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A. H. (Alexander Henry) Green.

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IRLF




LIBRARY

OF THE

UNIVERSITY OF CALIFORNIA.



Class



GEOLOGY



TO THB

of
THE EEY. W. H. COLEMAN

MY FIRST TEACHER IN GEOLOGY




PEEFACE.



'PHE 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, Palaeontology, 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 composition,
classes together rocks which differ totally in their origin or
manner of occurrence ; the Palseontologist 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.



Vlll PREFACE.

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
feeling 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.



I 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, 1876.




PBEFACE TO SECOND EDITION.



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 expe-
rience 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.

THE AIM AND SCOPE OF GEOLOGY, WITH A SKETCH OF
ITS RISE AND PROGRESS, pp. 110.



CHAPTER II.
DESCRIPTIVE GEOLOGY, pp. 1187.

SECTION I. GENERAL RESULTS ARRIVED AT BY A LITHOLOGICAL
EXAMINATION OF ROCKS.

PAGE

Descriptive Geology or Petrography . . . < >.v, 4 .. , . 11

Historical Geology or Geogonie 11

Subdivisions of Descriptive Geology, Lithology, and Petrology . 12

Instances of the Lithological Examination of Rocks . . 13

Definition of a Mineral . . . . _* J ..', .-. -k'. ' : '. . 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, Crys-
talline Forms . . . . . . .19

Cleavage . . . ... '*,, , . " . 20

Fundamental Form . .'';'.'' . . ... . 20

Axes of Crystals 22

Enumeration of Fundamental Forms 22

Classification of Fundamental Forms 25

Laws of Crystalline Form .26

External Form and Internal Structure of Minerals. 2nd,

Amorphous Forms 29

Other Properties of Minerals 30



XIV CONTENTS.



SECTION III. ENUMERATION AND DESCRIPTION OF ROCK-FORMING
MINERALS.

MM

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) Mag mesian Micas . . . .. . . .38

B (3). Hornblendic 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 . .. ' v 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 54

A c. Granites 57

Chemical and Mineral identity of Acidic Rocks . . . '' ? . 58

Textural Varieties pass into one another 58

B. INTERMEDIATE ROCKS 60

C. BASIC ROCKS 62

Ga. Diorites 63

Gb.Melaphyres G4

Qc. Basalts 64

Ud.Corsite . . .66



SECTION VI. NON-CRYSTALLINE ROCKS.

Texture f . 67

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 75



CONTENTS. XV

SECTION VII. PETROLOGY.

PAGB

Stratification or Bedding 82

Relation between Stratification and Crystalline or Non-crystalline

Texture . . 82

Fossiliferous and Unfossiliferous Kocks 84

Petrological Classification of Kocks 84

Terms connected with Stratification 84

Descriptive Geology. Summary . . . ... -85

CHAPTEE III.

DENUDATION, pp. 88-117-

SECTION I. PRINCIPLES ON WHICH THE INQUIRY INTO THE ORIGIN
OF ROGKS IS BASED. EXAMPLES OF THE APPLICATION OF THE8B
PRINCIPLES. DEFINITION OF DENUDATION AND ENUMERATION OP
DENUDING AGENTS.

Principles on which the Origin of Rocks is determined . . 89

Example of the Determination of the Origin of a Rock . . 90

Denudation 94

Enumeration of Denuding Agents 94

SECTION II. How DENUDING AGENTS WOEK.

1. RAIN . . . . * * v: '. -V > . 91

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 Ice 109

Ground Ice 109

4. ACTION OF WIND 110

5. ORGANIC DENUDING AGENTS . . . . . .111

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.



CHAPTEE IY.

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. 118178.

SECTION I. MATTER MECHANIC A.LLY CARRIED.

PAGE

Arrangf ment of Mechanical Deposits according to Size and

Weight . . _ . . ^ 119

Arrangement of Mechanical Deposits according to Mineral Com-
position 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 . 126

Summary of Characteristics of Shallow Water Deposits . . 127
General character of Deposits of finely-divided Matter . .127

Stratification, and Thickness of Beds 128

Parallel between Modern 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 AGKNCY.

