Page:Geological Evidences of the Antiquity of Man.djvu/316

296 In 1841, I advanced, jointly with Mr. Darwin, the theory that the erratics may have been transferred by floating ice to the Jura, at the time when the greater part of that chain, and the whole of the Swiss valley to the south, was under the sea. We pointed out, that if at that period the Alps had attained only half their present altitude, they would yet have constituted a chain as lofty as the Chilian Andes, which, in a latitude corresponding to Switzerland, now send down glaciers to the head of every sound, from which icebergs, covered with blocks of granite, are floated seaward. Opposite that part of Chili where the glaciers abound, is situated the island of Chiloe, one hundred miles in length, with a breadth of thirty miles, running parallel to the continent. The channel which separates it from the main land is of considerable depth, and twenty-five miles broad. Parts of its surface, like the adjacent coast of Chili, are overspread with recent marine shells, showing an upheaval of the land during a very modern period; and beneath these shells is a boulder deposit, in which Mr. Darwin found large blocks of granite and syenite, which had evidently come from the Andes.

A continuance in future of the elevatory movement, now observed to be going on in this region of the Andes and of Chiloe, might cause the former chain to rival the Alps in altitude, and give to Chiloe a height equal to that of the Jura. The same rise might dry up the channel between Chiloe and the main land, so that it would then represent the great valley of Switzerland.

Sir Roderick I. Murchison, after making several important geological surveys of the Alps, proposed, in 1849, a theory agreeing essentially with that suggested by Mr. Darwin and myself, viz. that the erratics were transported to the Jura, at a time when the great strath of Switzerland, and