THE DISTRIBUTION OF LOESS FOSSILS I 35 



ence of others which are normally inhabitants of dry regions, 

 suggest a climate sufficiently dry that during a part of the year 

 at least, clouds of dust could be taken up by the winds. 



4. The overwhelming preponderance of land snails in the 

 loess must always be borne in mind. This however does not 

 prove that the loess regions were entirely devoid of lakes and 

 streams, but rather that the loess proper was deposited chiefly 

 upon higher grounds, for, if by any agency fine material were to 

 be uniformly deposited over all of Iowa today, covering the 

 successive generations of our present molluscan fauna, there 

 would be a much greater proportion of aquatic and moisture- 

 loving species than we find anywhere in the loess. 



5. The amount of material carried by the winds need not 

 have been so great as is sometimes assumed. The estimate 

 made by the writer 1 for the rate of deposition for eastern loess 

 (1 mm per year), and that made by Keyes 2 for western loess 

 (to to t °f an i ncn )> would be sufficient to form most of these 

 deposits respectively in the 8000 years, usually computed, since 

 the recession of the glaciers. 



The objection made by Dr. Chamberlin 3 that "the aeolian 

 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 abrupt slopes, 

 are scarcely eroded by even the most violent precipitation of 

 moisture. Professor Udden's recent admirable report 4 also bears 

 on this question, and should not be overlooked by the student of 

 loess problems. 



6. No distinction can be made between the origin of eastern and 

 western loess. The finer quality and lesser thickness of the 

 former rather suggest that there had been more moisture (z. e., 



'Proc. Iowa Acad. Sci., Vol. Ill, p. 88. 



2 Am. Jour, of Sci., Vol. VI, pp. 301, 302. 



3 Jour. Geol., Vol. V, p. 801. 



4 The Mechanical Composition of Wind Deposits, 1898. 



