clay. +-- Noricus, Speeton. +-- macrocephalus, Oolite. +-- margaritatus, Lias. +-- planorbis, Lias. +-- rhotomagensis, Chalk marl. + +Amphibole group of minerals. + +Amphistegina Hauerina, Vienna basin. + +Amphitherium Broderipii, in Stonesfield. +-- Prevostii, Stonesfield slate. + +Ampullaria glauca. + +Amygdaloid. + +Analcime. + +Anamesite, a variety of basalt. + +Ananchytes ovatus, White chalk. +--, with crania attached. + +Ancillaria subulata, Eocene. + +Ancyloceras gigas. +-- spinigerum, Gault. +-- Duvallei, Neocomian. + +Ancylus velletia (A. elegans). + +Andalusite. + +Andes, Plutonic rocks of the. + +Andreasburg, metalliferous vein of. + +Angelin, on Cambrian of Sweden. + +Angiosperms. +-- of the Coal. + +Anglesea, dike cutting through shale in. + +Anodonta Cordierii. +-- Jukesii, Upper Old Red. +-- latimarginata. + +Anoplotherium commune, Binstead. +-- gracile, Paris basin. + +Anorthite. + +Annularia sphenophylloides, Coal. + +Antholithes, coal-measures. + +Anthracite, conversion of coal into. + +Anticlinal and synclinal curves. + +Antrim, Chalk altered by a dike in. +--, Lower Miocene, volcanic rocks of. + +Antwerp Crag. + +Apateon pedestris, a carboniferous reptile. + +Apatite. + +Apennines, Northern, metamorphic rocks of. + +Apes, fossil of the Upper Miocene. + +Apiocrinites rotundus, Bradford. + +Appalachians, long lines of flexures in. +--, vast thickness of successive strata in. + +Aptychus, part of ammonite. + +Aqueous rocks defined. + +Araucaria sphaerocarpa, Inferior Oolite. + +Arbroath, section of Old Red at. + +Archaeopteryx macrura, Solenhofen. + +Archegosaurus minor and A. medius, coal measures. + +Archiac, M. de, on nummulites. +--, on chalk of France. + +Arctic Miocene Flora. + +Area of the Wealden. + +Areas, permanence of continental. + +Arenaceous rocks described. + +Arenicolites linearis, Arenig beds. + +Arenig or Stiper-Stones group. +--, volcanic formations of. + +Argile plastique. + +Argillaceous rocks described. + +Argillite, Argillaceous schist. + +Argyll, Duke of, on Isle of Mull leaf-beds. + +Armagh, bone-beds in Mountain Limestone at. + +Arran, amygdaloid filled with spar near. +--, erect trees in volcanic ash of. +--, Greenstone dike in. + +Arthur's seat, trap rocks of. + +Arvicola, tooth of. + +Asaphus caudatus, Silurian. +-- tyrannus, A. Buchii. + +Ascension, lamination of volcanic rocks in. + +Ash, Mr., on fossils of Tremadoc beds. + +Ashby-de-la-Zouch, fault in coal field of. + +Aspidura loricata, Muschelkalk. + +Astarte borealis (=A. arctica = A. compressa). +-- Omalii, Crag. + +Asterophyllites foliosus, Coal. + +Astrangia lineata (Anthophyllum lineatum). + +Astraea basaltiforme, Carboniferous. + +Astropecten crispatus, London clay. + +Atherfield clay. + +Atlantic mud, composition of. + +Atrypa reticularis, Aymestry. + +Aturia ziczac (Nautilus ziczac). + +Augite. + +Auricula, recent. + +Austen, Mr. Godwin, on marine deposit of Selsea Bill. +--, on boulders in chalk. + +Australian cave breccias. + +Australia, auriferous gravel of. + +Auvergne, alluvium in. +--, chain of extinct volcanoes in. +--, granite veins in. +--, Lower Miocene of. +--, Miocene volcanic rocks of. +--, Post-pliocene volcanic eruptions in. +--, springs from spent volcanoes in. + +Aveline Mr., on Tarannon shales. + +Avicula contorta, Rhaetic beds. +-- cygnipes, Lias. +-- inaequivalvis, Lias. +-- socialis, Muschelkalk. + +Aviculopecten papyraceus, coal measures. +-- sublobatus, mountain limestone. + +Aymestry Limestone. + +Azoic period, supposed. + +Azores, Miocene lavas with shells. + +Bacillaria paradoxa. + +Baculites anceps, Lower Chalk. +-- Fauiasii, chalk. + +Baffin's Bay, formation of drift in. + +Bagshot sands. + +Baiae, Bay of, subterranean igneous action in. + +Bakewell, Mr., on cleavage in Swiss Alps. + +Bala and Caradoc beds. + +Balistidae, defensive spine of. + +Bangor, or Longmynd group. + +Banksia, seed and fruit of, Lower Miocene. + +Barmouth sandstones. + +Barnes, Mr. J., on insects in American coal. + +Barnstaple, Upper Devonian of. + +Barrande, M. Joachim, his "Primordial Zone." +--, on metamorphosis of trilobites. + +Barrett, Mr., on bird in Blackdown beds. + +Barton series sands and clays. +-- shells, percentage of, common to London clay. + +Basalt, columnar. +--, composition of. + +Basaltic rocks, poor in silica. +--, specific gravity of minerals in. + +Basilosaurus, Eocene, United States. + +Basset, term explained. + +Basterot, M. de, on Bordeaux tertiary strata. + +Bath Oolite. + +Batrachian reptiles in coal. + +Bay of Fundy, denudation in coalfield in. + +Bean, Mr., on Yorkshire Oolite. + +Bear Island carboniferous flora. + +Beaumont, M. E. de, on island in Cretaceous sea. +--, on mineral veins. +--, on Jurassic plutonic rocks. +--, on formation of granite. + +Beckles, Mr. S.H., on footprints in Hastings sands. +-- on Mammalia of Purbeck. + +Belemnitella mucronata, Chalk. + +Belemnites hastatus, Oxford clay. +-- Puzosianus, Oxford clay. + +Belgium, Lower Miocene of. + +Bellerophon costatus, Mountain Limestone. + +Belosepia sepioidea, Sheppey. + +Belt, Mr., on subdivision of Lingula Flags. + +Bembridge beds, Yarmouth. + +Berger, Dr., on rocks altered by dikes. + +Berlin, Miocene strata near. + +Bernese Alps, gneiss in the. + +Berthier on isomorphism. + +Bertrich-Baden, columnar basalt of. + +Beyrich on term Oligocene for Lower Miocene. + +Billings, Mr., on trilobites. + +Binney, Mr., on Sigillariae in volcanic ash. +--, on Stigmaria, the root of Sigillaria. + +Biotite. + +Bird in argile plastique. + +Bischoff, Professor, on Nile and Rhine mud. +--, on conversion of coal into anthracite. +--, on hydrothermal action. + +Blackdown beds. + +Blacklead of Borrowdale. + +Bog-iron-ore. + +Bohemia, Cambrian rocks of. +--, silver veins in. + +Bolderberg, in Belgium, Upper Miocene of. + +Bone-bed of fish remains, Armagh. +-- of Upper Ludlow. +-- of the Trias. + +Boom, Lower Miocene of. + +Bordeaux, Upper Miocene of. + +Borrowdale, blacklead of. + +Bosquet, M. on chalk fossils. +--, on Maestricht beds. + +Botanical nomenclature. + +Boucher de Perthes on Abbeville alluvium. + +Boulder-clay, whether formed by icebergs or land-ice. + +Boulder-clay of Canada. +-- fauna of. + +Boulders and pebbles in chalk. + +Bournemouth beds (Lower Bagshot). + +Bovey Tracey, lignites and clays of. + +Bowerbank, Mr., on fossil fruits of London Clay. +--, on fossil fruits of Sheppey. + +Bowman, Mr., on uniting of distinct coal-seams. + +Brachiopoda, preponderance of, in older rocks. +--, mode of recognising shells of. + +Bracklesham beds and Bagshot Sands. + +Bradford encrinites. + +Breccias of Lower Permian. + +Brick-earth or fluviatile loam. + +Bridlington drift. + +Bristol, dolomitic conglomerate of. + +Bristow, Mr., on volcanic minerals. + +Brixham cave near Torquay. + +Brocchi on Italian tertiary strata. +-- on subapennine strata. + +Brockenhurst, corals and shells of. + +Brodie, Reverend P.B., on Lias insects. + +Brodie, Mr. W.R., on Purbeck mammalia. + +Brongniart, M. Adolphe, on botanical nomenclature. +--, on Lias plants. +--, on flora of the Bunter. +--, on flora of the coal. +--, on fruit of Lepidodendron. +--, M. Alex., on Tertiary series. + +Bronteus flabellifer, Devonian. + +Brora, oolitic coal formation of. + +Brown, Mr. Richard, on Stigmaria. +--, on carboniferous rain-prints. + +Brown, Robert, on Eocene protaceous fruit. + +Brown, Reverend T., on marine shells in Scotch drift. + +Brown-coal of Germany. + +Bryce, Mr., on Scotch till. + +Bryozoa of Mountain Limestone. +-- and polyzoa, terms explained. + +Buch, von. See Von Buch. + +Buckland, Dr., on Kirkdale cave. +--, on violent death of saurians. +--, on spines of fish. +--, on Eocene oysters. +--, on pot-stones in chalk. + +Buddle, Mr., on creeps in coal-mines. + +Bulimus ellipticus, Bembridge. +-- lubricus, Loess. + +Bullock, Captain, R.N., on Atlantic mud. + +Bunbury, Sir C., on leaf-bed of Madeira. +--, on ferns of the Maryland coal + +Bunter of Germany. +-- or Lower Trias of England. + +Buprestis? Elytron of, Stonesfield. + +Burmeister on trilobites. + +Cainozoic, term defined. + +Caithness, fish beds of. + +Calamite, root of. + +Calamites Sucowii, coal, and restored stem. + +Calamophyllia radiata, Bath Oolite. + +Calcaire de la Beauce, age of the. +-- grossier, fossils of the. +-- siliceux of France. + +Calcareous matter poured out by springs. +-- rocks described. +-- nodules in Lias. + +Calcarina rarispina, Eocene. + +Calceola sandalina, Devonian. +--, schiefer of Germany. + +California, aurifrous gravel of. +--, gold in petrified wood of age of alluvium. + +Calymene Blumenbachii, Silurian. + +Cambrian Group, classification of the. + +Cambrian, Upper. +--, Lower. +--, of Sweden and Norway. +--, strata of Bohemia. +--, of North America. +--, volcanic rocks. + +Campophyllum flexuosum. + +Canada, Cambrian of. +--, Devonian of. +--, trap-rocks of. + +Canadian drift. + +Canary, Grand, shelly tuffs of. + +Cantal, Lower Miocene of the. + +Cape Breton, rain-prints in coal-measures of. + +Cape Wrath, granite veins in gneiss at. + +Caradoc and Bala beds. + +Carbonate of lime in rocks, how tested. + +Carboniferous Group, subdivisions of the. +-- flora. +-- limestone, thickness of. +--, marine fauna of the. +-- Period, trap-rocks of. +-- plutonic rocks. +-- reptiles. +-- insects. + +Carcharodon angustidens, Bracklesham. + +Cardiganshire, section of slaty cleavage in. + +Cardiocarpon Ottonis, Permian. + +Cardita (Venericardia) planicosta. +-- sulcata, Barton. + +Cardium dissimile, Portland Stone. +-- rhaeticum, Rhaetic Beds. +-- striatulum, Kimmeridge clay. + +Carne, Mr. N., on Cornish lodes. + +Carpenter, Dr., on Atlantic mud. +--, on Eozoon Canadense. + +Carrara, marble of. + +Carruthers, Mr., on Eocene proteaceous fruit. +--, on cycads of the Purbeck. +--, on leaves of calamite. +--, on spores of carboniferous Lycopodiaceae. +--, on structure of sigillaria. +--, on trees in volcanic ash. + +Cashmere, recent formations in. + +Cassian, St., Triassic strata of. + +Castrogiovanni, curved strata near. + +Catania, laterite formed in. +--, Tertiary beds in. + +Catillus