Page:Geological Evidences of the Antiquity of Man.djvu/318

298 great clearness before the British public in 1852 by Mr. Charles Maclaren, who had himself visited Switzerland for the sake of forming an independent opinion on a theoretical question of so much interest, and on which so many eminent men of science had come to such opposite conclusions.

M. Guyot had endeavoured to show that the Alpine erratics, instead of being scattered at random over the Jura and the great plain of Switzerland, are arranged in a certain determinate order, strictly analogous to that which ought to prevail if they had once constituted the lateral, medial, and terminal moraines of great glaciers. The rocks chiefly relied on as evidence of this distribution consist of three varieties of granite, besides gneiss, chlorite-slate, euphotide, serpentine, and a peculiar kind of conglomerate, all of them mineral compounds, foreign alike to the great strath between the Alps and Jura, and to the structure of the Jura itself. In these two regions, limestones, sandstones, and clays of the secondary and tertiary formations alone crop out at the surface, so that the travelled fragments of Alpine origin can easily be distinguished, and in some cases the precise localities pointed out from whence they must have come.

The accompanying map or diagram, slightly altered from one given by Mr. Maclaren, will enable the reader more fully to appreciate the line of argument relied on by M. Guyot. The dotted area is that over which the Alpine fragments were spread by the supposed extinct glacier of the Rhone. The site of the present reduced glacier of that name is shown at. From that point, the boulders may first be traced to, or Martigny, where the valley takes an abrupt turn at right angles to its former course. Here the blocks belonging to the right side of the river, or derived from c, d, e, have not crossed over to the left side at, as they should