Page:Popular Science Monthly Volume 49.djvu/379

Rh pebbles from near the sources of the Mississippi to the shore of the Gulf of Mexico, Prof. E. W. Hilgard thinks that the preglacial uplift, inaugurating the Ice age, was four thousand or five thousand feet more in the central part of the continent than at this river's mouth.

Although the adequacy of the preglacial epeirogenic elevation of this continent to produce its Pleistocene ice sheet was tardily recognized, it was distinctly claimed by Dana in 1870 that the Champlain subsidence of the land beneath its ice load, supposing it to have been previously at a high altitude, must have brought climatic conditions under which the ice would very rapidly disappear. The depression would be like coming from Greenland to southern Canada and New England. In Prof. Dana's words: "Such an extended change of climate over the glacier area was equivalent in effect to a transfer from a cold, icy region to that of a temperate climate and melting sun. The melting would therefore have gone forward over vast surfaces at once, wide in latitude as well as longitude."

Such explanations as these, accounting for the gradual accumulation and comparatively rapid dissolution of the North American ice sheet, are also found to be applicable to the ice sheets of other regions. The fiords of the northern portions of the British Isles and of Scandinavia show that the drift-bearing northwestern part of Europe stood in preglacial time one thousand to four thousand feet higher than now; while, on the other hand, late glacial marine beds and strand lines of sea erosion testify that when the ice disappeared the land on which it had lain was depressed one hundred to six hundred feet below its present height, or nearly to the same amount as the Champlain depression in North America. Mr. T. F. Jamieson appears to have been the first in Great Britain or Europe to attribute the ice accumulation to altitude of the land, and to hold the view (which I receive from him) that the submergence of glaciated lands, when they were loaded with ice, was caused directly by this load pressing down the earth's crust upon its fused interior, and that the subsequent re-elevation was a hydrostatic uplifting of the crust by underflow of the inner mass when the ice was melted away. Just the same evidences of abundant and deep fiords and of marine beds overlying the glacial drift to heights of several hundred feet above the sea are found in Patagonia, as described by Darwin and Agassiz. On these three continental areas the widely separated chief drift-bearing regions of the earth are found to have experienced in connection with their glaciation in each case three great epeirogenic movements of similar character and sequence—first, a comparatively long-continued uplift, which in its culmination appears to have given a high plateau