ACTION OF FROST 83 



the Delaware Water Gap, and wherever cliffs or peaks of naked 

 rock are exposed to severe cold. Many mountain passes are so 

 bombarded by falling stones as to be extremely dangerous ; in the 

 Sierra Nevada of California talus slopes as much as 4000 feet high 

 are reported, all the work of frost. At Sherman, where the Union 

 Pacific railroad crosses the "continental divide," the ground is 

 covered for miles with small, angular fragments of granite broken 

 up by the frost. 



In the polar regions frost is probably the most important of the 

 disintegrating agents. In Spitzbergen Beechy found that in sum- 

 mer the mountain slopes absorb quantities of water, which freezes 

 in winter with very destructive effect. " Masses of rock were, in 

 consequence, repeatedly detached from the hills, accompanied by 

 a loud report, and falling from a great height, were shattered to 

 fragments at the base of the mountain, there to undergo more 

 rapid disintegration." Similar phenomena are reported from the 

 Aleutian Islands of Alaska. 



The action of frost is, in itself, purely mechanical, no chemical 

 change is occasioned by it, and the smallest fragments into which a 

 block may be riven are sharp and angular, and the minerals have 

 unaltered and shining faces. But, on the other hand, frost pre- 

 pares the way for the more rapid action of rain and percolating 

 waters. The effects of these agents are produced upon the surface 

 of the rocks and the walls of the crevices which run through them. 

 By breaking up the blocks, the frost greatly increases the surface 

 and thus facilitates the work of the rain. An example will make 

 this clear. A cube of stone, measuring one foot each way, has six 

 sides, each of 144 square inches, and its total superficies is thus 

 144 x 6 = 864 square inches. Suppose this block to be riven by 

 the frost into pieces of one cubic inch each ; of such small cubes 

 there will be 1728, each with six square inches of surface, giving 

 10,368 square inches of superficies for all the cubes. A breaking 

 up of the cubic foot into cubic inches thus multiplies the exposed 

 surface by 12. 



Rain and frost are agents whose effects are most important in 

 regions of moist climate and abundant rainfall, for both are forms 



