The Origin of Mountain Ranges considered experimentally ... and in relation to their geological history

Preface

. The Origin of Mountain-Ranges is a problem that has interested geologists ever since the structure of the crust of the Earth has been systematically studied. Notwithstanding this, I know of no work in the English language devoted entirely to a consideration of this important subject, or containing what may be considered a systematic theory of mountain-building. The line of geological thought in this country has hitherto run more in stratigraphical than dynamical grooves. The American geologists, forced into contact with the stupendous mechanical problems presented by the geology of their continent and great mountain-ranges, have devoted considerable practical attention to dynamical geology. Still, scattered through the works of the great masters of geology who have flourished in our own country are to be found the germs of most true geological reasoning, dynamical and otherwise.

An attempt such as mine to provide a systematic treatise will probably be considered an ambitious one. Whether the theory here presented to the geological world be the true one or not will hardly be determined by such a consideration. The true test, as of everything geological, is time ; aud if it be found that the views here set forth best explain the manifold aspects of Nature, and otherwise meet the conditions of the problem, the ideas will live ; if not, they may still survive as partial truths. In the mean time I have thought that the presentation of the subject in as complete and practical a form as is possible to me at the present time would have its value, and this must partly be my excuse for publishing. Throughout the book I have attempted to do justice, by means of extracts and notes and references, to previous workers in the same field.

This is perhaps the most difficult duty of a scientific author; and if in this particular I have failed, as I feel that I must have done in many cases, I trust it will be attributed to nothing more than inadvertence. Such are the rapid strides with which our favourite study advances that, although the writing of this work has been continuous, I have constantly had to refer in footnotes to important work done, even since the type has been set up. The subject is in many respects difficult and even abstruse. To render the information more readily available, and to enable the reader to grasp the ideas in the concrete, I have taken some pains to illustrate the text. The sketches of scenery have in every case been drawn from Nature, and generally during my holiday rambles. The shaded views are ink-photos of sepia-drawings prepared by me from the original sketches.

The line-drawings, both of scenery and structure, are photo-lithographs from drawings I have specially prepared for this work, and also are in all cases from Nature. Some of the illustrations, as will be seen by the descriptions, are reproductions from reports published by the United States Geological Survey and the Second Geological Survey of Pennsylvania; the remainder of the diagrams and maps are mostly from my own drawings. V It now becomes my pleasant duty to thank those who have, during the progress of the work, afforded me aid and sympathy. To Prof. Powell, Mr. M'Gee, Prof. Lesley, and the Governments they represent, my best thanks are due for the presentation of various important reports on the Geology of the United States, and for permission to use the maps and sections. From the Liverpool Free Public Library Committee, and Mr. Cowell, their librarian, I have received important help through the loan of valuable works of reference, and even to the extent of their adding to the Library at my suggestion.

With thanks to these gentlemen and numerous kind friends, I conclude, for the present, work which, though arduous, has at the same time been as pleasant as it was engrossing. T. MELLARD READE. Park Corner. Blundellsands, LiTerpool, Sept. 30th, 1886. CONTENTS.

Chapter I

Variability in the Temperature of the Crust of the Earth, and its Cause. Pages Increase of heat with depth. — Variations in its rate. — Existing variations not constant in time. — Crust responds to internal changes of temperature. — Variations in the internal heat as the crust is approached shown by volcanic activity. — Mallet's view of the origin of volcanic heat controverted. — The cause of volcanic activity. — Speculations relating to a hypothetical zone of molten rock between a solid nucleus and a solid crust. — No incompatibility between a solid spheroid and the known facts of Geology. — Interior potentially molten though solid 1-7

Chapter II

Effects of Differential Heating on the Earth's Crust. Influence of a change of temperature on the Earth's crust. — Coefficient of expansion of average rock. — Calculation of cubical expansion. — Effects of expansion 8-11 CHAPTER HI. Mechanical Effects of Expansion by Heat on various Materials, as known through common experience. Effects of expansion on chimneys, kilns, &c. — Expansion of metal work in buildings. — Effects of ordinary changes of temperature on engineering structures — rails, bridges, roofs, girders, and masonry 12-16 CONTENTS.

