CHAPTER I. The Time Problem and Its History

THE TIME PROBLEM AND ITS HISTORY

Primitive races and their creation myths—The chronologies of Eastern sages—The demands of geology opposed by theological prejudice—Bishop Ussher’s date of the creation of the world—The Deluge and the Doctrine of Catastrophism—Ludicrous conceptions of fossils—The rise and eventual success of Uniformitarianism—Leonardo da Vinci—Steno—Generelli—Desmarest—Hutton—Scrope—Lyell—Ussher’s chronology rejected—Geological time no longer limited—The hour-glass of denudation and deposition—Darwin’s excessive drafts on time—The conservation of energy—Geology limited once again—Thomson on the age of the earth and sun, 1862—The great controversy—Huxley, 1869—Perry and the correspondence in Nature, 1895—Sir G. Darwin and the birth of the moon—Clarence King’s estimate of the age of the earth—Time implied by the stratified rocks—Joly on the salinity and age of the oceans—The significance of radioactivity—Emission of energy by radium and its widespread distribution—Radioactivity provides a new measure of time—Controversy again—Methods of dealing with the problem—The inadequate testimony of biology.

The most primitive races of mankind, strenuously engaged in the daily struggle for existence, appear to have given little thought to the antiquity of the world on which they lived. Even at the present day there exist barbaric tribes to whom it has never occurred that the earth may have had a beginning. The conception of creation, of the production of order from chaos, indicates a marked intellectual advance, but into the myths and legends of which this idea was so often the inspiration, the element of time did not usually enter.

The East African natives have traditions of the upheaval of mountains and of the freeing of the earth from bondage, legends which may have had a foundation in the phenomena exhibited by volcanoes and glaciers. If the question “When?” is asked, the natural reply is merely a look of astonishment, and persistent enquiry elicits nothing more definite than a vague “Long ago.” Anything else, of course, could not be expected from a people whose conceptions of the development of the universe are limited to a recognition of the alternation of seasons and the insistent mysteries of life and death.

The priests and philosophers who flourished during the ancient civilizations of the East, speculated in greater detail and devoted some attention to the elaboration of a chronology of earth history. The Chaldeans had a well-deserved reputation for astronomy, and their known observations go back for more than 6000 years. Cicero relates that their venerable priesthood had records of stellar observations stretching back for 470,000 years; a fanciful period which tallies with the date assigned by the Chaldeans to the origin of Man. According to the same remarkable system, the earth had already existed for 215 myriads of years. The Persian sages, led by Zoroaster believed that the total duration of the world’s existence was limited to 12,000 years. The Hebrew chronological tables are familiar to everyone, and the restricted interpretation which was placed upon them during the Middle Ages, when they affected European beliefs so powerfully, will be presently referred to. Opposed to these ideas of a definite beginning were the more abstract conceptions of Indian philosophers, who regarded Time and the Earth as eternal.

To determine the age of the earth, or to express the actual problem with more accuracy, to measure the duration of geological time, became a definite scientific aspiration only during the last century. The ultimate purpose of Geology is essentially to elucidate the history of the earth, a record of which is imperfectly written in the stratified and igneous rocks to which access is possible. As the characters and sequence of the various formations gradually became better understood, it was found that the story they disclosed was one of successive changes of life and scene of the most impressive kind. The immensity of time which seemed to be indicated was at first a fruitful source of confusion and prejudice, for it brought geology into disrepute at an early period, owing to the widely prevalent idea that the writings of Moses fixed the antiquity of the universe beyond dispute. It is indicated by a marginal reference in most English Bibles that the creation of the world took place in the year 4004 B.C. This famous estimate, which probably represents the most limited period ever assigned to the past duration of our planet, was put forward in 1650 by Bishop Ussher. Some such date as this had been generally believed in during the Middle Ages as marking the epoch of transition from chaos to an ordered world. The corresponding Byzantine date was 5509 B.C. The whole of geological history had therefore to be squeezed into about six or seven thousand years, and this limitation naturally demanded some extraordinary hypotheses to uphold it. As Prof. Sollas says, “In the days when Geology was young, it found a careful foster-mother in Theology, who watched over its early growth with anxious solicitude, and stored its receptive mind with the most beautiful stories which the young science never tired of transforming into curious fancies of its own, which it usually styled ‘Theories of the Earth.’”

At the time of the revival of learning at the close of the 15th century, men’s ideas of the earth’s past history were largely dominated by the exaggerated effects ascribed to the Noachian Deluge. This devastating catastrophe was the type of a succession of destructive cataclysms which was believed to have preceded it. Supported by Jewish cosmogony, and in harmony with the scriptures, this view prevailed as the Doctrine of Catastrophism until well into the 19th century. It was believed by some writers that all sediments were deposited during the Flood; others, impressed by the succession of different types of deposits found far from the present sea-level, and often containing fossils, considered that one flood was insufficient. As far back as the 9th century Rhabanus Maurus, in explaining this phenomenon, called to his aid three great universal floods, the two later ones being contemporaneous with Jacob and Moses respectively, but these floods apparently were not as convincing as that of the time of Noah, for they appear no more in geological literature. Marine sedimentation of antediluvial times was believed by some to have been extraordinarily active, and this was added to the effects of the Deluge to relieve the latter of the enormous amount of work for which it was held responsible.

Fossils were regarded with suspicion and jealousy, and most of the early naturalists resolutely set themselves against the obvious deduction to be drawn from them. About 300 B.C., Theophrastus had failed to see in them the evidence of past life, and according to this philosopher a “plastic virtue latent in the earth” was supposed in some mysterious way to have given them birth. The process was occasionally explained as being akin to crystallisation, but other authors, gifted with more fascinating powers of imagination though with less philosophic insight, called to their aid the occult powers of “lapidifying juice” and the obscure consequences of “tumultuous movements of terrestrial exhalations.” There were even those who thought that fossils were the work of the devil, subtly designed to draw believers away from the faith.

Nevertheless, there were, from time to time, more rational thinkers to whom such ideas were both repugnant and untenable. Endowed with a keener perception than their fellows, and with a more critical temperament, they felt compelled to regard fossils as organic remains. Leonardo da Vinci (1452-1519) and Steno (1631-1686) were amongst the most illustrious of these early observers. The work of Steno, published in 1669, shows how seriously he was handicapped by the orthodox belief that only a few thousand years had elapsed since the beginning of the world. Any ideas which demanded longer periods were contrary to revealed truth and were therefore bitterly opposed. Steno demonstrated not only the true nature of fossils, but also the orderly succession of stratified deposits. He traced the structure of the rocks to their subsequent movements, and their surface features to the destructive agencies of denudation. These remarkable investigations distinguished Steno as an observer far in advance of his age. The revelation of a tiny chapter of the earth’s past history which was thus afforded him must surely have suggested the necessity of calling more liberally upon time. However, whether or not he recognised how hopelessly inadequate was the period at his disposal, he remained the victim of theological prejudice, and cautiously avoided speculations as to the antiquity of the earth.

Gradually, as the result of careful and patient work like that of Steno, the foundation stones of modern geology were laid in the face of a bitter antagonism. Generelli in Italy in 1749, and Desmarest in France in 1777 established the importance of the slow but ever active processes at work in the evolution of the earth’s surface features. It was found no longer legitimate to evoke forces more intense, upheavals more violent, or catastrophes more devastating than those of present experience.

Closely following these courageous authors, and independently of their influence, came in 1785 the Theory of the Earth of Hutton. In this epoch-making work the principle was defined which made dynamical geology possible and which has proved to be of the greatest assistance in wresting from the rocks their history. It was asserted that the activity of the agencies which had moulded the earth’s surface in the past and brought it to its present condition still remained undiminished in intensity. In existing causes lay the key with which to unlock the secrets of the past.

The doctrine of Uniformitarianism, as it came to be called, naturally implied that the earth’s age should be restricted no longer by dogmatic obstinacy. To Hutton time scarcely presented a difficulty. He found it at his unlimited disposal both in past and future, and he concluded his enquiries with the assurance that he found “no vestige of a beginning—no prospect of an end.” He did not, however, infer that the world had neither beginning nor end, a view for which some of his critics held him responsible. On the contrary, he carefully pointed out that in tracing back the course of events we are at last limited in our retrospect, and that beyond the dim horizon of those early times stretches an unknown past. Concerning this past, Hutton preserves an open mind. It is not time which fails but data; and as he says “to reason without data is nothing but delusion.”

