Geology for students and general readers. Physical Geology ... Second edition

Front matter

GEOLOGY " That delightful labour 01 the imagination which is not mere arbitrariness, but the exercise of disciplined power — combining and constructing with the clearest eye for probabilities and the fullest obedience to knowledge ; and then, in yet more energetic alliance with impartial Nature, standing aloof to invent tests by which to try its own work."— George Eliot. GEOLOGY jpor Sbtu&mts anU (Kftural Uea&m PHYSICAL GEOLOGY By A. H. GREEN, M.A., F.G.S. PROFESSOR OF GEOLOGY IN THE YORKSHIRE COLLEGE OF SCIENCE, LEEDS; LECTURER ON GEOLOGY AT THE SCHOOL OF MILITARY ENGINEERING, CHATHAM ; OF H.M. GEOLOGICAL SURVEY . SOMETIME A SENIOR FELLOW OF GONVILLE AND CAIUS COLLEGE, CAMBRIDGE SECOND EDITION LONDON DALDY, ISBISTER & CO. 56, LUDGATE HILL loxdoh : printed bt virtue anl. co., limited, city eoad. TO THE JtttmorB of THE EEV. W. H. COLEMAN MY FIRST TEACHER IN GEOLOGY

Preface

. •THE Science of Geology, though barely yet a century old, •*- covers already so wide a field and takes in so great a diversity of subjects, that few, if any, men can hope thoroughly to master the whole of it. Mineralogy, Petrology, Stratigraphical Geology, Terres trial Cosmogony, Palæontology, and other lines of research, though they may fairly be looked upon as subdivisions of Geology, are fast becoming separate Sciences. But while it has become almost an absolute necessity for most Geologists to concentrate their attention on some one department of the Science and be content with a less perfect grasp of the rest, there is yet a certain basis or ground work, with which every one who meddles with Geology, whatever be the branch to which he specially devotes him self, must be acquainted if his work is to be sound.

For want of a knowledge of this groundwork the Petrologist, looking merely to chemical and mineralogical conijuosition, classes together rocks which differ totally in their origin or manner of occurrence ; the Palæontologist pure and simple is apt to force into an unnatural connection, on account of similarity in fossils, formations which physical evidence shows ought to be kept widely apart ; the Field Geolo gist is content with tracing boundaries on his map, and forgets to ask himself how his lines were produced and what they mean. It is this fundamental groundwork of which I have attempted to give an outline in the present volume, and in default of a better name I have called it Physical Geology.

PREFACE.

. VIII k I have had it in my mind to produce a book which will supply the requirements of two classes of readers. I wished to draw up a manual which would serve the purpose of those students who, without going very deeply into the subject, desire to know as much of the Science as any man of culture may be reasonably expected to possess. To be familiar with only thus much of Geology affords many opportunities of agreeable intellectual amusement, for some of its branches, such as the connection between the scenery of a country and its geological structure, can be 'understood without any special knowledge, and may be mastered and enjoyed by any one who can use his eyes, and reason in a very common-sense way about what he sees around him. But I have been still more anxious to produce a text book for the School and Lecture-room ; and in the hope that the book may be found suitable for educational pur poses, I am tempted to venture a few remarks on the rank which Natural Science is entitled to hold as an instrument for training the mind to reflect and reason, a subject just now of somewhat brisk controversy.

As a means of cul tivating the faculty of observation its superiority is unques tioned ; but it is not so generally allowed that it is as powerful an engine for developing the reasoning powers as the older studies of Mathematics and Classics. If Natural Science is ever to take rank beside these, it must show that it is equal to them in this very im portant respect ; and any work on Natural Science, which is intended for educational use, must not only state clearly the results arrived at, but must also put forth the methods by which they have been obtained. For this reason I have dwelt, with I hope not wearisome minuteness, on the logical processes by which the conclusions of Geology have been reached. One point more perhaps deserves notice. In spite of the elementary character of the work, I have not thought it desirable to shut out altogether those speculative branches of the subject, in which we are at present only f eeling our way darkly along, and have not yet been able

PREFACE.

