1882.] The Loess of North America. 375 



here made to the almost or quite vertical planes of cleavage. 

 Wherever streams, both great and small, have eroded channels 

 through the deposit, or when they undermine the resulting cliffs, 

 the masses that become detached break off in planes parallel 

 to the original cleavage planes. This is especially remarkable 

 since the material of the loess is not cohesive, and not at all plas- 

 tic, unless thoroughly saturated with water. The use to which 

 this feature has been put is well illustrated by the great work of 

 Richthofen on China. In our country it is most common to meet 

 with bluffs that are more or less rounded, a condition due to the 

 action of rains and frosts. 



Microscopical and Chemical Features.— The soil of the loess 

 presents an unusually beautiful field when viewed with a good 

 working microscope. A number of such examinations were 

 made (1) of soil as taken in situ, in which were presented minute 

 granules of pure silica of an average diameter of ^ to ^V* of 

 an inch ; (2) of soil after treatment with strong nitric acid, when the 

 same features were prominent, the silica granules merely appear- 

 ing somewhat brighter. None of the olive-green crystalline par- 

 ticles, found by Pumpelly in the Chinese loess, were to be found, 

 while in our examinations, as in his, there were no remains of 



were devoid of the sharp angles which recently detached particles 

 of silicious rocks give. This may, in part at least, be due to the 

 action of the acids mentioned above, and in part to attrition 

 against one another. They were all irregularly ovoid and some- 

 what translucent bodies, but occasionally discolored by some one 

 or another of the iron oxides. 



Numerous analyses made by several observers, rate the ap- 

 proximate quantity of silica in the loess soil at from seventy- five 

 to eighty per cent. Blow-pipe analysis, conducted solely with a 

 view to qualitative ends, gave, as constituents of the soil from the 

 Missouri river and Des Moines valley deposits, water, phosphorus 



and silica. To present more clearly the nature of the soil, its 

 value agriculturally, and as anticipatory of its mode of origin, the 

 following table will be found useful and instructive. I give, also, 



