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THE BIRTH-TIME OF THE WORLD AND OTHER SCIENTIFIC ESSAYS
by
J. JOLY, M.A., Sc.D., F.R.S., PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF DUBLIN
E. P. DUTTON AND COMPANY 681 FIFTH AVENUE NEW YORK
Cover
Title page
CONTENTS PAGE
I. THE BIRTH-TIME OF THE WORLD - - - - - - - - - - - 1
II. DENUDATION - - - - - - - - - - - - - - - - - - 30
III. THE ABUNDANCE OF LIFE - - - - - - - - - - - - 60
IV. THE BRIGHT COLOURS OF ALPINE FLOWERS - - - - - 102
V. MOUNTAIN GENESIS - - - - - - - - - - - - - - - 116
VI. ALPINE STRUCTURE - - - - - - - - - - - - - - - 146
VII. OTHER MINDS THAN OURS - - - - - - - - - - - - 162
VIII. THE LATENT IMAGE - - - - - - - - - - - - - - 202
IX. PLEOCHROIC HALOES - - - - - - - - - - - - - - 214
X. THE USE OF RADIUM IN MEDICINE - - - - - - - - - 244
XI. SKATING - - - - - - - - - - - - - - - - - - - 260
XII. A SPECULATION AS TO A PRE-MATERIAL UNIVERSE - 288
LIST OF ILLUSTRATIONS
PLATE I. LAKE OF LUCERNE, LOOKING WEST FROM BRUNNEN - Frontispiece
PLATE II. "UPLIFTED FROM THE SEAS." CLIFFS OF THE TITLIS, SWITZERLAND - to face p. 4
PLATE III. AN ALPINE TORRENT AT WORK—VAL D'HERENS, SWITZERLAND - to face p. 31
PLATE IV. EARTH PILLARS—VAL D'HERENS - to face p. 34
PLATE V. "SCENES OF DESOLATION." THE WEISSHORN SEEN FROM BELLA TOLA, SWITZERLAND - to face p. 40
PLATE VI. ALLUVIAL CONE—NICOLAI THAL, SWITZERLAND. MORAINE ON ALETSCH GLACIER SWITZERLAND - to face p. 50
PLATE VII. IN THE REGION OF THE CROCI; DOLOMITES. THE ROTHWAND SEEN FROM MONTE PIANO - to face p. 60
PLATE VIII. FIRS ASSAILING THE HEIGHTS OF THE MADERANER THAL, SWITZERLAND - to face p. 73
PLATE IX. LIFE NEAR THE SNOW LINE; THE BOG-COTTON IN POSSESSION. NEAR THE TSCHINGEL PASS, SWITZERLAND - to face p. 80
PLATE X. THE JOY OF LIFE. THE AMPEZZO THAL; DOLOMITES - to face p. 93
PLATE XI. "PINES SOLEMNLY QUIET." DUeSSISTOCK; MADERANER THAL - to face p. 100
PLATE XII. ALPINE FLOWERS IN THE VALLEYS - to face p. 105
PLATE XIII. ALPINE FLOWERS ON THE HEIGHTS - to face p. 106
PLATE XIV. MOUNTAIN SOLITUDES; VAL DE ZINAL. FROM LEFT TO RIGHT ROTHHORN; BESSO; OBERGABELHORN; MATTERHORN; PIC DE ZINAL (THROUGH CLOUD); DENT BLANCHE - to face p. 116
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PLATE XV. SECTOR OF THE EARTH RISE OF ISOGEOTHERMS INTO A DEPOSIT EVOLVING RADIOACTIVE HEAT - to face p. 118
PLATE XVI. "THE MOUNTAINS COME AND GO." THE DENT BLANCHE SEEN FROM THE SASSENEIRE - to face p. 133
PLATE XVII. DIAGRAMMATIC SECTIONS OF THE HIMALAYA - to face p. 140
PLATE XVIII. RESIDUES OF DENUDATION. THE MATTERHORN SEEN FROM THE SUMMIT OF THE ZINAL ROTHHORN - to face p. 148
PLATE XIX. THE FOLDED ROCKS OF THE MATTERHORN, SEEN FROM NEAR HOeHBALM. SKETCH MADE IN 1906 - to face p. 156
PLATE XX. SCHIAPARELLI'S MAP OF MARS OF 1882, AND ADDITIONS (IN RED) OF 1892 - to face p. 166
PLATE XXI. GLOBE OF MARS SHOWING PATH OF IN-FALLING SATELLITE - to face p. 188
PLATE XXII. CANALS MAPPED BY LOWELL COMPARED WITH CANALS FORMED BY IN-FALLING SATELLITES - to face p. 192
PLATE XXIII. HALOES IN MICA; CO. CARLOW. HALO IN BIOTITE CONTAINED IN GRANITE - to face p. 224
PLATE XXIV. RADIUM HALO, MUCH ENLARGED. THORIUM HALO AND RADIUM HALO IN MICA - to face p. 228
PLATE XXV. HALO ROUND CAPILLARY IN GLASS TUBE. HALOES ROUND TUBULAR PASSAGES IN MICA - to face p. 230
PLATE XXVI. ALETSCH GLACIER, SWITZERLAND - to face p. 282
PLATE XXVII. THE MIDDLE ALETSCH GLACIER JOINING THE GREAT ALETSCH GLACIER. GLACIERS OF THE LAUTERBRUNNEN THAL - to face p. 285
PLATE XXVIII. PERCHED BLOCK ON THE ALETSCH GLACIER. GRANITE ERRATIC NEAR ROUNDWOOD, CO. WICKLOW; NOW BROKEN UP AND REMOVED - to face p. 286
And Fifteen Illustrations in the Text.
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PREFACE
Tins volume contains twelve essays written at various times during recent years. Many of them are studies contributed to Scientific Reviews or delivered as popular lectures. Some are expositions of views the scientific basis of which may be regarded as established. Others—the greater number—may be described as attempting the solution of problems which cannot be approached by direct observation.
The essay on The Birth-time of the World is based on a lecture delivered before the Royal Dublin Society. The subject has attracted much attention within recent years. The age of the Earth is, indeed, of primary importance in our conception of the longevity of planetary systems. The essay deals with the evidence, derived from the investigation of purely terrestrial phenomena, as to the period which has elapsed since the ocean condensed upon the Earth's surface. Dr. Decker's recent addition to the subject appeared too late for inclusion in it. He finds that the movements (termed isostatic) which geologists recognise as taking place deep in the Earth's crust, indicate an age of the same order of magnitude
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as that which is inferred from the statistics of denudative history.[1]
The subject of Denudation naturally arises from the first essay. In thinking over the method of finding the age of the ocean by the accumulation of sodium therein, I perceived so long ago as 1899, when my first paper was published, that this method afforded a means of ascertaining the grand total of denudative work effected on the Earth's surface since the beginning of geological time; the resulting knowledge in no way involving any assumption as to the duration of the period comprising the denudative actions. This idea has been elaborated in various publications since then, both by myself and by others. "Denudation," while including a survey of the subject generally, is mainly a popular account of this method and its results. It closes with a reference to the fascinating problems presented by the inner nature of sedimentation: a branch of science to which I endeavoured to contribute some years ago.
Mountain Genesis first brings in the subject of the geological intervention of radioactivity. There can, I believe, be no doubt as to the influence of transforming elements upon the developments of the surface features of the Earth; and, if I am right, this source of thermal energy is mainly responsible for that local accumulation of wrinkling which we term mountain chains. The
[1] Bull. Geol. Soc. America, vol. xxvi, March 1915.
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paper on Alpine Structure is a reprint from "Radioactivity and Geology," which for the sake of completeness is here included. It is directed to the elucidation of a detail of mountain genesis: a detail which enters into recent theories of Alpine development. The weakness of the theory of the "horst" is manifest, however, in many of its other applications; if not, indeed, in all.
The foregoing essays on the physical influences affecting the surface features of the Earth are accompanied by one entitled The Abundance of Life. This originated amidst the overwhelming presentation of life which confronts us in the Swiss Alps. The subject is sufficiently inspiring. Can no fundamental reason be given for the urgency and aggressiveness of life? Vitality is an ever-extending phenomenon. It is plain that the great principles which have been enunciated in explanation of the origin of species do not really touch the problem. In the essay—which is an early one (1890)—the explanation of the whole great matter is sought—and as I believe found—in the attitude of the organism towards energy external to it; an attitude which results in its evasion of the retardative and dissipatory effects which prevail in lifeless dynamic systems of all kinds.
Other Minds than Ours? attempts a solution of the vexed question of the origin of the Martian "canals." The essay is an abridgment of two popular lectures on the subject. I had previously written an account of my views which carried the enquiry as far as it was in
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my power to go. This paper appeared in the "Transactions of the Royal Dublin Society, 1897." The theory put forward is a purely physical one, and, if justified, the view that intelligent beings exist in Mars derives no support from his visible surface features; but is, in fact, confronted with fresh difficulties.
