108 IOWA ACADEMY OF SCIENCES. 



The presence of this shell-bearing stratum suggests that for 

 the period during which it formed the surface soil, and while 

 it was slowly accumulating, the conditions in this particular 

 locality were more favorable to the growth of land-snails than 

 now. There was, probably more vegetation, and hence the 

 surface was not so frequently storm-swept as at present. This 

 does not necessarily signify that general climatic conditions 

 were different, but that these particular banks or bluffs w^ere 

 more heavily timbered, with the Missouri river, probably flow- 

 ing at its base, its surface conditions being similar to those of 

 many timbered hills and knolls between Omaha and Nebraska 

 City, west of the Missouri. 



It is interesting to note that between Iowa and Nebraska, 

 the Missouri river now flows along the western side of its broad 

 valley, and that the adjacent western bluffs are more heavily 

 timbered and contain all the living species of molluscs herein 

 recorded, with the exception of Nos. 1 and 16, while the more 

 remote eastern bluffs are more barren and rugged. The shell- 

 bearing band may simply represent the period during which 

 the river in its shif tings occupied the eastern part of the valley. 



The foregoing facts lend support to the seolian theory of the 

 origin of the loess, as is shown by the following considerations: 



First. — The general manner of distribution of the modern and 

 fossil molluscs is essentially the same, this fact indicating that 

 they were not carried by waters, but w^ere quietly buried in 

 dust. Had they formed a part of river-drifts, they would be 

 more frequently heaped together, — not scattered as we find 

 them in the loess — and fluviatile shells would be more or less 

 intermingled. Moreover in many years' experience in dredg- 

 ing in ponds and streams, the writer has seldom seen a land- 

 shell which had been carried with the finest sediment into 

 ponds or lakes though such shells are sometimes found in sand 

 and other coarse material. Currents of water which could 

 carry most of the shells now found fossil, would also carry 

 coarser material than that which makes up the loess. 



Another fact which bears out this conclusion is the presence 

 of opercula in fossil shells of HeUcina occulta in the northern 

 loess and Helk-ina orbiculata in the southern loess. As the 

 operculum so readily falls from the decaying animal, it would 

 scarcely remain in place if the shell had been transported any 

 distance. 



Second. — The occurrence of fossiliferous loess chiefly in the 



