130 ICE AXD GLACIERS. 



are imbedded in the lower surface of the mass of ice, and 

 which 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 appearance. 



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 characteristic. 

 There also we find the celebrated polished plates, over 

 wliich 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 



