380 The Loess of North America. [May, 



valleys, and the high lands immediately adjacent ; that of the 

 fineness of its material, its composition, its rounded or tritu- 

 rated form, the fossils imbedded in it, and the unmistakable action 

 of water in assorting ; that of general continental depression syn- 

 chronous with its formation ; that of the vast quantity of the ma- 

 terial and its deposition alike on hill and in valley. These sev- 

 erally and together are fatal to the hypothesis of Von Richtho- 

 (cn. 1 It is nevertheless beyond question that the loess, after de- 

 position, has been somewhat modified by the action of strong 

 winds, but the evidences of such action are purely local. The 

 great dust-storms of Western Iowa, extending far beyond the 

 central portions of the State, which occurred in the spring of 

 1880, will long be remembered in the annals of Iowa. For days 

 the air was filled with fine dust, coming from the south-west, the 

 locality of the greatest areas of loess and the prevailing quarter 

 of the winds. That much of this fine material was carried miles 

 further away I have no doubt. The main effect however, of such 

 wind storms, would be the denudation of the windward, and the 

 deeper covering of the leeward bases and sides of hills. 



Fossils. — The mollusks of the loess belong, with perhaps a 

 single exception, to genera which now flourish in regions adjacent 

 to the formation. They are Lirnncm, Physa, Planorbis, Segmcntina, 

 Pomatiopsis, Valvata, Amnicola, Splncrium, Anodonta, among 

 fresh-water forms, and Hyalina, Sienotrema, Heticodiscus, Conidus, 

 Strobila, Helicina, Patida, Mcsodon, Vallonia, Macrocyclu, Pupa, 

 Succinea, Vertigo, and Cwnella, among the land forms. Unto is 



Th<: 



-tenth the wh 



; ' Report on the Mississippi I< 



loess deposition, the amount of sedime 

 is a fact beyond question, the material 

 ing of glaciers. These consideration^ 



