242 ON GLACIERS IN GENERAL. 



glaciers. The Swiss peasants attribute to them an intrinsic 

 power of rejecting impurities. The fact is, that year by year, 

 and month by month, fresh thicknesses of virgin ice become 

 revealed by the fusion of the surface. That ice, -formed in 

 the highest mountain hollows, never was or could be impure. 

 The rocks and earth have fallen upon the surface since ; and, 

 by the conditions which we have mentioned, once there, there 

 they remain. Even those blocks which fall into the crevasses 

 are usually arrested at no great depth, and by the general 

 lowering of the glacier-surface, soon attain its level. 



The superficial waste of a glacier is thus a very important 

 phenomenon. Owing to it the body of the ice has its vertical 

 thickness rapidly diminished during the heats of summer, and, 

 as we have already intimated, the lower end of a glacier has its 

 position determined by the amount of this waste. Suppose a 

 glacier to move along its bed at the rate of 300 feet per annum, 

 and imagine (merely for the sake of illustration) its yearly 

 superficial waste to be 20 feet : then the thickness of the glacier 

 will diminish by 20 feet for every 300 feet of its length, or at 

 the rate of 360 feet per mile ; * so that the longitudinal section of 

 a glacier has the form of a wedge ; and however enormous its 

 original thickness, after a certain course we must at length come 

 to the thin end of the wedge, and that the more rapidily as the 

 causes of melting increase towards the lower extremity. These 

 causes are indeed so various that it is difficult to estimate them 

 with accuracy. We have (1) the direct solar heat ; (2) the 

 contact of warm air ; (3) the washing of rain. All these causes 

 act on the surface, and produce the ablation of the surface. 

 Besides these, the ice of the glacier wastes somewhat beneath 

 by the contact of the soil and the washing of the inferior streams. 

 This may be called its subsidence. Further, the natural slope of 

 the rocky bed of the glacier causes any point of the surface to 

 stand absolutely lower each day in consequence of the progres- 

 sive motion. These three causes united produce the geometrical 

 depression of the surface. I have elsewhere shown how the 



* [ More correctly 352 feet per mile.] 