. IX to arrive at any conclusion whatever. It has been objected to Natural Science in general, and the objection applies with special force to Geology, that it is unsuited for an instrument of education, because much* of it is uncertain and liable to be upset by new discoveries, and much of it at present little more than a blank. But it is this very circumstance which seems to me to constitute one of the chief claims of Science to rank high among educational tools. The multiplicity of its unsettled points causes it to make constant calls on the imagination, and so to fill a corner hitherto unoccupied in the educational programme ; for the results of Mathematics are too certain, and those of Classics too stereotyped, to leave much scope for imagina tive ingenuity. In short, let us have Mathematics with its severe logic to develop our reasoning faculty, Litera ture and Art with their elegancies to form our taste, and Natural Science with its vexed questions and unsettled problems to stimulate and at the same time guide our imagination,* and we shall have a curriculum with every requisite for developing the intellect all round, and pro ducing that highest result of culture, a many-sided mind.

A work like this affords little scope for originality, and I doubt whether there is in the book a single thing from beginning to end that can be said to be new. I have borrowed right and left ; in many cases my obligations are so obvious, that it would have been unnecessarily burdening the pages with references to have acknowledged the sources of my information ; in fact I have as a rule given references only in those cases where I wished the student to go more fully into the subject than I had room for. But whether I have recognised my debt or not, I beg to offer my best thanks to those numerous brethren of the hammer of whose labours I have availed myself without scruple and without stint. I must also content myself with a general acknowledg ment of the not inconsiderable help I have received from * I would not be understood to mean that this is the only function of the study of Natural Science, or this the only way in which the imagination may be awakened. X

PREFACE.

. private sources ; but I cannot forbear offering my special thanks to my father for his assistance in the revision of the proofs, and to my friend Mr. L. C. Miall for a similar service, as well as for a host of suggestions which have had the effect of materially adding to any value the book may possess. Leeds, January,

PREFACE.

TO SECOND EDITION. fPHOUGH it may seem ungrateful to those who have done me the honour to be readers of my book, I must confess that the somewhat rapid sale of its first impression has not been a source of unmixed satisfaction to me. I would gladly have tried to correct the many deficiencies which experience has proved it to possess, but the pressure of other engagements has left me time to do no more than put right a few obvious slips and make a few small additions. Most of these corrections have been suggested by friends, or by readers who, though they are personally unknown to me, I venture to reckon among my friends on account of the kindly interest they have shown in my work. A. H. G. Leeds, November 15, 1876. CONTENTS.

Chapter I. Ihe Aim And Scope Of Geology, With

A SKETCH OF ITS RISE AND PROGRESS, pp. 1—10.

Chapter II Descriptive Geology

pp. 11—87. Section I. — General Results arrived at by a Lithological Examination op Rocks. PAGE Descriptive Geology or Petrography 11 Historical Geology or Geogonie ...... 11 Subdivisions of Descriptive Geology, Lithology, and Petrology . 12 Instances of the Lithological Examination ol Rocks . . 13 Definition of a Mineral . . . . . . . .14 Definition of a Rock 14 Section II. — Mineralogy. Number of Rock-forming Minerals ...... 15 Chemical Composition of Rock-forming Minerals ... 16 External Form and Internal Structure of Minerals. — 1st, Crystalline Forms 19 Cleavage ........... 20 Fundamental Form 20 Axes of Crystals 22 Enumeration of Fundamental Forms ...... 22 Classification of Fundamental Forms 2-5 Laws of Crystalline Form ........ 26 External Form and Internal Structure of Minerals. — 2nd, Amorphous Forms ........ 29 Other Properties of Minerals . . . ' . . . .30 CONTENTS, Section HI. — Enumeration and Description of Rock-forming Minerals.

