THE great problems of the origin and final destiny of things have always loomed large in the minds of thoughtful men. Was there a beginning and will there be an end of the world in which we dwell, are questions of perennial interest to-day, as they were in the time of Plato. From these age-long speculations of the philosophers, aided to a greater or less degree by the observations and experiments of scientists, there arose two antagonistic theories which have long dominated the course of thought. According to the one, the world is of comparatively recent origin, and it proceeded from the mind of the Creator full grown, like Minerva from the head of Jupiter. The other looked upon the world in its present form as a single link in an endless chain of transformation. Of this series there was “no indication of a beginning and no prospect of an end.” Either hypothesis not only renders futile any attempt to calculate the age of the earth, but indeed, involves an admission of the uselessness of such an investigation. But to both of these theories philosophical objections of the most convincing kind can be offered. If there be any meaning in the phenomena of nature which the human mind can decipher, we can read in the records of the rocks a past history extending back for millions of years. On the other hand, the evidence of astronomy and geology alike reveal to us a world and a universe which are slowly but inevitably running down. The final end and catastrophe of the world may be slow in coming, but come it must. And if an end, then a beginning. And since there are certain phenomena which are susceptible of accurate examination, and which seem to offer a measure for geological time, the problem ceases to be a transcendental one, and becomes the legitimate subject for scientific investigation and speculation. It is not surprising therefore that so profound a problem should have attracted the attention of men of science. It has been most carefully investigated from various aspects, and several of these studies have promise of important results. While it is impossible as yet, to arrive at any definite conclusion as to the number of years which have passed since the earth first took its present form, yet it is of interest to review 'the advance of human knowledge in this direction, and to weigh the probabilities of an eventual answer to the great question. But we shWlld first have a clear conception of what it is that we are really trying to solve. Apparently we have no means at present of calculating the time since first, in obedience to creative laws, the earth began its course. It may be that this is one of the questions which the human mind can never answer. And so we do well to confine ourselves to a discussion of what is called geological time, or that period which has elapsed since the geological forces as we kpow them, weathering, river erosion and the rest, first began to be exerted. This is of special interest as it also represents the time during which conditions have been favorable for the ex- istenee of life. Three lines of investigation merit our attention, even in so short a study of the problem as the present. The first is based upon the results of physical research, the second draws its facts from the observations of the geologists, while the third and most recent of the three depends upon the newly discovered phenomena of radioactivity. n There is but one fund of energy in the universe. The faintest ray of light which reaches us from a star in the Milky Way, and the agreeable warmth that radiates from our own fireside are but separate manifestations of the one great, eternal force. At the present time this energy is concentrated at certain points within the universe which we call stars. This concentration is, however, hut temporary, for the stars are ceaselessly radiating light and heat and thus reducing themselves to the surrounding dead level. Our star, the sun, has no refuge from this inexorable fate—it is not eternal. Sooner or later its fierce heat will all have been dissipated into space, and it will become cold and dead, “And that old common arbitrator, Time, Will one day end it.” And the life of our planet is bound up with that of its parent sun. From this luminary is derived the force by virtue of which rivers run and waves rise and fall and those geological processes are carried on which result in the destruction of old rocks and the formation of new: without it water would cease to flow, the oceans would congeal to continents of ice and life would become extinct. It necessarily follows that the age of the sun is a measure of the maximum life of the earth. Is it, then, possible to measure in years the length of time during which our sun could continue to radiate heat at approximately the same rate as we now experience, neither too hot nor too cold for the existence of life? Lord Kelvin's affirmative answer to this question in 1862 was so incisive and so surprising that the scientific world was at once roused to vigorous argument. It is evident that if we know the temperature of the sun and the rate at which its heat is being dissipated into space, we have a means of determining how long the atmosphere of the earth has been cool enough for the existence of living things. But it is not so simple a matter as this statement of the case would indicate. It is believed that the cooling of the sun is accompanied by a corresponding shrinkage. Paradoxical as it may seem, this, in turn, gives rise to additional heat, and so the rate of cooling is retarded. Now, taking this into consideration, and putting the present temperature of the sun at 6,000° F, Kelvin concluded that it has been only a matter of about 18,300,^W years that the sun has been at its present degree of heat. It has been conceded that the rate of radiation in the past may have been somewhat different, which might result in a possible lengthening of the time to years. Many attempts have been made to discredit this result, and many and ingenious have been the suggestions of possible sources of energy which would serve to extend the life of the sun and earth, but it must be confessed that none of them stand the test of rigid analysis, and this result still stands unshaken. iii When we consider the endless diversity of living forms, and the imperceptible rate at which change has taken place during historic time, we are impressed with