Page:Popular Science Monthly Volume 49.djvu/375

Rh 30° south, and in India only 20° north, of the equator. Such evidences of late Palæozoic glaciation are also very clearly exhibited on the Varanger Fiord, in the extreme northern part of Norway, beyond the Arctic Circle. During all the earth's history before the Ice age of Pleistocene time, no other such distinct indications of general or interrupted and alternating glaciation have been found. Geologic exploration reveals only these two glacial periods, and they are separated in time by the vast Mesozoic and Tertiary eras, together estimated by Dana and others to comprise some ten to fifteen or twenty million years.

It is especially suggestive, in our inquiry concerning the causes of the Ice age, that both the Palæozoic and the Quaternary glacial periods were characterized by very unusual and exceptional oscillations in the height of continental areas and by the formation or renewed uplifting of great mountain ranges. Epochs in which certain mountain belts came into existence, or, after being partly or chiefly worn away, were restored by great uplifts, have alternated with far longer periods and eras of comparative repose. Between the epochs of mountain-building, the slow wearing and gnawing of rain, frost, and chemical decay have striven to carry away the mountains to the plains and the sea. At two times of the birth or rejuvenation of the grandest mountain chains of the world, with the most remarkable upward and downward movements of continents, the accumulation of glaciers and ice sheets has been closely associated.

Each of these periods of mountain formation, continental uplifts, and widespread glaciation was geologically short; but they were separated by a lapse of time so long that it can be adequately imagined only through the aid of a mathematical or geometric illustration on an almost infinitely reduced scale. Let the duration of a lifetime of seventy years be represented by a span, or nine inches. A century on this scale will be denoted by a foot, a thousand years by ten feet, and a million years by about two miles. The whole duration of the earth's existence since the beginning of life upon its surface, if between fifty and a hundred million years, as estimated by Dana, Walcott, and others, would then be represented by a distance of about one hundred, one hundred and fifty, or two hundred miles. In accordance with the probable ratios of the several great eras of geology, which are determined through comparisons of their thicknesses of sedimentary rocks and their progress in evolutionary changes of faunas and floras, we may place the Palæozoic glacial period at a distance of twenty to forty miles back from the present day, corresponding to some ten to twenty million years. That glacial period may have been no longer than the Ice age recently ended—that is, twenty-five thousand to fifty thousand years, more or less. The