410 



GEOLOGY. 



of fluvial origin; but some investigators, while assenting to this con- 

 clusion, would exclude such deposits from the loess proper. Some, 

 indeed, would so define the loess as to make it an colian product. The 

 distribution of the loess along the rivers naturally suggests a genetic 



* 



^# 



9 



' / 



X- 



j i 



Pig. 526. — Loess Shells, a-b, Zonitoides minusculus (Binney); c-d, Euconulus fulvus 

 (Drap.); -/, Strobilops labyrinthica (Say); g, Polygyra clausa (Say); h, P. mul- 

 tilineata (Say); i-j, Succinea obliqua Say; k, S. avara Say; l-m, Polygyra 

 monodon (Rack); n, Bifidaria pentodon (Say); o, B. corticaria (Say); p, B.mus- 

 corum (Linn.); q, B. armifera (Say). The small figures adjacent to some of the 

 large ones show the natural size of the shells. 



relation to them. This is conceded, without proving that the loess 

 is fluvial. 



By the aqueous hypothesis, the loess is assigned to direct deposi- 

 tion by the rivers, or their lake-like expansions. To make this possible, 

 it is necessary to suppose that the waters stood- at elevations 200 to 

 600 feet higher than now, relative to adjacent surfaces. This involves 

 difficulties that have never been satisfactorily met, for great areas 



