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upheaved. That the main features of the land, such as the great | beginning of things, but must be content to date its first chapter mountain-chains, had been produced by gigantic plication of the from the
earliest period of wbich any record has been preserved fractures and folds had probably initiated many of the valleys. among the rocks. But those who realized most vividly the momentous results achieved Nevertheless, though geology in its usual restricted sense has by ages of subaerial denudation perceived that, as Hutton showed, been, and musi ever be, unable to reveal the carliest history of even without the aid of underground agency, the mere flow of water in streams across a mass of land must in course of time carve out
our planet, it no longer ignores, as mere speculation, what is just such a system of valleys as may anywhere be seen.
It was attempted in this subject by its sister sciences. Astronomy, J. B. Jukes who, in 1862, first revived the Huttonian doctrine, physics and chemistry have in late years all contributed to cast and showed how completely it explained the drainage-lines in the light on the earlier stages of the earth's existence, previous to south of Ireland. Other writers followed in quick succession until,
the beginning of what is commonly regarded as geological history. in a few years, the doctrine came to be widely recognized as one of the established principles of modern geology. Much help was derived
But whatever extends our knowledge of the former conditions from the admirable illustrations of land-sculpture and river-erosion of our globe may be legitimately claimed as part of the domain of supplied from the Western Territories and States of the American geology. If this branch inquiry, therefore, is to continue Union. Another branch of physiographical gcology which could only come
worthy of its name as the science of the earth, it must take into existence after most of the other departments of the science cognizance of these recent contributions from other sciences. had made large progress, deals with the evolution of the framework It must no longer be content to begin its annals with the records of each country and of the several continents and oceans of the globe. of the oldest rocks, but must endeavour to grope its way through It is now possible, with more or less confidence, to trace backward the ages which preceded the formation of any rocks. Thanks the history of every terrestrial area, to see how sea and land have there succeeded each other, how rivers and lakes have come and
to the results achieved with the telescope, the spectroscope and gone, how the crust of the earth has been ridged up at widely the chemical laboratory, the story of these earliest ages of our separated intervals, each movement determining some line of earth is every year becoming more definite and intelligible. mountains or plains, how the boundaries of the oceans have shisted Up to the present time no definite light has been thrown by again and again in the past, and thus how, pfter so prolonged a series of revolutions, the present topography of each country, and of the physics on the origin and earliest condition of our globe. The globe as a whole, has been produced. In the prosecution of this famous nebular theory (9.0.) of Kant and Laplace sketched the subject maps have been constructed to show what is conjectured supposed evolution of the solar system from a gaseous nebula, to have been the distribution of sea and land during the various slowly rotating round a more condensed central portion of its geological periods in different parts of the world, and thus to indicate the successive stages through which the architecture of the land has
mass, which eventually became the sun. As a consequence of been gradually evolved. The most noteworthy contribution to this increased rapidity of rotation resulting from cooling and condepartment of the science is the Antlitz der Erde of Professor Suess traction, the nebula acquired a more and more lenticular form, of Vienna. This important and suggestive work has been translated until at last it threw off from its equatorial protuberance a ring into French and English.