Pleochroic Haloes is a popular exposition of an inconspicuous but very beautiful phenomenon of the rocks. Minute darkened spheres—a microscopic detail—appear everywhere in certain of the rock minerals. What are they? The discoveries of recent radioactive research—chiefly due to Rutherford—give the answer. The measurements applied to the little objects render the explanation beyond question. They turn out to be a quite extraordinary record of radioactive energy; a record accumulated since remote geological times, and assuring us, indirectly, of the stability of the chemical elements in general since the beginning of the world. This assurance is, without proof, often assumed in our views on the geological history of the Globe.
Skating is a discourse, with a recent addition supporting the original thesis. It is an illustration of a common experience—the explanation of an unimportant action involving principles the most influential considered as a part of Nature's resources.
The address on The Latent Image deals with a subject which had been approached by various writers before the time of my essay; but, so far as I know, an explanation
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based on the facts of photo-electricity had not been attempted. Students of this subject will notice that the views expressed are similar to those subsequently put forward by Lenard and Saeland in explanation of phosphorescence. The whole matter is of more practical importance than appears at first sight, for the photoelectric nature of the effects involved in the radiative treatment of many cruel diseases seems to be beyond doubt.
It was in connection with photo-electric science that I was led to take an interest in the application of radioactivity in medicine. The lecture on The Use of Radium in Medicine deals with this subject. Towards the conclusion of this essay reference will be found to a practical outcome of such studies which, by improving on the methods, and facilitating the application, of radioactive treatment, has, in the hands of skilled medical men, already resulted in the alleviation of suffering.
Leaving out much which might well appear in a prefatory notice, a word should yet be added respecting the illustrations of scenery. They are a small selection from a considerable number of photographs taken during my summer wanderings in the Alps in company with Henry H. Dixon. An exception is Plate X, which is by the late Dr. Edward Stapleton. From what has been said above, it will be gathered that these illustrations are fitly included among pages which owe so much to Alpine inspiration. They illustrate the
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subjects dealt with, and, it is to be hoped, they will in some cases recall to the reader scenes which have in past times influenced his thoughts in the same manner; scenes which in their endless perspective seem to reduce to their proper insignificance the lesser things of life.
My thanks are due to Mr. John Murray for kindly consenting to the reissue of the essay on The Birth-time of the World from the pages of Science Progress; to Messrs. Constable & Co. for leave to reprint Pleochroic Haloes from Bedrock, and also to make some extracts from Radioactivity and Geology; and to the Council of the Royal Dublin Society for permission to republish certain papers from the Proceedings of the Society.
Iveagh Geological Laboratory, Trinity College, Dublin.
July, 1915.
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THE BIRTH-TIME OF THE WORLD [1]
LONG ago Lucretius wrote: "For lack of power to solve the question troubles the mind with doubts, whether there was ever a birth-time of the world and whether likewise there is to be any end." "And if" (he says in answer) "there was no birth-time of earth and heaven and they have been from everlasting, why before the Theban war and the destruction of Troy have not other poets as well sung other themes? Whither have so many deeds of men so often passed away, why live they nowhere embodied in lasting records of fame? The truth methinks is that the sum has but a recent date, and the nature of the world is new and has but lately had its commencement."[2]
Thus spake Lucretius nearly 2,000 years ago. Since then we have attained another standpoint and found very different limitations. To Lucretius the world commenced with man, and the answer he would give to his questions was in accord with his philosophy: he would date the birth-time of the world from the time when
[1] A lecture delivered before the Royal Dublin Society, February 6th, 1914. Science Progress, vol. ix., p. 37
[2] De Rerum Natura, translated by H. A. J. Munro (Cambridge, 1886).
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poets first sang upon the earth. Modern Science has along with the theory that the Earth dated its beginning with the advent of man, swept utterly away this beautiful imagining. We can, indeed, find no beginning of the world. We trace back events and come to barriers which close our vista—barriers which, for all we know, may for ever close it. They stand like the gates of ivory and of horn; portals from which only dreams proceed; and Science cannot as yet say of this or that dream if it proceeds from the gate of horn or from that of ivory.
In short, of the Earth's origin we have no certain knowledge; nor can we assign any date to it. Possibly its formation was an event so gradual that the beginning was spread over immense periods. We can only trace the history back to certain events which may with considerable certainty be regarded as ushering in our geological era.
Notwithstanding our limitations, the date of the birth-time of our geological era is the most important date in Science. For in taking into our minds the spacious history of the universe, the world's age must play the part of time-unit upon which all our conceptions depend. If we date the geological history of the Earth by thousands of years, as did our forerunners, we must shape our ideas of planetary time accordingly; and the duration of our solar system, and of the heavens, becomes comparable with that of the dynasties of ancient nations. If by millions of years, the sun and stars are proportionately venerable. If by hundreds or thousands of millions of
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years the human mind must consent to correspondingly vast epochs for the duration of material changes. The geological age plays the same part in our views of the duration of the universe as the Earth's orbital radius does in our views of the immensity of space. Lucretius knew nothing of our time-unit: his unit was the life of a man. So also he knew nothing of our space-unit, and he marvels that so small a body as the sun can shed so much, heat and light upon the Earth.
A study of the rocks shows us that the world was not always what it now is and long has been. We live in an epoch of denudation. The rains and frosts disintegrate the hills; and the rivers roll to the sea the finely divided particles into which they have been resolved; as well as the salts which have been leached from them. The sediments collect near the coasts of the continents; the dissolved matter mingles with the general ocean. The geologist has measured and mapped these deposits and traced them back into the past, layer by layer. He finds them ever the same; sandstones, slates, limestones, etc. But one thing is not the same. Life grows ever less diversified in character as the sediments are traced downwards. Mammals and birds, reptiles, amphibians, fishes, die out successively in the past; and barren sediments ultimately succeed, leaving the first beginnings of life undecipherable. Beneath these barren sediments lie rocks collectively differing in character from those above: mainly volcanic or poured out from fissures in
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the early crust of the Earth. Sediments are scarce among these materials.[1]
There can be little doubt that in this underlying floor of igneous and metamorphic rocks we have reached those surface materials of the earth which existed before the long epoch of sedimentation began, and before the seas came into being. They formed the floor of a vaporised ocean upon which the waters condensed here and there from the hot and heavy atmosphere. Such were the probable conditions which preceded the birth-time of the ocean and of our era of life and its evolution.
It is from this epoch we date our geological age. Our next purpose is to consider how long ago, measured in years, that birth-time was.
That the geological age of the Earth is very great appears from what we have already reviewed. The sediments of the past are many miles in collective thickness: yet the feeble silt of the rivers built them all from base to summit. They have been uplifted from the seas and piled into mountains by movements so slow that during all the time man has been upon the Earth but little change would have been visible. The mountains have again been worn down into the ocean by denudation and again younger mountains built out of their redeposited materials. The contemplation of such vast events
[1] For a description of these early rocks, see especially the monograph of Van Hise and Leith on the pre-Cambrian Geology of North America (Bulletin 360, U.S. Geol. Survey).
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prepares our minds to accept many scores of millions of years or hundreds of millions of years, if such be yielded by our calculations.
THE AGE AS INFERRED FROM THE THICKNESS OF THE SEDIMENTS
The earliest recognised method of arriving at an estimate of the Earth's geological age is based upon the measurement of the collective sediments of geological periods. The method has undergone much revision from time to time. Let us briefly review it on the latest data.
The method consists in measuring the depths of all the successive sedimentary deposits where these are best developed. We go all over the explored world, recognising the successive deposits by their fossils and by their stratigraphical relations, measuring their thickness and selecting as part of the data required those beds which we believe to most completely represent each formation. The total of these measurements would tell us the age of the Earth if their tale was indeed complete, and if we knew the average rate at which they have been deposited. We soon, however, find difficulties in arriving at the quantities we require. Thus it is not easy to measure the real thickness of a deposit. It may be folded back upon itself, and so we may measure it twice over. We may exaggerate its thickness by measuring it not quite straight across the bedding or by unwittingly including volcanic materials. On the other hand, there
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may be deposits which are inaccessible to us; or, again, an entire absence of deposits; either because not laid down in the areas we examine, or, if laid down, again washed into the sea. These sources of error in part neutralise one another. Some make our resulting age too long, others make it out too short. But we do not know if a balance of error does not still remain. Here, however, is a table of deposits which summarises a great deal of our knowledge of the thickness of the stratigraphical accumulations. It is due to Sollas.[1]
Feet.
Recent and Pleistocene - - 4,000 Pliocene - - 13,000 Miocene - - 14,000 Oligocene - - 2,000 Eocene - - 20,000 63,000
Upper Cretaceous - - 24,000 Lower Cretaceous - - 20,000 Jurassic - - 8,000 Trias - - 7,000 69,000
Permian - - 2,000 Carboniferous - - 29,000 Devonian - - 22,000 63,000
Silurian - - 15,000 Ordovician - - 17,000 Cambrian - - 6,000 58,000
Algonkian—Keeweenawan - - 50,000 Algonkian—Animikian - - 14,000 Algonkian—Huronian - - 18,000 82,000
Archaean - - ?
