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THE MOST SALIENT EXCERPTS
(if you don’t want to tackle the whole)
The Age of the Earth
THE TIME PROBLEM AND ITS HISTORY
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….
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 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…
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….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….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….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….
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….
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 reconcilation 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…afforded a valuable time-index….But the record was perplexingly difficult to read, and the time units…were little more than guesses, uncertain and even misleading….
Regarding the earth as a globe which had gradually cooled down, [Thomson] 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….
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.”
…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 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….
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….[I]t was generally held that the sedimentary succession implied a period three times as long….
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….
With these discoveries the long controversy was finally buried, and Kelvin’s treatment of the problem was proved to have been fallacious….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 super-abundance….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….[W]e 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….
TIME AND TIDE IN THE SOLAR SYSTEM
Speculative fancies concerning the origin of the world form the subject-matter of many of the earliest writings on record, and throughout the intellectual history of mankind the problem has proved to be one of supreme fascination. It was not, however, until quite recent times that the efforts of imagination gave place to reasoned hypotheses, tempered by a more sober regard for physical probabilities. At first, on having attained the status of a science, geology steadfastly refused to consider seriously the cosmogonic fantasies then current. It was Hutton, who by advocating the direct observation of nature in place of the old scholastic arguments, first delivered geology from the inevitable wranglings that would necessarily have arisen from so premature a discussion of the beginning of things. Cosmogony, in spite of this, continued to receive attention from workers in other sciences, and while to-day we are still unable from geological facts alone to trace back with confidence the details of the earth’s beginning, yet the uncertainty which justified Hutton in entirely disregarding the earth’s genesis no longer exists. Astronomy, physics, and chemistry have all contributed to the elucidation of what may be called the prehistoric period, and have done much to remove our modern ideas from the dangerous quicksands of speculation.
It is becoming more and more evident that many of the fundamental problems of geology can be solved only with reference to the processes involved in the making of the earth and in its subsequent evolution. The dynamic agencies at work to-day are genetically the outcome of the ancestral forces which first moulded our planet, and theoretically, the tectonics and constitution of the earth’s crust should lead back to a more complete understanding of its initial condition. But the mind of man is impatient of delay; shrewd guesses are made and gradually adjusted to known facts, wih the result that many geological doctrines are founded not on observation alone, but also in part on fundamental hypotheses….
REVIEW OF THE EVIDENCE
Of the various methods which have been devised to solve the problem of the earth’s age, only two, the geological and the radioactive, have successfully withstood the force of destructive criticism….From the mists of controversy which for half a century have hung over the subject, the two hour-glass methods alone emerge, and the final issue must be fought out between them. In the one the world itself is the hour-glass, and the accumulating materials are salt, the sedimentary rocks and calcium-carbonate. Three concordant sets of results may be drawn from this triple scheme of measurement, but it must not be supposed that they are altogether independent. Each set of data is intimately related to the others and all stand or fall together. In the other case the accumulating materials are helium and lead, and the hour-glass is constituted by the minerals in which they collect. Provided that the field-evidence is clear and convincing and that the relative geological age of a mineral specimen can be determined, the construction of an exact and precise time-scale is a task which can be dealt with successfully in the laboratory. The problem has advanced from the qualitative to the quantitative stage, and for the first time in historical geology accurate measurement founded on delicate experimental work has become possible….The only assumption which can reasonably be called into question is that of uniformity, and it is involved equally in both calculations….
There can be no doubt that agreement will never be brought about by the more convincing testimony of experimental demonstration. It must be almost entirely a question of interpretation. An attempt has been made to show that in the geological evidence there is nothing impossibly at variance with the dictates of the radioactive minerals. With the acceptance of a reliable time-scale, geology will have gained an invaluable key to further discovery. In every branch of the science its mission will be to unify and correlate, and with its help a fresh light will be thrown on the more fascinating problems of the Earth and its Past.
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