340 HELMHQLTZ 



have partly fallen there through crevasses, and may partly 

 have been detached from the bottom of the valley. For 

 these stones are gradually pushed with the ice along the 

 base of the valley, and at the same time are pressed against 

 this base by the enormous weight of the superincumbent ice. 

 Both the stones imbedded in the ice as well as the rocky 

 base are equally hard, but by their friction against each 

 other they are ground to powder with a power compared to 

 which any human exertion of force is infinitely small. The 

 product of this friction is an extremely fine powder, which, 

 swept away by water, appears lower down in the glacier 

 brook, imparting to it a whitish or yellowish muddy ap- 

 pearance. 



The rocks of the trough of the valley, on the contrary, 

 on which the glacier exerts year by year its grinding power, 

 are polished as if in an enormous polishing machine. They 

 remain as rounded, smoothly polished masses, in which are 

 occasional scratches produced by individual harder stones. 

 Thus we see them appear at the edge of existing glaciers, 

 when after a series of dry and hot seasons the glaciers have 

 somewhat receded. But we find such polished rocks as 

 remains of gigantic ancient glaciers to a far greater extent 

 in the lower parts of many Alpine valleys. In the valley 

 of the Aar more especially, as far down as Meyringen, the 

 rock-walls polished to a considerable height are very charac- 

 teristic. There also we find the celebrated polished plates, 

 over which the way passes, and which are so smooth that 

 furrows have had to be hewn into them and rails erected to 

 enable men and animals to traverse them in safety. 



The former enormous extent of glaciers is recognised by 

 ancient moraine-dykes and by transported blocks of stone, 

 as well as by these polished rocks. The blocks of stone 

 which have been carried away by the glacier are distin- 

 guished from those which water has rolled down, by their 

 enormous magnitude, by the perfect retention of all their 

 edges which are not at all rounded off, and finally by their 

 being deposited on the glacier in exactly the same order in 

 which the rocks of which they formed part stand in the 

 mountain ridge; while the stones which currents of water 

 carry along are completely mixed together. 