Total - - 335,000 feet.
[1] Address to the Geol. Soc. of London, 1509.
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In the next place we require to know the average rate at which these rocks were laid down. This is really the weakest link in the chain. The most diverse results have been arrived at, which space does not permit us to consider. The value required is most difficult to determine, for it is different for the different classes of material, and varies from river to river according to the conditions of discharge to the sea. We may probably take it as between two and six inches in a century.
Now the total depth of the sediments as we see is about 335,000 feet (or 64 miles), and if we take the rate of collecting as three inches in a hundred years we get the time for all to collect as 134 millions of years. If the rate be four inches, the time is soo millions of years, which is the figure Geikie favoured, although his result was based on somewhat different data. Sollas most recently finds 80 millions of years.[1]
THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS
In the above method we obtain our result by the measurement of the linear dimensions of the sediments. These measurements, as we have seen, are difficult to arrive at. We may, however, proceed by measurements of the mass of the sediments, and then the method becomes more definite. The new method is pursued as follows:
[1] Geikie, Text Book of Geology (Macmillan, 1903), vol. i., p. 73, et seq. Sollas, loc. cit. Joly, Radioactivity and Geology (Constable, 1909), and Phil. Mag., Sept. 1911.
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The total mass of the sediments formed since denudation began may be ascertained with comparative accuracy by a study of the chemical composition of the waters of the ocean. The salts in the ocean are undoubtedly derived from the rocks; increasing age by age as the latter are degraded from their original character under the action of the weather, etc., and converted to the sedimentary form. By comparing the average chemical composition of these two classes of material—the primary or igneous rocks and the sedimentary—it is easy to arrive at a knowledge of how much of this or that constituent was given to the ocean by each ton of primary rock which was denuded to the sedimentary form. This, however, will not assist us to our object unless the ocean has retained the salts shed into it. It has not generally done so. In the case of every substance but one the ocean continually gives up again more or less of the salts supplied to it by the rivers. The one exception is the element sodium. The great solubility of its salts has protected it from abstraction, and it has gone on collecting during geological time, practically in its entirety. This gives us the clue to the denudative history of the Earth.[1]
The process is now simple. We estimate by chemical examination of igneous and sedimentary rocks the amount of sodium which has been supplied to the ocean per ton of sediment produced by denudation. We also calculate
[1] Trans. R.D.S., May, 1899.
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the amount of sodium contained in the ocean. We divide the one into the other (stated, of course, in the same units of mass), and the quotient gives us the number of tons of sediment. The most recent estimate of the sediments made in this manner affords 56 x 1016 tonnes.[1]
Now we are assured that all this sediment was transported by the rivers to the sea during geological time. Thus it follows that, if we can estimate the average annual rate of the river supply of sediments to the ocean over the past, we can calculate the required age. The land surface is at present largely covered with the sedimentary rocks themselves. Sediment derived from these rocks must be regarded as, for the most part, purely cyclical; that is, circulating from the sea to the land and back again. It does not go to increase the great body of detrital deposits. We cannot, therefore, take the present river supply of sediment as representing that obtaining over the long past. If the land was all covered still with primary rocks we might do so. It has been estimated that about 25 per cent. of the existing continental area is covered with archaean and igneous rocks, the remainder being sediments.[2] On this estimate we may find valuable
[1] Clarke, A Preliminary Study of Chemical Denudation (Washington, 1910). My own estimate in 1899 (loc. cit.) made as a test of yet another method of finding the age, showed that the sediments may be taken as sufficient to form a layer 1.1 mile deep if spread uniformly over the continents; and would amount to 64 x 1018 tons.
[2] Van Tillo, Comptes Rendues (Paris), vol. cxiv., 1892.
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major and minor limits to the geological age. If we take 25 per cent. only of the present river supply of sediment, we evidently fix a major limit to the age, for it is certain that over the past there must have been on the average a faster supply. If we take the entire river supply, on similar reasoning we have what is undoubtedly a minor limit to the age.
The river supply of detrital sediment has not been very extensively investigated, although the quantities involved may be found with comparative ease and accuracy. The following table embodies the results obtained for some of the leading rivers.[1]
Mean annual Total annual Ratio of discharge in sediment in sediment cubic feet thousands to water per second. of tons. by weight.
Potomac - 20,160 5,557 1 : 3.575 Mississippi - 610,000 406,250 1 : 1,500 Rio Grande - 1,700 3,830 1 : 291 Uruguay - 150,000 14,782 1 : 10,000 Rhone - 65,850 36,000 1 : 1,775 Po - 62,200 67,000 1 : 900 Danube - 315,200 108,000 1 : 2,880 Nile - 113,000 54,000 1 : 2,050 Irrawaddy - 475,000 291,430 1 : 1,610
Mean - 201,468 109,650 1 : 2,731
We see that the ratio of the weight of water to the
[1] Russell, River Development (John Murray, 1888).
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weight of transported sediment in six out of the nine rivers does not vary widely. The mean is 2,730 to 1. But this is not the required average. The water-discharge of each river has to be taken into account. If we ascribe to the ratio given for each river the weight proper to the amount of water it discharges, the proportion of weight of water to weight of sediment, for the whole quantity of water involved, comes out as 2,520 to 1.
Now if this proportion holds for all the rivers of the world—which collectively discharge about 27 x 1012 tonnes of water per annum—the river-born detritus is 1.07 x 1010 tonnes. To this an addition of 11 per cent. has to be made for silt pushed along the river-bed.[1] On these figures the minor limit to the age comes out as 47 millions of years, and the major limit as 188 millions. We are here going on rather deficient estimates, the rivers involved representing only some 6 per cent. of the total river supply of water to the ocean. But the result is probably not very far out.
We may arrive at a probable age lying between the major and minor limits. If, first, we take the arithmetic mean of these limits, we get 117 millions of years. Now this is almost certainly excessive, for we here assume that the rate of covering of the primary rocks by sediments was uniform. It would not be so, however, for the rate of supply of original sediment must have been continually diminishing
[1] According to observations made on the Mississippi (Russell, loc. cit.).
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during geological time, and hence we may assume that the rate of advance of the sediments on the primary rocks has also been diminishing. Now we may probably take, as a fair assumption, that the sediment-covered area was at any instant increasing at a rate proportionate to the rate of supply of sediment; that is, to the area of primary rocks then exposed. On this assumption the age is found to be 87 millions of years.
THE AGE BY THE SODIUM OF THE OCEAN
I have next to lay before you a quite different method. I have already touched upon the chemistry of the ocean, and on the remarkable fact that the sodium contained in it has been preserved, practically, in its entirety from the beginning of geological time.
That the sea is one of the most beautiful and magnificent sights in Nature, all admit. But, I think, to those who know its story its beauty and magnificence are ten-fold increased. Its saltness it due to no magic mill. It is the dissolved rocks of the Earth which give it at once its brine, its strength, and its buoyancy. The rivers which we say flow with "fresh" water to the sea nevertheless contain those traces of salt which, collected over the long ages, occasion the saltness of the ocean. Each gallon of river water contributes to the final result; and this has been going on since the beginning of our era. The mighty total of the rivers is 6,500 cubic miles of water in the year!
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There is little doubt that the primeval ocean was in the condition of a fresh-water lake. It can be shown that a primitive and more rapid solution of the original crust of the Earth by the slowly cooling ocean would have given rise to relatively small salinity. The fact is, the quantity of salts in the ocean is enormous. We are only now concerned with the sodium; but if we could extract all the rock-salt (the chloride of sodium) from the ocean we should have enough to cover the entire dry land of the Earth to a depth of 400 feet. It is this gigantic quantity which is going to enter into our estimate of the Earth's age. The calculated mass of sodium contained in this rock-salt is 14,130 million million tonnes.
If now we can determine the rate at which the rivers supply sodium to the ocean, we can determine the age.[1] As the result of many thousands of river analyses, the total amount of sodium annually discharged to the ocean
[1] Trans. R.D.S., 1899. A paper by Edmund Halley, the astronomer, in the Philosophical Transactions of the Royal Society for 1715, contains a suggestion for finding the age of the world by the following procedure. He proposes to make observations on the saltness of the seas and ocean at intervals of one or more centuries, and from the increment of saltness arrive at their age. The measurements, as a matter of fact, are impracticable. The salinity would only gain (if all remained in solution) one millionth part in Too years; and, of course, the continuous rejection of salts by the ocean would invalidate the method. The last objection also invalidates the calculation by T. Mellard Reade (Proc. Liverpool Geol. Soc., 1876) of a minor limit to the age by the calcium sulphate in the ocean. Both papers were quite unknown to me when working out my method. Halley's paper was, I think, only brought to light in 1908.