Lamarckii, White Chalk. + +Caucasus, absence of lakes in the. + +Caulopteris primaeva, Coal. + +Cave-breccias of Australia. + +Cavern deposits with human and animal remains. + +Caves of Kirkdale and Brixham. + +Celts described. + +Cementing of strata. + +Cephalaspis Lyelli, Old Red. + +Ceratites nodosus, Muschelkalk. + +Cerithium concavum, Headon. +-- elegans, Hempstead beds. +-- (Terebra) Portlandicum. +-- plicatum, Hempstead beds. +-- melanoides. + +Cervus alces, tooth of. + +Cestracion Phillippi, Recent. + +Chabasite. + +Chalk, composition, extent, and origin of. +-- of Faxoe. +-- flints, origin of. +-- fossils of the White. +--, iceborne boulders in the. +-- of North and South Europe. +--, Lower White, without flints. +-- marl, fossils of the. +-- Period, popular error concerning. + +Chalk-pit with pot-stones, view of. + +Chama squamosa, Barton. + +Champoleon, junction of granite with Jurassic strata near. + +Chara elastica, C. medicaginula. +-- tuberculata, Bembridge. + +Charpentier, M., on Alpine glaciers. +--, on depression of Alps in Glacial Period. + +Chatham coal-field. + +Cheirotherium, footprints of. + +Chemical deposits in veins. +-- and mechanical deposits. + +Chiapa, fall of volcanic dust at. + +Chichester, erratics near. + +Chili, copper pyrites with gold in. +--, walls cracked by earthquake in. + +Chillesford and Aldeby beds. + +Chimaera monstrosa, Lias. + +Chlorite-schist. + +Chloritic series, or Upper Greensand. + +Christiania, Euritic porphyry at. +--, granite veins in Silurian strata of. +--, quartz vein in gneiss at. + +Chronological groups of formations. + +Chronology, test of, in rocks. + +Cinder-bed of the Purbeck. + +Cinnamomum polymorphum, Miocene. +-- Rossmassleri, Miocene. + +Claiborne beds, Eocene fossils of. + +Clarke County, United States, Zeuglodon of. + +Classification of Tertiary formations. +--, value of shells in. + +Clausilia bidens, Loess. + +Clay defined. +-- iron-stone defined. +--, plastic. +-- slate. +--, Weald. + +Cleavage explained. +--, crystalline theory of. +--, mechanical theory of. +-- of metamorphic rocks. + +Cleidotheca operculata. + +Clermont, metalliferous gneiss near. + +Climate of the Crags. +-- of the Coal. +-- of the Miocene in the Arctic regions. +-- of the Post-pliocene period. + +Clinkstone. + +Clinton group, fossils of the. + +Clyde, buried canoes in estuary of. +--, arctic marine shells in drifts of. + +Clymenia linearis, Devonian. + +Clymenien-Kalk of Germany. + +Coal, conversion into anthracite of. +-- a land and swamp formation. +--, cause of the purity of. +--, conversion of lignite into. +--, erect trees in. +--, structure of the. +--, vegetation of the. +--, air-breathers in the. + +Coal Period, climate of the. +-- field of Virginia. +-- measures of Nova Scotia. +-- measures, thickness of, in Wales. +-- pipes, danger of. +--, rainprints in. +-- seams, uniting of. + +Coalbrook-Dale, faults in. + +Cochliodus contortus. + +Cockfield Fell rocks, altered by dikes. + +Coelacanthus granulatus, Permian. + +Coleoptera of Oeningen beds. + +Collyrites ringens, Inferior Oolite. + +Columnar structure of volcanic rocks. +-- basalt in the Vicentin. + +Compact feldspar. + +Concretionary structure. + +Cone of Tartaret. +-- of Come. + +Cones and craters described. +--, absence of, in England. + +Conformable stratification. + +Conglomerate or pudding-stone. +--, Dolomitic, of Bristol. + +Coniferae of the coal-measures. + +Connecticut Valley, New Red Sandstone of. + +Conocephalus striatus. + +Conocoryphe striata. + +Conrad, Mr., on age of American cretaceous rocks. + +Consolidation of strata. + +Continents and oceans, permanence of. + +Contorted strata, in drift. + +Conularia ornata, Devonian. + +Conulus priscus, Coal. + +Conus deperditus, Bracklesham. + +Conybeare and Phillips on ninety-fathom dike. + +Conybeare, Mr., on reptiles of the Lias. + +Copper lode near Redruth. + +Coprolite bed of Chloritic Series. +-- beds of Red and Coralline crags. + +Coprolites of fish from the chalk. + +Coral Rag, fossils of the. + +Coralline of White Crag. + +Corals of the Devonian. +-- of the Mountain Limestone. +--, Neozoic type of. +--, Palaeozoic type of. + +Corbicella (Cyrena) fluminalis. + +Corbula pisum, Hempstead beds. + +Corinth, corrosion of rocks by gases near. + +Cornbrash or Forest Marble. + +Cornwall, granite veins in. +--, lodes in. +--, mass of granite in. +--, vertical sections of veins in mine. + +Cosequina volcano, burying of organic remains by. + +Crag, term defined. +-- of Antwerp. +--, fauna of, its relation to that of present seas. +--, Norwich. +--, Coralline or White. +--, Red. +--, tables of marine testacea in. +-- deposits, climate of. + +Crania attached to a sea-urchin. +-- Parisiensis, White Chalk. + +Crassatella sulcata, Barton. + +Craters and cones described. +--, Theory of Elevation. + +Craven fault. + +Creeps in coal-mines. + +Cretaceous rocks of United States. +-- Period, error as to continuity of. +--, flora of the Upper. +-- volcanic rocks. +-- plutonic rocks. +-- Period, distinct mineral character of rocks in. +-- rocks, classification of. +-- strata, connection between Upper and Lower. + +Crinoidea of Mountain Limestone. + +Croatia, Lower Miocene beds of. + +Croll, Mr., on amount of subaerial denudation. + +Cromer forest-bed. + +Crop out, term explained. + +Crossopterygidae, or fringe-finned fish. + +Crowfoot, Mr., on shells of Aldeby beds. + +Crust of the earth defined. + +Crustaceans of Old Red Sandstone. + +Cryptodon angulatum, London Clay. + +Crystalline Limestone. +-- rocks defined. +-- schists, much alkali in the. +-- theory of cleavage. + +Cup and Star corals. + +Curved strata. + +Cutch, salt-layers in the Runn of. + +Cuvier, M., on fauna of the Paris basin. +--, on Mammalia of Paris gypsum. +--, on Tertiary series. + +Cyathocrinus caryocrinoides. +-- planus. + +Cyathophyllum caespitosum. + +Cyclopean isles, beds of tuff and clay in. +--, contorted strata in. + +Cyclopteris Hibernica, Old Red. + +Cyclostigma (Lepidodendron), Old Red. + +Cyclostoma elegans, Loess. + +Cylindrites acutus, Great Oolite. + +Cypress swamps of the Mississippi. + +Cyprides in the Weald Clay. + +Cypridina serrato-striata. + +Cypris in fresh-water deposits. +-- gibbosa, C. tuberculata, C. leguminella. +-- striato-punctata, C. fasciculata, C. granulata. +-- Purbeckensis, Cypris punctata. +-- spinigera, Weald Clay. + +Cyrena (Corbicella) fluminalis. +-- cuneiformis, Woolwich Clays. +-- obovata. +-- semistriata, Hempstead beds. + +Cystideae of Silurian rocks. + +Cythere inflata, coal-measures. + +Dadoxylon, fragment of coniferous wood. + +Dana, on volcanic minerals. + +Danish kitchen-middens. + +Dapedius monilifer, Lias. + +Darbishire on shells of Moel Tryfaen. + +Dartmoor, post-carboniferous granite of. +-- intrusive granite at. + +Darwin, Mr., on foliation and lamination. +--, on mammalia of South America. +--, on marine saurian. +--, on rise of part of South America. +--, on sinking of coral reefs. +--, on plutonic rocks of the Andes. +--, on relationship of extinct to living types. + +Dates of discovery of fossil vertebrata. + +Daubeny, Dr., on decomposition of trachytic rocks. + +Daubree, on formation of zeolites. +--, on alkaline waters of Plombieres. + +Davidson, Mr., on Spiriferina. + +Davis, Mr. E., on fossils of Lingula Flags. + +Dawkins, Mr. Boyd, on Hyaena spelaea. +--, on mammalia of Cromer Forest-bed. +--, on Triassic mammifer. + +Dawson, Dr., on Devonian flora and insects. +--, on Eozoon Canadense. +--, on Nova Scotia coal-measures. +--, on Nova Scotia coal-plants. +--, on Pupa vetusta. +--, on reptiles and shells in Nova Scotia coal. +--, on structure of calamite. +--, on structure of sigillaria. + +Deane, Dr., on footprints in Trias. + +Debey, Dr., on flora and fauna of Aix. + +Dechen, M. von, on organic remains of the brown coal. +--, on Cornish granite veins. + +De la Beche, Sir H., on granite of Dartmoor. +--, on Carrara marble. +--, on mineral veins. +--, on Redruth copper-mine. +--, on saurians of the Lias. +--, on trap-rocks of New Red. +--, on Welsh coal-measures. + +Delesse, on action of water in metamorphism. + +Deltas, strata accumulated in. + +Dendrerpeton in Coal. + +Denudation defined. +--, subaerial. +--, littoral. +--, submarine. +--, average annual amount of subaerial. +-- of carboniferous strata. +-- counteracting upheaval. +-- a means of exposing crystalline rocks. +--, trap-dikes cut off by. +-- and volcanic force antagonistic powers. + +Deposition, rate of, shown by fossils. + +Derbyshire, veins in Mountain Limestone. + +Derivative shells of the Red Crag. + +Desnoyers, M., on age of Faluns. +--, on Eocene fossil footprints. + +Desor, M., on Celtic coins in lake-dwellings. + +Devonian Period, Upper, Middle, Lower. +-- fossils of the Eifel. +-- of Russia. +-- of United States and Canada. +-- insects of Canada. +-- strata, classification of. + +Devonshire, cleavage of slate rocks in. + +Diabase. + +Diagonal, or cross-stratification. + +Diagram of fossiliferous rocks. +-- of plutonic and sedimentary formations. + +Diallage. + +Diastopora diluviana, Bath Oolite. + +Diatomaceae forming tripoli. + +Diceras Lonsdalii, Neocomian. + +Didelphys Azarae, Recent. + +Didymograpsus geminus. +-- Murchisonii. + +Dike cutting through shale, Anglesea. +-- cutting through chalk, Antrim. + +Dikelocephalus Minnesotensis. + +Dikes defined. +-- of Monte Somma. +-- in Palagonia, ground-plan of. +--, volcanic or trap. + +Diluvium, origin of term. + +Dinornis Palapteryx, of New Zealand. + +Dinotherium giganteum. + +Diorite. + +Dip and strike, terms explained. + +Diplograpsus folium, Llandeilo Flags. +-- pristis, Llandeilo beds. + +Dirt-bed of the Purbeck. + +Dogger-bank described. + +Dolerite, a variety of basalt. + +Dolomite defined. + +Dolomitic conglomerate of Bristol. + +Downs, escarpments of North and South. + +Downton Sandstone. + +Dowson, Mr., on shells of Aldeby beds. + +Drew, Mr., on Hastings Sands. + +Drift of Ireland. +-- of Norfolk cliffs. +-- of Scandinavia. +-- of Bridlington. +-- carried by icebergs. +-- shells in Canada. +--, contorted strata