Chapter IV

Mechanical Effects of Expansion by Heat as determined by Experiment on various Materials. Pages Linear expansion. —Superficial expansion. — Experiments on lead plates. — Deductions from experiments. — Effects of differential ex pansion on various stones. — What the experiments show 17-28

Chapter V

Geological Phenomena associated with Mountain-Ranges. General considerations. — Mountains of Great Britain. — Mountains of North America : Appalachians ; the Rocky-Mountain system ; the Basin-Range system ; the three Geological Provinces. — Geology of India and structure of the Himalayas : Peninsular India ; Mountain-Ranges of Extra-Peninsular India — Sind and Western Punjab, Himalayas; Geological Formations. — South America: The Andes 29-70

Chapter VI

General Review of the Geological Evidence associating Mountain Upheaval with previous Sedimentation. North America. — South America. — India. — Britain. — Switzerland. —Physical inferences 71-88

Chapter VII

Exciting Causes initiating Earth-movements and Mountain-making. Effect of heat on consolidated strata at great depths. — Faults . . 89-98 CHAPTER VIH. Normal Faulting. — Effects of Contraction on the previously expanded Sediments making up Mountain-Ranges. Contraction of rocks greatest at the base.— Low tensile stren°th of rocks.— Crushing strength.— Upper or surface beds destroyed by denudation. — Flowing of rocks by gravitative compression. Shear ing.—Why the hade is towards the downthrow.— Explanation by appeal to natural sections 99-106 CONTENTS. IX

Chapter IX

Competency of Expansion by Heat to produce all the phenomena of Mountain-structure. Pages Coefficients of expansion of rocks. — Table of coefficients as determined by the Author's experiments. — Ditto by A. J. Adie. — Ditto by W. H. 0. Bartlett and Col. Totten. — Cubical expansion and its effects on the Earth's crust. — Effective increase in cubic miles of a given heated area 107-117

Chapter X

The Globe as a Reservoir of Heat. Calculation of the volume of mountain uplifts during geological history. — Volume and increase of temperature of the rock required to produce the necessary expansion. — Proportion of the total heat of the globe used up iu this work. — Comparison with the loss of heat through cooling of the Earth's crust 118-120

Chapter XI

Examination of the Hypothesis that Mountain-Ranges are Ridges thrown up by the Compression induced in the Rigid Crust of the Globe by a Shrinkage of the Heated Nucleus. Important link in the chain of reasoning on the problem has been omitted. — Circumferential contraction of the underlayers of the crust. — Radial contraction. — Shell of compression. — Shell of con traction. — Analogy of the Moon : Features of the Moon's surface ; No evidence of linear ridging-up and folding, denudation, or sedi mentary deposits; Absence of appearances of horizontal pressure and folding accounted for 121-128 CHAPTER XH. Other Theories of Compression by Gravitation. Mr. O. Fisher's hypothesis of contraction by loss of water examined. — W. B. Taylor's theory of compression at the Equator by loss of rotational velocity by the Earth considered -. 129-131 X CONTENTS.

CHAPTER XHI. Application of the Theory of Local Expansions of the Earth's Crust to explain characteristic Features of Mountain-structure. Pages Instances of compression and movements of Archæan Rocks. — Repeated compression of the lowest rocks in the geological series. — Instances of repeated compression. — Flowing of the crystalline rocks : Examples cited. — A consideration of the forces to which the Archæan Series have been subjected 132-153

CHAPTER XIV.

Mechanical Effects ofthe Expansion of a Mass of Archæan and Granite Rocks overlaid by a great thickness of Sediment. Packing of sheets of rock by lateral pressure. — Manufacture of lead pipes as an illustration of flowing : Application of the illustration. — Pre-Palæozoic surface. — Crushing of gneissic rocks and granite. 154--161

CHAPTER XV.

Enormous Force developed by Expansion and its possible effect. Transfusion by pressure.— Mechanical effects of punching. — The flow of solids. — Extension of strata caused by detrusion : Applica tion to the length of folds in mountain-chains ; Difference between length of base-line and length over the folds not caused by the Earth's contraction 162-174

CHAPTER XVI.