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by all the rivers of the world is found to be probably not far from 175 million tonnes.[1] Dividing this into the mass of oceanic sodium we get the age as 80.7 millions of years. Certain corrections have to be applied to this figure which result in raising it to a little over 90 millions of years. Sollas, as the result of a careful review of the data, gets the age as between 80 and 150 millions of years. My own result[2] was between 80 and 90 millions of years; but I subsequently found that upon certain extreme assumptions a maximum age might be arrived at of 105 millions of years.[3] Clarke regards the 80.7 millions of years as certainly a maximum in the light of certain calculations by Becker.[4]
The order of magnitude of these results cannot be shaken unless on the assumption that there is something entirely misleading in the existing rate of solvent denudation. On the strength of the results of another and
[1] F. W. Clarke, A Preliminary Study of Chemical Denudation (Smithsonian Miscellaneous Collections, 1910).
[2] Loc. cit.
[3] "The Circulation of Salt and Geological Time" (Geol. Mag., 1901, p. 350).
[4] Becker (loc. cit.), assuming that the exposed igneous and archaean rocks alone are responsible for the supply of sodium to the ocean, arrives at 74 millions of years as the geological age. This matter was discussed by me formerly (Trans. R.D.S., 1899, pp. 54 et seq.). The assumption made is, I believe, inadmissible. It is not supported by river analyses, or by the chemical character of residual soils from sedimentary rocks. There may be some convergence in the rate of solvent denudation, but—as I think on the evidence—in our time unimportant.
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entirely different method of approaching the question of the Earth's age (which shall be presently referred to), it has been contended that it is too low. It is even asserted that it is from nine to fourteen times too low. We have then to consider whether such an enormous error can enter into the method. The measurements involved cannot be seriously impugned. Corrections for possible errors applied to the quantities entering into this method have been considered by various writers. My own original corrections have been generally confirmed. I think the only point left open for discussion is the principle of uniformitarianism involved in this method and in the methods previously discussed.
In order to appreciate the force of the evidence for uniformity in the geological history of the Earth, it is, of course, necessary to possess some acquaintance with geological science. Some of the most eminent geologists, among whom Lyell and Geikie[1] may be mentioned, have upheld the doctrine of uniformity. It must here suffice to dwell upon a few points having special reference to the matter under discussion.
The mere extent of the land surface does not, within limits, affect the question of the rate of denudation. This arises from the fact that the rain supply is quite insufficient to denude the whole existing land surface. About 30 per cent. of it does not, in fact, drain to the
[1] See especially Geikie's Address to Sect. C., Brit. Assoc. Rep., 1399.
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ocean. If the continents become invaded by a great transgression of the ocean, this "rainless" area diminishes: and the denuded area advances inwards without diminution. If the ocean recedes from the present strand lines, the "rainless" area advances outwards, but, the rain supply being sensibly constant, no change in the river supply of salts is to be expected.
Age-long submergence of the entire land, or of any very large proportion of what now exists, is negatived by the continuous sequence of vast areas of sediment in every geologic age from the earliest times. Now sediment-receiving areas always are but a small fraction of those exposed areas whence the sediments are supplied.[1] Hence in the continuous records of the sediments we have assurance of the continuous exposure of the continents above the ocean surface. The doctrine of the permanency of the continents has in its main features been accepted by the most eminent authorities. As to the actual amount of land which was exposed during past times to denudative effects, no data exist to show it was very different from what is now exposed. It has been estimated that the average area of the North American continent over geologic time was about eight-tenths of its existing area.[2] Restorations of other continents, so far as they have been attempted, would not
[1] On the strength of the Mississippi measurements about 1 to 18 (Magee, Am. Jour. of Sc., 1892, p. 188).
[2] Schuchert, Bull. Geol. Soc. Am., vol. xx., 1910.
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suggest any more serious divergency one way or the other.
That climate in the oceans and upon the land was throughout much as it is now, the continuous chain of teeming life and the sensitive temperature limits of protoplasmic existence are sufficient evidence.[1] The influence at once of climate and of elevation of the land may be appraised at their true value by the ascertained facts of solvent denudation, as the following table shows.
Tonnes removed in Mean elevation. solution per square Metres. mile per annum. North America - 79 700 South America - 50 650 Europe - 100 300 Asia - 84 950 Africa - 44 650
In this table the estimated number of tonnes of matter in solution, which for every square mile of area the rivers convey to the ocean in one year, is given in the first column. These results are compiled by Clarke from a very large number of analyses of river waters. The second column of the table gives the mean heights in metres above sea level of the several continents, as cited by Arrhenius.[2]
Of all the denudation results given in the table, those relating to North America and to Europe are far the
[1] See also Poulton, Address to Sect. D., Brit. Assoc. Rep., 1896.
[2] Lehybuch dev Kosmischen Physik, vol. i., p. 347.
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most reliable. Indeed these may be described as highly reliable, being founded on some thousands of analyses, many of which have been systematically pursued through every season of the year. These show that Europe with a mean altitude of less than half that of North America sheds to the ocean 25 per cent. more salts. A result which is to be expected when the more important factors of solvent denudation are given intelligent consideration and we discriminate between conditions favouring solvent and detrital denudation respectively: conditions in many cases antagonistic.[1] Hence if it is true, as has been stated, that we now live in a period of exceptionally high continental elevation, we must infer that the average supply of salts to the ocean by the rivers of the world is less than over the long past, and that, therefore, our estimate of the age of the Earth as already given is excessive.
There is, however, one condition which will operate to unduly diminish our estimate of geologic time, and it is a condition which may possibly obtain at the present time. If the land is, on the whole, now sinking relatively to the ocean level, the denudation area tends, as we have seen, to move inwards. It will thus encroach upon regions which have not for long periods drained to the ocean. On such areas there is an accumulation of soluble salts which the deficient rivers have not been able to carry to the ocean. Thus the salt content of certain of
[1] See the essay on Denudation.
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the rivers draining to the ocean will be influenced not only by present denudative effects, but also by the stored results of past effects. Certain rivers appear to reveal this unduly increased salt supply those which flow through comparatively arid areas. However, the flowoff of such tributaries is relatively small and the final effects on the great rivers apparently unimportant—a result which might have been anticipated when the extremely slow rate of the land movements is taken into account.
The difficulty of effecting any reconciliation of the methods already described and that now to be given increases the interest both of the former and the latter.
THE AGE BY RADIOACTIVE TRANSFORMATIONS
Rutherford suggested in 1905 that as helium was continually being evolved at a uniform rate by radioactive substances (in the form of the alpha rays) a determination of the age of minerals containing the radioactive elements might be made by measurements of the amount of the stored helium and of the radioactive elements giving rise to it, The parent radioactive substances are—according to present knowledge—uranium and thorium. An estimate of the amounts of these elements present enables the rate of production of the helium to be calculated. Rutherford shortly afterwards found by this method an age of 240 millions of years for a radioactive mineral of presumably remote age. Strutt, who carried
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his measurements to a wonderful degree of refinement, found the following ages for mineral substances originating in different geological ages:
Oligocene - 8.4 millions of years. Eocene - 31 millions of years. Lower Carboniferous - 150 millions of years. Archaean - 750 millions of years.
Periods of time much less than, and very inconsistent with, these were also found. The lower results are, however, easily explained if we assume that the helium—which is a gas under prevailing conditions—escapes in many cases slowly from the mineral.
Another product of radioactive origin is lead. The suggestion that this substance might be made available to determine the age of the Earth also originated with Rutherford. We are at least assured that this element cannot escape by gaseous diffusion from the minerals. Boltwood's results on the amount of lead contained in minerals of various ages, taken in conjunction with the amount of uranium or parent substance present, afforded ages rising to 1,640 millions of years for archaean and 1,200 millions for Algonkian time. Becker, applying the same method, obtained results rising to quite incredible periods: from 1,671 to 11,470 millions of years. Becker maintained that original lead rendered the determinations indefinite. The more recent results of Mr. A. Holmes support the conclusion that "original" lead may be present and may completely falsify results derived
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from minerals of low radioactivity in which the derived lead would be small in amount. By rejecting such results as appeared to be of this character, he arrives at 370 millions of years as the age of the Devonian.
I must now describe a very recent method of estimating the age of the Earth. There are, in certain rock-forming minerals, colour-changes set up by radioactive causes. The minute and curious marks so produced are known as haloes; for they surround, in ringlike forms, minute particles of included substances which contain radioactive elements. It is now well known how these haloes are formed. The particle in the centre of the halo contains uranium or thorium, and, necessarily, along with the parent substance, the various elements derived from it. In the process of transformation giving rise to these several derived substances, atoms of helium—the alpha rays—projected with great velocity into the surrounding mineral, occasion the colour changes referred to. These changes are limited to the distance to which the alpha rays penetrate; hence the halo is a spherical volume surrounding the central substance.[1]
The time required to form a halo could be found if on the one hand we could ascertain the number of alpha rays ejected from the nucleus of the halo in, say, one year, and, on the other, if we determined by experiment just how many alpha rays were required to produce the same
[1] Phil. Mag., March, 1907 and February, 1910; also Bedrock, January, 1913. See Pleochroic Haloes in this volume.
