It being apparent that glaciers were not adequate to produce the results which we find, the glacialists have fallen back upon an extraordinary hypothesis--to wit, that the whole north and south regions of the globe, extending from the poles to 35° or 40° of north and south latitude, were, in the Drift age, covered with enormous, continuous sheets of ice, from one mile thick at its southern margin, to three or five miles thick at the poles. As they find drift-scratches upon the tops of mountains in Europe three to four thousand feet high, and in New England upon elevations six thousand feet high, it follows, according to this hypothesis, that the ice-sheet must have been considerably higher than these mountains, for the ice must have been thick enough to cover their tops, and high enough and heavy enough above their tops to press down upon and groove and scratch the rocks. And as the _striæ_ in Northern Europe were found to disregard the conformation of the continent and the islands of the sea, it became necessary to suppose that this polar ice-sheet filled up the bays and seas, so that one could have passed dry-shod, in that period, from France to the north pole, over a steadily ascending plane of ice. No attempt has been made to explain where all this {p. 24} ice came from; or what force lifted the moisture into the air which, afterward descending, constituted these world-cloaks of frozen water. It is, perhaps, easy to suppose that such world-cloaks might have existed; we can imagine the water of the seas falling on the continents, and freezing as it fell, until, in the course of ages, it constituted such gigantic ice-sheets; but something more than this is needed. This does not account for these hundreds of feet of clay, bowlders, and gravel. But it is supposed that these were torn from the surface of the rocks by the pressure of the ice-sheet moving southward. But what would make it move southward? We know that some of our mountains are covered to-day with immense sheets of ice, hundreds and thousands of feet in thickness. Do these descend upon the flat country? No; they lie there and melt, and are renewed, kept in equipoise by the contending forces of heat and cold. Why should the ice-sheet move southward? Because, say the "glacialists," the lands of the northern parts of Europe and America were then elevated fifteen hundred feet higher than at present, and this gave the ice a sufficient descent. But what became of that elevation afterward? Why, it went down again. It had accommodatingly performed its function, and then the land resumed its old place! But _did_ the land rise up in this extraordinary fashion? Croll says: