Page:Popular Science Monthly Volume 30.djvu/15

Rh is unmistakable, and no one who has not learned the language in which it is written is warranted in taking part in the discussion; but he who has done so will find graven on the rocks of the Alps, the hills of New England, the basins of the Great Lakes, and the mountains of Colorado and Oregon, an inscription which is everywhere the same, which can have but one meaning, and bears a signature that can not be counterfeited.

While it is hopeless to expect that all men will agree upon this—or any other—subject, I think I am justified in saying that the facts which have been stated, and others of like import, constitute an indisputable record, not necessarily of the former existence of a great icecap over all the northern regions, but of the simultaneous prevalence of sheets of land-ice, i. e., glaciers, over great areas of our continent; and that these glaciers, forever in motion, holding imbedded in their substance sand, gravel, and bowlders, pressed against the underlying rock by their enormous weight (probably averaging fifty thousand pounds to the square foot), became powerful agents of erosion; general and uniform when they were broad, narrow and special when they were local. This is the reading of the facts now given by those who are best qualified to judge of the import of the phenomena in the Old World and in the New. Already the belief in an ice period and ancient glaciers is general—hereafter, with more complete knowledge of the subject, it must become universal.

Accepting the facts cited above as demonstrating the truth of the glacial hypothesis, and as proving beyond cavil the reality of an ice period, we now pass to consider the proximate and remote causes of the distinctive phenomena of this remarkable chapter in geological history.

With characteristic conservatism Lyell endeavored to account for the prevalence of glaciers over the northern hemisphere by supposing them to be due to a peculiar arrangement of land and sea; broad and elevated areas of land in the Arctic regions, low and narrow land surfaces in the tropics. I have elsewhere discussed this question at some length, and have shown that this theory is untenable, because: First, during the Tertiary age the land was high at the north, no marine Tertiary deposits being found there; Asia, Europe, and America were then connected by land, and the tropical currents were excluded from the Arctic Ocean, but in that age a warm climate