Hutton’s convictions were regarded with righteous horror by the official leaders of the day, most of whom combined the study of theology with that of their favourite science, and demanded in the latter a harmonious agreement with the scriptures. From their point of view Catastrophism had the advantage, and they were firmly persuaded of its truth. Fifty years had yet to elapse before the superiority of many of Hutton’s opinions came to be generally recognised, and even then the fallacies underlying the earlier doctrines were but grudgingly admitted.

The opening years of the 19th century were signalised by the work of Lamarck and Cuvier in palæontology. Lamarck recognised that fossil shells were most commonly of marine types and that in the gently accumulating sediments of the sea-bottom they were buried and preserved. In the succession of faunas which he studied, and in his belief that by some law of development they were all related, he found a cogent argument in favour of the great antiquity of the earth. Cuvier, however, was led by his researches to extend the conception of great world convulsions. Time after time the earth’s inhabitants had been destroyed and entombed, only to be replaced by the creation of fresh types after the force of the cataclysm had subsided. The Deluge was the last of these upheavals, and a scientific proof of its widespread effects was drawn from the superficial deposits found in so many parts of the globe.

Until the barren ideas of Catastrophism had been abandoned, the past provided little more than an exercise for the imagination. But the immature days of geology were passing away. The solid work of William Smith prepared the way for an accurate historical development of the science. Scrope drew attention to the importance of studying geological processes actually in operation, and arrived at conclusions essentially the same as those of Hutton. Finally, during the years 1830-1833, the first edition of Lyell’s Principles was issued, a work which set flowing the full tide of Uniformitarianism. Ten years previously Lyell had felt compelled to renounce the unsound doctrines of his teachers, and with the publication of these carefully reasoned volumes he became the champion of the rival position. He denied the former existence of catastrophes of an order of magnitude different from those of the present. In their place he demanded only time. A short but convulsive past was to be exchanged for a longer one, uniform and comparatively tranquil throughout.

Lyell’s views did not find many ready adherents among the older geologists. Buckland at Oxford and Sedgwick at Cambridge had long taught a system of geology which they believed to be in accordance with the first few chapters of Genesis. Eventually, however, they found that under more careful examination the evidence of a universal flood faded away, and with them and their generation the tendency to demand a literal agreement between geological theory and the writings of Moses gradually disappeared. A new fraternity of geologists arose whose ideas were limited by fewer prejudices, and who found the time barriers raised against them no longer.

Geology was now able to stand firm on its own evidence, and to insist without fear of contradiction that long periods must have elapsed during the slow accumulation of strata. The burden of reconciliation now fell upon the theologians. Happily, the days of malice and persecution had gone by, and in the light of a more broad-minded criticism it was found that the Church had nothing to lose by the rejection of Bishop Ussher’s chronology. The first three words of the Bible, “In the beginning,” were interpreted afresh, and from the indefinite lapse of time which they seemed to imply the geologist was allowed to draw at will.

From this period until 1862, when Thomson (Lord Kelvin) attacked the problem in an entirely new way, there was no necessity to apologise for the most extensive drafts on the bank of time, and no further restraint was felt in estimating the antiquity of the earth. But the data were still inadequate, and many mistakes were made. Most of the estimates put forward were afterwards regarded as wildly extravagant, and naturally, they have no intrinsic value to-day. The stratified rocks, representing the aggregate accumulation of material denuded from the lands, afforded a valuable time-index. It was recognised that the processes of denudation and deposition, like a gigantic hour-glass, had been recording time since land and sea were first defined. But the record was perplexingly difficult to read, and the time units, based on estimates of present rates of erosion and sedimentation, were little more than guesses, uncertain and even misleading.