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amount of colour alteration as we perceive to extend around the nucleus.
The latter estimate is fairly easily and surely made. But to know the number of rays leaving the central particle in unit time we require to know the quantity of radioactive material in the nucleus. This cannot be directly determined. We can only, from known results obtained with larger specimens of just such a mineral substance as composes the nucleus, guess at the amount of uranium, or it may be thorium, which may be present.
This method has been applied to the uranium haloes of the mica of County Carlow.[1] Results for the age of the halo of from 20 to 400 millions of years have been obtained. This mica was probably formed in the granite of Leinster in late Silurian or in Devonian times.
The higher results are probably the least in error, upon the data involved; for the assumption made as to the amount of uranium in the nuclei of the haloes was such as to render the higher results the more reliable.
This method is, of course, a radioactive method, and similar to the method by helium storage, save that it is free of the risk of error by escape of the helium, the effects of which are, as it were, registered at the moment of its production, so that its subsequent escape is of no moment.
[1] Joly and Rutherford, Phil. Mag., April, 1913.
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REVIEW OF THE RESULTS
We shall now briefly review the results on the geological age of the Earth.
By methods based on the approximate uniformity of denudative effects in the past, a period of the order of 100 millions of years has been obtained as the duration of our geological age; and consistently whether we accept for measurement the sediments or the dissolved sodium. We can give reasons why these measurements might afford too great an age, but we can find absolutely no good reason why they should give one much too low.
By measuring radioactive products ages have been found which, while they vary widely among themselves, yet claim to possess accuracy in their superior limits, and exceed those derived from denudation from nine to fourteen times.
In this difficulty let us consider the claims of the radioactive method in any of its forms. In order to be trustworthy it must be true; (1) that the rate of transformation now shown by the parent substance has obtained throughout the entire past, and (2) that there were no other radioactive substances, either now or formerly existing, except uranium, which gave rise to lead. As regards methods based on the production of helium, what we have to say will largely apply to it also. If some unknown source of these elements exists we, of course, on our assumption overestimate the age.
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As regards the first point: In ascribing a constant rate of change to the parent substance—which Becker (loc. cit.) describes as "a simple though tremendous extrapolation"—we reason upon analogy with the constant rate of decay observed in the derived radioactive bodies. If uranium and thorium are really primary elements, however, the analogy relied on may be misleading; at least, it is obviously incomplete. It is incomplete in a particular which may be very important: the mode of origin of these parent bodies—whatever it may have been—is different to that of the secondary elements with which we compare them. A convergence in their rate of transformation is not impossible, or even improbable, so far as we known.
As regards the second point: It is assumed that uranium alone of the elements in radioactive minerals is ultimately transformed to lead by radioactive changes. We must consider this assumption.
Recent advances in the chemistry of the radioactive elements has brought out evidence that all three lines of radioactive descent known to us—i.e. those beginning with uranium, with thorium, and with actinium—alike converge to lead.[1] There are difficulties in the way of believing that all the lead-like atoms so produced ("isotopes" of lead, as Soddy proposes to call them) actually remain as stable lead in the minerals. For one
[1] See Soddy's Chemistry of the Radioactive Elements (Longmans, Green & Co.).
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thing there is sometimes, along with very large amounts of thorium, an almost entire absence of lead in thorianites and thorites. And in some urano—thorites the lead may be noticed to follow the uranium in approximate proportionality, notwithstanding the presence of large amounts of thorium.[1] This is in favour of the assumption that all the lead present is derived from the uranium. The actinium is present in negligibly small amounts.
On the other hand, there is evidence arising from the atomic weight of lead which seems to involve some other parent than uranium. Soddy, in the work referred to, points this out. The atomic weight of radium is well known, and uranium in its descent has to change to this element. The loss of mass between radium and uranium-derived lead can be accurately estimated by the number of alpha rays given off. From this we get the atomic weight of uranium-derived lead as closely 206. Now the best determinations of the atomic weight of normal lead assign to this element an atomic weight of closely
[1] It seems very difficult at present to suggest an end product for thorium, unless we assume that, by loss of electrons, thorium E, or thorium-lead, reverts to a substance chemically identical with thorium itself. Such a change—whether considered from the point of view of the periodic law or of the radioactive theory would involve many interesting consequences. It is, of course, quite possible that the nature of the conditions attending the deposition of the uranium ores, many of which are comparatively recent, are responsible for the difficulties observed. The thorium and uranium ores are, again, specially prone to alteration.
25
207. By a somewhat similar calculation it is deduced that thorium-derived lead would possess the atomic weight of 208. Thus normal lead might be an admixture of uranium- and thorium-derived lead. However, as we have seen, the view that thorium gives rise to stable lead is beset with some difficulties.
If we are going upon reliable facts and figures, we must, then, assume: (a) That some other element than uranium, and genetically connected with it (probably as parent substance), gives rise, or formerly gave rise, to lead of heavier atomic weight than normal lead. It may be observed respecting this theory that there is some support for the view that a parent substance both to uranium and thorium has existed or possibly exists. The evidence is found in the proportionality frequently observed between the amounts of thorium and uranium in the primary rocks.[1] Or: (b) We may meet the difficulties in a simpler way, which may be stated as follows: If we assume that all stable lead is derived from uranium, and at the same time recognise that lead is not perfectly homogeneous in atomic weight, we must, of necessity, ascribe to uranium a similar want of homogeneity; heavy atoms of uranium giving rise to heavy
[1] Compare results for the thorium content of such rocks (appearing in a paper by the author Cong. Int. de Radiologie et d'Electricite, vol. i., 1910, p. 373), and those for the radium content, as collected in Phil. Mag., October, 1912, p. 697. Also A. L. Fletcher, Phil. Mag., July, 1910; January, 1911, and June, 1911. J. H. J. Poole, Phil. Mag., April, 1915
26
atoms of lead and light atoms of uranium generating light atoms of lead. This assumption seems to be involved in the figures upon, which we are going. Still relying on these figures, we find, however, that existing uranium cannot give rise to lead of normal atomic weight. We can only conclude that the heavier atoms of uranium have decayed more rapidly than the lighter ones. In this connection it is of interest to note the complexity of uranium as recently established by Geiger, although in this case it is assumed that the shorter-lived isotope bears the relation of offspring to the longer-lived and largely preponderating constituent. However, there does not seem to be any direct proof of this as yet.
From these considerations it would seem that unless the atomic weight of lead in uraninites, etc., is 206, the former complexity and more accelerated decay of uranium are indicated in the data respecting the atomic weights of radium and lead[1]. As an alternative view, we may assume, as in our first hypothesis, that some elementally different but genetically connected substance, decaying along branching lines of descent at a rate sufficient to practically remove the whole of it during geological time, formerly existed. Whichever hypothesis we adopt
[1] Later investigation has shown that the atomic weight of lead in uranium-bearing ores is about 206.6 (see Richards and Lembert, Journ. of Am. Claem. Soc., July, 1914). This result gives support to the view expressed above.
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we are confronted by probabilities which invalidate time-measurements based on the lead and helium ratio in minerals. We have, in short, grave reason to question the measure of uniformitarianism postulated in finding the age by any of the known radioactive methods.
That we have much to learn respecting our assumptions, whether we pursue the geological or the radioactive methods of approaching the age of our era, is, indeed, probable. Whatever the issue it is certain that the reconciling facts will leave us with much more light than we at present possess either as respects the Earth's history or the history of the radioactive elements. With this necessary admission we leave our study of the Birth-Time of the World.
It has led us a long way from Lucretius. We do not ask if other Iliads have perished; or if poets before Homer have vainly sung, becoming a prey to all-consuming time. We move in a greater history, the landmarks of which are not the birth and death of kings and poets, but of species, genera, orders. And we set out these organic events not according to the passing generations of man, but over scores or hundreds of millions of years.
How much Lucretius has lost, and how much we have gained, is bound up with the question of the intrinsic value of knowledge and great ideas. Let us appraise knowledge as we would the Homeric poems, as some-
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thing which ennobles life and makes it happier. Well, then, we are, as I think, in possession today of some of those lost Iliads and Odysseys for which Lucretius looked in vain.[1]
[1] The duration in the past of Solar heat is necessarily bound up with the geological age. There is no known means (outside speculative science) of accounting for more than about 30 million years of the existing solar temperature in the past. In this direction the age seems certainly limited to 100 million years. See a review of the question by Dr. Lindemann in Nature, April 5th, 1915.
