392 



SOILS. 



of both alkaline and earthy bases in acid-soluble silicate combinations. 

 But much farther research is needed to explain fully the marked dis- 

 crepancies observed in this respect between soils not only occurring in 

 closely contiguous localities, but also showing marked similarities in 

 their general composition. 



Ferric Hydrate. There is no obvious reason, from the 

 chemical standpoint, why iron, that is, ferric hydrate or iron 

 rust, should be more abundant in the soils of the arid regions, 

 as the averages given in the table suggest ; moreover, the fact 

 does not impress itself upon the eye, since the orange or reddish 

 tints are by far more common in the humid than in the arid 

 regions of the United States at least. The California average 

 is considerably influenced by the very highly ferruginous soils 

 from the foothills of the Sierra Nevada, and by the black 

 (magnetite) sand so commonly present; that of Oregon by the 

 black, highly ferruginous country rock (basalts), from which 

 they are partly derived. The average for Montana is not 

 higher than that of three states of the humid region, and less 

 than that of Kentucky. \Ye might imagine a cause for deple- 

 tion of iron in the soils of the humid areas in the frequency 

 with which humid moisture and high temperature will during 

 the summers concur toward the bringing about of a reducing 

 process in the soil, which by getting the iron into proto-car- 

 bonate solution would make it liable to be leached into the sub- 

 soil, as is frequently the case; yet the resulting " black gravel " 

 or bog ore, in its various forms, is of not infrequent occurrence 

 in the arid regions also. A constant quantitative difference due 

 to climatic conditions does not appear to be shown by the data 

 thus far at command, but the finer distribution of the ferric 

 hydrate in the humid temperate as well tropical regions is 

 obvious to the observer, from the frequent redness of humid 

 and tropical soils. 



Manganese. An unexpected and apparently well-defined 

 contrary relation appears to be shown as regards the related 

 metal manganese; the average percentage of which is in all 

 cases less in the arid than in the humid region. The cause of 

 this relation is altogether obscure; it is too frequent to be ac- 

 cidental. 



Phosphoric Acid. As regards that highly important soil 