PAGS A. — Minerals composed of Silica ...... 31 B. — Minerals composed mainly of Silicates .... 33 B (1). — Felspar Group ........ 33 B (2).— Mica Group 37 (1) Non-magnesian Micas ........ 37 (2) — Magnesian Micas ........ 38 B (3). — Sornblendic or Augitic Group ...... 38 B (4).— Talc and Chlorite Group 40 C. — Compounds of Lime 41 Section IV. — Lithological Classification of Rocks, Lithological Classification of Rocks .43 Crystalline Rocks 43 Non-crystalline Rocks 44 Section V. — Crystalline Rocks, Texture of Crystalline Rocks 45 Structure ...... 46 Subdivisions of the Crystalline Rocks ...... 46 Acidic Rocks .......... 48 Basic Rocks .......... 48 Characters of the three classes of Crystalline Rocks ... 48 Imperfections of the above Classification 50 A. — Acidic Rocks 51 A a. — Quartzose Trachytes ........ 52 A b. — Felstones .......... 64 A c. — Granites 57 Chemical and Mineral identity of Acidic Rocks .... 58 Textural Varieties pass into ons another ..... 58 B. — Intermediate Rocks ........ 60 C. — Basic Rocks 62 C a. — Diorites 63 C b. — Meluphyres 64 Ou. — Basalts .......... 64 C d. — Corsite 66 Section VI. — Non-crystalline Rocks.

Texture gy Subdivisions of the Non-crystalline Rocks 67 1. — Arenaceous or Sandy Rocks 68 2. — Argillaceous or Clayey Rocks 69 3. — Calcareous Rocks or Limestones 72 4. — Carbonaceous Rocks CONTENTS. XV Section VII. — Petrology. Stratification or Bedding 82 Relation between Stratification and Crystalline or Non-crystalline Texture .......... 82 Fossiliferous and Unfossiliferous Rocks 84 Petrological Classification of Rocks 84 Terms connected with Stratification 84 Descriptive Geology. Summary 85 chapter rn. DENUDATION, pp. 88—117. Section* I. — Principles on which the Inquiry into the Origin of Rocks is based. — Examples of the Application of these Principles. — Definition of Denudation and Enumeration of Denuding Agents. Principles on which the Origin of Rocks is determined . . 89 Example of the Determination of the Origin of a Rock . . 90 Deuudation 94 Enumeration of Denuding Agents 94 Section II. — How Denuding Agents Work. 1.— Rain 94 Mechanical Action of Rain 94 Chemical Action of Rain 95 2. — Running Water 99 Rivers as Carriers of Sediment ....... 99 Denudation wrought by Rivers directly ..... 101 Underground Streams ........ 101 3.— Frost and Ice 103 Frozen Water 103 Glaciers ........... 104 Continental Ice-sheets ........ 108 Coast lee 109 Ground Ice 109 4. — Action of Wind 110 5. — Organic Denuding Agents Ill General View of Subaerial Denudation . . . .112 Formation of Soil 112 Removal of Soil from higher to lower Levels . . . .114 6. — Marine Denudation 115 Relative Importance of Subaerial and Marine Denudation . . 116' CONTENTS.

Chapter IV. What Becomes Of The Waste Produced And Carried Off By

DENUDATION.— THE METHOD OF FORMATION OF BEDDED ROCKS, AND SOME STRUC- TURES IMPRESSED ON THEM AFTER THEIR FORMA- TION, pp. 118—178. Section I. — Matter Mechanically Carried. PAGE Arrangement of Mechanical Deposits according to Size and Weight 119 Arrangement of Mechanical Deposits according to Mineral Composition . . . . . . . . . .119 General Arrangement of Mechanical Deposits . . . .120 Horizontal Growth of Coarse Deposits . . . . .122 Vertical Growth of Coarse Deposits ...... 122 Drift or Current Bedding ........ 123 Ripple-drift 124 Contemporaneous Erosion ........ 125 Ripple-marks, Rain-drops, Sun-cracks, and Animal Tracks . 12Q Summary ot' Characteristics of Shallow Water Deposits . . 127 General character of Deposits of finely-divided Matter . . 127 Stratification, and Thickness of Beds ...... 128 Parallel between Mudern Bedded Deposits and Stratified Rocks . 129 Section II. — Matter Carried in Solution and thrown down by Precipitation.