Foraminifera 133

Coral >. 134

Other Limestone-secreting Animals 141

Origin of Pure Limestones and Inference from their presence . 141

Place of Limestone in the Sea Bed 141

Animals and Plants secreting Silica . . . . - * . . 142

Red Clay of the Atlantic 142

SECTION IV. TERRESTRIAL DEPOSITS.

Soil and Rain-wash . '. . . ' ... 145

Screes - .-..-. 147

Blown Sand . . .149

Rocks of Vegetable Origin 149

Coal 150

Subaqueous Coal . . 151

Cannel Coal . 153

Partings in Coal Seams 154



CONTENTS.
SECTION V. DEPOSITS OF ICE-FORMED DETRITUS.

PAGE

Distinctive characters of Ice-borne Detritus . . . .156

Forms of Glacial Deposits -157

Till 15$

Moraines 159

Glacial Mud 160

Boulder Clays 160

Erratics and Perched Blocks 161

Rearranged Glacial Beds 162

Rocks and Deposits of Glacial Origin 162

SECTION VI. How SEDIMENT is COMPACTED INTO ROCK.

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 . . 176

Secretionary Nodules 177

CHAPTEE Y.

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. 179213.

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 Glacial Formations 183

A. MARINE ROCKS . . .' 184

Littoral Rocks 184

Thalassic Rocks 185

Normal Oceanic Rocks , t . .186

Erratics in Oceanic Deposits 187

Chemical Deposits in Oceanic Areas 188

B. KSTUARINE ROCKS 189

Shape in Section of Deltas 191

Fossils of Estuarine Beds 191

Deposits formed by the Union of Deltas 192

b



CONTENTS.

MM

Example of an Estuarine Group ....... 192

C. LACUSTRINE ROCKS 195

Fresh-water Lacustrine Deposits 197

Salt-water Lacustrine Rocks 198

Red Colour of Inland Sea Deposits 200

Processes by which Chemical Deposits may have been foimed 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



CHAPTEE VI.
VOLCANIC ROCKS, pp. 214260.

SECTION I. CAUSE OF CRYSTALLINE TEXTURB.

Origin of Crystalline Rocks . . . . .- r . . . 214

SECTION II. PHENOMENA AND PRODUCTS OP 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 now 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 . . . * v 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 . . . . ,,,. V- . 236

SECTION III. REMNANTS OF OLD VOLCANOES.

Ancient Volcanic Cones . . ,. '. , . . ,. 237

Remains of Central Phig of Lava . . . . . 238

Other Proofs of Old Volcanic Action 238



CONTENTS. XIX

ft&M

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

CEAPTEE VH.
METAMORPHIC ROCKS, pp. 261308

SECTION I. GENERAL VIEW AND INSTANCES OF METAMOBPHISM.

General Description . . '' . . ...,.. 261

Metamorphic Rocks of Carrara 263

Metamorphic Rocks of County Donegal 266

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 . . V v. 271

a] Siliceous Members . >..' . . . . . ' . 271

Argillaceous Members ' . . . . . 273

c] 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 284

Artificial Production of Cleavage Foliation .... 286

Crumpled Laminae 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 '300



XX CONTENTS.

PA OH

Heated Vapours 300

Water .;.,-.. 300

Pressure and Depth . . v '"* . 302

Experiments of Daubree * 303

Researches of Sterry Hunt 304

Observations of Mr. Sorby 304

Pseudomorphism ... ...... 305

Variations in amount of Metamorphism 306

Subsidiary Metamorphosing Agencies 306

Summary . . . . . . . . W 306

Metamorphism no Proof of Antiquity . f . . : , 307



CHAPTEK Vin.

GRANITIC ROCKS, pp. 309336,

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



CHAPTEK IX.

HOW THE ROCKS CAME INTO THE POSITIONS IN
WHICH WE NOW FIND THEM, pp. 337406.