The only interpretation worthy of serious consideration was that of John Phillips, who concluded in 1860 that the time required for the deposition of the complete succession of strata lay between 38 and 96 million years. As an example of one of the more extravagant demands, mention may be made of Darwin’s estimate in 1859 of the time elapsed since the latter part of the Cretaceous period. From the rate of chalk erosion in Kent, he deduced that the excavation of the Wealden valleys had required a period of 300 million years. Referring to this estimate, Jukes gave his opinion that while it might be a hundred times too great, it was equally probable that it was a hundred times too small. Evidently 30,000 million years, in the absence of more reliable data, was not considered an absurdly excessive period for the erosion of the Weald. It should be noticed, however, that after the publication of Thomson’s views, Darwin withdrew this estimate from the Origin of Species.

We must now return to consider the problem as it appeared to the leading physicists of half a century ago. In 1842 Mayer, and during the following year Joule, had placed the conception of the conservation of energy on a firm experimental basis, and started the principles of thermo-dynamics on their brilliant career through the sciences. Twenty years later, Thomson invaded the domain of Geology, hoping to reform its speculations and bring them into accordance with the doctrines of the conservation and degradation of energy, doctrines which were apparently set in defiance by the orthodox tenets of the science.

As early as 1852 he had shown that under the laws to which matter and energy are subject, the earth within a finite period of past time must have been unfit as a habitation for life. This general conclusion he afterwards worked out in detail, determined to protest against what he considered the immoderate application of the principle of Uniformity. So profoundly did he influence geological opinion, that for nearly fifty years the question of the earth’s antiquity centred around his name. A short account of his contributions to the physical aspect of the subject, and of the famous controversy which they aroused, cannot therefore be dispensed with.

In 1855 Thomson indicated the way in which observations of underground temperatures might be applied to the determination of absolute dates in geological history. Already the origin and maintenance of the sun’s heat had been discussed by Mayer and Helmholtz, and during the years 1860 and 1861 Thomson dealt with their respective suggestions. During the following year, 1862, came the epoch-making papers On the Age of the Sun’s Heat, and On the Secular Cooling of the Earth.

The age of the sun’s heat evidently restricts geological time in a very decided way, and Thomson drew attention to the necessity of the fact that unless the sun were a perpetual miracle, it could not have continued to radiate heat-energy for an unlimited period. His guarded conclusion was that the sun most probably has not illuminated the earth for 100 million years, and almost certainly not for 500 million years.

Regarding the earth as a globe which had gradually cooled down, he tried to show that the principles of thermo-dynamics had been disregarded by geologists. From the earth’s present store of heat, as revealed by underground temperature gradients, he calculated that the consolidation of the crust took place about 100 million years ago. Owing to the uncertainty of much of the data on which this estimate was based, he allowed wide limits. Had a solid crust formed permanently less than 20 million years ago, underground heat should be greater than is actually observed. Had it formed at a date more remote than 400 million years ago, then the temperature gradient near the surface should have been notably less than it is.

In 1865 appeared a remarkable little paper in which attention was drawn to the earth’s thermal history, and particularly to its more energetic youth, the object again being to refute the doctrine of Uniformity. During the next year Thomson delivered the Rede Lecture on The Dissipation of Energy, and showed the importance of the tides in terrestrial dynamics. All over the ocean the effect of the friction set up by moving water is either to accelerate or to retard the earth’s rotation. The final result was, he asserted, to retard the earth as though a gigantic friction brake were being slowly applied. The ultimate tendency was towards a state when relative motion between earth and moon should be no more.

In 1868, in an address on Geological Time, Thomson gathered together his three limiting criteria, and further developed the application of tidal friction to the question. In his discussion the earth’s figure was supposed to be a survival from the period of consolidation; a permanent record of terrestrial conditions at that time. This being so, the rate of rotation could be computed under which that form would be in fluid equilibrium.

The three maximum time limits which Thomson drew from his arguments were respectively 500, 400, and 1000 million years, with a lower limit of 20 million years. The final conclusion was “that the existing state of things on the earth ... must be limited within some such period of past time as one hundred million years.”

The reply to this series of attacks on what was supposed to be orthodox geological opinion, came from Huxley in 1869. He did not deny the consequences of the dissipation of energy in the cases to which attention had been directed, but decided in favour of the geological evidence, which indicates, as time has more and more substantiated, that “they have made no practical difference to the earth during the period of which a record is preserved in the stratified rocks.”

