LOESS FOSSILS. 175 



Their presence rather only serves to emphasize my contention that the 

 fauna of the loess is in all cases very similar to the fauna of the surface in 

 the same region. Pond species are more abundant and of greater variety 

 in eastern Iowa and Illinois, especially along river courses, than they are 

 westward. Probably during the deposition of the loess "the same conditions 

 existed, and the agency chiefly concerned in the work of deposition, whether 

 wind or water, buried more of these aquatic species in the eastern loess, 

 simply because there were more of them, proportionately, upon the surface. 

 The following quotation 1 bears upon this point: 



No distinction cau 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 (i. 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. 



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. 



In addition to these notes furnished by Professor Shimek, a few remarks 

 are taken from his discussion of the loess in a recent paper published in 

 Proceedings of the Iowa Academy of Sciences: 2 



The majority of the geologists who have given attention to the loess of the 

 Mississippi Valley have ascribed its deposition to water in lakes or sluggish streams 

 Some have also contended that this occurred in a glacial climate, or at least in a 

 climate much colder than that of to-day in the same region. 



That fresh water has been regarded as the agent of deposition is due in no small 

 degree to the belief that a very considerable portion of the species and individuals 

 found in the deposit consist of aquatic or semiaquatic forms, although the fact has 

 long been recognized that terrestrial species prevail. There is, however, absolutely 

 nothing in the loess fauna to indicate that the loess land surfaces were more moist, or 

 to any extent more widely or more deeply covered with waters, than are the surfaces 



1 Shimek: Proc. la. Acad. Sci., vol. vi, 1898, p. 110. 

 2 Proc. Iowa Acad. Sci. for 1897, Vol. V, 1898, pp. 32-45. 



