THE ATTACK OF THE WEATHER 



155 



The rock mantle and its shield in the mat of vegetation. - 

 Through the action of weathering, the rocks, as we have seen, 

 lose their integrity within a surface layer, which, though it may be 

 as much as a hundred feet or more 

 in thickness, must still be accounted 

 a mere film above the underlying bed 

 rock. The mechanical agents of the 

 breakdown operate only within a few 

 feet of the surface, and the agents of 

 rock decomposition, derived as they 

 are from the atmosphere, become 

 inert before they have descended to 

 any considerable depth. The surface 

 layer of incoherent rock is usually 

 referred to as the rock mantle (Fig. 

 163). Where the rock mantle is rel- 

 atively deep, as it is in the states 

 south of the Ohio in the eastern 

 United States, there is found, deep 

 below the outer layer of soil, a partially decomposed and disin- 

 tegrated rock, of which the unaltered minerals lie unchanged in 

 position but separated by the new minerals which have resulted 



from the breakdown of their more 

 susceptible associates. While thus 

 in a certain sense possessing the 

 original structure, this altered ma- 

 terial is essentially incoherent and 

 easily succumbs to attack by the 

 pick and spade, so that it is only 

 at considerably greater depths 

 that the unaltered rock is en- 

 countered. 

 Because of the tendency of 



FIG. 162. A large glacial bowlder 

 split by a growing tree near East 

 Lansing, Michigan (after a pho- 

 tograph by Bertha Thompson). 



FIG. 163. Rock mantle consisting of 

 broken rock, above which is soil and 



a vegetable mat. Coast of California mantle rock to creep down upon 



(after a photograph by Fairbanks). . 



slopes it is generally found thicker 



upon the crests and at the bases of hills and thinnest upon their 

 slopes (Fig. 164). 



In the transformation of the upper portion of the mantle rock 

 into soil, additional chemical processes to those of weathering 