Means by which Precipitation is brought about .... 129 Conditions necessary for Chemical Precipitation . . . 131 Section III. — Dissolved Matters Extracted by Organic Agency. Foraminifera ......... , 133 Coral .......... \ 13, Other Limestone-secreting Animals j42 Origin of Pure Limestones and Inference from their presence . 141 Place of Limestone in the Sea Bed 14I Animals and Plants secreting biliia [ Rtd Clay of the Atlantic [ j42 Section IV. — Terrestkial Deposits. Soil and Rain-wash ......... 145 Screes ........... 147 Blown Sand ........ 149 Rocks of Vegetable Origin 149 Coal .150 Subaqueous Coal 1,51 Cannel Coal 153 Partings in Coal Seams CONTENTS. XVII Section V. — Deposits op Ice-formed Detritus. PAGE Distinctive characters of Ice-borne Detritus .... 155 Forms of Glacial Deposits . . . . . . . . ici ™.- • • ill Moraines jgg Glacial Mud Igg Boulder Clays .......... igo Erratics and Perched Blocks .......

Rearranged Glacial Beds ........ ig2 Rocks and Deposits of Glacial Origin 162 Section VI. — How Sediment is Compacted into Rogk, Weight of Overlying Masses 163 Deposition of Cement 163 Chemical Reactions ......... 163 Internal Heat 163 Pressure 164 Section VII. — Some Structures Impressed on Rocks after their Formation. Cleavage 166 Jointing ........... 169 Concretions . . . . . . . . . . .174 Concretionary Structure in Rocks . . . . . .176 Oolitic Structure . . 1.6 Secretionary Nodules 177

Chapter V. Definition And Classification Op Derivative Rocks

.- AND HOW FROM A STUDY OF THEIR CHARACTERS WE CAN DETERMINE THE PHYSICAL GEOGRAPHY OF THE EARTH AT DIFFERENT PERIODS OF ITS PAST HISTORY, pp. 179—213. Derivative Rocks and their Classification ..... 180 General Classification of Derivative Rocks . . . 181 Importance of learning the Conditions under which Rocks were formed 182 Teaching of G!.. •! Formations 183 A. — Marine Rock 184 Littoral Rocks 184 Thalassic Rocks . . 185 Normal Oceanic Rocks ....... 186 Erratics in Oceanic Deposits ...... 187 Chemical Deposits in Oceanic Arc 188 B. — Estuarine Rocks 189 Shape in Section of Deltas 191 Fossils of Estuarine Beds 191 Deposits formed by the Union of Deltas 192 1, CONTENTS. XVIII PAGE Example of an Estuarine Group J92 C— Lacustrine Rocks J™ Fresh-water Lacustrine Deposits J»' Salt-water Lacustrine Rocks *°° Red Colour of Inland Sea Deposits . . ■ • ■ • -«K» Processes by which Chemical Deposits may have been formed . 202 Rock Salt, Dolomite, and Gypsum . . ■ • • '203 Sources of the Materials for Chemical Deposits . . . • 207 Example of Chemically-formed Deposits 208 D.— Terrestrial Rocks .... • 210 Application to a particular instance 210

Chapter VI. Volcanic Rocks

pp. 214—260. Section I. — Cause of Crystalline Texture, Origin of Crystalline Rocks 214 Section IL— Phenomena and Products of Volcanic Action. Producing Causes of Volcanic Eruption 219 Structure of Single Cone 219 Cessation and Repetition of Eruption 220 Truncation and Breaching of Cone : Production of Crater and Cone within it ........ 221 Subsidence after Cessation of Eruptions 224 Dispersion of Ash and flow of Lava beyond the Cone : Prolonged Dykes 224 Submarine Eruptions • 225 Volcanic Products : (1) Lavas 225 Fluidity 225 Motion of Lava Streams, and Texture of their different Parts . 228 Composition of Lava 229 Texture of Lava 230 Bedded Structure 230 Laminated Structure . • .231 Jointing and Columnar Structure 231 Concretionary Structure . . . . . . . .231 Parallel between Lava and Crystalline Rocks in Structure . . 232 (2) Fragmental Products 233 Structure of Subaerial Ashy Deposits 234 Structure of Subaqueous Ashy Deposits ..... 235 (3) Gaseous Products 236 Section III. — Remnants of Old Volcanoes.

