ARTICLE VII.

GEOLOGIC TIME RATIOS, AND ESTIMATES OF THE EARTH'S AGE AND OF MAN'S

ANTIQUITY.

BY WARREN UPHAM, ASSISTANT ON THE GEOLOGICAL SURVEYS OF NEW HAMPSHIRE, MINNESOTA, AND THE UNITED STATES.

THE ancient Greek philosophers reasoned backward to primal cosmic conditions which they called Chaos; and the inspired author of the sublime first chapter of Genesis saw the earth, at the beginning of his earliest vision, "without form and void, and darkness was upon the face of the deep." In obedience to the Creator there came, in the first Mosaic vision, light, and the division of day and night; in the second vision, a world-wide ocean, and the gathering of a dense cloud-bank above a stratum of open air; in the third vision, areas of land, clothed with vegetation; in the fourth, the appearance of sun, moon, and stars, when rifts were first made in the previously continuous envelope of clouds; in the fifth, swimming and flying animals; and in the sixth and last vision, lowly and higher land animals, succeeded by God's crowning work, man and woman, endowed with the lofty capabilities of the human mind. So completely is this sequence in accord with the history revealed in the rocks to the geologist that Dana, the most eminent of Americans in this science, declares the record of Genesis "profoundly philosophical, . . . true and divine, . . . a declaration of authorship, both of Creation and the Bible."

Heathen sages, in all times, of whatever nation or religion, and modern scientists, whether Christian or agnostic,

have sought to penetrate the mysteries of the origin of man and of the earth, and to conjecture or measure their antiquity. Prominent among the contributions to these inquiries during the past year 1892 are Prof. G. F. Wright's series of Lowell Lectures on the "Antiquity and Origin of the Human Race," given in Boston during February and March; the treatment of the question of geologic time by Sir Archibald Geikie in his presidential address1 before the British Association for the Advancement of Science; another consideration of the same question by Mr. W J McGee in a paper entitled "Comparative Chronology," read before the section of anthropology in the American Association; an article by Prof. James Geikie, "On the Glacial Succession in Europe;" and Professor Wright's second and abridged work on the Ice age, bearing the title, "Man and the Glacial Period."

These writers and others, as Dana, Haughton, Wallace, and Alexander Winchell, weigh the geologic evidences of the earth's age. They are all approximately in agreement as to the ratios of the several great divisions of geologic time, but differ widely in their estimates of its aggregate duration. According to Sir A. Geikie, the known rates of deposition of sediments imply that for the formation of all the stratified rocks of the earth's crust a duration somewhere between 73 millions and 680 millions of years must be required. Most geologists would doubtless regard the lowest of these estimates as a minimum of the time needed for the processes of deposition and erosion revealed by their study of the rocks, and for the concurrent changes of the earth's floras and faunas from their beginning to the present time. But to some geologists

1 Nature, Aug. 4, 1892, Vol. xlvi. pp. 317–323.

2 Am. Anthropologist, Oct., 1892, Vol. v. pp. 327-344, with a plate showing relative durations of natural time units, historical eras, and geologic periods.

8 Trans., Royal Society of Edinburgh, 1892, Vol. xxxvii. pp. 127-149, with map.

these figures seem far too small, among whom McGee, the most extravagant of all, reasoning from similar premises of geologic observations, would claim about seven thousand millions of years as the more probable measure of the part of the earth's duration since its earliest fossiliferous rocks were formed, and probably twice as long since the earth began its planetary existence.

On the other hand, the most eminent writers who have considered this subject from the standpoint of physical experiment and theory and their relationship with astronomy, including Thomson, Tait, Newcomb, Young, and Ball, tell us that geologists can be allowed probably no more than one hundred millions of years, and perhaps only about ten millions, since our earth was so cooled as to permit the beginning of life upon it.

