WEATHERING 113 
United States, where the temperature often falls below 
the freezing-point, the water contained in the crevices 
of the rocks must freeze. When water freezes, it in- 
ereases in bulk; and therefore, if confined in a cavity, 
it exerts great pressure on the walls that enclose it. 
An iron ball or sphere, filled with freezing water and 
tightly sealed, will burst. The water that is in the 
rock cavities exerts like pressure, when frozen; and 
so, whenever the temperature of the earth’s surface 
goes below the freezing-point, the action of the frost 
tears off tiny fragments of rock. This is one of the 
means by which rocks are made to slowly crumble. 
On all bare ledges, and particularly upon exposed 
mountain tops (Fig. 55), this action of frost is very 
important, and the rocks rapidly disintegrate. In dry, 
desert countries, the surface strata do not contain water 
in appreciable quantity, and so here the action of frost 
is nearly absent; and in tropical lands, where the tem- 
perature never descends below the freezing-point, there 
is no frost action. 
Liffect of Heat. Another effect of temperature 
change, is that caused by the difference between day 
and night. When a rock is warmed it expands, and 
as it cools it contracts, so that when warm days are 
followed by cold nights, there is daily change. In 
some of the hot countries, the dark-colored rocks 
become so warm that it is painful to touch them; but 
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