110 IOWA ACADEMY OF SCIENCES. 



for western loess (one-tenth to one-fourth of an inch) 

 would be sufficient to form most of these deposits respec- 

 tively in the 8,000 years, usually computed, since the reces- 

 sion of the glaciers. 



The objection made by Doctor Chamberlin* that " the eolian 

 deposits are measured, not by the quantity of silt borne by the 

 winds and lodged on the surface, but by the difference between 

 such lodgment and the erosion of the surface/' is met, at least 

 in part, by the theory offered, for it is a well-known fact that 

 timbered areas, even when very rough and with abundant 

 slopes, are scarcely eroded by even the most violent precipi- 

 tation of moisture. Professor Udden's recent admirable 

 reportf also bears on this question, and should not be over- 

 looked by the student of loess- problems. 



Sixth. — No distinction can be made between the origin of 

 eastern and western loess. The finer quality and lesser thick- 

 ness of the former rather suggest that there had been more 

 moisture (/. e., a shorter dry period during each year) and, 

 hence, less dust; that the winds were less violent, and -that 

 there were greater areas completely covered with vegetation, 

 this resulting in the necessity of transporting dust much 

 greater distances, which would therefore be finer. J 



It should be borne in mind that the above noted differences 

 between the regions in question actually exist to-day. There 

 is more rain, — there are larger areas closely covered with 

 vegetation, and less violent winds prevail in eastern Iowa and 

 eastward, — and considering the position of mountain chains and 

 seas, the same differences must have existed for a long time. 

 That they did exist, during the deposition of the loess, is also 

 indicated by the proportionately somewhat larger number of 

 species in the eastern loess which prefer or require moist 

 habitats. But the fauna of the eastern or Mississippi river 

 loess is essentially a terrestrial fauna. The great fluviatile 

 groups, now everywhere common in the streams of eastern 

 Iowa, are wanting in the loess, and the few fossil aquatic 

 species are such as to-day prefer ponds, and are often found 

 even in those which dry up during the summer. 



It may again be emphasized that the fossils show no greater 

 difference, between the surface conditions which existed during 

 the deposition of the loess of the eastern and western parts of 



* Jour, of GeoL, Vol. V, p. 801. 



tThe Mechanical Composition of Wind Deposits. 1898. 



•fSee Udden, I. c. pp. 56, 57 and 67. 



