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125 HOW CROPS FEED. 
less ice-fields, but they are frozen rivers, rising in perpet 
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 it- 
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 ice, 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 ere- 
vasses thus formed blocks of stone fall to the bottom, and 
water me!ted from the surface in hot days flows down and 
finds a channel beneath the ice. The middle of the glacicr 
moves most rapidly, the sides and bottom being retarded 
by friction. The ice is thus rubbed and rolled upon itself, 
and the stones imbedded in it crush and grind each other 
to smaller fragments an: 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 AND AIR, 
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