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Page:Popular Science Monthly Volume 49.djvu/377

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CAUSES, STAGES, AND TIME OF THE ICE AGE.

Chandler's investigations, estimates that the place of the pole since the Glacial period, and from even earlier geologic times, has been without greater changes of position than would lie inside the area of a block or square inclosed by the intersecting streets of a city.

We come now to the wholly terrestrial or geologic theory of the causes of the Ice age, which in terms varying with increasing knowledge has been successively advocated by Lyell, Dana, Le Conte, Wright, and the present writer. According to this explanation, the accumulation of the ice sheets was due to uplifts of the land as extensive high plateaus receiving snowfall throughout the year. Geology has received from Gilbert, in his monograph on Lake Bonneville for the United States Geological Survey, the terms epeirogeny and epeirogenic (continent-producing), to designate the broad movements of uplift and subsidence which affect the whole or large portions of continental areas or of the oceanic basins. This view, accounting for glaciation by high altitude, may therefore be very properly named the epeirogenic theory. It is adversely criticised by Prof. James Geikie, who calls it "the earth-movement hypothesis."

So early as 1830 Lyell pointed out the intimate dependence of climate upon the distribution of areas of land and water and upon the altitude of the land. In 1855 Dana, reasoning from the prevalence of fiords in all glaciated regions, and showing that these are valleys eroded by streams during a formerly greater elevation of the land previous to glaciation, and from the marine beds of the St. Lawrence Valley and basin of Lake Champlain belonging to the time immediately following the glaciation, announced that the formation of the drift in North America was attended by three great continental movements: the first upward, during which the ice sheet was accumulated on the land; the second downward, when the ice sheet was melted away; and the third, within recent time, a re-elevation, bringing the land to its present height. But with the moderate depth of the fiords and submarine valleys then known, the amount of preglacial elevation which could be thus affirmed was evidently too little to be an adequate cause for the cold and snowy climate producing the ice sheet. The belief that this uplift was three thousand feet or more, giving sufficiently cool climate, as Prof. T. G. Bonney has shown, to cause the ice accumulation, has been only reached within the past ten years through the discovery, by soundings of the United States Coast Survey, that on both the Atlantic and Pacific coasts of the United States submarine valleys evidently eroded in late Tertiary and Quaternary time reached to profound depths, two thousand to three thousand feet below the present sea level.