the implication of a long series of ages. This was clearly perceived by the great biologists who adorned Great Britain at the time when Kelvin's results were first published, and from that period on their voices and those of their successors have been constantly raised in protest against the ihadequacy of the time allowed them by the physicists. Let me illustrate the point of view of the biologists who insist upon the necessity of having very much greater resources of time in order to enable them to account for the development of species, hy reference to the best known family history, that of the horses. In this case we have a record of approximate completeness, extending from the early Eocene to the present. The eaj'liest number of the family was called Eohippus, a small animal, some eleven inches in height, whose remains have been found in the Bad Lands of the Western States. In the rocks immediately overlying those in which its bones occur, there is a series of fossil horses each species of which is a littli) larger than its predecessor until we have in the modern horse a height up to 64 inches. The total increase is therefore some 53 inches. If the growth still continues, it is so slow as not to show any perceptible results within historic time. Now if we allot time according to a conservative geological system, the Eocene appears to be between 5,00,00 and 6,00,00 years before the modern horse appeared, or allowing five or six years to a generation of horses, we would have about 1,(00,000 generations. This means that there would he an increase of about 1 inch in 100,00 years or 20,000 generations, which would be gradual enough to satisfy the biologists. But if this inch be allowed there is a mile which should be conceded for consistency's sake. The horses stand very near the head of the animal kingdom. They represent perhaps its highest physical development, and if it has taken five or six million years to add this cubit to their stature, in what terms shall we estimate the years that must have been required to have produced all this wonderful complexity of form and function from the seed of an ancestor void of organs and microscopic in size? A similar vision of the immensity of past time is given to the geologist, as he contemplates the work of the cosmic forces as recorded in the rocks. He reads there the story from birth to maturity of lofty ranges of mountains. He sees their summits raising themselves five miles above the sea by degrees so gradual as to he imperceptible. He sees them gradually disappear, for the everlasting hills themselves are not proof “against the tooth of time, And rasure of oblivion.” Little by little the solid rock is crumbled to sand. Grain by grain that sand is carried by wind and rain and river to the great sea, until where once the mountains stood there stretches a rolling plain. Even this great cycle of changes does not represent the whole extent of geological time, for the growth and decay of mountains has taken place many times since the lie- ginning, and in fact, may be completed within a comparatively short division of the whole. How inconceivably long then have been the ages which have passed since first the rivers began to flow and the earth became an abode of living things. And it is surprising that both the biologist and the geologist should have been so impressed by these concrete facts that even the apparently flawless reasoning of the physicists has failed to convince them? In order to attain some measurable representation of the extent of geological time, recourse was had to two different geological processes: the formation of stratified rock and the accumulation of salt in the oceans, and both of these have been studied with the greatest care and with results of steadily increasing accuracy. The calculation of age from the thickness of sedimentary rocks is based upon the fact that the material of which they are composed was carried down by the rivers and deposited under the shallow water which surrounds the continents. If we could measure the total depth of all such accumulations, and if we could gage the average load of mud and sand and gravel that goes down to the sea with each year's quota of river water, the problem that we are trying to solve would resolve itself into one of simple division. But the measurement of this total thickness is an exceedingly slow and complicated process. As long as the accumulated sediments continue to be submerged, their depth cannot be determined, but as the sea level is constantly changing, what was once the bed of the ocean, now becomes dry land. Even under these conditions it is not possible to attain our objective until a river has cut a gorge, or the hand of man has quarried away portions, so that the edges of the rock layers come into view. If we could find somewhere such an exposure of rock which had been accumulating ever since the sea first'received the contributions of the streams, it would, be an easy matter to measure the total thickness of sedimentary rock, but no such simple measuretllfnt is at our disposal. The ohservations of innumerable small sections must be laboriously fitted together to construct one comprehensive whole, and thus the total accumulation of sediments determined. The latest and hest figures availahle put it at 335,^W feet, or about 64 miles. We have now to determine the rate at which these sediments are accumulated. This is a problem even more diffieult than the previous one. It involves careful observations of the amount of solids carried each year by the rivers. These studies are still being carried on notably by the American Geological Survey, and constantly bring us closer to the standard of essential accuracy. Next we require to know the total area over which these sediments are being deposited, and thus the annual rate attained. This has been set hy some observers at 3 inches per century, which would make the time requisite to form the total 13-! million years; by others it has been placed at i inches per century, which would give us 100 million, and by others still at five inches, with a consequent reduction of time to 80 millions of years. The other method, a most ingenious one, was first made use of by Professor Joly of Dublin University in a paper published in 1899. It is based upon the theory that the saltness of the sea is due to the fact that ever since they began to flow, the