in. +--, marine shells in Scotch. + +Dudley Limestone. + +Dufrenoy, M., on granite of Pyrenees. + +Dumont, Professor, on Belgian Lower Eocene. + +Duncan, Dr., on Neozoic corals passing down to Devonian. + +Dundry Hill, near Bristol, section of. + +Dunker, Dr., on wealden of Germany. + +Dura Den, yellow sandstone of. + +Earth's crust defined. + +Echinoderms of Suffolk Crag. + +Echinosphaeronites balticus. + +Egerton, Sir P., on fish of Headon series. +--, on fish of the Permian. +--, on fish of Penarth beds. + +Ehrenberg, Professor, on term Bryozoum. +--, on Silurian foraminifera. +--, on infusoria. + +Eifel Limestone. +--, Lake-craters of. +-- Miocene, volcanic rocks of. +-- Pliocene, volcanoes of the. +--, trass of the. + +Elephas antiquus, molar of. +-- meridionalis, molar of. +-- primigenius, molar of. + +Elevation craters, theory of. + +Elvans, term explained. +-- of Ireland and Cornwall. + +Elytron of Buprestis? Stonesfield. + +Emmons, Professor, on jaws of Triassic quadruped. +--, on Dromatherium. + +Encrinites of Bradford. + +Encrinus liliiformis, Muschelkalk. + +Endogens, term explained. + +Engihoul cave, human and animal remains in. + +England and Wales, glaciation of. + +Enstatite. + +Eocene areas of Europe, map of. +-- foraminifera. +-- formations of France. +-- of England. +-- period, volcanic rocks of. +--, plutonic rocks of the. +--, metamorphic rocks of the. +-- of France, footprints in. +-- and Miocene, line between the. +--, term defined. +-- of the United States. + +Eozoon Canadense, oldest known fossil. + +Epidote. + +Eppelsheim, Dinotherium of. + +Equisetaceae of the Coal. + +Equisetites columnaris, Keuper. + +Equus caballus, tooth of. + +Erratic blocks, nature of. +-- of Greenland. +-- near Chichester. +-- in the Red Crag. + +Erratics, Alpine. + +Escarpments explained. + +Eschara disticha, White Chalk. + +Escharina oceani, White Chalk. + +Estheria minuta, Trias. +-- ovata, Richmond, Virginia. + +Ethridge, Mr., on Atlantic mud. +--, on Devonian series, in Devon. +--, on Devonian fauna. +--, on mollusca of Bracklesham. +--, on St. Cassian fossils. + +Etna, built up since Newer Pliocene. +--, Pliocene lavas of. + +Ettingshausen on Sheppey Eocene fruit. + +Eunomia radiata, Bath Oolite. + +Eunotia bidens, Atlantic mud. + +Euomphalus pentangulatus. + +Eurite. + +Euritic porphyry of Norway. + +Evans, Mr., on Archaeopteryx. + +Exogens. + +Exogyra virgula, Kimmeridge Clay. + +Extracrinus (Pentacrinus) Briareus, Lias. + +Falconer, Dr., on Miocene fauna of Siwalik Hills. +--, on Brixham Cave flint knives. +--, on Purbeck mammalia. + +Faluns of Loire, recent shells in. +-- of Touraine. + +Farnham, phosphate of lime near. + +Fascicularia aurantium, Coralline crag. + +Faults in coal-measures of Coalbrook Dale. +-- described. +-- often the result of repeated movements. + +Fauna of the crag, its relation to that of our present seas. +-- of the Mountain Limestone. +-- of the Paris basin. + +Favosites cervicornis, Devonian. +-- Gothlandica, Silurian. + +Favre, M. E., on glaciers and moraines of the Caucasus. + +Faxoe, chalk of. + +Feldspar-porphyry. + +Feldspar, varieties of. + +Feldstone. + +Felis tigris, tooth of. + +Fenestella retiformis, Magnesian Limestone. + +Ferns of the coal. + +Fife, trap-dike in. + +Fish, fossil of the Carboniferous. +--, Eocene of Monte Bolca. +--, oldest known fossil. +--, number of living. +--, fresh-water and marine. +-- of the Upper Ludlow. +-- of the Old Red Sandstone. +-- of the Permian marl slate. +-- of the brown coal. +-- of the Lias. + +Fisherton, Greenland lemming in drift of. + +Fissures, filled with metallic matter. + +Fitton, Dr., on the Neocomian strata. + +Fleming, Dr., on Parka decipiens. +--, on trap-dike in Fife. + +Flints in the Chalk. + +Flisk dike of Fife. + +Flora of the Carboniferous. +--, Devonian, compared to Carboniferous. +-- of the Subapennines. +--, Lower Miocene of Switzerland. +--, Miocene of the Arctic Regions. +--, Older Pliocene of Italy. +-- of the Permian. +-- of the Upper Cretaceous. +--, Upper Miocene of Switzerland. +-- of the Wealden. + +Fluvio-marine or Norwich Crag. + +Flysch of the Alps. +--, plutonic rocks invading. + +Folding and denudation of Nova Scotia Carboniferous rocks. + +Folds of parallel strata, arrangement and direction of. + +Foliation of crystalline rocks. +--, irregularities in. + +Folkestone and Hythe beds. + +Fontainebleau, Gres de. + +Footprints in Potsdam sandstone. +-- of reptiles in Coal-measures. +--, fossil in New red. +-- in Paris gypsum. + +Foraminifera, Eocene. +-- of Mountain Limestone. +-- of the Chalk. + +Forbes, Mr. David, on glass cavities in quartz. +--, on planes of foliation. +--, on specific gravity of quartz. +--, on volcanic minerals. + +Forbes, Professor E., on fossils of Bembridge beds. +--, on Hempstead beds. +--, on shells of the crag. +--, on sphaeronites. +--, on subdivisions of the Purbeck. +--, on testacea of the Faluns. +--, on thickness of Upper Neocomian. + +Forest-bed at Cromer. +-- marble or cornbrash. +--, submerged. +--, fossil in Coal. +--, fossil of Isle of Portland. + +Forfarshire, Cephalaspis beds of. +--, contorted strata in. + +Formation, term defined. + +Fossil, term defined. +-- trees erect in coal. +-- Fish of Old Red Sandstone. + +Fossiliferous groups, table of succession of. + +Fossils, arrangement of, in strata. +--, destruction of, in older formations. +--, fresh-water and marine. +-- obliterated by metamorphic action. +--, recent, and Post-pliocene. +-- of the drift. +-- of the Crags. +--, Upper Miocene. +--, Lower Miocene of Switzerland. +-- of the Hempstead Beds. +--, Eocene. +-- of the Barton Clay. +-- of the White Chalk. +-- of the Neocomian. +-- of the Oolite. +-- of the Stonesfield Slate. +-- of the Lias. +-- of the Trias. +-- of the Magnesian Limestone. +-- of the Coal. +-- plants of the Coal. +-- of the Mountain Limestone. +--, Devonian. +--, Silurian. +--, Cambrian. +-- Laurentian. + +Fournet, M. on metalliferous gneiss. +--, on veins in granite. + +Fox, Reverend D., on Isle of Wight Eocene fossils. + +Fox, Mr. R., on lodes in Cornwall. + +Fractures of strata, and faults. + +Fragments, included, a test of age of plutonic rocks. +--, included, a test of age of strata. +-- a test of age in volcanic rocks. + +France, Eocene formations of. +--, Lower Miocene of. +--, Upper Miocene of. + +Freshfield, Mr., on absence of lakes in the Caucasus. + +Fresh-water strata, how distinguished from marine. +-- formation of Auvergne. + +Fucoid sandstones of Sweden. + +Fulgur canaliculatus, Maryland. + +Fuller's earth, fossils of the. + +Fundy, Bay of, fossil trees exposed in cliffs at. + +Fusilina cylindrica. + +Fusion of quartz. + +Fusus contrarius (Trophon antiquum). +-- quadricostatus, Maryland. + +Gabbro. + +Gaillonella ferruginea, and G. distans. + +Galapagos Islands, living marine saurian in. + +Galeocerdo latidens, Bracklesham. + +Galerites albogalerus, White Chalk. + +Galestes in Middle Purbeck. + +Ganoids, the type of Old Red Sandstone fish. +-- of the Wealden. +-- of the Trias. + +Gaps in the sequence of fossil remains. + +Garnet. + +Gases, corrosion of rocks by. + +Gaudin on Lower Miocene of Switzerland. +-- on Pliocene flora of Italy. +-- on Proteaceae in Bournemouth Eocene. + +Gault, thickness and fossils of. + +Geikie, Mr. A., on Ayrshire Permian trap-rocks. +--, on subaerial denudation. +--, on ice erosion of lake-basins. +--, on Isle of Mull volcanic rocks. +--, on Pentland Old Red volcanic rocks. +--, on Silurian metamorphic rocks. +--, on syenite of Skye. + +Geinitz, M., on Permian flora. + +Gemunder Maar, volcanic rocks of. + +Geneva, Lower Miocene of. + +Geology defined. + +Gergovia, tuffs and associated lacustrine strata of. + +Germany, Lower Miocene of. +--, Triassic fauna of. + +Gers, Upper Miocene of. + +Gervillia anceps, Neocomian. +-- socialis, Muschelkalk. + +Giant's Causeway basalt, age of. +--, laterite of the. +--, columnar basalt of. + +Girgenti, Newer Pliocene of. + +Glacial drift, distribution and nature of. +-- epoch in the Post-pliocene. +-- formations of Pliocene age. + +Glaciation of Russia and Scandinavia. +-- of Scotland. +-- of Wales and England. +-- of North America. + +Glaciers, transporting and abrading power of. + +Glasgow, marine strata near. + +Glauconie grossiere. + +Glen Tilt, junction of granite and schist at. + +Globiform pitchstone. + +Globigerina bulloides. + +Globular structure of volcanic rocks. + +Glyptostrobus, Europaeus, Oeningen. + +Gneiss, granite veins traversing. +-- defined and figured. +--, fundamental, of Scotland. + +Gold mines of Australia and Chili. +-- veins of Russia. +-- of California, of age of alluvium. + +Goldenberg, Professor, on Saarbruck coal insects. + +Goldfuss, Professor, on reptiles in coal. + +Goniatites crenistria. +-- Listeri, coal-measures. + +Goppert, on American forms in Swiss Miocene flora. +-- on petrification. +-- on plants of coal-measures. + +Gorgonia infundibuliformis, Permian. + +Graham's Island, forming ashy conglomerate. + +Grampians, Old Red conglomerates of. +--, trap-rocks of the. +--, former glaciers in the. + +Grand Canary, Upper Miocene, shelly tuffs of. + +Granite, composition of. +--, graphic and columnar. +--, how far connected with trap-rocks. +--, hydrothermal action in formation of. +-- metamorphosing fossiliferous strata. +--, porphyritic. +--, oldest. +--, protrusion of solid. +--, passage of, into trap. +--, schorly. +-- veins. +-- veins in talcose gneiss. + +Granton, angiosperm found in coal at. + +Graptolites of Llandeilo flags. + +Graptolites Murchisonii. Llandeilo flags. + +Graptolithus priodon, Silurian. + +Gray's, Essex, pachyderms found at. + +Great (or Bath) Oolite. + +Greece, Upper Miocene formations of. + +Greenland, continental ice of. +--, gradual sinking of. + +Greenstone. + +Gres de Beauchamp, Paris basin. + +Gres de Fontainebleau, age of the. + +Griffiths, Sir R., on yellow sandstone of Ireland. + +Grit defined. + +Groups, older, rise highest above the sea. +-- why the newest to be studied first. + +Gryllacris lithanthraca, coal. + +Gryphaea coated with serpulae. +-- columba, Chloritic