42 IOWA ACADEMY OP SCIENCES. 



dant, so there are deposits of loess without fossils, adjacent to 

 those which are fossilif erous. As the lands, • igh or low, lying 

 adjacent to larger streams have greater numbers of molluscs 

 to-day than the outljang prairies, so the loess bordering these 

 streams is usually much more fossiliferous than that which 

 covers more remote areas, — but the distribution of the fossils 

 is not in bands, as if drifted, but is similar to that of the mod- 

 ern specimens at the surface. Summing up the evidence of the 

 fossils we may assert that it points to conditions not unlike 

 those which exist to-day, and that geologists, in seeking for 

 the cause and manner of the deposition of the loess must give 

 up the assumption of widely submerged areas over which fos- 

 siliferous loess now occurs, and of a cold climate. 



That the loess is not everywhere uniform in composition is 

 recognized.* Especially marked is the difference between the 

 loess in the counties in Iowa designed as eastern, and those in 

 Iowa and Nebraska referred to as western, the former eing 

 finer and washing readily, while the latter is more silicious and 

 persists in vertical sections for a much longer time. These 

 differences may possibly suggest a difference in mode of deposi- 

 tion, but so far as evidence is furnished by the molluscs, the 

 climatic and surface conditions in both regions were essentially 

 the same, — that is, they did not differ more than at present. 

 A comparison of the species and number of specimens of each 

 from the eastern and western loess, as given in Table I, shows 

 that but few are not common to both, and these are mostly the 

 rarer species. There are now equivalent differences between 

 the faunas of the two regions, but the great majority of fossils 

 as well as of recent forms is the same in both. The differences 

 which exist between the two deposits are probably due to the 

 different sources of material rather than to different agencies 

 of deposition. 



It seems evident that the loess materials originated largely 

 or wholly in drift,* and as the comparatively recent investiga- 

 tions by members of the Iowa geological survey have demon- 

 strated the presence of several drift sheets in this state, and as 

 Nebraska has at least two such sheets, an interesting problem 

 is suggested to geologists, namely: the determination of the 

 relation which the various deposits of loess bear to those drift 



*W. .J. MoG^e.-Rep. U. ^. Geol. Sur., Vol. Xf, pp. 293 and 295, etc. 

 *R. D. Salisbury,— A.rk. Getl. Sar., Vol. H, pp. 3^5-6; S. Calvin,— Iowa Geol. Sur., Vol 

 VII. p. 89. 