Thomson returned to the attack a month or two later, protesting once more against this attitude. At the close of his address, he said: “A large proportion of British popular geologists of the present day have been longer contented than other scientific men to look upon the sun as Fontenelle’s roses looked upon their gardener. ‘Our gardener,’ say they, ‘must be a very old man; within the memory of roses he is the same as he has always been; it is impossible he can ever die, or be other than he is.’”

There were no further contributions to the problem until 1876, when Thomson revised the former conclusion which he had based on the distribution of underground heat, and narrowed his limits to 50 and 90 million years. In later years he reduced both these limits still further, though he was always more prudent than Tait, who with little justification cut down the time allowance in a most alarming way. Tait wrote in 1875, “Ten million years is about the utmost that can be allowed from the physical point of view for all the changes that have taken place on the earth’s surface since vegetable life of the lowest known form was capable of existing there.” Assertions such as this were among the most embarrassing circumstances that geologists had to face. The late Sir George Darwin was more kindly disposed, although his contributions to the question were considered to support the physical arguments. Believing that the moon had been derived from the earth by the separation of an enormous tidal wave, he calculated that since this stupendous event at least 56 million years must have passed.

The physical evidence appeared at first to be irrefutable, and the estimates based upon it equally certain. Yet earth history could not comfortably be squeezed into less than 100 million years. The stratified rocks were there in undoubted succession; mile after mile of thickness with no indication of more rapid accumulation than that of modern deposits. In spite of this, however, a prejudice in favour of short estimates was gradually aroused, and some attempt was made to hurry up geological activities in the past in renunciation of the principles of Uniformitarianism. But many geologists refused to give way and vigorously attacked the physical stronghold, searching out and exposing all the assumptions, and noting with satisfaction the uncertainty of much of the data and its doubtful applicability. A few physicists themselves denied that the limited estimates were securely founded.

In 1895 the controversy was re-opened by the publication in Nature of a correspondence initiated by Prof. Perry. While Perry had previously accepted Kelvin’s conclusions, he now challenged the validity of all three. The strongest criticism was directed against Kelvin’s assumption of a homogeneous earth. It was shown that if the rocks of the interior, under high temperatures and pressures, were to conduct heat ten times as well as those near the surface, then Kelvin’s figures would need to be increased fifty-six times. To settle this point adequate data were not then forthcoming, but Perry stated his belief that, if geologists had sound reasons for demanding long periods of time, he saw nothing which denied them four times the greatest (1000 million years) of Kelvin’s estimates.

Kelvin wrote that he would rather know the date of the consistentior status than that of the Norman Conquest, so interesting did he find the subject. After remarking that 100 million years were ample to satisfy Geikie, he said, “I should be exceedingly frightened to meet him now with only 20 million in my mouth.” However, Kelvin carefully examined the data referring to the conductivities of rocks and decided that they were not favourable to Perry’s suggestion. Thus fortified, he lent his support to the independent estimate of 24 million years which had been arrived at in 1893 by Clarence King.

Kelvin’s last pronouncement of his views was in 1897, when he delivered an address on The Age of the Earth as an Abode Fitted for Life. He then narrowed down his earlier estimates to 20 and 40 million years. To most geologists these limits were seriously in conflict with the requirements of their science. A feeble attempt was made to force an agreement, but it was generally held that the sedimentary succession implied a period three times as long. An independent mode of reckoning geological time was developed by Prof. Joly in 1899. He made a careful study of the amount of sodium annually removed from the land by solvent denudation, and calculated the time during which the total quantity of sodium now held by the oceans could have accumulated. The period closely approached 100 million years, and thus further support was added to the testimony of the rocks, for with the birth of the ocean their deposition must have commenced.

Among many of the more optimistic geologists there was a prevalent expectation that some flaw would ultimately be found in the physical arguments. Their anticipation was realised ten years ago in the most surprising and unexpected way. In 1903 came the discovery by Curie and Laborde that radium maintains a temperature above that of its environment owing to the spontaneous evolution of heat involved in its disintegration. Other investigators found that radium and its radioactive associates were widely distributed in the earth’s surface materials. In all waters and gases of natural origin, and all rocks and soils, traces of these elements have been detected. We owe a great deal to the careful researches of Prof. Strutt, which have shown conclusively that the earth can no longer be regarded merely as a cooling body. A newly recognised source of heat must now be taken into account, and indeed, so relatively abundant is the supply, that our present difficulty is to understand why the earth is not hotter than we actually find it.