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DENUDATION
THE subject of denudation is at once one of the most interesting and one of the most complicated with which the geologist has to deal. While its great results are apparent even to the most casual observer, the factors which have led to these results are in many cases so indeterminate, and in some cases apparently so variable in influence, that thoughtful writers have even claimed precisely opposite effects as originating from, the same cause. Indeed, it is almost impossible to deal with the subject without entering upon controversial matters. In the following pages I shall endeavour to keep to broad issues which are, at the present day, either conceded by the greater number of authorities on the subject, or are, from their strictly quantitative character, not open to controversy.
It is evident, in the first place, that denudation—or the wearing away of the land surfaces of the earth—is mainly a result of the circulation of water from the ocean to the land, and back again to the ocean. An action entirely conditioned by solar heat, and without which it would completely cease and further change upon the land come to an end.
To what actions, then, is so great a potency of the
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circulating water to be traced? Broadly speaking, we may classify them as mechanical and chemical. The first involves the separation of rock masses into smaller fragments of all sizes, down to the finest dust. The second involves the actual solution in the water of the rock constituents, which may be regarded as the final act of disintegration. The rivers bear the burden both of the comminuted and the dissolved materials to the sea. The mud and sand carried by their currents, or gradually pushed along their beds, represent the former; the invisible dissolved matter, only to be demonstrated to the eye by evaporation of the water or by chemical precipitation, represents the latter.
The results of these actions, integrated over geological time, are enormous. The entire bulk of the sedimentary rocks, such as sandstones, slates, shales, conglomerates, limestones, etc., and the salt content of the ocean, are due to the combined activity of mechanical and solvent denudation. We shall, later on, make an estimate of the magnitude of the quantities actually involved.
In the Swiss valleys we see torrents of muddy water hurrying along, and if we follow them up, we trace them to glaciers high among the mountains. From beneath the foot of the glacier, we find, the torrent has birth. The first debris given to the river is derived from the wearing of the rocky bed along which the glacier moves. The river of ice bequeaths to the river of water—of which it is the parent—the spoils which it has won from the rocks
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The work of mechanical disintegration is, however, not restricted to the glacier's bed. It proceeds everywhere over the surface of the rocks. It is aided by the most diverse actions. For instance, the freezing and expansion of water in the chinks and cracks in those alpine heights where between sunrise and sunset the heat of summer reigns, and between sunset and sunrise the cold of winter. Again, under these conditions the mere change of surface temperature from night to day severely stresses the surface layers of the rocks, and, on the same principles as we explain the fracture of an unequally heated glass vessel, the rocks cleave off in slabs which slip down the steeps of the mountain and collect as screes in the valley. At lower levels the expansive force of vegetable growth is not unimportant, as all will admit who have seen the strong roots of the pines penetrating the crannies of the rocks. Nor does the river which flows in the bed of the valley act as a carrier only. Listening carefully we may detect beneath the roar of the alpine torrent the crunching and knocking of descending boulders. And in the potholes scooped by its whirling waters we recognise the abrasive action of the suspended sand upon the river bed.
A view from an Alpine summit reveals a scene of remarkable desolation (Pl. V, p. 40). Screes lie piled against the steep slopes. Cliffs stand shattered and ready to fall in ruins. And here the forces at work readily reveal themselves. An occasional wreath of white smoke among
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the far-off peaks, followed by a rumbling reverberation, marks the fall of an avalanche. Water everywhere trickles through the shaly debris scattered around. In the full sunshine the rocks are almost too hot to bear touching. A few hours later the cold is deadly, and all becomes a frozen silence. In such scenes of desolation and destruction, detrital sediments are actively being generated. As we descend into the valley we hear the deep voice of the torrents which are continually hurrying the disintegrated rocks to the ocean.
A remarkable demonstration of the activity of mechanical denudation is shown by the phenomenon of "earth pillars." The photograph (Pl. IV.) of the earth pillars of the Val d'Herens (Switzerland) shows the peculiar appearance these objects present. They arise under conditions where large stones or boulders are scattered in a deep deposit of clay, and where much of the denudation is due to water scour. The large boulders not only act as shelter against rain, but they bind and consolidate by their mere weight the clay upon which they rest. Hence the materials underlying the boulders become more resistant, and as the surrounding clays are gradually washed away and carried to the streams, these compacted parts persist, and, finally, stand like walls or pillars above the general level. After a time the great boulders fall off and the underlying clay becomes worn by the rainwash to fantastic spikes and ridges. In the Val d'Herens the earth pillars are formed
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of the deep moraine stuff which thickly overlies the slopes of the valley. The wall of pillars runs across the axis of the valley, down the slope of the hill, and crosses the road, so that it has to be tunnelled to permit the passage of traffic. It is not improbable that some additional influence—possibly the presence of lime—has hardened the material forming the pillars, and tended to their preservation.
Denudation has, however, other methods of work than purely mechanical; methods more noiseless and gentle, but not less effective, as the victories of peace ate no less than those of war.
Over the immense tracts of the continents chemical work proceeds relentlessly. The rock in general, more especially the primary igneous rock, is not stable in presence of the atmosphere and of water. Some of the minerals, such as certain silicates and carbonates, dissolve relatively fast, others with extreme slowness. In the process of solution chemical actions are involved; oxidation in presence of the free oxygen of the atmosphere; attack by the feeble acid arising from the solution of carbon dioxide in water; or, again, by the activity of certain acids—humous acids—which originate in the decomposition of vegetable remains. These chemical agents may in some instances, e.g. in the case of carbonates such as limestone or dolomite—bring practically the whole rock into solution. In other instances—e.g. granites, basalts, etc.—they may remove some of the
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constituent minerals completely or partially, such as felspar, olivine, augite, and leave more resistant substances to be ultimately washed down as fine sand or mud into the river.
It is often difficult or impossible to appraise the relative efficiency of mechanical and chemical denudation in removing the materials from a certain area. There can be, indeed, little doubt that in mountainous regions the mechanical effects are largely predominant. The silts of glacial rivers are little different from freshly-powdered rock. The water which carries them but little different from the pure rain or snow which falls from the sky. There has not been time for the chemical or solvent actions to take place. Now while gravitational forces favour sudden shock and violent motions in the hills, the effect of these on solvent and chemical denudation is but small. Nor is good drainage favourable to chemical actions, for water is the primary factor in every case. Water takes up and removes soluble combinations of molecules, and penetrates beneath residual insoluble substances. It carries the oxygen and acids downwards through the soils, and finally conveys the results of its own work to the rivers and streams. The lower mean temperature of the mountains as well as the perfect drainage diminishes chemical activities.
Hence we conclude that the heights are not generally favourable to the purely solvent and chemical actions. It is on the lower-lying land that soils tend to accumulate,
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and in these the chief solvent and the chief chemical denudation of the Earth are effected.
The solvent and chemical effects which go on in the finely-divided materials of the soils may be observed in the laboratory. They proceed faster than would be anticipated. The observation is made by passing a measured quantity of water backwards and forwards for some months through a tube containing a few grammes of powdered rock. Finally the water is analysed, and in this manner the amount of dissolved matter it has taken up is estimated. The rock powder is examined under the microscope in order to determine the size of the grains, and so to calculate the total surface exposed to the action of the water. We must be careful in such experiments to permit free oxidation by the atmosphere. Results obtained in this way of course take no account of the chemical effects of organic acids such as exist in the soils. The quantities obtained in the laboratory will, therefore, be deficient as compared with the natural results.
In this manner it has been found that fresh basalt exposed to continually moving water will lose about 0.20 gramme per square metre of surface per year. The mineral orthoclase, which enters largely into the constitution of many granites, was found to lose under the same conditions 0.025 gramme. A glassy lava (obsidian) rich in silica and in the chemical constituents of an average granite, was more resistant still; losing but 0.013 gramme per square metre per year. Hornblende, a mineral
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abundant in many rocks, lost 0.075 gramme. The mean of the results showed that 0.08 gramme was washed in a year from each square metre. Such results give us some indication of the rate at which the work of solution goes on in the finely divided soils.[1]
It might be urged that, as the mechanical break up of rocks, and the production in this way of large surfaces, must be at the basis of solvent and chemical denudation, these latter activities should be predominant in the mountains. The answer to this is that the soils rarely owe their existence to mechanical actions. The alluvium of the valleys constitutes only narrow margins to the rivers; the finer debris from the mountains is rapidly brought into the ocean. The soils which cover the greater part of continental areas have had a very different origin.
In any quarry where a section of the soil and of the underlying rock is visible, we may study the mode of formation of soils. Our observations are, we will suppose, pursued in a granite quarry. We first note that the material of the soil nearest the surface is intermixed with the roots of grasses, trees, or shrubs. Examining a handful of this soil, we see glistening flakes of mica which plainly are derived from the original granite. Washing off the finer particles, we find the largest remaining grains are composed of the all but indestructible quartz.
[1] Proc. Roy. Irish Acad., VIII., Ser. A, p. 21.