Sand. +-- convexa, Chalk. +-- incurva (G. arcuata). +-- virgula, Kimmeridge clay. + +Gryphite Limestone. + +Guadaloupe, glass cavities in quartz of. + +Gulf-Stream, probable abrading power of. + +Gumbel, M., on Rhaetic beds. + +Gunn, Mrs., on pot-stones in the chalk. + +Gutbier, Colonel, on Permian flora. + +Gymnogens, term explained. + +Gypseous marls of Auvergne. + +Gypsum and gypseous marl defined. + +Gyrolepis tenuistriatus, Rhaetic beds. + +Haime, Mr., on palaeozoic corals. + +Hakea silicina, and Hakea saligna, Oeningen. + +Hall, Captain Basil, on Cyclopean Isles. + +Hall, Sir James, on curved strata. + +Hall, Mr. J., on Appalachian palaeozoic rocks. + +Hallstadt and St. Cassian beds. + +Halysites catenularis, Silurian. + +Hamilton, Sir W., on eruption of Vesuvius. + +Hamites spiniger, Gault. + +Hancock, Mr., on Protosaurus in Permian. + +Harkness, Professor, on Silurian metamorphic rocks. + +Harlech grits, fossils of the. + +Harris, Major, on the Salt Lakes. + +Harpactor maculipes, Oeningen. + +Harpe, M. de la, on Bournemouth Eocene flora. + +Hartung, Mr., cited. + +Hartz mountains, mineral veins of. +--, Bunter Sandstein of. + +Hastings Sands, subdivisions of the. + +Hautes Alpes, granite of the. + +Hauy on isomorphism. + +Headon series, fossils of the. + +Heat, powerful in consolidating rocks. +--, rocks upraised and folded by. + +Hebert, M., on age of Sables de Bracheux. +--, comparison of Sables Moyens and Barton shells. +--, on pisolitic limestone. + +Hebrides, dikes in the. + +Heer, Professor, on American genera in Swiss Miocene. +--, on age of Madeira leaf-bed. +--, on Arctic Miocene flora. +--, on Bear Island flora. +--, on Bovey Tracey Miocene flora. +--, on fossil plants of Switzerland. +--, on Lower Miocene plants of Mull. +--, on Monte Bolca Eocene plants. +--, on Proteas of Lower Miocene. +--, on plants of Hempstead beds. +--, on plants of coal-field, Virginia. +--, on Swiss Miocene insects. +--, on supposed Proteaceae of Oeningen beds. +--, on Superga fossil plants. + +Heidelberg, varieties of granite near. + +Heliolites porosa, Devonian. + +Helix hispida (plebeia). +-- labyrinthica, Headon. +-- occlusa, Bembridge. +-- Turonensis, faluns. + +Hemicidaris Purbeckensis, Purbeck. + +Hemipneustes radiatus, Chalk. + +Hemitelites Brownii, Inferior Oolite. + +Hempstead beds, subdivisions of the. + +Henry, on absorption of carbonic acid gas in water. + +Henslow, Professor, on dike in Anglesea. +--, on Red Crag coprolite bed. + +Herschel, Sir J., on slaty cleavage. + +Hertfordshire pudding-stone. + +Heterocercal tail of fish. + +Hicks, Dr., on fossils of Arenig beds. +--, on fossils of Harlech grits. +--, on Menevian beds. + +Himalaya, shells 18,000 feet high in. +--, Upper Miocene of. + +Hippopodium ponderosum, Lias. + +Hippopotamus, tooth of. + +Hippurite Limestone. + +Hippurites organisans, Chalk. + +Histioderma hibernica. + +Hitchcock, Professor, on Trias footprints. + +Holoptychius nobilissimus, scale of, and restoration. + +Homalonotus Delphinocephalus. +-- armatus, Devonian. + +Homfray, Mr., on fossils of Tremadoc beds. + +Homocercal tail of fish. + +Hooghly River, analysis of water. + +Hooker, Dr., on coniferae. +--, on structure of sigillaria. +--, on sporangia of Silurian plant. + +Horizontality of strata. + +Horizontal strata, upheaval of. + +Hornblende. + +Hornblende-schist. + +Hornes, Dr., on fossil mollusca of Vienna basin. + +Horstead, pot-stones at. + +Hour-glass illustrating the destruction and renovation of land. + +Howse, Mr., on Protosaurus in Permian. + +Hubbard, Professor, on granite of White Mountains. + +Hudson River Group, fossils of the. + +Hughes, Mr. T. McKenny, cited. +--, on slaty cleavage. +--, on protrusion of solid granite. + +Hull, Mr. E., on breccias in Permian. +-- on carboniferous of Lancashire. +--, on carboniferous rocks of north of England. +--, on faults in Lancashire coal-field. +--, on anticlinals and synclinals, Lancashire. +--, on thickness of the Upper Trias. +--, on thickness of Permian. +--, on three lines of flexure since the coal in Lancashire. + +Human remains of Recent Period. +-- in cavern deposits. + +Humboldt, on mineral character of rocks. + +Humphrey and Abbot on Mississippi denudation. + +Hungary, trachyte of. + +Hunt, Sterry, on action of water in metamorphism. + +Huronian series, thickness of the. + +Huxley, Professor, on Atlantic chalk-mud. +--, on affinity between reptiles and birds. +--, on batrachians of the coal. +--, on fish of Old Red Sandstone. +--, on Pteraspis. + +Hyaena den of Kirkdale cave. + +Hyaena spelaea, tooth of. + +Hybodus plicatilis, Rhaetic beds. +-- reticulatus, Lias. + +Hydrothermal action producing metamorphism. +-- in formation of granite. +-- forming granite veins. + +Hymenocaris vermicauda. + +Hyperodapedon Gordoni, Trias. + +Hypersthene. +-- rock. +-- rocks of Skye. + +Hypogene rocks, uniformity of mineral character in. +-- rocks, term defined. + +Hypsiprymnus Gaimardi, molar of recent. + +Hythe, Neocomian beds of. + +Ice, erosion of lake-basins considered. +--, abrading power of. +--, continental, of Greenland. + +Icebergs, drift carried by. +-- stranded in Baffin's Bay. + +Ice-borne erratics at Chichester. + +Iceland, glass cavities in quartz of. +--, flow of lava in. + +Ichthyosaurus communis, Lias. + +Idocrase. + +Ichthyodorulite of the Lias. + +Iguanodon Mantelli, Weald Clay. + +Ilfracombe Group of Devon. + +Inclined strata. + +India, Miocene formations of. + +India, Upper Miocene of. + +Inferior Oolite, thickness and fossils of. + +Infusoria in tripoli. + +Inland sea-cliffs. + +Inoceramus Lamarckii, White Chalk. + +Insect in American coal. +-- beds of the Lias. +-- wing of neuropterous. + +Insects, Devonian, of Canada. +-- in European coal. +--, Miocene, of Croatia. +--, Upper Miocene, at Oeningen. + +Intrusion, a test of age of Plutonic rocks. +--, a test of age of volcanic rocks. + +Inundation mud of rivers. + +Ireland, glacial drift of. +--, yellow sandstone of. + +Iron pyrites. +-- weapons of Swiss lake-dwellings. + +Isastraea oblonga, Portland Sand. + +Isle of Bourbon, lava current of the. +-- Wight, Hempstead beds. +-- Wight, Eocene beds. +-- Mull, Miocene leaf-bed of. +-- Mull, volcanic rocks. + +Isomorphism, theory of. + +Italy, Lower Miocene of. +--, Older Pliocene volcanoes of. +--, Pliocene of. +--, Older Pliocene flora of. +--, Upper Miocene strata of. + +Jamieson, Mr. T.F., on Scotch glacial drift. + +Jaws of mammalia in Purbeck. + +Jeffreys, Mr. Gwyn, on Atlantic mud. + +Jointed structure of metamorphic rocks. + +Jones, Dr. Rupert, on Eozoon Canadense. + +Jorullo, lava stream of. + +Judd, Mr., on Speeton clay. + +Jukes, Mr., on Tarannon shales. + +Jura, erratic blocks on the. +--, structure of the. + +Kangaroo, jaws of. + +Kasegrotte, Bertrich Baden, Basaltic pillars of. + +Kaup, Professor, on footprints of the Trias. + +Keilhau, Professor, on granite veins. +--, on planes of foliation. +--, on Silurian granite of Norway. +--, on protrusion of granite. + +Keller, Dr. F., on lake-dwellings. + +Kelloway Rock, percentage of Oxford clay fossils in. + +Kentish Rag. + +Keuper, of Germany. +-- or Upper Trias of England. + +Kilkenny, fossil plants of. + +Killas, altered by granite in Cornwall. + +Kiltorkan, yellow sandstone of, with Anodonta. + +Kimmeridge Clay. + +King, Dr., on reptile footprints in coal. +--, Mr., on Permian fossils. + +Kirkdale cave, hyaena's den of. + +Kitchen-middens of Denmark. + +Kleyn Spawen beds. + +Konen, Baron von, on Brockenhurst shells. + +Koninck, M. de, on Mountain Limestone fish. +--, on shells of Mayence basin. + +Koninckia Leonhardi, Hallstadt. + +Labrador rock. +-- series. + +Labradorite. + +Labyrinthodon Jaegeri, section of tooth. +--, tooth of. + +Labyrinthodonts of Coal. + +Lake-craters of the Eifel. + +Lake districts, southern limits of the. + +Lake-dwellings, scarcity of human remains in. +-- of Switzerland. + +Lakes, deposits in. +--, connection of, with glacial action. + +Lamarck on bivalve mollusca. + +Lamination of clay slate. + +Lamna elegans, Bracklesham. + +Lancashire, vast thickness of rocks without corresponding altitude in. + +Land, balance of dry, how preserved. +-- has been raised, not the sea lowered. +--, mean height of, above the sea. +--, rise of, in Sweden. +--, rise and fall of, affecting denudation. + +Land-ice, action of, in Greenland. + +Land's End, columnar granite at. +--, porphyritic granite at. + +La Roche, recent deposits in estuary of. + +Lartet, M., on mammalia of Faluns. +--, on Gastornis Parisiensis. +--, on reindeer period. + +Lastraea stiriaca, Monod. + +Lateral compression causing curved strata. + +Laterite of Giant's Causeway. + +Laurentian gneiss of Scotland. +-- Group, Upper and Lower. +-- metamorphic rocks. +-- volcanic rocks. + +Lava. +-- consolidating on slopes. +-- currents of Auvergne. +-- streams, effect of. +-- of La Coupe d'Ayzac. +-- of Jorullo. + +Lead veins, age of. + +Leaf-bed of Madeira in basalt and scoriae. +--, Isle of Mull Miocene. + +Leda amygdaloides, London Clay. +-- Deshayesiana (Nucula Deshayesiana). +-- lanceolata (L. oblonga), Scotch drift. +-- truncata, Scotch drift. + +Lee, Mr. J.E., on Pteraspis of Lower Ludlow. + +Leidy, Dr., on fossil quadrupeds of Nebraska. + +Leperditia inflata, coal-measures. + +Lepidodendron Griffithsii. +-- corrugatum, carboniferous. +-- Sternbergii, coal-measures. + +Lepidolite. + +Lepidostrobus ornatus, Coal. + +Lepidotus gigas, Lias. +-- Mantelli, Wealden. + +Leptaena depressa, Wenlock. +-- Moorei, Lias. + +Level of surface altered by change of subterranean heat. + +Lewis, hornblendic gneiss of. + +Lias, fishes of the. +--, fossils of the. +-- and Oolite, origin of the. +--, reptiles of the. +--, insects of the. +--, plants of the. +--, plutonic rocks of the. +--, subdivisions of the. +--, volcanic rocks of the. + +Liebig, on conversion of coal into anthracite. +--, on origin of stalactite. + +Liege, limestone caverns at. + +Lightbody, Mr., on Lower Ludlow shales. + +Lignite, conversion of into coal. + +Lima giganteum. +-- Hoperi, Chalk. +-- spinosa, White Chalk. + +Limagne