SOIL-FORMING PROCESSES 



29 



ported. The former remains in place, being the rock residuum 

 in which organic matter accumulates. Residual clay is an 

 example. The second group, on the other hand, in addition 

 suffers transportation and is represented by the soils arising 

 from glacial drift, alluvial accumulations, aeolian deposits, 

 and the like. In the first case, the soil is derived from a single 

 lithologic unit ; in the second place, the assorted and blended 

 materials are from many sources. A general statement of 

 the formation of a residual soil^ is obviously the easier to 



SOIL 



Fig. 5. — The gradual transition of country rock into residual soil by 



weathering in situ. 



make. Such a statement adequately covers every process in 

 the production of a transported soil except the disintegra- 

 tion, assortment, and solution due to translocation. (See 

 Fig. 5.) 



'*The changes that a rock undergoes in forming a residual 

 soil are first a physical breaking down, accompanied by certain 

 chemical transformations, which consist in the hydration of 

 a portion of the feldspars, micas and similar minerals; the 



^Buckman, H. O., The Formation of Eesidual Clay; Trans. Amer. 

 Cer. Soe., Vol. XIII, p. 362. Feb., 1911. 



