E. W. Hilgard — Peculiarities of Rock - Weathering. 267 



Average Composition of Soils in the Humid and Arid Regions 

 of the United States* 



No. analyzed. 



Humid. 

 696 



Insoluble residue 84 



Soluble silica ... 4 



Sum of insoluble residue and soluble 



silica . ._ 8V 



Potash - 



Soda 



Lime 



Magnesia 



Br. ox. of manganese 



Peroxide of iron 3 



Alumina 3 



Phosphoric acid 



Sulphuric acid .. 



Water and organic matter 4 



Hygroscopic moisture 



Humus 1 



Nitrogen in humus 5 



Nitrogen in soil .. 



17 

 04 



68 

 21 

 14 

 19 

 29 

 L3 

 88 

 66 

 12 

 05 

 40 



22 

 00 

 17 



Arid. 



573 



69-16 



6-71 



75 87 





7 





35 



1 



43 



1 



27 





11 



5 



48 



7 



21 





16 





06 



5 



15 



5 



4 



1 



12 



15 



23 







14 



The table before you shows these peculiarities, which I have 

 somewhat elaborately discussed elsewhere. A glance shows 

 the main points of difference to lie just where they would be 

 expected as resulting from a diminished rainfall, causing a cor- 

 responding diminution of the leaching-out process, which in 

 the humid climates is the necessary consequence not only of a 

 large rainfall, but especially of its occurrence during the warm 

 season, when the concurrent effects of warmth and the consequent 

 evolution of carbonic acid, both from the roots of vegetation, 

 and from the oxidation or fermentation of dead vegetable 

 matter, can be exerted. Hence we see that the lime which in 

 the humid region is constantly and abundantly leached out into 

 the drainage and streams, is in the arid retained to an extent 

 amounting to from ten to thirteen times the average content 

 found in humid soils not directly derived from calcareous 

 formations. A similar condition is found as regards magnesia, 

 through to a less degree. Potash averages the triple amount. 

 It may seem strange that a similar relation is not shown here as 

 regards soda; the reason is that for fairness of comparison, alkali 

 soils are not included in the list. It is well known that in the 

 case of zeolitic silicates, such as are known to exist in soils, pot- 

 ash is retained more tenaciously than soda. The latter is 

 therefore washed down to the lower ground, and there gives 



* Soils directly derived from calcareous formations have, equally in both 

 regions, been excluded from this comparison. 