Movements of Definitely Bedded Rocks, as seen in Mountains of Palæozoic Age, in some Pre- Cambrians, and in Mesozoic and Tertiary Formations in Britain. Examples op Contortions in Bedded Rocks.— Skipton anticlinal.—Folded Silurians, Wigtonshire.— Contorted Silurian strata in Eastern Berwickshire. — Contorted Lower Silurians at Aberystwyth, Central Wales. —Strike of rocks and trend of the coast-line.— Domed form of anticlinals — Saddles. — Core of anticlinal. — Synclinals hollowed out by denudation. — Complex convolutions. — Small anticlinal. —Horizontal drag.— Horizontal throws of faults. — Faults by com- CONTENTS. XI Pages pression and shearing. — Compression fault. — Slate rock at Machynlleth. — Foldings of massive rocks. Anglesey Crystalline Schists. — Cemmaes Bay to Bull Bay and Point Ælianus. — Holyhead Island. — Details of convoluted structure.

Coal-Basins. — Panther-Creek Basin. — Floor of the Mammoth Bed. — Variations in thickness of beds by pressure. — Pinching-out of Coal-beds. — Surface areas of Coal-basins as compared with the areas of the beds. — Internal changes involved in the folding of beds : Simple flexure ; Squeezing-out or stretching of overfolds. — Anticlinal of Brady's Bend. — Anticlinal and synclinal axes in Indiana county. — Ligonier valley. — Great synclinals, enclosing groups of sub-anticlinals and synclinals. — Packing of strata in synclinals. Contortion in Mesozoic and Tertiary Rocks in Britain. — Contortions in Chalk at Flamboro' Head. — In Magnesian Limestone, Nottingham. — In Tertiary strata, Isle of Wight 175-219 CHAPTER XVH. The Cordilleras of the Western United States. Mesozoic and Cainozoic Upthrows. Description of section of the Rocky Mountains. — High Plateaus of Utah.— The Zuni uplift.— The Black Hills of Dakota.— The Henry Mountains 220-234 CHAPTER XVIH.

Faults of the Plateau System and the Great Basin. East Musinia fault. — Sevier fault. — Hurricane fault. — Faulting in the Great Basin. — Discussion of the nature of these faults, whether upthrows or downthrows, and their cause 235-241

CHAPTER XIX.

Uinta Mountains. Nature of the uplift. — Type intermediate between the true anticlinal and the monoclinal plateau. — Axis at right angles to the general trend of the Ranges of North America ; What this indicates. — Faulting. — Examination of Prof. Powell's theory 242-247 CONTENTS.

CHAPTER XX.

Volcanism in the High Plateaus. Pages Periods of activity. — Explanation of uplifts and flexures. — Faults later than the uplifts. — Two-period displacement of faults .... 248-252

CHAPTER XXI.

The Connection of Volcanic Action with Mountain-building. History of the investigation of volcanic phenomena. Hutton's views. — Volcanic rocks ofthe Deccan. — Discussion ofthe connection between volcanic phenomena and mountain-building. — Hypothetical zone of semi-fluid rock will not explain volcanic phenomena. — Extra vasation of lava due to potentially molten rock being forced up into the crust by compression arising from expansion. — Lava accumulating in reservoirs in the crust is at intervals pumped up to the surface bythe expansion and contraction of the solid crust. — Surface influence on volcanic action ; Examples given 253-265

CHAPTER XXII.

The Larger Bendings of the Earth's Crust. — Regional Subsidence and Elevation. Examination of the theory that elevation and depression are due to sedimentary loading and lightening of the Earth's crust. — Extra vasation of lava creates a flow of the heated magma below the Earth's crust ; Interaction that results. — Instances of continental submersion and elevation and their cause. — Limit to distortion of the Earth's form. — Mean height of continents 266-274

CHAPTER XXIII.

■ Lateral Movements not confined to Highly Plicated Rocks. Horizontal slickensides in Triassic rocks; in Carboniferous; in Chalk.— What they indicate 275-277 CONTENTS. XIII

CHAPTER XXIV.