of matter. Subsequently the same process was repeated, and
other similar rings successively separated from the parent mass. PART II.-COSMICAL ASPECTS
Each ring went through a corresponding series of changes until Before geology had attained to the position of an inductive it ultimately became a planet, with or without one or more science, it was customary to begin investigations into the attendant satellites. The intimate relationship of our earth history of the earth by propounding or adopting some more to the sun and the other planets was, in this way, shown. But or less fanciful hypothesis in explanation of the origin of our there are some serious physical difficulties in the way of the planet, or even of the universe. Such preliminary notions were acceptance of the nebular hypothesis. Another explanation looked upon as essential to a right understanding of the manner is given by the meteoritic hypothesis, according to which, out in which the materials of the globe had been put together. One of the swarms of meteorites with which the regions of space are of the distinguishing features of Hutton's Theory of the Earth crowded, the sun and planets have been formed by gradual consisted in his protest that it is no part of the province of accretion. geology to discuss the origin of things. He taught that in the According to these theoretical views we should expect to find materials from which geological evidence is to be compiled a general uniformity of composition in the constituent matter there can be found " no traces of a beginning, no prospect of an of the solar system. For many years the only available evidence end." In England, mainly to the influence of the school which on this point was derived from the meteorites (9.0.) which so he founded, and to the subsequent rise of the Geological Society constantly fall from outer space upon the surface of the earth. of London, which resolved to collect facts instead of fighting These bodies were found to consist of elements all of which had over hypotheses, is due the disappearance of the crude and been recognized as entering into the constitution of the earth. unscientific cosmologies by which the writings of the earlier But the discoveries of spectroscopic research have made known geologists were distinguished
a far more widely serviceable method of investigation, which But there can now be little doubt that in the reaction against can be applied even to the luminous stars and nebulae that lie those visionary and often grotesque speculations, geologists far beyond the bounds of the solar system. By this method were carried too far in an opposite direction. In allowing information has been obtained regarding the constitution of the themselves to believe that geology had nothing to do with sun, and many of our terrestrial metals, such as iron, nickel and questions of cosmogony, they gradually grew up in the conviction magnesium, have been ascertained to exist in the form of inthat such questions could never be other than mere speculation, candescent vapour in the solar atmosphere. The present interesting or amusing as a theme for the employment of the condition of the sun probably represents one of the phases fancy, but hardly coming within the domain of sober and through which stars and planets pass in their progress towards inductive science. Nor would they soon have been awakened becoming cool and dark bodies in space. If our globe was at out of this belief by anything in their own science. It is still first, like its parent sun, an incandescent mass of probably true that in the data with which they are accustomed to deal, gaseous matter, occupying much more space than it now fills, as comprising the sum of geological evidence, there can be we can conceive that it has ever since been cooling and contractfound no trace of a beginning, though the evidence furnished ing until it has reached its present form and dimensions, and that by the terrestrial crust shows a general evolution of organic it still retains a high internal temperature. Its oblately spheroidal forms from some starting-point which cannot be seen. The form is such as would be assumed by a rotating mass of matter oldest rocks which have been discovered on any part of the in the transition from a vaporous and self-luminous or liquid globe have probably been derived from other rocks older than condition to one of cool and dark solidity. But it has been themselves. Geology by itself has not yet revealed, and is little claimed that even a solid spherical globe might develop, under likely ever to reveal, a trace of the first solid crust of our globe. the influence of protracted rotation, such a shape as the carth If, then, geological history is to be compiled from direct evidence at present possesses. furnished by the rocks of the earth, it cannot begin at the The observed increase of temperature downwards in our
planet bas hitherto been generally accepted as a relic and proof | in the case of rivers which flow in a meridional direction. It has of an original high temperature and mobility of substance. been asserted that those, which in the northern hemisphere Recently, however, the validity of this proof has been challenged flow from north to south, like the Volga, by continually passing on the ground that the ascertained amount of radium in the into regions where the velocity of rotation is increasingly greater, rocks of the outer crust is more than sufficient to account for are thrown more against their western than their eastern banks, the observed downward increase of temperature. Too little, while those whose general course is in an opposite direction, like however, is known of the history and properties of what is the Irtisch and Yenesei, press more upon their eastern sides. called radium to afford a satisfactory ground on which to There cannot be any doubt that the tendency of the streams discard what has been, and still remains, the prevalent belief must be in the directions indicated. But when the comparatively on this subject.
slow current and constantly meandering course of most rivers An important epoch in the geological history of the earth are taken into consideration, it may be doubted whether the was marked by the separation of the moon from its mass (see influence of rotation is of 'much practical account so far as TIDE). Whether the severance arose from the rupture of a river-erosion is concerned. surrounding ring or the gradual condensation of matter in such One of the cosmical relations of our planet which has been a ring, or from the ejection of a single mass of matter from the more especially prominent in geological speculations relates to rapidly rotating planet, it has been shown that our satellite the position of the earth's axis of rotation. Abundant evidence was only a few thousand miles from the earth's surface, since has now been obtained to prove that at a comparatively late when it has retreated to its present distance of 240,000 m. Hence geological period a rich flora, resembling that of warm climates the influence of the moon's attraction, and all the gcological at the present day, existed in high latitudes even within less than effects to which it gives rise, attained their maximum far back 9° of the north pole, where, with an extremely low temperature in the development of the globe, and have been slowly diminish- and darkness lasting for half of the year, no such vegetation could ing throughout geological history.
