Page:Popular Science Monthly Volume 30.djvu/14

4 Hind; but there are good reasons for believing that it reaches northward to the Arctic Ocean, and that the great lakes of the north, like those of the St. Lawrence chain, Superior, Huron, Michigan, etc., are pre-glacial river-valleys scooped out and modified by ice.

From the facts already gathered, it is a justifiable inference that fully half the Continent of North America and nearly all north of the fortieth parallel was at one time covered with ice or perpetual snow, and, so far as we can now judge, the glaciation of all the North American localities enumerated was synchronous.

Some writers have attempted to prove that a large part of the glacial phenomena described above is really the work of icebergs and shore-ice, and one of the consequences of a great continental subsidence; but no man who has studied the inscriptions made by glaciers will hold to this theory when he has traversed much of the glaciated areas east or west of the Mississippi. To all the mountainous region of the West it is evident that the iceberg theory is inapplicable, and, when the enormous glaciers of the West are conceded, it is difficult to see why they should be denied to the East. Since, however, the iceberg theory is insisted upon for this section, it may be well enough to say that it is demonstrated untrue by four unanswerable arguments:

1. The inequalities of level in the fancied water-line formed by the margin of the drift area are irreconcilable. Mount Washington, Mount Marcy, Mount Mansfield, must have been totally submerged—because their tops are worn and striated—while the shore-line was at New York at the sea-level, in Pennsylvania twenty-one hundred feet, and at Cincinnati three hundred feet higher. South of the drift-line, high lands and low were alike beyond the reach of the flood, while in Wisconsin it spared a special district not above the general level, and all around it the rocks are scored and strewed with débris.

2. The direction of the ice-scratches and the derivation of the bowlders would require the submergence of all the northern portion of the continent, so that icebergs (which had no land to start from, and therefore could not have existed) could float southward over all the Canadian highlands; and the local variations of direction (southwest by south, in the basin of Lake Erie, south in that of Lake Huron, south-southwest in Lake Michigan, southwest in Lake Superior, and southeast in New England) show an incomprehensible tangle of ocean-currents.

3. The complete absence of marine shells from the great drift area of the interior, while they are abundant in the Champlain and bowlder clays on the coast, is incompatible with this theory.

4. The inscription left by the eroding agent is altogether sui generis, and characteristic of glacial action, and not at all that which could be effected by dragging masses of ice over the sea-bottom. This in itself is a conclusive refutation of the theory. The record made by a glacier