Slaty Cleavage and Foliation. Pages Discussion of the nature of slaty cleavage. — Slaty cleavage not entirely due to extension along the planes of cleavage, as shown by the contortions of the beds not influencing the direction of the cleavage-planes. — Cleavages of Upper Silurian slates in the neigh bourhood of Llangollen. — Prof. William King's theory. — Darwin, Forbes, DaubrCe, and Sorby on foliation 278-285

CHAPTER XXV.

Earthquakes and their Causes. Discussion of the cause of earthquakes. — Earthquakes, away from volcanic foci, occur with most violence in the newer formations ; Cause of this. — Sudden release of accumulated strains brought on by variations of temperature taking place in the deep-seated rocks. — Origin in the rigid crust. — Earthquakes and earth-tremors the necessary consequence of the readjustment of portions of the Earth's crust to altered conditions of bulk produced by local expansions and contractions 286-291

CHAPTER XXVI.

Sedimentation, Upheaval, and Subsidence. Sedimentation preceding the uplift of the Western North-American mountain-ranges. — Mountain-building a slow process. — Effects of fluctuations of temperature during the building of a mountain range. — Upheaval and faulting. — History of the Western North- American uplifts. — Supposed effects of removal of weight by denudation 292-299

CHAPTER XXVII.

Whence the Materials for Mountain-building come. Continental denudation.— Land-area to the eastward during the deposition of the Cretaceous of the Rocky Mountains. — Appalachian range then constituted the "great divide."— Origin of the great river-systems in the Tertiary period. — North and South America distinct continents in Cretaceous and Tertiary times 300-302 CONTENTS. CHAPTER XXVHI. Time required for Mountain-building. Pages Approximate calculation of the time which has elapsed since the Carboniferous rocks of Western America, finally eroded, became an area, of deposit, and the commencement of the uplift of the Rocky Mountains. — Approximate calculation of the time that has elapsed since the commencement of the uplift to the present time 303-305 CHAPTER XXTX. Permanence of Mountain- Chains.

Being built up by the thickening of the crust, or actual transference of material by expansion-creeps, mountain-ranges can only be removed by denudation. — Mountain-chains map out upon the Earth's surface certain fixed boundaries, forming permanent drainage areas or geological provinces. — These areas form the loci of future mountain-building 306-307

CHAPTER XXX.

Mountain-building in the Future. Great deposits now taking place parallel to the western shores of the Atlantic Ocean. — Submarine bench of deposits. — Submarine slope one of deposition. — Possible geosynclinals. — Probable that these deposits will form the locus of mountain-building in the future. — Permanence of land-areas the permanence of continuity, not of location. — Coast of Spain and Africa, inequalities of the bottom. — Deposits in the Gulf of Guinea. — Contours of the Atlantic bottom 308-313

CHAPTER XXXI.

Physical and Geological History of the British Isles. Structure of Great Britain geologically complex. — Evidence here destroyed by denudation preserved in America. — On the other hand, denudation throws light upon certain earlier stages of mountain building more fully in Britain than America. — British Isles represent the deeply truncated base of a stupendous mountain-system up heaved out of Cambrian and Silurian sediments, — Strata rest on a CONTENTS. XV Pages floor of Archæan rocks. — The Murchisonian succession. — Archæan rocks of the Highlands not an isolated phenomenon ; The folding and plications were impressed on them while part of a grand series of deeply buried sediments. — Archæan areas connected. — Cambro- Silurian strata were of enormous thickness and extent. — How the knowledge of the dependence of mountain-building on sedimentation aids us to clearer views. — Development of energy in Silurian times. — Centripetal compression. — History of the mutations of the land. — Continental extension. — Sedimentation and denudation, with their normal successions and developments. — Periods of volcanic energy and repose, and the relation of these to mountain-upheaval during Silurian, Old Red, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Tertiary times. — The geological cycle of recurrent periods. — Where to seek for the key of geological difficulties 314-325

CHAPTER XXXII.

