264 E. W. Hilgard — Peculiarities of Rock - Weathering. 



as in swamps or marshes, humid conditions exist in the arid 

 region as well. 



Since kaolinization, which must precede the formation of 

 colloidal, plastic clay material, is essentially a process of hydra- 

 tion, the presumption is that it will progress more slowly in 

 regions of deficient rainfall. That this is actually the case can 

 hardly be questioned by any one who has studied comparatively 

 the results of rock decay in both the arid and humid regions ; 

 the contrary assertion, lately made, cannot, so far as I know, be 

 supported by any facts properly observed and interpreted. 

 The contrasts between the condition of the granites and grano- 

 diorites on the upper portions and the lower slopes of the 

 Sierra Nevada and of the Sierra Madre of southern California, 

 and still more between the latter and the southern Alleghenies, 

 are so strongly accentuated as hardly to require discussion. 

 The enormous deposits of almost fresh granitic sand at the 

 base of the southern arid ranges, so coarse that the trees and 

 seeds must be planted far below the surface in order to main- 

 tain life, while in the Alleghenies a clay subsoil formed in place 

 lies within a foot of the surface on which the planting is done, 

 are a practical demonstration of the greater intensity of clay- 

 formation in the humid regions. 



These differences in characters of the regolith of the two 

 regions may be schematically illustrated in a diagram. This 

 would show that in the humid region the hnmous "soil" as 

 a rule extends only from six to nine inches from the surface, these 

 being the depths prescribed by the Eastern Experiment stations 

 for the taking of samples of " surface soil " ; beneath which 

 follows almost invariably a more clayey subsoil, which as a 

 rule is unadapted to the growth of crops, so that not more 

 than half an inch of it is allowed to be plowed up to the 

 surface at once, on pain of spoiling the seed-bed for the suc- 

 ceeding season at least. No such restriction applies to the 

 typical arid soils, in which the " surface " or humous soil is 

 rarely less than three feet in thickness, and frequently extends 

 to a much greater depth even with a considerable humus- 

 content. This state of things is illustrated by the columns 

 of California soils, in two of which the humus has been 

 determined as reaching to the depth of twelve feet, as shown 

 in the table here presented ; and doubtless to at least one 

 foot beyond. 



The root of a hop plant here before you illustrates one of 

 the practically most important results of this great depth and 

 pulverulent texture of soils in the arid region. This root at 

 ten feet is still over a millimeter in diameter, and of course 

 went at least two"feet deeper than the specimen shows. Now 



