116 ICE AND GLACIERS. 



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 con- 

 siderable height are very characteristic. 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 distinguished from 

 those which water has rolled down, by their enormous magni- 

 tude, 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. 



From these indications, geologists have been able to prove 

 that the glaciers of Chamouni, of Monte Rosa, of the St. Gott- 

 hard, and the Bernese Alps, formerly penetrated through the 

 valley of the Arve, the Rhone, the Aar, and the Rhine to the 

 more level part of Switzerland and the Jura, where they have 

 deposited their boulders at a height of more than a thousand 