possibly now exist. It has accordingly been maintained by The sun by virtue of its vast size has not yet passed out of many geologists that the axis of rotation must have shifted, the condition of glowing gas, and still continues to radiate heat and that when the remarkable Arctic assemblage of fossil plants beyond the farthest planet of the solar system. The earth, lived the region of their growth must have lain in latitudes much however, being so small a body in comparison, would cool down nearer to the equator of the time. much more quickly. Underneath its hot atmosphere a crust The possibility of any serious displacement of the rotational would conceivably begin to form over its molten surface, though axis since a very early period in the carth's history has been the interior might still possess a high temperature and, owing strenuously denied by astronomers, and their arguments have to the feeble conducting power of rocks, would remain intensely been generally, but somewhat reluctantly, accepted by geologists, hot for a protracted series of ages.
who find themselves confronted with a problem which has Full information regarding the form and size of the earth, hitherto seemed insoluble. That the axis is not rigidly stable, and its relations to the other planetary members of the solar however, has been postulated by some physicists, and has now system, will be found in the articles PLANET and SOLAR SYSTEM. been demonstrated by actual observation and measurement, For the purposes of geological inquiry the reader will bear in It is admitted that by the movement of large bodies of water mind that the equatorial diameter of our globe is estimated to the air over the surface of the globe, and more particularly by be about 7925 m., and the polar diameter about 7899 m.; the the accumulation of vast masses of snow and ice in different difference between these two sums representing the amount of regions, the position of the axis might be to some extent shifted; Kattening at the poles (about 26 m.). The planet has been more serious effects might follow from widespread upheavals compared in shape to an orange, but it resembles an orange or depressions of the surface of the lithosphere. On the assumpwhich has been somewhat squeezed, for its equatorial circum- tion of the extreme rigidity of the earth's interior, however, the ference is not a regular circle but an ellipse, of which the major general result of mathematical calculation is to negative the axis lies in long. 8° 15' W.-on a meridian which cuts the north- supposition that in any of these ways within the period reprewest corner of America, passing through Portugal and Ireland, sented by what is known as the “ geological record,” thai is, and the north-east corner of Asia in the opposite hemisphere. since the time of the oldest known sedimentary formations, the
The rotation of the earth on its axis exerts an important rotational axis has ever been so seriously displaced as to account influence on the movements of the atmosphere, and thereby for such stupendous geological events as the spread of a luxuriant afíects the geological operations connected with these movements. vegetation far up into polar latitudes. If, however, the inside The influence of rotation is most marked in the great aerial of the globe possesses a great plasticity than has been allowed, circulation between the poles and the equator. Currents of the shifting of the axis might not be impossible, even to such an air, which set out in a meridional direction írom high latitudes extent as would satisfy the geological requirements. This towards the equator, come from regions where the velocity due question is one on which the last word has not been said, and to rotation is small to where it is greater, and they conscquently regarding which judgment must remain in suspense. fall behind. Thus, in the northern hemisphere a north wind, In recent years fresh information bearing on the minor devagaas it moves away from its northern source of origin, is gradually tions of the pole has been obtained from a series of several deflected more and more towards the west and becomes a north- thousand careful observations made in Europe and North east current; while in the opposite hemisphere a wind making America. It has thus been ascertained that the pole wanders from high southern latitudes towards the equator becomes, with a curiously irregular but somewhat spiral movement, from the same cause, a south-east current. Where, on the within an amplitude of between 40 and 50 st., and completes other hand, the air moves from the equatorial to the polar regions its erratic circuit in about 428 days. It was not supposed that its higher velocity of rotation carries it eastward, so that on the its movement had any geological interest, but Dr John Milne south side of the equator it becomes a north-west current and has recently pointed out that the times of sharpest curvature on the north side a south-west current. It is to this cause that in the path of the pole coincide with the occurrence of large the easting and westing of the great atmospheric currents are earthquakes, and has suggested that, although it can hardly be 10 be attributed, as is familiarly exemplified in the trade winds. assumed that this coincidence shows any direct connexion
The atmospheric circulation thus deflected influences the between earthquake frequency and changes in the position of circulation of the ocean. The winds which persistently blow the earth's axis, both effects may not improbably arise from from the north-east on the north side of the equator, and from the same redistribution of surface material by ocean currents the south-east on the south side, drive the superficial waters and meteorological causes. onwards, and give rise to converging oceanic currents which If for any reason the earth's centre of gravity were sensibly unite to form the great westcrly equatorial current.