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This also is from the granite. Some few of the grains are of chalky-looking felspar; again a granitic mineral. What is the finer silt we have washed off? It, too, is composed of mineral particles to a great extent; rock dust stained with iron oxide and intermixed with organic remains, both animal and vegetable. But if we make a chemical analysis of the finer silt we find that the composition is by no means that of the granite beneath. The chemist is able to say, from a study of his results, that there has been, in the first place, a large loss of material attending the conversion of the granite to the soil. He finds a concentration of certain of the more resistant substances of the granite arising from the loss of the less resistant. Thus the percentage amount of alumina is increased. The percentage of iron is also increased. But silica and most other substances show a diminished percentage. Notably lime has nearly disappeared. Soda is much reduced; so is magnesia. Potash is not so completely abstracted. Finally, owing to hydration, there is much more combined water in the soil than in the rock. This is a typical result for rocks of this kind.
Deeper in the soil we often observe a change of texture. It has become finer, and at the same time the clay is paler in colour. This subsoil represents the finer particles carried by rain from above. The change of colour is due to the state of the iron which is less oxidised low down in the soil. Beneath the subsoil the soil grows
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again coarser. Finally, we recognise in it fragments of granite which ever grow larger as we descend, till the soil has become replaced by the loose and shattered rock. Beneath this the only sign of weathering apparent in the rock is the rusty hue imparted by the oxidised iron which the percolating rain has leached from iron-bearing minerals.
The soil we have examined has plainly been derived in situ from the underlying rock. It represents the more insoluble residue after water and acids have done their work. Each year there must be a very slow sinking of the surface, but the ablation is infinitesimal.
The depth of such a soil may be considerable. The total surface exposed by the countless grains of which it is composed is enormous. In a cubic foot of average soil the surface area of the grains may be 50,000 square feet or more. Hence a soil only two feet deep may expose 100,000 square feet for each square foot of surface area.
It is true that soils formed in this manner by atmospheric and organic actions take a very long time to grow. It must be remembered, however, that the process is throughout attended by the removal in solution: of chemically altered materials.
Considerations such as the foregoing must convince us that while the accumulation of the detrital sediments around the continents is largely the result of activities progressing on the steeper slopes of the land, that is,
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among the mountainous regions, the feeding of the salts to the ocean arises from the slower work of meteorological and organic agencies attacking the molecular constitution of the rocks; processes which best proceed where the drainage is sluggish and the quiescent conditions permit of the development of abundant organic growth and decay.
Statistics of the solvent denudation of the continents support this view. Within recent years a very large amount of work has been expended on the chemical investigation of river waters of America and of Europe. F. W. Clarke has, at the expense of much labour, collected and compared these results. They are expressed as so many tonnes removed in solution per square mile per annum. For North America the result shows 79 tonnes so removed; for Europe 100 tonnes. Now there is a notable difference between the mean elevations of these two continents. North America has a mean elevation of 700 metres over sea level, whereas the mean elevation of Europe is but 300 metres. We see in these figures that the more mountainous land supplies less dissolved matter to the ocean than the land of lower elevation, as our study has led us to expect.
We have now considered the source of the detrital sediments, as well as of the dissolved matter which has given to the ocean, in the course of geological time, its present gigantic load of salts. It is true there are further solvent and chemical effects exerted by the sea water
40
upon the sediments discharged into it; but we are justified in concluding that, relatively to the similar actions taking place in the soils, the solvent and chemical work of the ocean is small. The fact is, the deposited detrital sediments around the continents occupy an area small when contrasted with the vast stretches of the land. The area of deposition is much less than that of denudation; probably hardly as much as one twentieth. And, again, the conditions of aeration and circulation which largely promote chemical and solvent denudation in the soils are relatively limited and ineffective in the detrital oceanic deposits.
The summation of the amounts of dissolved and detrital materials which denudation has brought into the ocean during the long denudative history of the Earth, as we might anticipate, reveals quantities of almost unrealisable greatness. The facts are among the most impressive which geological science has brought to light. Elsewhere in this volume they have been mentioned when discussing the age of the Earth. In the present connection, however, they are deserving of separate consideration.
The basis of our reasoning is that the ocean owes its saltness mainly if not entirely to the denudative activities we have been considering. We must establish this.
We may, in the first place, say that any other view at once raises the greatest difficulties. The chemical composition of the detrital sediments which are spread over
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the continents and which build up the mountains, differs on the average very considerably from that of the igneous rocks. We know the former have been derived from the latter, and we know that the difference in the composition of the two classes of materials is due to the removal in solution of certain of the constituents of the igneous rocks. But the ocean alone can have received this dissolved matter. We know of no other place in which to look for it. It is true that some part of this dissolved matter has been again rejected by the ocean; thus the formation of limestone is largely due to the abstraction of lime from sea water by organic and other agencies. This, however, in no way relieves us of the necessity of tracing to the ocean the substances dissolved from the igneous rocks. It follows that we have here a very causa for the saltness of the ocean. The view that the ocean "was salt from the first" is without one known fact to support it, and leaves us with the burden of the entire dissolved salts of geological time to dispose of—Where and how?
The argument we have outlined above becomes convincingly strong when examined more closely. For this purpose we first compare the average chemical composition of the sedimentary and the igneous rocks. The following table gives the percentages of the chief chemical constituents: [1]
[1] F. W. Clarke: A Preliminary Study of Chemical Denudation, p. 13
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Igneous. Sedimentary. Silica (SiO2) - 59.99 58.51 Alumina (Al2O3) - 15.04 13.07 Ferric oxide (F2O3) - 2.59 3.40 Ferrous oxide (FeO) - 3.34 2.00 Magnesia (MgO) - 3.89 2.52 Lime (CaO) - 4.81 5.42 Soda (Na2O) - 3.41 1.12 Potash (K2O) - 2.95 2.80 Water (H2O) - 1.92 4.28 Carbon dioxide (CO2) - — 4.93 Minor constituents - 2.06 1.95 100.00 100.00
In the derivation of the sediments from the igneous rocks there is a loss by solution of about 33 per cent; i.e. 100 tons of igneous rock yields rather less than 70 tons of sedimentary rock. This involves a concentration in the sediments of the more insoluble constituents. To this rule the lime-content appears to be an exception. It is not so in reality. Its high value in the sediments is due to its restoration from the ocean to the land. The magnesia and potash are, also, largely restored from the ocean; the former in dolomites and magnesian limestones; the latter in glauconite sands. The iron of the sediments shows increased oxidation. The most notable difference in the two analyses appears, however, in the soda percentages. This falls from 3.41 in the igneous rock to 1.12 in the average sediment. Indeed, this
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deficiency of soda in sedimentary rocks is so characteristic of secondary rocks that it may with some safety be applied to discriminate between the two classes of substances in cases where petrological distinctions of other kinds break down.
To what is this so marked deficiency of soda to be ascribed? It is a result of the extreme solubility of the salts of sodium in water. This has not only rendered its deposition by evaporation a relatively rare and unimportant incident of geological history, but also has protected it from abstraction from the ocean by organic agencies. The element sodium has, in fact, accumulated in the ocean during the whole of geological time.
We can use the facts associated with the accumulation of sodium salts in the ocean as a means of obtaining additional support to the view, that the processes of solvent denudation are responsible for the saltness of the ocean. The new evidence may be stated as follows: Estimates of the amounts of sedimentary rock on the continents have repeatedly been made. It is true that these estimates are no more than approximations. But they undoubtedly are approximations, and as such may legitimately be used in our argument; more especially as final agreement tends to check and to support the several estimates which enter into them.
The most recent and probable estimates of the sediments on the land assign an average thickness of one mile of
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secondary rocks over the land area of the world. To this some increase must be made to allow for similar materials concealed in the ocean, principally around the continental margins. If we add 10 per cent. and assign a specific gravity of 2.5 we get as the mass of the sediments 64 x 1016 tonnes. But as this is about 67 per cent. of the parent igneous rock—i.e. the average igneous rock from which the sediments are derived—we conclude that the primary denuded rock amounted to a mass of about 95 x 1016 tonnes.
Now from the mean chemical composition of the secondary rocks we calculate that the mass of sediments as above determined contains 0.72 x1016 tonnes of the sodium oxide, Na2O. If to this amount we add the quantity of sodium oxide which must have been given to the ocean in order to account for the sodium salts contained therein, we arrive at a total quantity of oxide of sodium which must be that possessed by the primary rock before denudation began its work upon it. The mass of the ocean being well ascertained, we easily calculate that the sodium in the ocean converted to sodium oxide amounts to 2.1 x 1016 tonnes. Hence between the estimated sediments and the waters of the ocean we can account for 2.82 x 1016 tonnes of soda. When now we put this quantity back into the estimated mass of primary rock we find that it assigns to the primary rock a soda percentage of 3.0. On the average analysis given above this should be 3.41 per cent. The agreement,
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all things considered, more especially the uncertainty in the estimate of the sediments, is plainly in support of the view that oceanic salts are derived from the rocks; if, indeed, it does not render it a certainty.