It is comparatively easy to determine the ratios or relative lengths of the successive geologic eras, but confessedly very difficult to decide beyond doubt even the approximate length in years of any part of the records of the rock strata. The portions for which we have the best means of determining their lengths are the Glacial and Recent periods, the latter extending from the Champlain epoch or closing stage of the Ice age to the present time, while these two divisions, the Glacial or Pleistocene period and the Recent, make up the Quaternary era. If we can only ascertain somewhat nearly what has been the duration of this era, from the oncoming of the Ice age until now, it will serve as a known quantity to be used as the multiplier for giving us the approximate or probable measures in years for the recedingly earlier and far longer Tertiary, Mesozoic or Secondary, Paleozoic or Primary, and Archæan or Beginning eras, which last takes us back almost or quite to the time when the cooling molten earth became first enveloped with a solid crust. This series of eras, however, comprising all the story of geology, is only a part of the earth's age as a separate planet.

Beyond the beginning of geology, the physicist and astronomer see our solar system in the process of slow but very grand evolution, from a globular nebulous mass with as great circumference as the orbit of the outermost planet whose mass became first detached from the revolving nebula, followed successively by the inner planets to Mars, the Earth, Venus, and Mercury, till finally the chief residual part of the nebula became the splendidly luminous sun. In like manner, as the planets became condensed to their present forms, most of them suffered a like severance of comparatively small portions of their masses to form moons and for Saturn both moons and rings. From measurements of the rate of radiation of the sun's heat, from the rate of increase of the earth's heat observed in deep mines and borings, and from experiments in the laboratory, aided by elaborate mathematical calculations, the physicist determines, to his own satisfaction, how long the sun can have been the centre of light and life for his attendant planets, how long time has passed since the moon's mass was thrown off from our whirling world, then revolving more rapidly than today, and the duration of the ages since the earth became encrusted and so far cooled that its ocean ceased to boil and vegetation and animal life began.

Geology, studying the stratified rocks and their fossils and the earlier and later volcanic rocks, tells us of the encrusted earth's history; the sciences of physics, chemistry, and astronomy go farther and bring before our imagination preceding æons through which the nebula had become condensed to the sun, planets, and satellites; but before the nebula we must believe that time was and "in the beginning God." Vast as may seem to our finite comprehension the antiquity of our race and of our earth, they are perhaps, nay, apparently must be, only a small and infinitesimal portion of the past eternity, just as the magnitude of our earth and solar system seems little in contrast with the infinitude

of universal space, containing its myriads of stars, each a blazing sun, and probably frequently or commonly attended with a retinue of worlds. In our inquiries concerning the earth's duration we may profitably first consider briefly what the physicist thinks of it in the light of his observations of nature, experiments in the laboratory, and mathematical computations, and afterwards what the geologist has learned from his reading the "sermons in stones."

Sir William Thomson (now Lord Kelvin) long ago estimated, from his study of the earth's internal heat, its increase from the surface downward, and the rate of its loss by radiation into space, that the time since the consolidation of the surface of the globe has been somewhere between 20 millions and 400 millions of years, and that most probably this time and all the geologic record must be limited within 100,000,000 years. Prof. George H. Darwin computes, from the influence of tidal friction in retarding the earth's rotation, that probably only 57,000,000 years have elapsed since the moon's mass was shed from the revolving molten earth, long before the formation of its crust. From the same arguments and the rate at which the sun is losing its store of heat, Prof. Guthrie Tait affirms that apparently 10,000,000 years are as much as physical science can allow to the geologist. Professor Newcomb, summing up the results of these physical and astronomic researches, writes: "If the sun had, in the beginning, filled all space, the amount of heat generated by his contraction to his present volume would have been sufficient to last 18,000,000 years at his present rate of radiation. . . . 10,000,000 years . . . is, therefore, near the extreme limit of time that we can suppose water to have existed on the earth in the fluid state." Not only the earth but even the whole solar system, according to Newcomb, "must have had a beginning within a certain number of years which we cannot yet calculate with