Ancient Volcanic Cones . 237 Remains of Central Plug of Lava 238 Other Proofs of Old Volcanic Action CONTENTS. XIX PAGE Example of Arthur's Seat ........ 238 Ancient Volcanoes of North Wales 243 Section IV. — Petrology of Volcanic Rocks. Distinction into Intrusive and Contemporaneous . . . 245 Alteration of Neighbouring Rocks 247 Included Fragments ......... 247 Fragmental Interbedded Rocks , 248 Necks of Agglomerate . . . . . . . .248 Instances of the Modes of Occurrence of Volcanic Rocks . . 248 Subdivisions of Igneous Rocks into Volcanic and Trappean . 256 CHAPTER Vn. METAMORPHIC ROCKS, pp. 261—308 Section I. — General View and Instances of Metamorphism. General Description . . . . . . . . .261 Metamornhic Rocks of Carrara ....... 263 Metamorphic Rocks of County Donegal ..... 26.5 Effects of Metamorphism ........ 268 Subdivisions of Metamorphic Rocks 268 Section II. — Description of the Principal Varieties of -ihe Metamorphic Rocks. 1st Class. — Those which still retain Traces of Bedding and other proofs of their originally derivative Condition . . . . ' . . . . . . 271 (a) /Siliceous Members . . . . . . . . .271 (b) Argillaceous Members ........ 273 (<;) Calcareous Members ........ 274 (d) Carbonaceous Members ........ 281 2nd Class. — Foliated or Schistose Rocks .... 282 Nature of Foliation 282 Degrees of Foliation . 283 What determines the Planes of Foliation .....

U84 Artificial Production of Cleavage Foliation .... 286 Crumpled Laminæ ......... 287 Intrusive Schistose Rocks . 287 (Summary ........... 289 Description of Foliated Rocks 290 Early Theories about Crystalline Schists . . . . .294 3rd Class. — Amorphous Crystalline Rocks .... 295 Serpentine 298 Section III. — Causes of Metamorphism. Local Metamorphism by Intrusive Igneous Rocks . . .. 299 Heat one Agent 299 Heat alone not enough XX CONTENTS. PAGE Heated Vapours 3°° Water 300 Pressure and Depth ax*t Experiments of Daubree ....•••• ■•>"••* Researches of Sterry Hunt ....•••• £*J* Observations of Mr. Sorby SOi Pseudomorphism ......... "u" Variations in amount of Metamorphism ..... 306 Subsidiary Metamorphosing Agencies ..... 306 Summary ........... ■-""•> Metamorphism no Proof of Antiquity 307 CHAPTER Vin.

GRANITIC ROCKS, pp. 309—336. Difference between Granitic and Volcanic Rocks . . . 310 Petrological Modes of Occurrence of Granite .... 310 Granite of the Pyrenees . . . . . . . .311 Metamorphic Granite ........ 312 Donegal 313 Brittany 313 Priestlaw 314 South-west of Scotland ........ 316 Intrusive Granite of Devon and Cornwall ..... 317 Intrusive Granite of Brittany ....... 319 Granite Veins . 320 Included Blocks in Granite ....... 321 Contact-Metamorphism by Granite 322 The Origin of Plutonic and Trappean Rocks .... 324 Objections to Metamorphic Theory ...... 326 Reasoning extended to other Plutonic Rocks .... 327 Summary and Conclusions ........ 328 Classification of the Crystalline Rocks based on the Metamorphic Theory 333

