126 uo^y CROPS feed. 



less ice-fields, but they are frozen rivers, rising in pei'pct^ 

 ual snows and melting into water, after having reached 

 half a mile or a mile below the limits of frost. The snow 

 that accumulates on the frozen peaks of high mountains, 

 which are bathed by moist winds, descends the slopes by 

 its own weight. The rate of descent is slow, — a few 

 inches, or, at the most, a few feet, daily. The motion i:- 

 self is not continuous, but intermittent by a succession of 

 pushes. In the gorges, where many smaller glaciers unite, 

 the mass has often a depth of a mile or more. Under the 

 pressure of accumulation the snow is compacted to ice. 

 Mingled with the snows are masses of rock broken off 

 the higher pinnacles by the weight of adhering ioe, or 

 loosened by-alternate freezing and thawing, below the line 

 of perpetual frost. The rocks thus falling on the edge of 

 a glacier become a part of the latter, and partake its mo- 

 tion. When the moving mass bends over a convex sur- 

 face, it cracks vertically to a great depth. Into the cre- 

 vasses thus formed blocks of stone fall to the bottom, and 

 water melted from the surface in hot days flows down and 

 finds a channel beneath the ice. The middle of the glacier 

 moves most rapidly, the sides and bottom being retarded 

 by fiiction. The ice is thus rubbed and rolled upon itself, 

 and the stones imbedded in it crush and grind each other 

 to smaller fragments and to dust. The rocky bed of the 

 glacier is broken, and ploughed by the stones frozen into 

 its sides and bottom. The glacier thus moves until it 

 descends so low that ice cannot exist, and gradually dis- 

 solves into a torrent whose waters are always thick with 

 mud, and whose course is strewn with worn blocks of 

 stone (boulders) for many miles. 



The Rhone, which is chiefly fed from the glaciers of the 

 Alps, transports such a volume of rock-dust that its muddy 

 waters may be traced for six or seven miles after they 

 have poured themselves into the Mediterranean. 



3, — Chemical Action of Water axd Aib, 