d'Auvergne, Lower Miocene mammalia of the. + +Limburg beds. + +Lime, scarcity of, in metamorphic rocks. +-- in solution, source of. + +Limestone, block of striated. +--, brecciated. +-- of chemical and organic origin. +--, compact. +--, Hippurite. +--, magnesian. +--, metamorphic or crystalline. +--, Mountain, and its fossils. +--, striated. + +Limnaea longiscata. + +Lingula beds, volcanic tuffs of the. + +Lingula Credneri, Permian. +-- Flags, fossils of the. +-- Dumortieri, Crag. +-- Lewisii, Ludlow. + +Lingulella Davisii. + +Lipari Isles, tufas in. + +Liquidambar europaeum. + +Lithrostrotion basaltiforme, Carboniferous. + +Lits coquilliers. + +Littoral denudation defined. + +Lituites giganteus, Ludlow. + +Llanberis slates. + +Llandeilo Flags, fossils of the. + +Llandeilo formation, thickness of the. +--, Lower. + +Llandovery Group, classification of the. +-- Rocks, thickness of the Lower. + +Loam defined. + +Lodes, shells and pebbles in. +-- See Mineral Veins. + +Loess of fluviatile loam described. +--, fossil shells of the. + +Logan, Sir W., on Eozoon Canadense. +--, on Gaspe sandstones. +--, on Huronian and Laurentian. +--, on stigmaria in under-clays. +--, on thickness of Nova Scotia coal. +--, on thickness of Laurentian in Canada. + +Loire, faluns of the. + +London Clay, fossils of the. + +Longevity, relative, of mammalia and testacea. + +Longmynd Group, fauna of the. + +Lonsdale, Mr., on corals of America. +--, on Devonian fossils. +--, on Stonesfield slate. +--, on United States Miocene corals. + +Lonsdaleia floriformis, Carboniferous. + +Lowe, Reverend R.T., on Mogador shells. + +Lubbock, Sir J., on the two stone-periods. + +Lucina serrata, Bracklesham. + +Ludlow formation, Upper. +-- Lower. +--, bone-bed of the Upper. + +Lulworth Cove, dirt-bed of. + +Lycett, Mr., on fossils of the Great Oolite. + +Lycopodiaceae of Coal. + +Lycopodium densum, living species. + +Lym-fiord, mingled fresh-water and marine strata of. + +Lymnea caudata, Headon. +-- longiscata, Bembridge. + +Lynton Group of Devon. + +Maclaren, Mr., on Pentland Hills, volcanic rocks. + +Macclesfield, marine shells 1,200 feet high at. + +MacClintock, Sir L., on Atlantic mud. + +MacCulloch, Dr., on Aberdeenshire granite. +--, on basaltic columns in Skye. +--, on formation of hornblende-schist. +--, on trap. + +MacMullen, Mr. J., on Eozoon Canadense. + +Macropus atlas, lower jaw of. +-- major (living), lower jaw of. + +Madeira, beds of laterite in. +--, dike in valley in. +--, Pliocene leaf-bed and shells in lavas of. +--, Miocene volcanic rocks of. +--, wind, removing scoriae in. + +Maestricht beds and their fossils. + +Maffiotte, Don Pedro, cited. + +Magas pumila, White Chalk. + +Magnesian Limestone defined. +-- and marl-slate. + +Magnetite. + +Maidstone, Upper Cretaceous fossils of. + +Malacolite. + +Malaise, Professor, on Engihoul cave. + +Mammalia, anterior to Paris gypsum, table of. +--, extinct, coeval with man. +--, fossil, of Middle Purbeck. +--, fossil, in Pliocene in Val d'Arno. +--, fossil, in the Crag. +--, fossil, of Vienna basin. +-- of the Limagne d'Auvergne. +-- of Siwalik Hills. +-- of the Stonesfield slate. +--, teeth of Post-pliocene. + +Mammalia and testacea, comparative longevity of. + +Mammoth, rude carving of in Perigord cave. +-- in Scotch till. +-- See Elephas primigenius. + +Man, antiquity of. + +Manfredi on amount of subaerial denudation. + +Mantell, Dr., on iguanodon of Wealden. +--, on Oxford Clay belemnites. +--, on Wealden fossils. + +Mantellia nidiformis, Purbeck. + +Map of Chalk formation in France. +-- of Eocene tertiary basins. +-- of Hallstadt and St. Cassian beds. + +Marble defined. +-- of Carrara, metamorphic. + +Marcou, M., on age of Wealden beds. + +Margaric acid. + +Marine fauna of the Carboniferous. +-- beds underlying the London Clay. +-- and brackish-water strata in coal. +-- strata, how distinguished from fresh-water. + +Marl from Lake Superior. +-- and marl-slate defined. +--, red, green, and white, of Auvergne. +-- slate of Middle Permian. + +Marsupials, extinct, of Australia. + +Marsupites Milleri, White Chalk. + +Massachusetts, plumbago of. + +Mastodon arvernensis, molar of, Norwich crag. +-- giganteus, in United States after the drift. + +Mayence basin tertiaries. + +May-Hill Sandstone. + +Mechanical and chemical deposits. +-- theory of cleavage. + +Mediterranean, one zoological province. + +Megalodon cucullatus, Devonian. + +Melania inquinata (Cerithium melanoides). + +Melania turritissima, Bembridge. + +Melanopsis buccinoidea. + +Melaphyre, a variety of basalt. + +Menevian beds and their fossils. + +Mesozoic, term explained. +-- and Cainozoic periods, gap between the. +-- and Palaeozoic rocks, limits of the. + +Metals, relative age of different. + +Metamorphic limestone. +-- strata, origin of. +-- theory, objections to, considered. +-- rocks defined. + +Metamorphic rocks. +--, cleavage of. +--, scarcity of lime in. +--, ages of. +--, order of succession of. +--, uniformity of mineral character in. + +Metamorphism, hydrothermal action producing. + +Metamorphosis of trilobites. + +Meteorites, minerals in. + +Mexico, Gulf of, terrestrial remains washed into. + +Meyer, Mr. Karl, on fossil shells of Madeira. +--, M. H. von, on reptiles in coal. +--, on Wealden of Germany. + +Miascite. + +Mica and its varieties. +--, how deposited. +-- schist or micaceous schist. + +Micaceous sandstone, origin of. + +Micraster cor-anguinum. + +Microconchus carbonarius, coal-measures. + +Microlestes antiquus, Upper Trias. + +Migrations of quadrupeds. + +Miliolite limestone. + +Miller, Hugh, on Old Red Sandstone fish. +--, on salt lakes. + +Milne Edwards, Mr., on Palaeozoic corals. + +Minchinhampton, Great Oolite of. + +Mineral composition a test of age of volcanic rocks. +-- a test of age of plutonic rocks. +-- a test of age of strata. +-- character of hypogene rocks. +-- springs of Auvergne. + +Mineral veins. +-- formed in fissures. +--, successive formation of. +--, swelling and contraction of. +--, relative age of. +--, pebbles in. + +Mineralisation of organic remains. + +Minerals in meteorites. +--, table of the most abundant in hypogene rocks. + +Miocene of Bordeaux and south of France. +-- and Eocene, line between the. +--, Lower, of England. +--, Lower, of Germany and Croatia. +--, Lower, of Central France. +--, Lower, of Italy. +--, Lower, of Nebraska, United States. +--, term defined. +--, Upper, of the Bolderberg. +--, Upper, of France. +--, Upper, of Italy. +--, Upper, of Greece. +--, Upper, of India. +--, Upper, of Vienna basin. + +Mississippi, sediment of, used as a test of denudation by rivers. +-- valley, deposition and denudation in the. + +Mitchell, Mr., on Aralia fruit in Alum Bay, Eocene. + +Mitchell, Sir T., on Wellington caves. + +Mitchell, Reverend Hugh, on Pteraspis. + +Mitra Scabra, Barton clay. + +Mitscherlich, on Isomorphism. + +Modiola acuminata, Permian. + +Moel Tryfaen, shells found at. + +Mohs on isomorphism. + +Molasse, Lower, of Switzerland. +--, Middle, or Marine, of Switzerland. +--, Upper, fresh-water, of Switzerland. +--, term explained. + +Mollusca. See Shells. +--, longevity of species of. +-- of Hallstadt beds. +--, value of, in classification. +-- of the Carboniferous. + +Monitor of Thuringia. + +Monoclinic feldspars. + +Monod, flora of the Lower Molasse at. + +Mons, unconformable strata near. + +Montblanc, talcose granite of. +-- Dor, Auvergne, extinct volcanoes of. +--, age of volcano of. + +Monte Bolca, fossil fish of. +-- Calvo, section of cross stratification. +-- Mario, age of volcanic deposits of. +-- Nuovo, formed 1538. + +Montmartre, gypseous series of. + +Monts Dome, Auvergne, extinct volcanoes. + +Moore, Mr. C., on Rhaetic beds. +--, on Upper Trias quadrupeds. + +Moraines described. + +Morea, cretaceous volcanic rocks of. + +Mortillet, M. de, on ice-erosion of lake-basins. + +Morton, Dr., on age of American cretaceous rocks. + +Mosasaurus Camperi, Chalk. + +Mountain Limestone, fossils of the. + +Mull, Isle of, leaf-bed. + +Munster, Count, on fossils of Solenhofen. + +Murchison, Sir R., on brackish-water strata in coal. +--, on Devonian series. +--, on Devonian ichthyolites. +--, on Eocene igneous rocks. +--, on Llandovery beds. +--, on Laurentian gneiss of Scotland. +--, on metamorphic rocks of North Highlands. +--, on Monte Bolca fish-beds. +--, on name Permian. +--, on Old Red Sandstone. +--, on Palaeozoic strata, Queenaig. +--, on protrusion of solid granite. +--, on Silurian. +--, on Tertiary volcanic rocks of Italy. +--, on thickness of chalk in Russia. +--, on thickness of the Trias. +--, on the Upper "Old Red." + +Murchisonia gracilis. + +Murex vaginatus. + +Muschelkalk, fossils of the. + +Muscovite, or common mica. + +Musk-ox, fossil, in Thames valley. + +Myliobates Edwardsi, Bracklesham. + +Mytilus septifer, Permian. + +Naples, Post-pliocene volcanic rocks of. +--, escape of carbonic acid near. + +Natica clausa, Scotch drift. +-- helicoides, Chillesford beds. + +Natrolite. + +Nautilus centralis, London Clay. +-- Danicus, Faxoe Chalk. +-- plicatus, Hythe beds. +-- truncatus, Lias. +-- ziczac (Aturia ziczac). + +Nebraska, Miocene strata of. + +Necker, M., on "underlying" igneous rocks. +--, on dikes in Vesuvius. + +Neocomian, Upper. +--, Middle. +--, Lower. +--, use of the term. + +Neolithic era. + +Neozoic type of corals. + +Nerinaea Goodhallii, Coral Rag. + +Nerinaean limestone. + +Nerita conoidea (N. Schmidelliana). +-- costulata, Great Oolite. +-- granulosa. + +Neritina concava. Headon. +-- globulus. + +Neufchatel, coins and iron tools in lake of. + +Newberry, Dr., on flora of American cretaceous rocks. + +Newcastle coal-field, faults in. + +Newfoundland bank described. + +New Jersey, mastodon in. + +New Madrid, "Sunk Country" in. + +New Red sandstone of Connecticut Valley. +--, trappean rocks of the. + +New York, Devonian strata of. +--, Cambrian strata of. +--, Silurian strata of. +--, Laurentian strata of. + +Niagara Limestone, fossils of the. + +Nidau, iron tools in lake of. + +Nile, homogeneous mud of the. + +Ninety-fathom dike in coal. + +Nipadites