With these discoveries the long controversy was finally buried, and Kelvin’s treatment of the problem was proved to have been fallacious. Kelvin lived just long enough to know something of the first attempts to utilise the principles of radioactivity in solving the vexed problem of time. The discovery of radium did not only destroy the validity of the older thermal arguments; but also, it led directly to the elaboration of a new and more refined method. As we shall see in the sequel, every radioactive mineral can be regarded as a chronometer registering its own age with exquisite accuracy. The record is not always completely preserved, but a few attempts have been made to read it, and in the more favourable cases, periods of enormous duration have been revealed. Indeed, if our interpretation is correct, some of the oldest Archean rocks must date back 1600 million years.

Not many years ago geologists were dissatisfied with the shortness of their time allowance; to-day they are confronted with an embarrassing superabundance. Certainly, it has been dogmatically denied that radioactive minerals can have the great ages which have been attributed to them, or that they can account for more than a small proportion of the earth’s loss of heat. But such denials do not help to remove the difficulty; they rather tend to aggravate it. In the face of two rival and mutually inconsistent systems of earth chronology, neither of which can be rejected with impunity, it becomes essential to examine most carefully the fundamental assumptions underlying each method, in the hope of detecting the subtle errors which are responsible for so glaring a disagreement. In this way alone can a means of reform be indicated and the road made clear for the construction of a time-scale which will be acceptable to all.

It is obvious that as yet we cannot measure the earth’s absolute age if by that expression is meant the time which has elapsed since our planet first existed. Kelvin’s work most nearly approached this desideratum, but, as we have seen, it was doomed to ultimate failure, though not indeed, to discredit. Sir George Darwin’s calculation of the lapse of time since the birth of the moon stands apart; his results are limited only by a definite minimum, and otherwise are sufficiently elastic to meet any reasonable demand. But this convenience, added to the doubtful validity of the hypothesis on which his estimates were based, detracts from their value. It would be unfair to expect too much of them, for they were only put forward in support of a hypothesis which would have been immediately disproved if ridiculously long or short time periods had been involved.

As we shall see in the following chapter we are still far from understanding the sequence of events which led up to the origin of the earth. All we can hope to do is to fix the dates of critical periods of its history and assign its origin to a point still more remote. The different methods which have been elaborated to deal with the problem are all based on a common principle. The rates of certain changes at the present day are determined as accurately as possible, and in imagination, the respective processes are traced backward in time, until limiting conditions are arrived at. Thus, Kelvin takes us back to a time when the earth was not yet a solid globe; Darwin traces back the moon’s history until he finds it revolving close to the earth; Joly bids us imagine the oceans in their original freshness, free, or nearly so, from salt; Geikie finds an end at last to the long succession of stratified rocks and seeks to estimate the time they represent. Last of all, and most brimful of promise, there lies in the mechanism of radioactivity an elegant method for assigning a date to the period of crystallisation of every igneous rock in which suitable minerals can be found.

The testimony of biology scarcely calls for consideration here, for as Huxley pointed out, biology must take its time-scale from geology. The procession of life forms shows broadly that the time involved must have been very great; and, moreover, where we first meet it in the Cambrian rocks, it is already far on its journey. This is usually taken to signify that pre-Cambrian time must be at least as long as that which has elapsed since. Such a statement can only be regarded as very conservative.

Prof. Poulton considers that the process of evolution must have required much longer periods of time than those estimated by the geologists. Sollas, on the contrary, is satisfied that 26 millions of years would be ample to meet all the demands of biology. It is obvious that a numerical estimate cannot be derived from the succession of organic forms, for the rate of change of species is as yet known only relatively. The influence of changing environment is complex beyond the possibility of exact analysis, and any method of chronology based on the development and ultimate extinction of past types would necessarily give results of very unequal value. The little shell Lingula has retained its individuality with but little deviation from its original form throughout the time represented by the fossiliferous strata, but the same record is eloquent of the gradual unfolding of fauna after fauna, culminating at last in the highly specialised organisms of to-day.