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Page:Popular Science Monthly Volume 49.djvu/380

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climate with abundant snowfall, forming an ice sheet whose duration extended until the land sank somewhat lower than now, leading to amelioration of the climate and the departure of the ice, followed by re-elevation to the present level. The coincidence of these great earth movements with glaciation naturally leads to the conviction that they were the direct and sufficient cause of the ice sheets and of their disappearance; and this conclusion is confirmed by the insufficiency and failure of the other theories which have been advanced to account for the Ice age.

The end of the Tertiary era and the subsequent Glacial period were exceptionally characterized by many great oscillations of continental and insular land areas. Where movements of land elevation took place in high latitudes, either north or south, which received abundant precipitation of moisture, ice sheets were formed; and the weight of these ice sheets seems to have been a chief cause, and often probably the only cause, of the subsidence of these lands and the disappearance of their ice.

The general contemporaneousness of the Glacial period on the opposite sides of the North Atlantic Ocean had been long accepted as probable, but its demonstration and the identification of the corresponding parts of the Ice age, having the same sequence on the two continents, were first made known less than two years ago by the studies of Geikie and Chamberlin in the new third edition of The Great Ice Age, and by their later papers in the Journal of Geology. According to the subdivision recognized by these authors, the time of principal accumulation of marginal moraines is regarded as an epoch distinct from the previous portions of the Ice age; and Chamberlin has named the earlier divisions of this period, when the North American ice sheet reached its culmination, the Kansan and Iowan stages, while the later moraine-forming time is called the Wisconsin stage, from the magnificent development of the moraines in eastern Wisconsin. Between these glacial stages, which appear well recognizable and synchronous in North America and Europe, these authors suppose that there were prolonged interglacial epochs, when the ice sheets were in large part or wholly melted away. To the most important of the warm intervals, separating the Kansan and Iowan stages of ice accumulation and advance, the name Aftonian is given by Chamberlin, from Afton in Iowa, where a thick bed of peat, formed during that time, lies between deposits of glacial drift.

Instead of this view of distinct epochs of glaciation, the Ice age seems to me, while accepting the successive stages here noted, to have been still essentially a single and continuous glacial period, with moderate fluctuations of the ice borders during both the growth and wane of the ice sheet. The marginal moraines I