ellipticus, Sheppey. + +Nodules in strata, how formed. + +Noeggerathia cuneifolia, Permian. + +Nomenclature of rocks. +-- of volcanic minerals. + +Norfolk cliffs, drift of. + +North America. See America. + +Norway, Cambrian of. +--, foliation of crystalline schists in. +--, granite veins in gneiss of. +--, granite altering fossiliferous strata in. + +Norwich, or Fluvio-marine crag. + +Nova Scotia coal-measures. +-- coal, reptiles and shells in. +--, folding and denudation of beds in. + +Nucula Cobboldiae, Crag. + +Nummulites laevigata, Bracklesham. +-- Puschi, Pyrenees. +-- variolaria, Bracklesham. + +Nummulitic formations. + +Obolus Apollinis, in Russian grit. + +Obsidian. + +Oceanic areas, permanence of. + +Oeningen, Upper Miocene beds of. + +Oeynhausen, M. von, on Cornish granite veins. + +Ogygia Buchii. + +Oldhamia radiata: O. antiqua. + +Old Red Sandstone, Upper. +--, Middle, with fish. +--, Lower. +--, trap of the. +--, classification of. + +Olenus micrurus. + +Oligocene, term for Lower Miocene. + +Oligoclase. + +Oliva Dufresnii, Bolderberg, Belgium. + +Olivine. + +Omphyma turbinatum, Silurian. + +Onchus tenuistriatus, Silurian. + +Oolite, classification and physical geography of the. +--, defined. +--, Inferior, fossils of the. +-- and Lias, origin of the. +-- and Chalk, Palaeontological break between. + +Oolitic strata, palaeontological relations of. +-- volcanic rocks. + +Ophioderma tenuibrachiata, Lias. + +Oppel on zones of Lias. + +Orbigny, Alcide de, on foraminifera of Vienna basin. +--, on orbitoidal limestone. +--, on Pisolitic limestone. +--, on Senonian. + +Oreodaphne Heerii, Italian Pliocene. + +Organic remains, mineralisation of. +--, tests of age of strata. +--, tests of age of volcanic rocks. +--, geological provinces of. + +Oriskany Sandstone. + +Orthis elegantula, Ludlow. +-- grandis, Caradoc beds. +-- tricenaria, Bala beds. +-- vespertilio, Bala beds. + +Orthoceras duplex. +-- Ludense, Silurian. +-- laterale. +-- ventricosum, Silurian. + +Orthoclase. + +Orthoclastic feldspars. + +Osborne or St. Helen's series, Eocene. + +Osteolepis, Old Red Sandstone. + +Ostraceon, spine of, Bracklesham. + +Ostrea acuminata, Fuller's earth. +-- carinata, Chalk marl. +-- columba, Chloritic sand. +-- gregarea, Coral Rag. +-- deltoidea, Kimmeridge clay. +-- distorta, Middle Purbeck. +-- expansa, Portland sand. +-- Marshii, Oolite. +-- vesicularis, Chalk. + +Otodus obliquus, Bracklesham. + +Outcrop of strata. + +Overlapping strata. + +Owen, Professor on Archaeopteryx. +--, on Eocene Zeuglodon. +--, on footprints in Trias. +--, on fauna of Sheppey. +--, on Gastornis Parisiensis. +--, on Labyrinthodon. +--, on mammalia of Stonesfield. +--, on Purbeck mammalia. +--, on reptiles of coal. +--, on zoological provinces of extinct animals. + +Ox, tooth of (recent). + +Oxford Clay, thickness and fossils of. + +Pagham, erratic block at. + +Palaeaster asperimus. + +Palaechinus gigas, Mountain Limestone. + +Palaeocoma tenuibrachiata, Lias. + +Palaeoniscus, Permian fish. +-- comptus, P. elegans, P. glaphyrus. + +Palaeotherium magnum. + +Palaeophis typhoeus, Bracklesham. + +Palaeozoic or Paleozoic, term defined. +-- Plutonic rocks. +-- rocks. +-- type of corals. + +Palagonia, dikes of lava in. + +Paleolithic era. +--, alluvial deposits of. + +Palm in Swiss Miocene. + +Palma, volcanic crater of. + +Paludina lenta, Hempstead beds. +-- orbicularis, Bembridge. + +Paradoxides Bohemicus. +-- Davidis, Lower Cambrian. + +Parallelism of folded strata for long distances. + +Paris basin, Tertiary group first studied in. +--, Tertiaries of the. + +Parka decipiens, "Old Red." + +Parkfield Colliery, ground-plan of. + +Patagonia, strata of, rich in soda. + +Patella rugosa, Great Oolite. + +Paterson, Dr., on angiosperm of the Coal. + +Peach, Mr. C, cited. +--, Pteraspis, found by. + +Pearlstone. + +Pebbles in mineral veins. +-- in chalk. + +Pecopteris elliptica, Coal. + +Pecten Beaveri, White Chalk. +-- cinctus, Neocomian. +-- islandicus, Scotch Drift. +-- jacobaeus, in tertiary of Sicily. +-- quinque-costatus. +-- Valoniensis, Rhaetic beds. + +Pegmatite. + +Penarth beds. + +Pengelly, Mr., on Bovey Tracey lignite. +--, on flint-knives of Brixham Cave. + +Pentacrinus Briareus, Lias. + +Pentamerus Knightii, Aymestry. +-- oblongus, and P. lirata. + +Pentland Hills, volcanic rocks of the. + +Perigord cave, carving of mammoth in. + +Permanence of continents and oceans. + +Permian Flora. +-- of Germany. +-- strata, thickness of, in north of England. +--, Upper and Middle. +--, Lower. + +Perna Mulleti, Neocomian. + +Petherwyn, Devonian fossils of. + +Petrifaction, process of. + +Petrophiloides Richardsoni, Sheppey. + +Pahcops caudatus, Silurian. +-- latifrons, Devonian. + +Phascolotherium Bucklandi. + +Phasianella Heddingtonensis, and cast. + +Phillippi, on tertiary shells of Sicily. + +Phillips, Professor, on fossils distorted by cleavage. +--, on ninety fathom dike. +--, on Wenlock limestone and shale. +--, on Yoredale series. + +Phillips, Mr. J. Arthur, on origin of gold of California. + +Phlebopteris contigua, Inferior Oolite. + +Phlogopite. + +Pholadomya fidicula, Inferior Oolite. + +Phonolite. + +Phorus extensus, London Clay. + +Phragmoceras ventricosum, Silurian. + +Physa Bristovii, Middle Purbeck. +-- columnaris. +-- hypnorum. + +Piedmont, absence of lakes in. + +Pile dwellings of Switzerland. + +Pilton, group of, Devon. + +Pinnularia in Atlantic mud. + +Pinus sylvestris in peat. + +Pisolitic limestone of France. + +Pitchstone. + +Placodus gigas, Muschelkalk. + +Placoids, rare in Old Red Sandstone. + +Plagiaulax Becklesii, jaw and molar of. + +Plagioclastic feldspars. + +Plagiostoma giganteum, Lias. +-- Hoperi, Chalk. + +Planorbis discus, Bembridge. +-- euomphalus. + +Plants of Bovey Tracey, Miocene. +--, fossil fresh-water. +-- of the Coal. +-- of the Lias. +-- of the Swiss Upper Miocene. + +Plas Newydd, rock altered by dike near. + +Plastic Clay, Eocene. + +Platanus aceroides, Miocene. + +Platystoma Suessii, Hallstadt. + +Playfair, on amount of subaerial denudation. +-- on faults. + +Plectrodus mirabilis, Ludlow. + +Plesiosaurus dolichodeirus, Lias. + +Pleurotoma attenuata, Bracklesham. +-- exorta, Eocene. + +Pleurotomaria anglica, and cast. +-- carinata (flammigera). +-- granulata, Inferior Oolite. +-- ornata, Inferior Oolite. + +Plieninger, Professor, on Triassic mammifer. + +Pliocene glacial formations. +-- Period. +-- plutonic rocks. +-- strata of Sicily. +--, term defined. +-- volcanic rocks. + +Plombieres, alkaline waters of. + +Plumbago of Massachusetts. + +Plutonic and sedimentary formations, diagram of. +--, origin of the term. +-- rocks, Mesozoic. +--, Recent and Pliocene. +--, Miocene and Eocene. +--, uncertain tests of age of. +-- defined. + +Podocarya Bucklandi, Oolite. + +Polypterus of the Nile. + +Polyzoa and Bryozoa, terms explained. + +Pomel, M., on fossil mammalia of the Limagne. + +Ponza Islands, globiform pitchstone of. + +Porites pyriformis, Devonian. + +Porphyritic granite. + +Porphyry. + +Portland, Cycads in dirt-bed of. +-- oolite and sand. + +"Portland screw," a cast of a shell. + +Porto Santo, marine shells in volcanic tuff of. + +Post-pliocene period, climate of the. +-- mammalia, teeth of. +--, term defined. +-- lakes of Switzerland. +-- volcanic rocks. + +Potamides cicntus. + +Pothocites Grantonii, coal-measures. + +Potsdam Sandstone. + +Pot-stones in the Chalk. + +Pottsville, coal seams of. + +Powrie, Mr., on Cephalaspis beds. +--, on Parka decipiens. + +Pratt, Mr., on Eocene Isle of Wight mammalia. + +Predazzo, altered rocks at. + +Pressure, solidifying rocks. + +Prestwich, Mr., on age of Sables inferieurs. +--, on Chillesford beds. +--, on Coalbrook Dale insects. +--, on Eocene strata. +--, on faults in coal-measure of Coalbrook Dale. +--, on shells of London clay. +--, on thickness of Coralline Crag. + +Prevost, M. Constant, on Paris basin. + +Primary Limestone. +-- rocks. +--, term defined. + +"Primordial Zone" of Bohemia. + +Productus horridus, Permian. +-- semireticulatus (antiquatus). + +Progressive development indicated by low grade of early mammals. + +Proteaceae of Aix-la-Chapelle flora. +-- of Lower Molasse, Switzerland. +-- of Oeningen beds. + +Protogine. + +Protosaurus of Thuringia. + +Protrusion of solid granite. + +Provinces of animals and plants. + +Psammodus porosus. + +Pseudocrinites bifasciatus, Silurian. + +Psilophyton princeps, Devonian. + +Pteraspis in Lower Ludlow shale. + +Pterichthys, Old Red Sandstone. + +Pterodactyl of Kentish chalk. + +Pterodactylus anglicus, Old Red. +-- crassirostris, Solenhofen. + +Ptychodus decurrens, White Chalk. + +Pudding-stone or conglomerate. +--, formation of. + +Pumice. + +Punfield beds, brackish and marine. + +Pupa muscorum. +-- tridens, Loess. +-- vetusta, Coal. + +Purbeck beds, Upper, Middle, and Lower. +--, fossil mammalia of the Middle. +-- marble. +--, subdivisions of the. + +Purity of coal, cause of. + +Purpura tetragona, Red Crag. + +Purpuroidea nodulata, Great Oolite. + +Puy de Come, cone and lava-current of. +-- de Tartaret, lava-current and cone of. +-- de Pariou, crater of the. + +Puzzuoli, elevation of land at. + +Pygopterus mandibularis, Permian. + +Pyrenees, chalk altered by granite in the. +--, curved strata in. +--, lamination of clay-slate in. + +Pyroxene group of minerals. + +Pyrula reticulata, Crag. + +Quader-sandstein, Cretaceous age of the. + +Quadrumana of Gers. + +Quadrupeds, extinct, in Paleolithic gravels. + +Quartz, specific gravity of. + +Quartzite or Quartz Rock. + +Queenaig, unconformable Palaeozoic strata at. + +Quenstedt on zones of Lias. + +Radaboj Miocene, brown coal of. + +Radiolites foliaceus, White Chalk. +-- Mortoni, White Chalk. +-- radiosa, White Chalk. + +Radnorshire, stratified trap in. + +Rain-prints with worm tracks in Coal. +--, carboniferous. + +Ramsay, Professor, on break between Upper and Lower Cretaceous. +--, on breccias in Permian. +--, on escarpments. +--, on denudation. +--, on ice-erosion of lake-basins. +--, on Lingula Flags. +--, on position of Tremadoc beds. +--, on Silurian metamorphic rocks. +--, on submergence in glacial period. +--, on thickness of the Lower Trias. +--, on thickness of Llandeilo beds. +--, on thickness of the Bala beds. +--, on volcanic tuffs of Snowdon. +--, on zones of the Lias. + +Rastrites peregrinus, Llandeilo Flags. + +Rath, Von, on Tridymite. + +Recent Period defined. +-- volcanic rocks. + +Record, imperfection of, in the earth's crust. + +Red Crag, older Pliocene. +-- Sandstone, Origin of. +-- Sea and Mediterranean, distinct species in. + +Redruth, Cornwall, section of veins in mine. + +Reindeer Period in South of France. + +Relistran mine, pebbles in tin of. + +Reptiles of the Coal. + +Reptiles of the Lias. + +Retepora flustracea, Permian. + +Rhaetic beds between Lias and Trias. + +Rhine, fresh-water strata of the. +--, loess of the. + +Rhinoceros in drift of Abbeville. +-- leptorhinus (megarhinus), molar of. +-- tichorhinus, molar of. + +Rhode Island, metamorphic rocks of. + +Rhynchonella navicula, Ludlow. +-- octoplicata, White Chalk. +-- spinosa, Inferior Oolite. +-- Wilsoni, Aymestry. + +Richmond, Virginia, Triassic coal-field of. + +Rigi and Speer, Lower Miocene of the. + +Rimula clathrata, Great Oolite. + +Rink, Mr., on Greenland land-ice. + +Ripple-marked sandstone, how formed. + +Rise and fall of land. + +Rissoa Chastelii, Hempstead beds. + +Rivers, denuding powers of. + +Roches moutonnees described. + +Rock, term defined. + +Rocks altered by volcanic dikes. +-- altered by subterranean gases. +--, analysis of minerals in. +--, aqueous or stratified. +--, classification of. +--, great thickness of palaeozoic. +--, glacial scorings on. +--, metamorphic, age of. +--, plutonic age of. +--, volcanic, age of. +--, trappean. +--, metamorphic, defined. +--, four classes of contemporaneous. +--, plutonic, defined. +--, tests of age of. +--, four contemporaneous classes of. +--, underlying, not always the oldest. +--, volcanic, defined. + +Rock-salt of Trias. +--, origin of. + +Rogers, Mr. H.D., on blending of coal-seams. +--, on Virginian fault. + +Rose, Gustavus, on isomorphism. +--, on Fifeshire dike. +--, on quartz in granite. + +Rosso antico, red porphyry of Egypt. + +Rostellaria (Hippocrenes) ampla, London Clay. + +Roth, M., on Miocene of Greece. + +Runn of Cutch, salt of. + +Rupelian beds of Dumont. + +Russia, glaciation of. +--, Devonian of. +--, Silurian strata of. + +Saarbruck, reptiles in coal-field of. + +Sabal major, Lower Miocene. + +Sables de Bracheux. +-- moyens, Paris basin. + +Sahlite. + +St. Abb's Head, curved strata of. +--, unconformable stratification at. + +St. Andrews, carboniferous trap-rocks of. + +St. Cassian, fossil mollusca of. +-- and Hallstadt beds. + +St. David's, Menevian beds of. + +St. Mary's, shells of. + +Salt, rock, origin of. + +Salter, Mr., on fossils of Arenig group. +--, on Menevian beds. +--, on Tremadoc fossils. + +Sandberger, Dr. F., on Mayence basin. + +Sandstone, New Red. +--, Old Red. +-- slab with cracks. +--, slab of ripple-marked. +-- slab with footprints. + +Sao hirsuta. + +Saurians of the Lias. +--, sudden destruction of. + +Saurichthys apicalis, Rhaetic Beds. + +Saussure, on vertical conglomerates. + +Saxicava rugosa, Scotch drift. + +Saxony, beds of minerals in. + +Scandinavia, glaciation of. + +Scaphites aequalis, Chloritic marl. + +Scapolite. + +Scheerer on action of water in metamorphism. + +Schist, mica. +--, argillaceous. +--, hornblende. + +Schizodus Schlotheimi, Permian. +-- truncatus, Permian. + +Schmerling, Dr., on Liege caverns. + +Schorl-rock, and schorly granite. + +Schwab, M., on Celtic coins in lake-dwellings. + +Scoliostoma, St. Cassian. + +Scoresby, on Arctic icebergs. + +Scoriaceous lava. + +Scoriae. + +Scotland, "Fundamental gneiss" of. +--, Old Red Sandstone of. +--, glaciation of. + +Screws, fossil, internal casts of shells. + +Scrope, Mr., on Isle of Ponza, globiform pitchstone. +--, on minerals in lava. +--, on water in lava. + +Scudder, Mr., on Devonian insects of Canada. + +Sea, apparent fall of, caused by rise of land. +--, denuding power of the. +--, deep soundings in. +--, mean depth of the. +-- cliffs, inland. + +Secondary and Tertiary, gap between the. +--, term defined. + +Section of Auvergne alluvium. +-- of carboniferous rocks, Lancashire. +-- of chalk and greensand. +-- of crags near Woodbridge, Suffolk. +-- of cross-stratification. +-- of curved strata of the Jura. +-- of dirt-bed in Isle of Portland. +-- of Forfarshire, showing curved strata. +-- of fossil tree, showing texture. +-- of folded and denuded carboniferous beds, Nova Scotia. +-- of the Oolitic strata. +-- of Recent and Post-pliocene alluvial deposits. +-- showing creeps in coal-mines. +-- of slaty cleavage. +-- showing valleys of denudation. +-- showing the Weald formation. +-- of strata thinning out. +-- of superimposed groups at Dundry Hill. +-- of unconformable strata near Mons. + +Sections illustrating faults. + +Sedgwick, Professor, on the Cambrian Group. +--, on classification of Arenig group. +--, on Devonian series. +--, on position of the May-Hill beds. +--, on protrusion of solid granite. +--, on slaty cleavage. +--, on garnet in altered rock. +--, on concretionary structure. + +Sediment, accumulation of, causing a shifting of the subterranean isothermals. + +Sedimentary beds of the Carboniferous. + +Selsea Bill, erratics at. + +Senarmont on action of water in metamorphism. + +Sequoia Langsdorfii. + +"Seraphim," head of Pterygotus anglicus. + +Serapis, marine littoral deposits of. + +Serpentine. + +Serpulae attached to Gryphaea. +-- attached to Spatangus. +-- attached to Apiocrinus. + +Shale defined. +-- of the Lower Ludlow. + +Sharpe, Mr. D., on American Silurian fossils. +--, on fossils distorted by cleavage. + +Shell-mounds of Denmark. + +Shells, Arctic, in Scotch drift. +--, derivative, in the Crag. +--, marine, found at great heights above the sea. +--, proportion of living, in the Crags. +--, value of, in classification. +--, fossil, of Virginia. +-- of the London clay. +-- of the mountain limestone. +-- of the Barton clay. +-- of the Oolite. +--, marine, of Moel Tryfaen. + +Sheppey, fauna and flora of. +--, Eocene fish of. + +Sherringham, erratic block at. + +Shetland, granite of. +--, hornblende-schist of. + +Sicily, fauna and flora of, older than the country itself. +--, newer Pliocene strata of. +--, subterranean igneous action in. +--, undulating gypseous marls of. +--, volcanic dikes of. + +Sidlaw Hills, trap of. + +Sigillaria in coal-measures. + +Sigillaria laevigata, coal-measures. + +Siliceous limestone defined. + +Silurian, derivation of the name. +--, granite of Norway. +--, metamorphic, of North Highlands. +-- rocks, classification of. +-- strata of the continent of Europe. +-- strata of United States. +-- volcanic rocks. + +Siphonotreta unguiculata, obolus grits. + +Siwalik Hills, fresh-water deposits of. + +Skaptar Jokul, flow of lava from. + +Skye, hypersthene rocks of. +--, Isle of, Miocene syenite of the. +--, trap dike in. + +Slaty cleavage. + +Slicken-sides, in opposite walls of veins. +--, term defined. + +Smilax sagittifera, Oeningen. + +Smith, Mr. W., on White Lias bed. + +Snowdon, volcanic tuffs of. + +Soissonnais sands. + +Solenastraea cellulosa, Brockenhurst. + +Solenhofen stone, fossils in the. + +Solfatara, decomposition of rocks in the. + +Somma, cone and dikes of. + +Sopwith, Mr. T., models of outcrop of strata. + +Sorby, Mr., on action of water in metamorphism. +--, on glass cavities in quartz. +--, on mechanical theory of cleavage. +--, on ripple-marks in mica schist. + +South Joggins, section of cliffs at. + +Spalacotherium, Purbeck. + +Spatangus radiatus, Chalk. +-- with serpula attached. + +Species, gradual change of. +-- older than the land they inhabit. +--, similarity of conditions causing reappearance of. + +Specific gravity of basalt and trachyte. + +Speer and Rigi, Lower Miocene of the. + +Speeton Clay. + +Sphaerexochus mirus, Silurian. + +Sphaerulites agariciformis, White Chalk. +-- of volcanic minerals. + +Sphenophyllum erosum, Coal. + +Sphenopteris gracilis, Hastings sands. + +Spheroidal concretions in limestone. + +Spicula of sponge, Atlantic mud. + +Spirifera disjuncta, Devonian. +-- alata, Permian. +-- mucronata. +-- trigonalis, and S. glabra. + +Spiriferina Walcotti, Lias. + +Spirolina stenostoma, Eocene. + +Spirorbis carbonarius, coal-measures. + +Spondylus spinosus, White Chalk. + +Sponge in flint from White Chalk. + +Sponges, vitreous, in the chalk. + +Springs, mineral of Auvergne. + +Staffa, age of columnar basalt of. + +Stalactite, origin of, explained. + +Starfish in Silurian strata. + +Stations of species affecting distribution of fossils. + +Stauria astraeiformis. + +Stereognathus of Stonesfield. + +Sternberg, Count, on insects in coal. + +Stigmaria attached to trunk of Sigillaria. +-- in coal-measures. +-- ficoides and surface showing tubercles, Coal. + +Stilbite. + +Stiper-Stones or Arenig Group. + +Stockwerk, assemblage of veins. + +Stonesfield slate, mammalia of the. + +Strata, term defined, alternations of fresh-water, and shallow and deep-sea. +--, alternations of marine and fresh-water. +--, curved, inclined, and vertical. +--, apparent horizontality of inclined. +--, contorted in drift. +--, contortion of, in Cyclopean Isles. +--, general table of fossiliferous. +--, horizontality of. +--, origin of metamorphic. +--, outcrop of. +--, overlapping. +-- repeated by being doubled back. +--, slow growth of, attested by fossils. +-- of organic origin. +--, tests of age of. +--, unconformability of. +--, vast thickness of, not forming high mountains. + +Stratification, diagonal or cross. +--, different forms described. +-- of metamorphic rocks considered. + +Stratified rocks, composition of. + +Striae, production of. + +Strickland, Mr., on thickness of the Trias. + +Stricklandinia lirata. + +Strike, term explained. + +Stringocephalus Burtini. + +Stromboli, lava of. + +Strophomena depressa, Wenlock. +-- grandis. + +Studer, Mr., on gneiss of the Jungfrau. + +Subaerial denudation, average annual amount of. + +Subapennine beds, proportion of recent species in. +-- strata, older