displaced, momentous geological changes would necessarily A more direct effect of terrestrial rotation has been claimed I ensue. That the centre of gravity does not coincide with the
centre of figure of the globe, but lies to the south of it, has long | yet anothing we know regarding the figure of the earth, and been known. This greater aggregation of dense material in the ihe disposition of land and water, would justify us in saying southern hemisphere probably dates from the early ages of the that a body consolidated when there was more centrifugal earth's consolidation, and it is difficult to believe that any force by 3% than now, might not now be in all respects like readjustment of the distribution of this material in the earth's the earth, so far as we know it at present." interior is now possible. But certain rearrangements of the 3. The third argument, based upon the age of the sun's heat, hydrosphere on the surface of the globe may, from time to time, is confessedly less to be relied on than the two previous ones. cause a shifting of the centre of gravity, which will affect the It proceeds upon calculations as to the amount of heat which level of the ocean. The accumulation of enormous masses of would be available by the falling together of masses from space, ice around the pole will give rise to such a displacement, and which gave rise by their impact to our sun. The vagueness of will thus increase the body of occanic water in the glaciated the data on which this argument rests may be inferred from hemisphere. Various calculations have been made of the effect the fact that in one passage P. G. Tait placed the limit of time of the transference of the ice-cap from one pole to the other, a during which the sun has been illuminating the earth as, "on revolution which may possibly have occurred more than once the very highest computation, not more than about 15 or 20 in the past history of the globe. James Croll estimated that if millions of years "; while, in another sentence of the same the mass of ice in the southern hemisphere be assumed to be volume, he admitted that, “ by calculations in which there is 1000 ft. thick down to lat. 60°, its removal to the opposite no possibility of large error, this hypothesis (of the origin of the hemisphere would raise the level of the sea 80 ft. at the north pole, sun's heat by the falling together of masses of matter) is while the Rev. Osmond Fisher made the rise as much as 409 it. thoroughly competent to explain 100 millions of years' solar The melting of the ice would still further raise the sea-level by radiation at the present rate, perhaps more." In more recently the addition of so large a volume of water to the ocean. To reviewing his argument, Lord Kelvin expressed himself in what extent superficial changes of this kind have operated in favour of more strictly limiting geological time than he had at geological history remains an unsolved problem, but their first been disposed to do. He insists that the time “ was more probable occurrence in the past has to be recognized as one of than 20 and less than 40 millions of years and probably much the factors that must be considered in tracing the revolutions of nearer 20 than 40." Geologists appear to have reluctantly the earth's surface.
brought themselves to believe that perhaps, after all, 100 millions The Age of the Earth.-Intimately connected with the relations of years might suffice for the evolution of geological history. of our globe to the sun and the other members of the solar system But when the time was cut down to 15 or 20 millions they is the question of the planet's antiquity-a subject of great protested that such a restricted period was insufficient for that geological importance, regarding which much discussion has evolution, and though they did not offer any effective criticism taken place since the middle of the 19th century. Though an of the arguments of the physicists they felt convinced that there account of this discussion necessarily involves allusion to depart. must be some flaw in the premises on which these arguments ments of geology which are more appropriately referred to in were based. later parts of this article, it may perhaps be most conveniently By degrees, however, there have arisen among the physicists included here.