Outline of the Theory of the Origin of Mountain-Ranges by Sedimentation accompanied by Local Changes of Temperature in the Earth's Crust. Sedimentation. — Rise of the isogeotherms. — Variations of temperature accompanied by expansion-creeps. — Cubical expansion. — Com pressive extension. — Accumulated effects. — Horizontal foldings and vertical uplifts are expressions of the same force, from which arise the various characteristic structures of mountain-ranges. — Normal faulting the result of contraction. — Shearing by decrease of bulk and gravitative settlement. — Accumulated effects of small recurrent causes. — Conclusion. — Experimental illustrations 326-333 PLATES. Page I. Diagrams of Experiments in Linear and Differential Expansion 19 II. Diagrams of Experiments in Linear, Differential, and Superficial Expansion 23 III.j IV ' ' of Superficial Expansion on sheets of Lead .... 28 VLJ VII.

Denudation Map of Great Britain and part of Ireland. . 37 VIII. Geological Map of the Appalachian .Region 41 IX. Section across Pennsylvania and the Appalachians. .. . 42 X. Geological Map of part of the Rocky-Mountain Region and the Plateau System of Colorado 49 XL Sections through the Rocky-Mountain Region westwards 50 XII. Giggieswick Scar; Snowdonia from Moelwyn 81 XIII. Geological Map of Switzerland 97 XIV. Section through the Central Alps; Section of the Mendip Hills ; Section of Snowdon 98 XV. Diagrams explanatory of Normal Faulting 101 XVI. Selected Sections exhihiting Normal Faulting 100 XVII. Diagrammatic Illustrations of several Effects of Cubical Expansion 117 XVIII. Diagram to illustrate the Effect of Secular Contraction on the Earth's Crust 123 XIX. The Lunar Apennines, Archimedes, &c 128 XX.

View of Mont-Blanc Massif from the Cantine in the Allee Blanche 141 XXI. View of the Matterhorn 149 XXII. Example of Tension caused by a Compvessive Force . . 109 XXIII. Diagrammatic Blustrations of several Types of Monoclinal and Anticlinal Flexures having Gneissic cores . 173 b PLATES. Page XXIV. Contorted Carboniferous Limestone in Draughton Quarry ; Synclinal in Wheetam-Rock Quarry 179 XXV. View of Folded Silurians, Wigtonshire, north of Garliestown 181 XXVI. View of Contorted Silurians, Isle of Whithorn, Wigtonshire 181 XXVII. View of Contorted Silurians, north of the Isle of Whithorn, Wigtonshire 181 XXVHI. I Sections of Contortions in Lower Silurians, Aberyst- XXIX. 1 wyth 187 XXX. View of Cliffs in Lower Silurian Rocks near Borth, north of Aberystwyth 187 XXXI. View ofthe Bwlch Drws, Ardudwy, Merionethshire . . 193 XXXII.

Analysis of Convoluted Structure in the Anglesey Crystalline Schists 197 XXXin. Frontispiece. View of Convoluted Schists opposite the South Stack, Holyhead (description page 197). ' > Contortions seen in Cliffs, Holyhead Island 202 XXXVI. Sections of some of the Anthracite Coal-fields of Penn* sylvania 207 XXXVII. Tlan and Model of the Floor of the Mammoth Beds, Pennsylvania. % ? 207 XXXVIII. Diagrammatic Illustrations of the Progress of Foiling and the Extension of Strata by Lateral Pressure, Chevron Folds 217 XXXIX. Structure Sections through the High Plateaus 228 XL. Structure Sections through the Uinta Mountains .... 245 XLI. Contoured Chart of the North- Atlantic Ocean 313 XLII. Various Diagrams representing Experimental Illustrations of Movements of Bedded Rocks, Folding, &c. . . THE ORIGIN OF MOUNTAIN-RANGES.

