Page:Geological Evidences of the Antiquity of Man.djvu/330

310 contorted positions, must also have given rise to many superficial inequalities, in some of which large bodies of water would collect. M. Desor, in a memoir on the Swiss and Italian lakes, suggested that they may have escaped being obliterated by sedimentary deposition, by having been filled with ice during the whole of the glacial period.

Subsequently to the retreat of the great glaciers, we know that the lake-basins have been to a certain extent encroached upon and turned into land by river deltas; one of which, that of the Rhone at the head of the lake of Geneva, is no less than twelve miles long and several miles broad, besides which there are many torrents on the borders of the same lake, forming smaller deltas.

M. Gabriel de Mortillet, after a careful study of the glacial formations of the Alps, agreed with his predecessors, that the great lakes had existed before the glacial period, but came to the opinion, in 1859, that they had all been first filled up with alluvial matter, and then re-excavated by the action of ice, which, during the epoch of intense cold, had by its weight and force of propulsion, scooped out the loose and incoherent alluvial strata, even where they had accumulated to a thickness of 2,000 feet. Besides this erosion, the ice had carried the whole mass of mud and stones up the inclined planes, from the central depths to the lower outlets of the lakes, and sometimes far beyond them. As some of these rock-basins are 500, others more than 2,000 feet deep, having their bottoms in some cases 500, in others 1,000 feet below the level of the sea, and having areas from twenty to fifty miles in length and from four to twelve in breadth, we may well be startled at the boldness of this hypothesis.

The following are the facts and train of reasoning which induced M. de Mortillet to embrace these views. At the lower ends of the great Italian lakes, such as Maggiore, Como, Garda and others, there are vast moraines which are proved