Pliocene. + +Submarine denudation. + +Subsidence of land must preponderate over upheaval. + +Succinea amphibia. +-- elongata. + +Suess, M., on fossils of St. Cassian beds. +--, on Vienna basin. + +Suffolk, Crag of. + +"Sunk country," New Madrid. + +Superga, Lower Miocene of the. + +Superior, Lake, marl in. + +Superposition of deposits, a test of age. +-- a test of age of volcanic rocks. + +Sutherlandshire, unconformable Palaeozoic strata in. + +Swanage, fossil mammalia found at. + +Sweden, Cambrian of. +--, slow rise of land in. +--, small thickness of Silurian strata in. + +Switzerland, lake-dwellings of. +--, Lower Molasse of. +--, Middle or Marine Molasse of. +--, Upper Miocene of, at Oeningen. + +Sydney coal-field, rain-prints in. + +Syenite, composition of. +--, how far connected with trap-rocks. + +Syenitic granite. + +Symonds, Reverend W.S., on Moel Tryfaen shells. + +Synclinal and anticlinal curves. + +Table of Botanical Nomenclature. +-- of St. Cassian fossil mollusca. +-- of Cretaceous formations. +-- of Devonian series in Devon. +-- of divisions of Hastings Sand. +-- of English and French Eocene strata. +-- of ages of fossil vertebrata. +-- of Neocomian strata. +-- of mammalia older than Paris gypsum. +-- of marine testacea in the Crag. +-- of Oolitic strata. +-- of volcanic minerals. +-- of Silurian strata of United States. +-- of Silurian rocks. +-- of Triassic strata. +-- of Cambrian strata. +-- of Permian of north of England. +-- of Welsh coal-measures. +-- of thicknesses of Carboniferous limestone. +--, general, of fossiliferous strata. + +Table Mountain, granite veins in clay-slate of. + +Tails of homocercal and heterocercal fish. + +Talcose granite. +-- gneiss. + +Tarannon shales. + +Tartaret cone, and lava of. + +Tate, Mr., on St Cassian fossils. + +Tealby series, Middle Neocomian. + +Teeth of extinct mammalia. + +Tellina balthica (T. solidula). +-- calcarea (T. proxima). +-- obliqua, Crag. + +Temnechinus excavatus. + +Temnopleurus excavatus. + +Tentaculites annulatus, Silurian. + +Terebellum fusiforme, Barton. +-- sopita, Barton. + +Terebratula affinis, Aymestry. +-- biplicata, White Chalk. +-- carnea, White Chalk. +-- digona, Bradford clay. +-- fimbria, Inferior Oolite. +-- hastata, Mountain Limestone. +-- sella, Neocomian. +-- Wilsoni, Aymestry. + +Terebratulina striata, White Chalk. + +Terebrirostra lyra, Chloritic Sand. + +Teredo navalis, boring wood. + +Tertiary formations, classification of. +-- strata, subdivisions of. +--, term defined. + +Testacea. See Shells. + +Thallogens. + +Thamnastraea, Coral Rag. + +Thanet sands. + +Theca operculata, Tremadoc beds. + +Thecodontosaurus, tooth of. + +Thecodus parvidens, Ludlow. + +Thecosmilia annularis, Coral Rag. + +Thirria, M., on Nerinaean limestone. + +Thompson, Dr., on Nummulites of Thibet. + +Thomson, Wyville, on Atlantic mud. +--, on sponges in chalk mud. + +Thuringia, monitor of. + +Thurmann, M., on Bernese Jura Oolite. +--, on structure of the Jura. + +Thylacotherium Prevostii, Stonesfield. + +Tile-stones of the Upper Ludlow. + +Tilgate forest, fossil Iguanodon in. + +Till described. +--, mammoth in Scotch. +-- of North America. + +Tin veins, age of, in Cornwall. + +Titanoferrite. + +Torell, Dr., on ice-action in Greenland. +--, on Swedish Cambrian fossils. + +Touraine, faluns of. + +Tourmaline. + +Trachytic rocks. +-- tuff. +-- porphyry. +-- lava, age of. + +Trap, term defined. +-- dike, intercepting strata. +-- dikes. +--, intrusion of, between strata. +-- rocks, ages of. +-- rocks passing into granite. +-- tuff described. + +Trappean rocks, nomenclature of. +-- rocks, their relation to active volcanoes. + +Trass of Lower Eifel. + +Travertin, how deposited. +--, inferieur of Paris basin. + +Tree ferns, living. + +Trees erect in coal, Nova Scotia. + +Tremadoc slates and their fossils. + +Tremolite. + +Trenton limestone, fossils of the. + +Trezza, volcanic rocks of. + +Trias, beds of passage between Lias and. +-- of England. +-- of Germany. +--, Saurians of the. +-- of the United States. + +Triassic mammifer, North Carolina. + +Triclinic feldspars. + +Tridymite, crystallised silica. + +Trigonellites latus, Kimmeridge clay. + +Trigonia caudata, Neocomian. +-- gibbosa, Portland stone. + +Trigonocarpum ovatum, and T. olivaeforme, Coal. + +Trigonotreta undulata, Permian. + +Trilobites of Bala and Caradoc beds. +--, metamorphosis of. +-- of primordial zone. + +Triloculina inflata, Eocene. + +Trimmer, Mr., on contorted strata. +--, on shells of Moel Tryfaen. + +Trinucleus concentricus, T. Caractaci. + +Trionyx, carapace of, Bembridge. + +Tripoli composed of diatomaceae. + +Trochoceras giganteus, Ludlow. + +Trophon antiquum (Fusus contrarius). +-- clathratum, Scotch drift. + +Tuff defined. +--, shelly, of the Grand Canary. +--, trappean, of Llandeilo rocks. +--, shelly, of Gergovia. + +Tupaia Tana, recent. + +Turner, Dr., on chemical decomposition. + +Turrilites costatus, Chalk. + +Turritella multisulcata, Bracklesham. + +Tuscany, mineral springs of. + +Tylor, Mr., on amount of subaerial denudation. + +Tyndall, Dr., on slaty cleavage. + +Tynedale fault. + +Tynemouth cliff, brecciated limestone of. + +Typhis pungens, Barton clay. + +Uncites Gryphus, Devonian. + +Unconformability of strata. + +Underlying, term applied to plutonic rocks. + +Unger on American forms in Swiss Miocene flora. +-- on Miocene plants of Croatia. + +Ungulite, or Obolus grit of Russia. + +Unio littoralis. +-- Valdensis, Hastings Sands. + +United States, Cambrian of the. +--, Cretaceous rocks of. +--, Devonian of. +--, Eocene strata in the. +--, footprints in Carboniferous of. +--, Lower Miocene of. +--, older Pliocene and Miocene formations of. +--, Silurian strata of. +--, Trias of the. + +Upheaval of land more than counteracted by subsidence. +--, power of denudation to counteract. + +Upper Greensand, or Chloritic series. + +Upsala, erratics on modern marine drift near. + +Ural Mountains, auriferous alluvium of. + +Uralite. + +Ursus spelaeus, tooth of. + +Urville, Captain de, on size of icebergs. + +Val d'Arno, Newer Pliocene of. + +Valleys, origin of. + +Valorsine, granite veins in talcose gneiss in. + +Valvata piscinalis. + +Vanessa Pluto, Lower Miocene, Croatia. + +Vegetation of the Coal. +-- of the Devonian of America. +--. See Plants. + +Veins, chemical deposits in. +--, granite rocks altered by. +--, different kinds of minerals. +--. See Mineral veins. + +Vein-stones. + +Venericardia planicosta. + +Venetz, M., on Alpine glaciers. + +Ventriculites radiatus, Chalk. + +Verneuil, M. de, on Russian Silurian. +--, on Permian flora. + +Vertebrata, progress of discovery of fossil. + +Vertical strata. + +Vesuvius, Recent and Post-pliocene volcanic rocks of. +--, basaltic lavas of. +--, tufaceous strata of. +--, dikes of. + +Vicarya Lujani, Punfield. + +Vicentin, columnar basalt of the. + +Vienna Basin, Upper Miocene beds of. + +Vine in Upper Miocene beds at Oeningen. + +Virginia, eighty miles of fault in. +--, coal-field of. + +Virlet, M, on corrosion of rocks near Corinth. +--, on Cretaceous traps of Greece. +--, on fossils in veins. +--, on volcanic rocks of the Morea. + +Volcanic ash or tuff. +-- breccia. +-- dikes. +-- force and denudation opposing powers. +-- mountains, structure and origin of. + +Volcanic rocks defined. +--, mineral composition of. +--, Recent and Post-pliocene. +--, Pliocene. +--, Miocene. +--, Eocene. +--, Cretaceous and Liassic. +--, New Red, Permian and Carboniferous. +--, Old Red Sandstone. +--, Silurian, Cambrian and Laurentian. +-- of Auvergne. +--, columnar and globular, structure of. +-- of Grand Canary. +-- of Silurian age. +--, special forms of structure of. +--, tests of age of. + +Volcanoes, extinct. +-- of Auvergne. + +Voltzia heterophylla, Bunter. + +Voluta ambigua, Barton clay. +-- athleta, Barton. +-- Lamberti, coralline and Red Crag. +-- Lamberti, faluns. +-- nodosa, London clay. +-- Selseiensis, Bracklesham. + +Von Buch, Leopold, on "elevation craters." +--, on Silurian plutonic rocks. + +Wacke described. + +Wagner, M., on Miocene of Greece. + +Walchia piniformis, Permian. + +Wales and England, glaciation of. + +Wallich, Dr., on Atlantic mud. + +Water, denuding power of running. +--, action of, in metamorphism. + +Watt, Gregory, on fusion of rock. + +Weald clay and its fossils. + +Wealden area, thickness of the. +-- formation. +-- flora. + +Webster, Mr. T., on Tertiary strata. + +Wellington Valley caves. + +Wenlock formation, fossils of the. +-- limestone. +-- shale. + +Werner on mineral veins in Saxony. +-- on isomorphism. + +Westwood, Mr., on Lias beetles. + +Wexford, veins of copper at. + +Whitaker, Mr., on subaerial origin of escarpments. + +White or coralline crag. +-- sand of Alum Bay. + +Whymper, Mr., on Arctic Miocene plants. + +Williams, Mr., on Cornish lodes. + +Williamson, Professor, on Conifers of the Coal. +--, on structure of calamite. + +Wind, denuding action of the. + +Wood, Mr. Searles, on Bridlington shells. +--, on Chillesford and Aldeby beds. +--, on shells of the Crags. +--, on shells of Crag and faluns compared. +--, on fish of Headon series. +--, table of marine testacea of the Crag. +--, on thickness of coralline crag. + +Woodward, Dr., on St. Cassian fossils. + +Woodward, Mr. H., on Pterygotus. + +Woolhope beds. + +Woolwich and Reading series. + +Wright, Dr., on Barton shells. +--, on zones of the Lias. + +Wunsch, Mr. E.A., on trees in volcanic ash. + +Wyville Thomson. See Thomson. + +Xiphodon gracile, Paris basin. + +Xylobius Sigillariae, Nova Scotia coal. + +Yoredale beds, thickness of the. + +Yorkshire, Oolite of. + +Young, Mr., on seeds washed out of mammoth tusks. + +Zechstein of Germany. + +Zeolites, secondary volcanic minerals. + +Zeuglodon cetoides, Eocene, United States. + +Zircon-syenite. + +Zoantharia rugosa and Z. aporosa. + +Zones of the Lias. + +Zonites priscus, Coal. + +Zoological provinces, great extent of. + +Zoophytes, fossil. +--. See Corals, Bryozoa, etc. + +Zurich, lake-dwellings in Lake of. + + +End of The Project Gutenberg Etext of The Student's Elements of Geology +by Sir Charles Lyell + |