Chapter I. Variability In The Temperature Of The Crust Of The Earth

AND ITS CAUSE. It has been long known that, as the crust of the Earth is penetrated, the temperature increases with the depth. Attempts have been made to educe a uniform rate of increase from the varied observations that have been made, but hitherto, I think, with no great amount of success. The temperature, we may say, universally increases with the depth, whether it be below the frozen soil of Siberia or in the regions of the torrid zone ■ but the rate of increase is inconstant, even taking into account the varying degrees of conductibility of the rocks penetrated. It appears to be inconstant even in the same bore-hole ; and in places not far apart the rate of increase varies con siderably. Thus the rate of increase at Astley Pit, Duckin* field, Cheshire, was, on the average, 1° for every 83'2 feet for a depth of 685 yards • while similar experiments at Rose Bridge Collieries, Ince, near Wigan, for a depth of 815 yards^ B V TEMPERATURE OF THE EARTH'S CRUST ; o gave an average increase of 1° in every 54*3 feet*.

In some of the Paris artesian wells the increase was found to be 1° for every 50 to 60 feet ; while the great well of La Chapelle, at Paris, showed, to a depth of 600 metres, an average increase of 1° in 9-i'3 feetf. In experiments made at Przibram, in Bohemia, by Herr Johann Grimm, Director of the School of Mines, in a depth of 1900 feet there was an increase of 1° in every 135 feet. The experiments at the Mont Cenis Tunnel show an increase of 1° in 93 feet, without correction for the convexity of the ground. The deepest artesian well in the world is one at Pesth, which had reached 951 metres when last reported upon, and yielded water at a temperature of 161° F. Unless we know the depth of the stratum from which this water was derived, it wo aid be impossible to arrive at the rate of increase ; but if this temperature was that of the rocks not much lower than the bottom of the bore, the increase would be as much as 1° for every 30 feet.

Professor Lebour, in reviewing the present state of our knowledge of underground temperature, gives a table of observed increases of temperature in various localities, 57 in all, in which the range is from 28-l feet in Anzin, in the north of France, to 1572 feet in the Minas Geraes Mines in Brazil, for every degree Fahrenheit. Allowing for all the difficulties which beset observations of this character, I think it will be conceded that the range of increase varies very considerably in different localities and areas of the Earth's crust. That these existing variations have been constant in time throughout these areas would scarcely be maintained by any one. If not, it follows that fluctuations of temperature occur in the solid crust of the Earth. That the rates of increase of heat vary during geolo- * British Association Report on Underground Temperature, 1870 pp 31, 32. t Ibid. 1873, p. 253.

ITS VARIABILITY AND CAUSE. 3 gical time for various localities would seem an obvious truth, even leaving out of account those greater revolutions of the Earth's surface by which seas have become lands and lands seas. Accepting as an inference of high probability that the increase of heat, whatever the rate, progresses downwards until the temperature of molten matter — or matter that would be molten if freed from the superincumbent pres sure — is reached, so enormous a reservoir of heat lies below, that it would seem puerile to doubt that the extremely thin cooled crust must respond to internal changes of tempe-, rature that in all probability take place in the enormous mass of heated interior rock. But it is unnecessary to rest this thesis on surmise and supposition. Volcanic activity shows that such redistribution and changes in the heat of the interior as the solid crust is approached are constantly, if with extreme slowness, taking place.

Even if we concede Mallet's view, which I am by no means prepared to do, that volcanic heat is a secondary product, a conversion of the mechanical energy of a collapsing crust, the fluctuation of heat in the outer envelope is admitted, if accounted for in another way. It is, however, rather unfortunate for the theory that, in the actual investi gation of the structure of mountains, no one has yet been able to trace any apparent connexion between the pressure to which the rocks have been subjected and the volcanic activity accompanying mountain-building. For instance, the enormous forces which have (as so clearly shown by Dr. Callaway, Professor Lapworth, and the officers of the Geological Survey *) folded the strata of the Northern Highlands in such a way as to superimpose Archæan Gneiss upon Silurian rocks, so as to deceive even the practised eye * " The Newer Gneissic Rocks of the Northern Highlands," Dr. Cal laway, Q. J. G. S. 1883, pp. 355-422.