themselves grave doubts as to the validity of the physical Geologists were for many years in the habit of believing that evidence on which the limitation of the earth's age has been no limit could be assigned to the antiquity of the planet, and that founded, and at the same time greater appreciation has been they were at liberty to make unlimited drafts on the ages of the shown of the signification and stength of the geological proofs past. In 1862 and subsequent years, however, Lord Kelvin of the high antiquity of our planet. In an address from the (then Sir William Thomson) pointed out that these demands were chair of the Mathematical Section of the British Association in opposed to known physical facts, and that the amount of time 1886, Professor (afterwards Sir) George Darwin reviewed the required for geological history was not only limited, but must controversy, and pronounced the following deliberate judgment have been comprised within a comparatively narrow compass. in regard to it: “ In considering these three arguments I have His argument rested on three kinds of evidence: (1) the internal adduced some reasons against the validity of the first (tidal heat and rate of cooling of the earth; (2) the tidal retardation friction), and have endeavoured to show that there are elements of the earth's rotation; and (3)the origin and age of the sun's of uncertainty surrounding, the second (secular cooling of the heat.
earth); nevertheless, they andoubtedly constitute a contribution 1. Applying Fourier's theory of thermal conductivity, Lord of the first importance to physical geology. Whilst, then, we Kelvin contended that in the known rate of increase of tempera- may protest against the precision with which Professor Tait ture downward and beneath the surface, and the rate of loss seeks to deduce results from them, we are fully justified in of heat from the earth, we have a limit to the antiquity of the following Sir William Thomson, who says that 'the existing planet. He showed, from the data available at the time, that state of things on the earth, life on the earth-all geological ihe superficial consolidation of the globe could not have occurred history showing continuity of life-must be limited within some less than 20 million years ago, or the underground heat would such period of past time as 100 million years?” Lord Kelvin have been greater than it is; nor more than 400 million years has never dealt with the geological and palaeontological objections ago, otherwise the underground temperature would have shown against the limitation of geological time to a few millions of years. no sensible increase downwards. He admitted that very wide But Professor Darwin, in the address just cited, uttered the limits were necessary. In subsequently discussing the subject, memorable warning: “At present our knowledge of a definite he inclined rather towards the lower than the higher antiquity, limit to geological time has so little precision that we should do but concluded that the limit, from a consideration of all the wrong summarily to reject theories which appear to demand evidence, must be placed within some such period of past time longer periods of time than those which now appear allowable.” as 100 millions of years.
In his presidential address to the British Association at Cape 2. The argument from tidal retardation proceeds on the Town in 1905 he returned to the subject, remarking that the admitted fact that, owing to the friction of the tide-wave, the argument derived from the increase of underground temperature rotation of the earth is retarded, and is, therefore, much slower seems to be entirely destroyed” by the discovery of the now than it must have been at one time. Lord Kelvin affirmed properties of radium. He thinks that "it does not seem ex• that had the globe become solid some 10,000 million years ago, iravagant to suppose that 500 to 1ooo million years may have or indeed any high antiquity beyond 100 million years, the elapsed since the birth of the moon.” He has always believed centrifugal force due to the more rapid rotation must have given that the geologists were more nearly correct than the physicists, the planet a very much greater polar flattening than it actually not withstanding the fact that appearances were so strongly possesses. He admitted, however, that, though 100 million against them," and he concludes thus: “It appears, then, that years ago that force must have been about 3% greater than now, the physical argument is not susceptible of a greater degree of
certainty than that of the geologists, and the scale of geological | geological activity, and even where they can be procured they time remains in great measure unknown" (see also Tide, chap. do not yet rest on a sufficiently wide collection of accurate and viï.).
