CAUSES, STAGES, AND TIME OF THE ICE AGE. 357 



30° south, and in India only 20° north, of the equator. Such evi- 

 dences of late Palaeozoic 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 peri- 

 ods, and they are separated in time by the vast Mesozoic and Ter- 

 tiary 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 Palaeozoic and the Quaternary glacial 

 periods were characterized by very unusual and exceptional oscil- 

 lations 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 alter- 

 nated 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 move- 

 ments of continents, the accumulation of glaciers and ice sheets 

 has been closely associated. 



Each of these periods of mountain formation, continental up- 

 lifts, and widespread glaciation was geologically short ; but they 

 were separated by a lapse of time so long that it can be adequate- 

 ly imagined only through the aid of a mathematical or geometric 

 illustration on an almost infinitely reduced scale. Let the dura- 

 tion 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 be- 

 ginning 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 hun- 

 dred and fifty, or two hundred miles. In accordance with the 

 probable ratios of the several great eras of geology, which are de- 

 termined through comparisons of their thicknesses of sedimentary 

 rocks and their progress in evolutionary changes of faunas and 

 floras, we may place the Palaeozoic 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 



