MECHANICAL ACTION OF WATEE AND ICE 179 



not seem to split up any further, for everywhere on the islands 

 where the rock consisted of the coarse sandstone, as in this place, 

 the talus consisted of these sharp-edged stones." 



Ice acts as a disintegrating agent in still other ways than 

 that mentioned. The phenomenon of the glacier is now so 

 well known that we need dwell upon it but briefly here. Long- 

 continued precipitation of snow upon regions of such elevation, 

 or in such latitudes as to preclude anything like an equally 

 rapid melting, gives rise to deep fields of snow, compacted in 

 the lower portions into the condition of ice. Advancing, it may 

 be, but an inch or several feet a day, now scarce moving at all, 

 or even retreating temporarily through a diminution in the 

 amount of their supplies, or an increase in the sun's heat, these 

 carry with them large quantities of fragmental rock material 

 fallen upon them from above, or picked up from the surfaces 

 over which they flow. Those fragments which remain upon the 

 upper surface, or frozen into the upper portions, are but trans- 

 ported to the low^er levels where, the temperature being suffi- 

 cient, the ice is melted and the load deposited in the form of a 

 moraine. 



Beneath, and frozen into the lower portion of the ice sheet, 

 there is, however, a variable amount of rock material, which, as 

 the glacier moves along, is crowded with all the weight of the 

 overlying mass, and all the resistless energy of the ice behind, 

 over the surface of the underlying rock. In virtue of this 

 material, this sand, gravel, and boulder aggregate, the glaciers 

 become converted into what we may compare to extremely 

 coarse files, to tear away the rocks over which they pass, and 

 grind and crush them into detritus of varying degrees of 

 fineness. The small streams which originate from the melting 

 of the glaciers become therefore charged to the point of tur- 

 bidity with the fine silt-like detritus ground from the ledges 

 and in part from the boulders themselves. Figure 3 of plate 20 

 shows a slab of limestone still bearing upon its surface the evi- 

 dences of the severity of the onslaught. A consideration of the 

 amount of detritus thus brought down either merely as transported 

 or as abraded material belongs properly to the chapter on trans- 

 portation, but a few illustrations are not without interest here. 

 The Aar in Switzerland is stated by Geikie to discharge every day 

 in August some 440,000,000 gallons of water, carrying some 

 280 tons of sand. A portion of this is in a state of such 