co-ordinated observations. But in some branches of dynamical In an address to the mathematical section of the American geology, material exists for, at least, a preliminary computation Association for the Advancement of Science in 1889, the vice of the rate of change. This is more especially the case in respect president of the section, R. S. Woodward, thus expressed himself of the wide domain of denudation. The observational records with regard to the physical arguments brought forward by Lord of the action of the sea, of springs, rivers and glaciers are becomKelvin and Professor Tait in limitation of geological time: ing gradually fuller and more trustworthy. A method of making " Having been at some pains to look into this matter, I feel use of these records for estimating the rate of denudation of bound to state that, although the hypothesis appears to be the the land has been devised. Taking the Mississippi as a general best which can be formulated at present, the odds are against type of river action, it has been shown that the amount of its correctness. Its weak links are the unverified assumptions of material conveyed by this stream into the sea in one year is an initial uniform temperature and a constant diffusivity. Very equivalent to the lowering of the general surface of the drainage likely these are approximations, but of what order we cannot basin of the river by ro'oo of a foot. This would amount to one decide. Furthermore, if we accept the hypothesis, the odds foot in 6000 years and 1000 ft. in 6 million years. So that at appear to be against the present attainment of trustworthy the present rate of waste in the Mississippi basin a whole connumerical results, since the data for calculation, obtained tinent might be worn away in a few millions of years. mostly from observations on continental areas, are far too It is evident that as deposition and denudation are simul. meagre to give satisfactory average values for the entire mass taneous processes, the ascertainment of the rate at which solid of the earth."
material is removed from the surface of the land supplies some Still more emphatic is the protest made from the physical necessary information for estimating the rate at which new side by Professor John Perry. He has attacked each of the sedimentary formations are being accumulated on the floor of three lines of argument of Lord Kelvin, and has impugned the the sea, and for a computation of the length of time that would validity of the conclusions drawn from them. The argument be required at the present rate of change for the deposition of all from tidak retardation he dismisses as fallacious, following in the stratified rocks that enter into the composition of the crust this contention the previous criticism of the Rev. Maxwell Close of our globe. If the thickness of these rocks be assumed to be and Sir George Darwin. In dealing with the argument based on 100,000 ft., and if we could suppose them to have been laid down the secular cooling of the earth, he holds it to be perfectly over as wide an area as that of the drainage basins from the allowable to assume a much higher conductivity for the interior waste of which they were derived, then at the present rate of of the globe, and that such a reasonable assumption would enable denudation their accumulation would require some 600 millions us greatly to increase our estimate of the earth's antiquity. of years. But, as Dr A. R. Wallace has justly pointed out, the As for the third argument, from the age of the sun's heat, he tract of sea-floor over which the material derived from the waste points out that the sun may have been repeatedly fed by a of the terrestrial surface is laid down is at present much less than supply of meteorites from outside, while the earth may have been that from which this material is worn away. We have no means, protected from radiation, and been able to retain much of its however, of determining what may have been the ratio between heat by being enveloped in a dense atmosphere. Remarking the two areas in past time. Certainly ancient marine sedimentary that “almost anything is possible as to the present internal rocks cover at the present day a much more extensive area than state of the earth," he concludes thus: “To sum up, we can that in which they are now being elaborated. If we take the find no published record of any lower maximum age of life on ratio postulated by Dr Wallace-1 to 19-the 100,000 ft. of the earth, as calculated by physicists, than 400 millions of years. sedimentary strata would require 31 millions of years for their From the three physical arguments Lord Kelvin's higher limits accumulation. It is quite possible, however, that this ratio may be are 1000, 400 and 500 million years. I have shown that we have much too high. There are reasons for believing that the proporreasons for believing that the age, from all these, may be very tion of coast-line to land area has been diminishing during geoconsiderably underestimated. It is to be observed that if we logical time; in other words, that in early times the land was exclude everything but the arguments from mere physics, the more insular and is now more continental. So that the 31 probable age of life on the earth is much less than any of the above millions of years may be much less than the period that would be estimates; but if the palaeontologists have good reasons for required, even on the supposition of continuous uninterrupted demanding much greater times, I see nothing from the physicists' denudation and sedimentation, during the whole of the time point of view which denies them four times the greatest of these represented by the stratified formations. estimates."
