RELATIONS OF DISINTEGRATION AND DECOMPOSITION. 497 



are potent influences in breaking up the rocks. By insolation the rocks 

 may be coarsely subdivided; by the wind, bearing sand, they may be 

 finely subdivided — both with very little chemical action. In many dry 

 regions the rainfall is largely concentrated into a small part of the year. 

 In some regions this rainfall may occur as severe local storms. Under 

 these circumstances the disintegrated products are stripped from the rocks 

 by the running water, thus exposing new surfaces to weathering. The 

 transported material may be carried to the sea or may be accumulated in 

 the lower levels between the steep slopes. The former case is illustrated 

 by the sedimentary rocks which are now forming in the Gulf of California. 

 McGee describes the granitic and other rocks of southern California as 

 being simply split apart and broken into their individual, grains, which are 

 transported at rare times of abundant rainfall by the process which he 

 calls sheet-flood erosion and deposited in the Gulf of California with 

 scarcely any chemical change Such rocks may have very nearly the 

 average chemical composition of the original rocks from which they are 

 derived." 



Very frequently the disintegrated material of arid regions does not reach 

 the sea, but accumulates in the lower areas. Hilg'ard notes that the soils of 

 such regions are "predominantly sandy or silty, with but a small propor- 

 tion of clay, unless derived directly or indirectly from preexisting forma- 

 tions of clay or clay shales." 6 Moreover, he notes that the soil and the 

 subsoil are substantial!}' the same. The deficiency of clay — that is, kaolin — 

 is explained by the lack of the decomposition of the feldspars. The 

 deficiency of clay is no less marked in these soils than the abundance of 

 alkalies and alkaline earths. Hilgard finds the proportions of these 

 elements in arid and humid regions to be as follows:" 



p. 17 



"McGee, W J, The formation of arkose: Science, new ser., vol. 4, 1896, pp. 962-963. 



& Hilgard, E. W., Relations of soil to climate: Bull. Weather Bureau No. 3, IT. S. Dept. Agric, 1892, 



' Hilgard, cit., p. 30. 



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