344 NATURAL HISTORY BULLETIN. 



to the surface of the whole flood plain; but in autumn, win- 

 ter, and spring, the diminished rivers flowed in comparative- 

 ly narrow channels, probably permitting the main part of the 

 flood plain to become more or less covered by grass and other 

 vegetation, and to be inhabited by air-breathing mollusks."* 

 It is fair to presume that under the climatic conditions here 

 assumed the winters were still long, and that much ice was 

 formed each season. If the streams were flooded four months 

 each year, as suggested, and much of the remainder of the 

 year was winter, when did the grass and snails grow? And, 

 furthermore, where do modern representatives of the loess 

 species of molluscs live under such conditions? 



If it is argued that the mass of loess was gradually accumu- 

 lated by a succession of floods, which periodically receded 

 sufficiently to expose land-surfaces, then it is necessary to con- 

 sider movements of enormous volumes of water in compara- 

 tively short time, for the loess-covered regions are not in re- 

 stricted depressions, and enormous floods would be required 

 to cover them. In Iowa, for example, these floods would have 

 covered the greater part of the state. The loess-topped hills 

 at Iowa City are lower than the loess border near the Missis- 

 sippi river, and the hill south of Carroll about 275 miles west 

 of Iowa City, and more than 600 feet higher, f and forming a 

 part of the great divide between the Mississippi and Missouri, 

 is covered with fossiliferous loess ! To periodically drain such 

 an area sufficiently to leave land areas exposed for a sufficient 

 length of time each year to enable a flora and a snail fauna 

 to develop, would require currents so strong that much coarse 

 material would be transported, and the loess would not be so 

 uniformly fine in texture. 



But the absurdity of the proposition that snails could grow 

 under such conditions, will appeal to everyone familiar with 

 their rate of growth and their habits. It may be that those 



* Substantially the same statement had been previously made by him 

 in Bull. Geol. Soc. of Am. , vol. 5, p. 94. 



+ See R. R. profiles, Iowa R. R. Commission, 1881. 