rivers have been carrying salt in solution down to the oceans, and while the water comes back in the form of the rain, the bulk of the salt still remains and so the sea ever becomes salter. It is evident that if we knew the amount of salt now in the ocean and the rate at which the rivers have been delivering it, the length of the time occupied by the process is a matter of very simple calculation. Unfortunately, however, the initial figures are most difficult of attainment. The best measurements at present available set the amount of sodium in theseas at 14,]30 billion tons, and each year the 6,500 cubic miles of water which the rivers contribute, have dissolved in them 175 million tons. These are th” basic figures, but certain allowances have to be made. The salt breezes do carry a small portion of salt with them, and a part of the accumulation of past ages does not remain in the sea but is stored away in the form of rock salt. After all necessary corrections have been made, the final result gives a period somewhere between 80,00,00 and 150,000,00 years, with the weight of evidence tending rather toward the smaller figure. The discovery of radioactivity has had many unexpected results in all branches of science, not the least interesting of which are those bearing upon geology. It even appears that we have in this way a method of determining the age in years of any given rock stratum. Among those elements which are known to undergo the mysterious change due to disintegration of the atom, is uranium. By giving off particles of helium at a constant and definite rate, uranium is believed to pass over into radium and lead. If in any given uranium- bearing mineral we can determine the relative proportions of uranium, radium and helium, and lead if it is present, knowing the rate at which these changes take place, we should be able to determine the age of the mineral itself. This method was first suggested by Sir Ernest Rutherford in 1906, who found, in a mineral he was studying, an amount of helium which would indicate an age of 241 million years. Unfortunately he did not state the geological horizon in which the mineral was found, so that the results, so far as our problem is concerned, were of value merely on account of their suggestive- ness. But the lack, which geologists felt was so much to be regretted in this research, was subsequently made good hy the Honorable It. J. Strutt, in the careful experiments which he made to determine the ages of a series of minerals whose positions in the geological time scale were definitely known. His results were somewhat startling in the unexpectedly great periods of time which they indicated. For instance he allotted the very respectable antiquity of 141 million years to some rocks which were found overlying carboniferous strata, or about half way down to the earliest fossilif- erous deposits. However, these first figures were not uniform, and it remained for subsequent investigators to add their quota. Of recent years these have been tabulated and indicate a certain amount of consistency, particularly in their unanimity in extending the reach of geological time to an extent undreamed of by the geologists. Who, for example, would have dared to suggest from geological evidence alone, that we have to do with periods of from 800 to 1,600 million years? It is perhaps too soon to accept unreservedly these methods and these deductions. No doubt the physicists are over sanguine when they say that “every uranium bearing mineral is like a clock ticking out its age in molecules of helium and lead.” So that our judgment on this work too must for the present remain in abeyance, waiting for fuller and time-tested information. We have thus obtained the opinion of three schools of investigators as to the extent of geological time. One tells us from 10 to 30 million years; the second, about 100 million and the third, anything up to million. We must admit that we have not advanced very far. The mean of 10, 100 and 1,000 is a figure of little value. But we have not yet reached finality in any one of the three investigations. The bounds of knowledge are yearly becoming wider and we may hope that even this profound and perplexing problem will one day be solved. But there is a sense in which 10, 100 and 1,(00 are approximately the same—that is when they are compared with infinity. The number of the years of time and space and force we believe is infinite. In the great ahyss there has floated for 100 or 1,000 millions of years, perhaps even more, the minute speck of matter which we call the earth but in the light of infinity this is but a momentary phase. “The created world is but a small parenthesis in eternity.” It has already been explained that thl! information at our disposal does not, at the present stage of human knowledge, offer us much hop3 of finally determining the time since the earth first came into existence. It does however give us in some slight measure a conception of what this may be. And following the reasoning of Professor Joly of Dublin, we are led to a most interesting and pregnant thought. To-night we see a myriad stars shining in the sky, and the photographic telescope reveals many millions more. It is true that there is evidence of dark bodies—dead stars, but most of them are still glowing. Since the life of a star is but an evanescent flash in the dark when compared with the infinity of time, it appears that the whole universe came into being at one time, and that all the stars, including our own—the sun, will be snuffed out together. Thus it follows that the measure of the age of our earth as the abode of life is a measure of the life of the universe, awl our study of geological chronology has will us to a contemplation of cosmic time. The investigation is yet in its infancy. Who can foretell how great may be the discoveries which science will yet wrest from the storehouse of Nature? What may we not learn about what Carlyle so eloquently •speaks of as “that great mystery of Time, were there no other, the illimitable, silent, never-resting thing called Time, rolling, rushing on, swift, silent, like an all-embracing ocean tide, on which we all and the Universe swim like exhalations, like apparitions, which are and then are not; this is forever very literally a miracle; a thing to strike us dumb—for we have no word to speak about it.” McMaster University, Toronto.