But no one who has made himself familiar with the actual A fresh line of argument against Lord Kelvin's limitation of composition of these formations and the detailed structure of the the antiquity of our globe has recently been started by the terrestrial crust can fail to recognize how vague, imperfect and remarkable discoveries in radio-activity. From the ascertained misleading are the data on which such computations are founded. properties of radium it appears to be possible that our estimates It requires no prolonged acquaintance with the earth's crust to of solar heat, as derived from the theory of gravitation, may impress upon the mind that one all-important element is omitted, have to be augmented ten or twenty times; that stores of and indeed can hardly be allowed for from want of sufficiently radium and similar bodies within the earth may have in- precise data, but the neglect of which must needs seriously definitely deferred the establishment of the present temperature impair the value of all numerical calculations made without it. gradient from the surface inward; that consequently the earth The assumption that the stratified formations can be treated as may have remained for long ages at a temperature not greatly if they consisted of a continuous unbroken sequence of sediments, different from that which it now possesses, and hence that the indicating a vast and uninterrupted process of waste and depositimes during which our globe has supported animal and vegetable tion, is one that is belied on every hand by the actual structure life may be very much longer than that allowed in the estimates of these formations. It can only give us a minimum of the time previously made by physicists from other data (see RADIO- required; for, instead of an unbroken series, the sedimentary ACTIVITY).
formations are full of "unconformabilities.”-gaps in the The arguments from the geological side against the physical sequence of the chronological records—as if whole chapters contention that would limit the age of our globe to some 10 and groups of chapters had been torn out of a historical work. or 20 millions of years are mainly based on the observed rates of It can often be shown that these breaks of continuity must have geological and biological changes at the present time upon land been of vast duration, and actually exceeded in chronological and sea, and on the nature, physical history and organic contents importance thick groups of strata lying below and above them of the stratified crust of the earth. Unfortunately, actual (see Part VI.). Moreover, even among the uninterrupted strata, numerical data are not obtainable in many departments of I where no such unconformabilities exist, but where the sediments
follow each other in apparently uninterrupted sequence, and thickness of the crust offering greater resistance to the stresses, might be thought to have been deposited continuously at the and giving rise to vaster plications, faults, thrust-planes and saine general rate, and without the intervention of any pause, it metamorphism, as this growing resistance had to be overcome. can be demonstrated that sometimes an inch or two of sediment The assertion that volcanic action must have been more much, on certain horizons, represent the deposit of an enormously violent and more persistent in ancient times than it is now has longer period than a hundred or a thousand times the same assuredly no geological evidence in its support. It is quite true amount of sediment on other horizons. A prolonged study of that there are vastly more remains of former volcanoes scattered these questions leads to a profound conviction that in many over the surface of the globe than there are active craters now, parts of the geological record the time represented by sedi- and that traces of copious eruptions of volcanic material can be mentary deposits may be vastly less than the time which is not followed back into some of the oldest parts of the geological so represented.
record. But we have no proof that ever at any one time in It has often been objected that the present rate of geological geological history there have been more or larger or more vigorous change ought not to be taken as a measure of the rate in past volcanoes than those of recent periods. It may be said that the time, because the total sum of terrestrial energy has been steadily absence of such proof ought not to invalidate the assertion until diminishing, and geological processes must consequently have a far wider area of the earth's surface has been geologically been more vigorous in former ages than they are now. Geo- studied. But most assuredly, as far as geological investigation logists do not pretend to assert that there has been no variation has yet gone, there is an overwhelming body of evidence to show or diminution in the activities of the various processes which that from the earliest epochs in geological history, as registered they have to study. What they do insist on is that the in the stratified rocks, volcanic action has manifested itself very present rate of change is the only one which we can watch and much as it does now, but on a less rather than on a greater scale. measure, and which will thus supply a statistical basis for any Nowhere can this subject be more exhaustively studied than in computations on the subject. But it has been dogmatically the British Isles, where a remarkably complete series of volcanic affirmed that because terrestrial energy has been diminishing eruptions has been chronicled ranging from the earliest Palaeozoic therefore all kinds of geological work must have been more down to older Tertiary time. The result of a prolonged study vigorously and more rapidly carried on in former times than of British volcanic geology has demonstrated that, even to now; that there were far more abundant and more stupendous minute points of detail, there has been a singular uniformity in volcanoes, more frequent and more destructive earthquakes, the phenomena from beginning to end. The oldest lavas and more gigantic upheavals and subsidences, more powerful oceanic ashes differ in no essential respect from the youngest. Nor have waves and tides, more violent atmospheric disturbances with they been erupted more copiously or more frequently. Many heavier rainfall and more active denudation.