CHAPTER IX. HOW THE ROCKS CAME INTO THE POSITIONS IN WHICH WE

NOW FIND THEM, pp. 337—406. Section I.— Nature of the Displacements which Rocks save UNDERGONE. Displacements which Submarine Beds have suffered . . . 337 Section II. — Vertical Elevation. Two possible Explanations of Elevation ..... 338 Arguments against a Lowering of the Sea-level .... 338 The Land has gone up, not the Sea gone down .... CONTENTS, XXI PAGE Denudation gives Proof of Elevation 339 Instances of observed Oscillation of Land ..... 339 Submergence produced by a Polar Icecap 341 Section III. — Displacement of the Rocks from their originally Horizontal position. Dip 342 Strike 343 Measurement of Dip 343 Outcrop 344 Undulations and Contortions 345 Anticlinal and Synclinal ; Dome and Basin .... 347 Anticlinal 349 Dome 350 Synclinal and Basin 351 Parallelism of Anticlinals 354 Classes of Anticlinals 354 Inversion 355 Outlier and Inlier 358 Section IV. — Faults.

Faults 362 Slickenside ........... 365 Hade of Faults 366 Course of Faults 366 Parallelism of Faults 367 Changes in Size and Dying out of Faults . 367 Effect of Faults on Outcrop 371 Indirect Evidence for Faults .* 372 Section V. — How the Displacements of the Rocks were produced. Character of the Movements 376 Folding would produce both Elevation and Dip .... 378 Direction of the Folding Force . . . . . ; . .378 Summary of the Evidence 385 Folding went on at great depths 386 Folding went on slowly 387 Contortions more frequent in Old than Recent Rocks . . . 388 Section VT.— Unconformity and Overlap. What constitutes Unconformity 388 Meaning of Unconformity 390 Deposition on Sinking Sea-bottoms 393 General Conclusions 393 Illustration of Unconformity 394 Incidental Proofs of Unconformity . . . . . .397 Deceptive Appearance of Unconformity owing to Underground Dissolution of Rock 398 Deceptive Conformity 398 Overlap 400 Practical Bearings CONTENTS.

CHAPTER X. HOW THE PRESENT SURFACE OF THE GROUND HAS BEEN PRODUCED,

pp. 406—480. Section I.— Proofs that the Shape of the Surface is due to Denudation. tage Surface due to Denudation ........ 406 Amount of Denudation ......•• 410 Section II. — The Share of each Denuding Agent ln producing the Shape of the Surface. Share of tbe Sea 413 Plain of Marine Denudation 414 Share of Subaerial Denuding Agents. — Rivers . . . .415 Cation of Colorado an Example of River Action .... 418 Other Subaerial Denuding Forces 421 Landslips ........... 421 Basin-shaped Lie of Outliers 423 Steps in the Formation of the Surface 424 Valleys determined by Joints ....... 427 Valleys determined by Faults ....... 427 Qualifications 427 Final Result9 of Subaerial Denudation 428 Anomalous Behaviour of Rivers explained ..... 429 History of the Idea of Subaerial Denudation .... 433 Section III. — How the Character and Lie of the Underlying Rocks affect the Shape of the Ground.

Relative Hardness 435 Other Qualities which enable Rocks to resist Denudation . . 438 Difference between Results of Marine and Subaerial Denudation 439 Effect of Natural Planes of Division 439 Effect of the Lie of the Beds on the Shape of the Surface . . 443 Escarpment and Dip-slope .......'. 443 Broadening of Valleys by River Action ..... 450 Cutting back of the Channels of Rivers 450 Section IV. — Features due to the Action of Ice. General Aspect of Ice-worn Districts ...... 452 Polished Surfaces 453 Scratches 453 Roches Moutonnees 455 Moraines ........... 457 Lakes CONTENTS. XXIII Section V. — Surfaces not wholly due to Denudation. PAOE Mountain Chains ......... 465 Volcanic Cones .......... 471 Eskers 471 Moraines 475 Sand Dunes 475 Lakes enclosed by heaped-up Mounds ..... 475 Alluvial Flats 476 River Flats .476 Old River Terraces 476 Sea-beaches 477 Raised Beaches 477 Surfaces of Deltas 478 Silted-up Lakes .......... 478 Prairies and Deserts ......... 478 Summary 478

