Page:Popular Science Monthly Volume 20.djvu/242

230 could not be set aside, some thought it the same with the flood of Noah, and others believed it to represent the chaos supposed to have pervaded space just before the advent of man. Then it was fashionable to believe in a submergence of the land by an ocean freighted with enormous icebergs, floating southerly from the pole. After this the battle raged fiercely between the advocates of icebergs and glaciers, the odds resulting in favor of the glacialists. Agassiz, with his polar-cap theory, advocated the view of a globe entirely encircled by ice, thickest at the poles, but covering the tropics with sufficient thickness to destroy the Tertiary life. Since these days of controversy, geologists have been accumulating facts in all glaciated countries; extreme views are being modified by their holders; and the time has come for consistent, reasonable generalizations in respect to the origin and extent of glacial phenomena.

Where the glacial markings have been most studiously investigated, it appears that an ice-sheet invariably occupies a definite area; that a mer de glace accumulates in the higher central portions, from which glaciers move outwardly in all directions, extending as far as the descending ground will permit, or the oceanic currents can convey the icebergs broken off from the frozen mass without melting; and that these ice-streams carry with them fragments of rock scoring the ledges as they pass along, thus affording the means for determining the exact dimensions of the glaciated areas, even after the climate has moderated and melted the ice. In Europe, the familiar Alpine district is constantly quoted as an illustration of the nature of a glaciated region, with its center of dispersion. Small ice-tributaries now cover the slopes of the Rhône Valley, where formerly the solid mass pressed down from the Bernese Oberland and the region of Mont Blanc, over Lake Geneva and the broad valley of Switzerland to the flanks of the Jura, forming an ice surface fifty miles wide, one hundred and fifty miles long, and two thousand feet deep. On the south, other streams have conveyed morainic materials equally far into the valley of the river Po. Hence we can assign to this area, with its center of dispersion, a definite number of square miles, linking together the actual glaciers with the traces of their former greater extension. No one will question the correctness of the generalizations enlarging the limits of the Alpine glaciated area.

We proceed next to consider the proper extent of the Scandinavian district, which will furnish an example more nearly analogous to that of Eastern America. Erdmann's map in Geikie's "Ice Age" represents striæ pushing northerly into the Arctic Ocean from Finland; southeast toward the White Sea and the Gulf of Finland; southerly in the south part of Sweden, turning to southwesterly toward the Cattegat; west, southwest, and northwest in Norway. Croll, in his work on "Climate and Time," supplements this map by drawing lines to indicate the direction taken by the ice after leaving Finland and