IOWA ACADEMY OF SCIENCES. iQy 



vicinity of streams is consistent with the tlieory of loess-forma- 

 tion presented by the writer before this academy.* 



Plants, and especially forests, develop chiefly and prima- 

 rily along- streams. This creates conditions favorable to land- 

 molluscs, and at the same time forms a trap for the dust, 

 carried from adjacent, more barren regions. The occurrence of 

 loess in the eastern part of Iowa, chiefly along the border of the 

 lowan drift sheet, may also be explained on the same ground. 

 After the melting of the ice, the terminal moraines offered the 

 first lodging-place for plants. Here, forests early developed, 

 and the conditions for entrapping the dust from adjacent, less- 

 favored territory, which was probably dry during a part of the 

 year, were here first developed We are in the habit of describ- 

 ing the lobed ridges of loess-regions as characteristic of loess 

 topography, yet they are quite as much characteristic of some 

 drift-areas; for example, along the Big- Sioux river in Iowa and 

 South Dakota. In eastern Iowa the surface of the loess is 

 largely shaped by the underlying moraines, which first 

 presented conditions suitable to the deposition of the loess, 

 and where, consequently, the deposit is best developed. The 

 loess at Natchez does not show this " loess -topography in the 

 same degree " 



Third. — The depauperation of some forms of shells, and the 

 presence 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. 



Fourth. — 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 was uniformly deposited over all of Iowa to-day, 

 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. 



Fifth. — The amount of material carried by the winds need not 

 have been so great as is sometimes assumed. The estimate 

 made by the writer f for the rate of deposition for east- 

 ern loess (1 mm. per year), and that made by Keyesl 



*Proc. Iowa Acad. Sci., Vol. Ill, p. 83 et seq. 

 tProc. Iowa Acad. Sci., Vol. Ill, p. 88. 

 :::Am. Jour, of Sci., (4), Vol. VI, pp. 301-303. 



