286 MOUNTAIN SLIDES. 
sometimes with a loud detonation; and it is a familiar ob- 
servation that the fire, in burning over newly cleared lands, 
breaks up and sometimes almost pulverizes the stones. This 
effect is due partly to the unequal expansion of the stone, partly 
to the action of heat on the water it contains in its pores. The 
sun, suddenly let in upon rock which had been covered with 
moist earth for centuries, produces more or less disintegration 
in the same way, and the stone is also exposed to chemical influ- 
ences from which it was sheltered before. But in the climate 
of the United States as well as of the Alps, frost is a still more 
powerful agent in breaking up mountain masses. The soil 
that protects the lime and sandstone, the slate and the granite 
from the influence of the sun, also prevents the water whic 
filters into their crevices and between their strata from freez- 
ing in the hardest winters, and the moisture descends, in a 
liquid form, until it escapes in springs, or passes off by deep 
subterranean channels. But when the ridges are laid bare, the 
water of the autumnal rains fills the minutest pores and veins 
and fissures and lines of separation of the rocks, then suddenly 
freezes, and bursts asunder huge, and apparently solid blocks 
of adamantine stone.* Where the strata are inclined at a con- 
siderable angle, the freezing of a thin film of water over a large 
and Nubia hisses audibly when thrown into water, from the escape of the air 
forced out of it by hydrostatic pressure and the capillary attraction of the 
pores for water. i 
Even the denser silicious stones are penetrable by fluids and the coloring 
matter they contain, to such an extent that agates and other forms of silex 
may be artificially stained through their substance. The colors of the stones 
cut at Oberstein are generally produced, or at least heightened, by art. This 
art was known to and practised by the ancient lapidaries, and it has been re- 
vived in recent times. 
* Palissy had observed the action of frost in disintegrating rock, and he 
thus describes it, in his essay on the formation of ice: ‘‘I know that the 
stones of the mountains of Ardennes he harder than marble. Nevertheless, 
the people of that country do not quarry the said stones in winter, for that 
they be subject to frost; and many times the rocks have been seen to fall 
without being cut, by means whereof many people have been killed, when 
the said rocks were thawing.” Palissy was ignorant of the expansion of 
water in freezing—in fact, he supposed that the mechanical force exerted by 