CHAPTER XI. ORIGINAL FLUIDITY AND PRESENT CONDITION OF THE INTERIOR OF THE

EARTH. CAUSE OF UPHEAVAL AND CONTORTION. ORIGIN OF THE HEAT RE- QUIRED FOR VOLCANIC ENERGY AND METAMOR- PHISM- REMARKS ON SPECULATIVE GEOLOGY, pp. 481—527. Section I. — The present Physical Condition of the Earth. Shape of the Earth 483 Mean Density of the Earth 484 Internal Temperature of the Earth ...... 486 Inferences from the foregoing Facts 487 Present State of the Earth's Interior . ... . . . 492 Doctrine of a Thin Crust 492 Argument from Precession ........ 596 Argument from Rigidity ... .... 498 Objections to tbe preceding Arguments ..... 500 Professor Hennessy's Views ....... 502 Chemistry of the Early History of the Earth .... 503 Section IL— Cause of Upheaval and Contortion. Sense in which Elevation is used 504 General Structure of Mountain Chains 505 Mr. Hopkins's Theory 505 Theory of Scrope and Babbage 507 Theory of Sir J. Herschel 507 Intrusion of Granite 508 Contraction Theory 609 Remarks on the Contraction Theory CONTENTS, Section III. — Origin of the Heat required for Volcanic Energy and Metamorphism.

PAG*** Metamorphism and Lava, both Effects of the same Cause . .513 Explanation on Hypothesis of a Thin Crust .... 513 Mr. Hopkins's Theory 513 Explanations of Mr. Scrope and Rev. 0. Fisher . . . .514 Sterry Hunt's Theory ......... 515 Mr. R. Mallet's Theory 516 Bearing of Mr. Mallet's Theory on Metamorphism . . . 522 Section IV. — Concluding Remarks on Speculative Geology. Geological Time 523 Former greater Intensity of Geological Action .... 524 CHAPTER XH. ON CHANGES OF CLIMATE, AND HOW THEY HAVB BEEN BROUGHT ABOUT, pp. 528—543.

PAGE Fio. 1. — Diagram to explain how we can infer from observations at the surface the constitution of the Earth's Crust deep underground ° Fig. 2. — Rhombohedron of Calcite " Fia. 3. — Cube of Fluor Spar 21 Fig. 4. — Net for Rhombic Dodecahedron .... 24 Fig. 5. — Net for Rhombohedron 25 Fig. 6. — Quarry in Stratified Rocks °* Fig. 7. — Clay-with-Flints resting on Chalk ■" Fig. 8. — Glacier with Lateral, Medial, and Terminal Moraines 107 Fig. 9. — Undercut Table of Gritstone . . . .110 Fig. 10. — Distribution of Mechanical, Deposits on the Sea-bottom 121 Fig. 11. — Formation of Current Bedding . . . .123 Fig. 12. — Quarry in Current-bedded Rock .... 124 Fig. 13. — Formation of Ripple-drift ..... 125 Fig. 14. — Contemporaneous Erosion 126 Fig. 15. — Section across a Fringing Reef .... 135 Fig. 16. — Section across a Barrier Reef .... 136 Fig. 17. — Section across an Atoll 137 Fig. 18. — View of an Atoll .... . . 138 Fig. 19. — Section across the Rim of a Coral-reef . . 140 Fig. 20. — Section of the Dirt Bed of the Dorsetshire Coast 146 Fig. 21. — Section showing the Position of the Dolomitic Conglomerate of the South-west of England 148 Fig. 22. — Coal resting on an Underclay with Stigmaria . 152 Fig. 23. — Erect Sioillaria with Stigmaria Roots • . 153 Fig. 24. — Diagram to explain the Thickening of a Parting in A Seam of Coal 155 Fig. 25. — Diagram to explain the gradual Replacement op Coal by Sandstone and Shale .... 156 Fig. 26. — Perched Block 161 ■ LIST OF WOODCUTS.

