80 THE DENUDANTS, OK CARVERS OF THE 



These operations, combined with those of cold, can 

 break up the hardest rocks into minute particles. 



Cold acts both as a destroyer and a preserver. If 

 matter is dry, it will, as an almost universal rule, 

 contract by cold ; if, however, it is wet, the moisture 

 by which it is permeated will freeze and expand. 

 Cracks opened by heat may be filled with rain or 

 moisture, cold will freeze and expand the water, and 

 thus widen the cracks. On the other hand, cold 

 will change rain into snow, the latter will form an 

 envelope on the ground, this while it lasts will pre- 

 vent meteoric abrasion. 



Wind is a denudant of no mean power. It works 

 differently from most of the others, for while they, in 

 general, carry the debris they form from higher to 

 lower levels, wind may carry it anywhere, even on to 

 higher ground. The effects of wind on a perpen- 

 dicular cliff are unique. Heat, frost, rain, or the sea, 

 acting on a cliff, will, with rare exceptions, form a 

 talus at the base, consisting of the detritus that is 

 constantly falling down. The sea eventually may 

 indeed carry away the debris ;. but while it is doing 

 so the upper portion of the cliff will probably be 

 weathered backward by meteoric denudation, and 

 it loses the perpendicular character; while heat, 

 frost, and rain, will change an inland cliff into a 

 slope, unless there is a lake or river at the bottom of 

 it. The character of wind denudation, however, is 