A leading and fundamental inference in the denudative history of the Earth thus finds support: indeed, we may say, verification. In the light of this fact the whole work of denudation stands revealed. That the ocean began its history as a vast fresh-water envelope of the Globe is a view which accords with the evidence for the primitive high temperature of the Earth. Geological history opened with the condensation of an atmosphere of immense extent, which, after long fluctuations between the states of steam and water, finally settled upon the surface, almost free of matter in solution: an ocean of distilled water. The epoch of denudation then began. It will, probably, continue till the waters, undergoing further loss of thermal energy, suffer yet another change of state, when their circulation will cease and their attack upon the rocks come to an end.
From what has been reviewed above it is evident that the sodium in the ocean is an index of the total activity of denudation integrated over geological time. From this the broad facts of the results of denudation admit of determination with considerable accuracy. We can estimate the amount of rock which has been degraded by solvent and chemical actions, and the amount of sediments which has been derived from it. We are,
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thus, able to amend our estimate of the sediments which, as determined by direct observation, served to support the basis of our argument.
We now go straight to the ocean for the amount of sodium of denudative origin. There may, indeed, have been some primitive sodium dissolved by a more rapid denudation while the Earth's surface was still falling in temperature. It can be shown, however, that this amount was relatively small. Neglecting it we may say with safety that the quantity of sodium carried into the ocean by the rivers must be between 14,000 and 15,000 million million tonnes: i.e. 14,500 x 1012 tonnes, say.
Keeping the figures to round numbers we find that this amount of sodium involves the denudation of about 80 x 1016 tonnes of average igneous rock to 53 x 1016 tonnes of average sediment. From these vast quantities we know that the parent rock denuded during geological time amounted to some 300 million cubic kilometres or about seventy million cubic miles. The sediments derived therefrom possessed a bulk of 220 million cubic kilometres or fifty million cubic miles. The area of the land surface of the Globe is 144 million square kilometres. The parent rock would have covered this to a uniform depth of rather more than two kilometres, and the derived sediment to more than 1.5 kilometres, or about one mile deep.
The slow accomplishment of results so vast conveys some idea of the great duration of geological time.
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The foregoing method of investigating the statistics of solvent denudation is capable of affording information not only as to the amount of sediments upon the land, but also as to the quantity which is spread over the floor of the ocean.
We see this when we follow the fate of the 33 per cent. of dissolved salts which has been leached from the parent igneous rock, and the mass of which we calculate from the ascertained mass of the latter, to be 27 x 1016 tonnes. This quantity was at one time or another all in the ocean. But, as we saw above, a certain part of it has been again abstracted from solution, chiefly by organic agencies. Now the abstracted solids have not been altogether retained beneath the ocean. Movements of the land during geological time have resulted in some portion being uplifted along with other sediments. These substances constitute, mainly, the limestones.
We see, then, that the 27 x 1016 tonnes of substances leached from the parent igneous rocks have had a threefold destination. One part is still in solution; a second part has been precipitated to the bottom of the ocean; a third part exists on the land in the form of calcareous rocks.
Observation on the land sediments shows that the calcareous rocks amount to about 5 per cent. of the whole. From this we find that 3 x 1016 tonnes, approximately, of such rocks have been taken from the ocean. This accounts for one of the three classes of material
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into which the original dissolved matter has been divided. Another of the three quantities is easily estimated: the amount of matter still in solution in the ocean. The volume of the ocean is 1,414 million cubic kilometres and its mass is 145 x 1016 tonnes. The dissolved salts in it constitute 3.4 per cent. of its mass; or, rather more than 5 x 1016 tonnes. The limestones on the land and the salts in the sea water together make up about 8 x 1016 tonnes. If we, now, deduct this from the total of 27 x 1016 tonnes, we find that about 19 x 1016 tonnes must exist as precipitated matter on the floor of the ocean.
The area of the ocean is 367 x 1012 square metres, so that if the precipitated sediment possesses an average specific gravity of 2.5, it would cover the entire floor to a uniform depth of 218 metres; that is 715 feet. This assumes that there was uniform deposition of the abstracted matter over the floor of the ocean. Of course, this assumption is not justifiable. It is certain that the rate of deposition on the floor of the sea has varied enormously with various conditions—principally with the depth. Again, it must be remembered that this estimate takes no account of solid materials otherwise brought into the oceanic deposits; e.g., by wind-transported dust from the land or volcanic ejectamenta in the ocean depths. It is not probable, however, that any considerable addition to the estimated mean depth of deposit from such sources would be allowable.
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The greatness of the quantities involved in these determinations is almost awe inspiring. Take the case of the dissolved salts in the ocean. They are but a fraction, as we have seen, of the total results of solvent denudation and represent the integration of the minute traces contributed by the river water. Yet the common salt (chloride of sodium) alone, contained in the ocean, would, if abstracted and spread over the dry land as a layer of rock salt having a specific gravity of 2.2, cover the whole to a depth of 107 metres or 354 feet. The total salts in solution in the ocean similarly spread over the land would increase the depth of the layer to 460 feet. After considering what this means we have to remember that this amount of matter now in solution in the seas is, in point of fact, less than a fifth part of the total dissolved from the rocks during geological time.
The transport by denudation of detrital and dissolved matter from the land to the ocean has had a most important influence on the events of geological history. The existing surface features of the earth must have been largely conditioned by the dynamical effects arising therefrom. In dealing with the subject of mountain genesis we will, elsewhere, see that all the great mountain ranges have originated in the accumulation of the detrital sediments near the shore in areas which, in consequence of the load, gradually became depressed and developed into synclines of many thousands of feet in depth. The most impressive surface features of the Globe originated
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in this manner. We will see too that these events were of a rhythmic character; the upraising of the mountains involving intensified mechanical denudation over the elevated area and in this way an accelerated transport of detritus to the sea; the formation of fresh deposits; renewed synclinal sinking of the sea floor, and, finally, the upheaval of a younger mountain range. This extraordinary sequence of events has been determined by the events of detrital denudation acting along with certain general conditions which have all along involved the growth of compressive stresses in the surface crust of the Earth.
The effects of purely solvent denudation are less easily traced, but, very probably, they have been of not less importance. I refer here to the transport from the land to the sea of matter in solution.
Solvent denudation, as observed above, takes place mainly in the soils and in this way over the more level continental areas. It has resulted in the removal from the land and transfer to the ocean of an amount of matter which represents a uniform layer of one half a kilometre; that is of more than 1,600 feet of rock. The continents have, during geological time, been lightened to this extent. On the other hand all this matter has for the greater part escaped the geosynclines and become uniformly diffused throughout the ocean or precipitated over its floor principally on the continental slopes before the great depths are reached. Of this material the ocean
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waters contain in solution an amount sufficient to increase their specific gravity by 2.7 per cent.
Taking the last point first, it is interesting to note the effects upon the bulk of the ocean which has resulted from the matter dissolved in it. From the known density of average sea water we find that 100 ccs. of it weigh just 102.7 grammes. Of this 3.5 per cent. by weight are solids in solution. That is to say, 3.594 grammes. Hence the weight of water present is 99.1 grammes, or a volume of 99.1 ccs. From this we see that the salts present have increased the volume by 0.9 ccs. or 0.9 per cent.
The average depth of the ocean is 2,000 fathoms or 3,700 metres. The increase of depth due to salts dissolved in the ocean has been, therefore, 108 feet or 33.24 metres. This result assumes that there has been no increased elastic compression due to the increased pressure, and no change of compressional elastic properties. We may be sure that the rise on the shore line of the land has not been less than 100 feet.
We see then that as the result of solvent denudation we have to do with a heavier and a deeper ocean, expanded in volume by nearly one per cent. and the floor of which has become raised, on an average, about 700 feet by precipitated sediment.
One of the first conceptions, which the student of geology has to dismiss from his mind, is that of the immobility or rigidity of the Earth's crust. The lane, we live on sways even to the gentle rise and fail of ocean tides
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around the coasts. It suffers its own tidal oscillations due to the moon's attractions. Large tracts of semi-liquid matter underlie it. There is every evidence that the raised features of the Globe are sustained by such pressures acting over other and adjacent areas as serve to keep them in equilibrium against the force of gravity. This state of equilibrium, which was first recognised by Pratt, as part of the dynamics of the Earth's crust, has been named isostasy. The state of the crust is that of "mobile equilibrium."
The transfer of matter from the exposed land surfaces to the sub-oceanic slopes of the continents and the increase in the density of the ocean, must all along have been attended by isostatic readjustment. We cannot take any other view. On the one hand the land was being lightened; on the other the sea was increasing in mass and depth and the flanks of the continents were being loaded with the matter removed from the land and borne in solution to the ocean. How important the resulting movements must have been may be gathered from the fact that the existing land of the Globe stands at a mean elevation of no more than 2,000 feet above sea level. We have seen that solvent denudation removed over 1,600 feet of rock. But we have no evidence that on the whole the elevation of land in the past was ever very different from what it now is. |
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