304 Charles Maclaren, Esq., on the 



a 6, The upper surface of the glacier. 



c d, A pile of blocks forming part of a medial moraine, resting 

 on the surface of the glacier, but a little raised above it. The 

 covering of stones protects the ice below, while the uncovered part 

 being exposed to fusion and evaporation, wastes away, and the mo- 

 raine is thus found riding on a ridge of ice, which Professor Forbes 

 informs us is sometimes 80 feet high. 



When the glacier was in progress towards final dissolution, its 

 surface a b gradually subsiding, would arrive in course of time at 

 the line e, and the blocks c' d! would then be deposited on the ter- 

 race in the position c d where we find them, 400 feet above the 

 bottom of the valley, except a few which slid over the declivity. 

 This explanation appears to me satisfactory, though our distin- 

 guished countryman, Sir Roderick Murchison, has raised some ob- 

 jections to it. We learn from Elie de Beaumont's Memoir that 

 there are glacial traces on the hills near Monthey, at an elevation 

 of 2350 feet (English) above the present bottom of the valley. The 

 left lateral moraine, therefore, of the great glacier would be at a, 

 probably 2000 or 3000 feet westward from c d. 



Having sketched the distribution of the travelled blocks, we recur 

 to the grand question — What were the means of their transporta- 

 tion ? And as we found that Charpentier's theory affords a plausi- 

 ble explanation of the erratic phenomena in the valley of the Aar, 

 let us inquire whether it is applicable to those we have been describ- 

 ing. The inquiry then presents itself in this form — whether the 

 magnitude and position of the ancient glacier which occupied the 

 Great Valley of the Upper Rhone, were such as, in accordance with 

 the laws of glacier motion, would enable it to transport the Alpine 

 blocks from their primitive sites to the Swiss plain, and the decli- 

 vities of Mount Jura ? 



The data for the solution of this problem are not so ample as 

 might be desired. The most important, so far as my information 

 extends, are supplied by the Memoir of Elie de Beaumont pre- 

 viously referred to, in which he gives the greatest elevation at which 

 polished rocks and erratics have been found at several points in the 

 Great Valley and across the Swiss plain to Jura. These traces in- 

 dicate the height and depth of the ancient glacier, and, when con- 

 nected by measuring the intervening spaces, enable us to deduce the 

 slope or inclination, which determined its progressive motion. The 

 following is Elie de Beaumont's table, with the metres converted 

 into English feet : — 



Upper limit of Polished, Bocks and, Erratics above the Sea. 



English 

 feet. 



1. Near the Grimsel (r in the map), . . . 7547 



2. Near Aernen (between r and b), . . . 5848 