PAGE Fig. 27.--Cleavaqe and Bedding Fig. 28. — Section of Rock before and after Cleavage . 167 Fig. 29.— Cleaved and Contorted Rocks, showing a Parallelism BETWEEN THE PLANES OF CLEAVAGE AND the Axes of the Folds ..... 1«° Fig. 30. — Quarry in a Jointed Rock l1° Fig. 31. — Concretions with lines of Bedding running through them 174 Fig. 32. — Concretionary Structure in Sandstone . .177 Fig. 33. — Diagram showing the Estuarine Beds of the Weald and their Marine Equivalents . .194 Fig. 34. — Vertical Section of single Volcanic Cone . . 220 Fig. 35. — Horizontal Section of single Volcanic Cone . 220 Fig. 36. — Ideal Section of a Volcanic Group . . . 222 Fig. 37. — Barren Island, after Scrope .... 223 Fig. 38. — Large angular Blocks embedded in Volcanic Ash, North Berwick 233 Figs. 39, 40. — Diagrammatic View and Section of Arthur's Seat 239 Fig. 41. — The Older Rocks of Arthur's Seat . . . 242 Fig. 42. — Section at Dunbar Castle, showing .an included mass of altered Sandstone in Greenstone . 248 Fig. 43. — Plan and Section of Intrusive Greenstone mass, Co. Donegal, Ireland .... . 249 Fig. 44. — Greenstone Dykes cutting through Shales and Limestones, west of Dunbar .... 250 Fig. 45. — Section across the Permian Basin of Ayrshire . 252 Fig. 46. — Dyke and Intrusive Sheet of Dolerite . . 255 Fig. 47. — Diagram illustrating the Metamorphism of the Rocks of Carrara ...... 264 Fig. 48. — Section across the Metamorphic District of Errigal, Co. Donegal, Ireland 266 Fig. 49. — Foliation parallel to Bedding .... 284 Fig. 50. — Crumpled Laminae of Foliation .... 286 Fig. 51. — Diagram to explain the cause of the crumpling of the laminæ of Foliation .... 287 Fig. 52. — Geological Sketch-map of Dartmoor . . . 318 Fig. 53. — Dykes of Granite cuttino through Limestone . 321 Fig. 54. — Diagram to illustrate the relations of Derivative, Metamorphic, Plutonic, and Volcanic Rocks 332 Fig. 55. — Diagram to illustrate the Definitions of Dip and Strike 343 Fig. 56. — Undulating and Contorted Beds with a general Dip in one Direction PAGE Fig. 57. — Contortions in Shale and Sandstone . . . 346 Fig. 58. — Contorted Limestone, Draughton, near Skipton . 348 Figs. 59, 60, 61. — Map and Sections of an Anticlinal . 347, 349 Figs. 62, 63, 64.— Map and Sections of a Dome . . 350, 351 Figs. 65, 66, 67. — Map and Sections of a Synclinal . 352, 353 Fig. 68. — General Section across the Appalachians . . 354 Fig. 69. — Inverted Beds near Milford Haven . . . 355 Figs. 70 and 71. — Inversion in the Jura . . . 356, 357 Fig. 72. — Diagram to illustrate the Production of Inversion 358 Figs. 73, 74, 75. — Map, Section, and View of an Outlier on Shutlingslow 359, 360 Figs. 76, 77. — Map and Section of an Inlier at Crich Hill 361, 362 Fig. 78. — Diagram of Faulted Strata 363 Fig. 79. — Faults unaccompanied by Disturbance or Contortion ......... 363 Fig. 80. — Contorted Beds in the Neighbourhood of a Fault 364 Fig. 81. — Section of a Main Fault and Smaller Faults adjoining it 364 Fig. 82.—