successive volcanic periods have followed each other after proIt is easy to make these assertions, and they look plausible; longed intervals of repose, each displaying the same general but, after all, they rest on nothing stronger than assumption. sequence of phenomena and similar evidence of gradual diminuThey can be tested by an appeal to the crust of the earth, in lion and extinction. The youngest, instead of being the feeblest, which the geological history of our planet has been so fully re were the most extensive outbursts in the whole of this prolonged corded. Had such portentous manifestations of geological series. activity ever been the normal condition of things since the If now we turn for evidence of the alleged greater activity beginning of that history, there ought to be a record of them in of all the epigene or superficial forces, and especially for proofs the rocks. But no evidence for them has been found there, of more rapid denudation and deposition on the earth's surface, though it has been diligently sought for in all quarters of the we scarch for it in vain among the stratified formations of the globe. We may confidently assert that while geological changes terrestrial crust. Had the oldest of these rocks been accumulated may quite possibly have taken place on a gigantic scale in the in a time of great atmospheric perturbation, of torrential rains, carliest ages of the earth's existence, of which no geological record colossal tides and violent storms, we might surely expect to find remains, there is no proof that they have ever done so since the among the sediments some proof of such disturbed meteorological time when the very oldest of the stratified formations were and geographical conditions. We should look, on the one hand, deposited. There is no need to maintain that they have always for tumultuous accumulations of coarse unworn detritus, rapidly been conducted precisely on the same scale as now, or to deny swept by rains, floods and waves from land to sea, and on the that they may have gradually become less vigorous as the general other hand, for an absence of any evidence of the tranquil and sum of terrestrial energy has diminished. But we may unhesitat- continuous deposit of such fine laminated silt as could only ingly affirm that no actual evidence of any such progressive settle in quiet water. But an appeal to the geological record diminution of activity has been adduced from the geological is made in vain for any such proofs. The oldest sediments, like record in the crust of the earth: that, on the contrary, no appear- the youngest, reveal the operation only of such agents and such ances have been detected there which necessarily demand the rates of activity as are still to be witnessed in the accumulation assumption of those more powerful operations postulated by of the same kind of deposits. If, for instance, we search the physicists, or which are not satisfactorily explicable by reference most ancient thick sedimentary formation in Britain-the to the existing scale of nature's processes.
Torridon Sandstone of north-west Scotland, which is older than That this conclusion is warranted even with regard to the innate the oldest fossiliferous deposits—we meet with nothing which energy of the globe itself will be seen if we institute a comparison might not be found in any Palacozoic, Mesozoic or Cainozoic between the more ancient and the more recent manifestations of group of similar sediments. We see an accumulation, at least that energy. Take, for example, the proofs of gigantic plication, 8000 or 10,000 st. thick, of consolidated sand, gravel and mud, fracture and displacement within the terrestrial crust. These, such as may be gathering now on the floor of any large mountainas they have affected the most ancient rocks of Europe, bave girdled lake. The conglomerates of this ancient series are not been worked out in great detail in the north-west of Scotland. pell-mell heaps of angular detritus, violently swept away from But they are not essentially different from or on a grcater scale the land and huddled promiscuously on the sea-ficor. They are, than those which have been proved to have affected the Alps, in general, built up of pebbles that have been worn smooth, and to have involved strata of so recent a date as the older rounded and polished by prolonged attrition in running water, Tertiary formations. On the contrary, it may be doubted and they follow each other on successive platforms with interwhether any denuded core of an ancient mountain-chain reveals vening layers of finer sediment. The sandstones are composed traces of such stupendous disturbances of the crust as those of well water-worn sand, some of which has been laid down só which have given rise to the younger mountain-chains of the tranquilly that its component grains have been separated out in globe. It may, indeed, quite well have been the rule that instead layers according to their specific gravity, in such manner that of diminishing in intensity of effect, the consequences of terres- they now present dark laminae in which particles of magnetic trial contraction have increased in magnitude, the augmenting I,iron, zircon and other heavy minerals have been sifted out