278 MOUNTAIN SLIDES. 



splits if thrown into a furnace, and sometimes witli a loud 

 detonation ; and it is a familiar observation that the fire, in bm-n- 

 ing 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 influences from which it was sheltered be- 

 fore. But in the climate of the United States as well as of the 

 Alps, frost is a still more powerful agent in breaking up moun- 

 tain masses. The soil that protects the Hme and sandstone, the 

 slate and the granite, from the influence of the sun, also prevents 

 the water which filters into their crevices and between their 

 strata from freezing in the hardest winters, so that 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 hues of separation of the rocks, then sud- 

 denly freezes, and bursts asunder huge and apparently sohd 

 blocks of adamantine stone.* Where the strata are inclined at a 

 considerable angle, the freezing of a thin film of water over a 



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 be 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 

 freezing-water was due to compression, not dilatation — and therefore he 

 ascribes to thawing alone effects resulting not less from congelation. 



Various forces combine to produce the stone avalanches of the higher Alps, 

 the fall of which is one of the greatest dangers incurred by the adventurous 

 explorers of those regions — the direct action of the sun upon the stone, the ex 

 pansion of freezing- water, and the loosening of masses of rock by the thawing 

 of the ice which supported them or held them together. 