" The Secret of the Highlands," Prof. Lapworth, Geol. Mag. 1883, pp. 120, 194, 337. " The Crystalline b2 TEMPERATURE OF THE EARTH'S CRUST; of a Murchison as to their succession, have not developed heat sufficient to melt any of the beds in question. Enor mous ruptures of the strata have taken place, beds being driven over beds for miles; rocks have been ground to powder, yet none of them, so far as we know, have been thereby fused. Volcanic activity accompanies these terrific but slow displays of force, as we shall see as the subject develops itself. But no direct connexion between the pres sure and fusion has yet been pointed out. In all cases the molten rock has welled up from great depths below the compressed materials of the mountain range. In other cases we find great displays of volcanic activity, such as Professor Judd so graphically describes in the basal wrecks of volcanic cones in the islands of Mull and Skye *, unaccompanied with anything like the amount of folding, pressure, and contortion that we find in the rocks of the Northern Highlands.

And here it may be as well to point out that the amount of lateral compression or shortening of the strata at the surface must, on the theory of a collapsing crust, be a measure ofthe mechanical energy which has been developed. Hence it follows, that if the volcanoes of Mull and Skye were due to tangential strains accumulating in those localities, the enormously greater strains to which the Archæan and Silurian rocks of the Highlands were subjected should have reduced most of these rocks to a state of fusion. This the pressure has not done; therefore we must seek another explanation of volcanic activity. But in Mr. Mallet's ingenious theory there lies another difficulty, of a sort we can contemplate without appealing to geology. It is evident that a collapsing crust cannot develop Rocks of the Scottish Highlands," by Dr. A. Geikie and Messrs. Peach and Home, - Nature,' Nov. 13, 1884, pp. 29-35. '• The Geology of Durness and Eriboll," by B. N. Peach and J. Home, British Association Report, 1885. * "The Secondary Rocks of Scotland," Q. J. G. S. 1878, pp. 660-743.

ITS VARIABILITY AND CAUSE. 5 more heat than is due to the distance it falls through ; hence, taking the crust as a whole and supposing the entire mechanical effect to be converted into heat, for every contraction of the radius of the Earth of 772 feet only sufficient heat would be generated to raise the mass of the falling crust 5° Fahr. It is true, if the total heat were concentrated on certain limited points or lines, the point of fusion would be reached ; but it appears to me that, in the enormous period required for a shortening of the Earth's radius by 772 feet, the whole of the secondary heat derived in this way would be dispersed into space. If, in addition to this, we consider how much of the mechanical energy would be dissipated in other work than that of heat -producing, we may form a con ception, without appeal to mathematics, of the utter inade quacy of such derivative heat to account for the enormous volcanic activities which geology shows us have been developed during the history of the Earth.

If, on the other hand, we assume, as we have fair grounds to do, that at a depth of about 30 miles the interior of the Earth reaches a temperature sufficient to melt rock (if it were at the surface), say 3000°, we find that this heated inner spheroid to the crust above bears the proportion of about 43 to 1. It does not enter into the scope of this work to specu late upon the cause of this internal heat. Its existence is accepted, and that it is subject to local fluctuations seems a reasonable supposition. Even if we admit that the Earth is solid from the circumference to the core, considering the multifarious materials and elements of which it is composed — judging by the crust — slow chemical inter changes, accompanied by increase or decrease of heat, must have been taking place during past geological time.

Unless we accept Mallet's theory in its entirety, volcanic activity cannot but be due to fluctuating and differential heating of the layers of heated rock lying immediately below TEMPERATURE OF THE EARTH'S CRUST ; the cooler crust. That this heated matter responds to changes slowly taking place on the crust itself it will be my object to show. So many and great have been the difficulties connected with the contractional hypothesis, that several eminent geo logists and physicists have assumed that a layer of molten matter lies between the solid outer shell and a solid inner spheroid. In this molten cushion is sought the flexibility supposed to be necessary to account for the plications and foldings which, more than crushings, we find in the solid crust. While a collapsing solid shell, floating on a more or less evenly distributed intercalated zone of molten rock, is hypothetically postulated by many geologists, and among others by no less an authority than Professor Prestwieh *, Mr. Osmund Fisher f, following out the theory to its logical conclusions, supports his mountain-ranges by floatation in a similar zone of heavier matter.