SECTION I. NATURE OP THE DISPLACEMENTS WHICH ROCKS HAVE

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 . . . . 333
The Land has gone up, not the Sea gone down .... 339



CONTENTS. XXI

PAGE

Denudation gives Proof of Elevation a>< *9

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 . ' V ". 'V* - - -342

Strike -''.'.*. . . .343

Measurement of Dip 343

Outcrop . . 344

Undulations and Contortions . . .'""' .345
Anticlinal and Synclinal; Dome and Basin . -,'. . 347
Anticlinal . . . " * * ' . 349
Dome. . . . i '. v* .- 4 . . 350

Synclinal and Basin 351

Parallelism of Anticlinals . . . . .-'/ 354
Classes of Anticlinals . ..-. t> *,;.. "*' \ 354
Inversion . . . . * : j 355
Outlier and Inlier . . -' . . '. *' -V 358

SECTION IV. FAULTS.

Faults . . . ' . '."... - . '.,' ' 362

Slickenside . ' , - .365

Hade of Faults 366

Course of Faults . . . . '. i* '''' 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 VI. UNCONFORMITY AND OVERLAP.

"What constitutes Unconformity 388

Meaning of Unconformity ;

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 .404



CONTENTS.



CHAPTEE X.

HOW THE PRESENT SURFACE OF THE GROUND HAS
BEEN PRODUCED, pp. 406480.

SECTION I. PROOFS THAT THE SHAPE OF THE SURFACE is DUE
TO DENUDATION.

TAOK

Surface due to Denudation ' 406

Amount of Denudation . . . . . .... 410

SECTION II. THE SHARE OF EACH DENUDING AGENT IN PRO-
DUCING THE SHAPE OF THE SURFACE.

Share of the Sea 413

Plain of Marine Denudation 414

Share of Subaerial Denuding Agents. Rivers . . . .415
Canon of Colorado an Example of River Action . . . .418

Other Subaerial Denuding Forces 421

Landslips 421

Basin-shaped Lie of Outliers 123

Steps in the Formation of the Surface 424

Valleys determined by Joints 427

Valleys determined by Faults 427

Qualifications ' . . . . 427

Final Results of Subaerial Denudation 428

Anomalous Behaviour of Rivera 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 v. . 436

Other Qualities which enable Rocks to resist Denudation . . 438
Difference between Results of Marine and Subaerial Denu-
dation f . . . . . 439

Effect of Natural Planes of Division . . . . V' . 439

Effect of the Lie of the Beds on the Shape of the Surface . . 443

Escarpment and Dip-slope t . 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 r + ... f ,. ,;* 453

Roches Moutonnees 455

Moraines 457

Lakes . . . . . . 457



CONTENTS. XX111

SECTION V. SURFACES NOT WHOLLY DUB TO. DENUDATION.

TAOB

Mountain Chains . . . 465

Volcanic Cones 471

Eskers 471

Moraines . . . . . ; , 475

Sand Dunes 475

Lakes enclosed by heaped-up Mounds ' . . . . . 475

Alluvial Flats . . . . . . 476

Eiver Flats. . . . V ; 476

Old River Terraces . . . ... -. . .476

Sea-beaches * *- . . ' 1 . . 477

Raised Beaches . . 477

Surfaces of Deltas 478

Silted-up Lakes ..** .478

Prairies and Deserts 478

Summary ...... 478

CHAPTEE 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 MET AMOR-
PHISM- REMARKS ON SPECULATIVE GEOLOGY, pp.
481527.

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 the preceding Arguments 500

Professor Hennessy's Views . . ' . . . . . 502

Chemistry of the Early History of the Earth .... 503

SECTION II. 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 509

Remarks on the Contraction Theory 510



XXIV CONTENTS.



SECTION III. ORIGIN OF THE HEAT REQUIRED FOR VOLCANIC
ENERGY AND METAMORPHISM.

Metamorphism and Lava, both Effects of the same Cause . . olli

Explanation on Hypothesis of a Thin Crust . . . . olu

Mr. Hopkins's Theory 513

Explanations of Mr. Scrope and Rev. O. Fisher . . . .514



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