CHAPTER IV. 



THE VARIOUS ROCKS AS SOIL-FORMERS. 



Rock-weathering in arid and humid Climates. From what 

 has been said in the preceding chapters of the physical and 

 chemical agencies concerned in rock-weathering, it is obvious 

 that climatic differences may materially influence the character 

 of the soils formed from one and the same kind of rock. Since 

 kaolinization is also a process of hydration, the presence of 

 water must greatly influence its intensity, and especially the 

 subsequent formation of colloidal clay ; so that rocks forming 

 clay soils in the region of summer rains may in the arid regions 

 form merely pulverulent soil materials. Many striking ex- 

 amples of these differences may be observed, c. g., in comparing 

 the outcome of the weathering of granitic rocks in the southern 

 Alleghenies with that of the same rocks in the Rocky Moun- 

 tains and westward, especially in California and Arizona. The 

 sharpness of the ridges of the Sierra Madre, and the roughness 

 of the hard granitic surfaces, contrasts sharply with the 

 rounded ranges formed by the " rotten " granites of the At- 

 lantic slope, where sound, unaltered rock can sometimes not 

 be found at a less depth than forty feet ; while at the foot of 

 the Sierra Madre ridges, thick beds of sharp, fresh granitic 

 sand, too open and pervious to serve as soils, cover the upper 

 slopes and the " washes " of the streams, causing the latter to 

 sink out of sight. A general discussion of the kinds of soils 

 formed from the various rocks must, therefore, take these 

 differences into due consideration. 



GENERAL CLASSIFICATION OF ROCKS. 



Rocks may be broadly classified into three categories, viz : 



1. Sedimentary rocks, formed by deposition in water and 

 hence more or less distinctly stratified. 



2. MctamorpJiic rocks, formed from rocks originally sedi- 

 mentary, by subterranean heat in presence of water. Usually 



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