304 C. R. Keyes — Eolian Origin of Loess. 



the same species now living in the same region presented a 

 marked depauperation, attesting a much more rigorous climate 

 in- glacial times than at present. The other writer, after ex- 

 amining a much greater amount of material, from a very much 

 larger area, shows that the loess fossils are not only not smaller 

 in size, but if anything slightly larger than existing indi- 

 viduals. 



Some fifty species of animal remains are credited to the loess 

 formations of the upper Mississippi Valley. Among these are 

 several vertebrates. Whether or not the latter really belong 

 to the loess or are to be regarded as having been incorporated 

 by slipping and over wash of the deposit, is not known. 



Molluscan shells form the great bulk of the loess fossils. 

 With the possible exception of half a dozen isolated exceptions 

 none of the species are bivalves. The large majority are land 

 forms, with a few water species that do, however, inhabit 

 small temporary pools. In the Bluff loess more than nine- 

 tenths of the total number of individuals belong to species 

 that are found only in unusually damp situations. They are 

 those species that to-day occur most abundantly in the loess 

 areas. The species having an optimum habitat that is not 

 excessively moist have not been observed to occur abundantly 

 in the Bluff loess, though they flourish in the country border- 

 ing the loess belts. These loess fossils are apparently those 

 forms that have lived among the plants of the belt. On the 

 death of the animals the shells simply dropped down on the 

 ground beneath and were covered. 



On the other hand, there occur in the loess of the ice fronts 

 shells of mollusks that do not now live in the region. Such 

 are Helicina occulta Say, Patula strigosa Gould, the boreal 

 species of Pupa, and other northern forms. The loess fossils 

 should be studied with reference to their possible use as a 

 means of discriminating between loess deposits having differ- 

 ent origins. 



The presence of vertical root remains and limonite tubules, 

 from the bottom to the top of the Bluff loess, the luxurious- 

 ness of vegetation covering the loess belts, the "dust" storms 

 on the larger silt-ladened streams, the peculiar relations existing 

 between the prevailing winds and the distribution of the loess 

 deposits, all point to fruitful sources of inquiry regarding one 

 of the most obstinate problems in Pleistocene geology. A 

 possible means is suggested of discriminating between deposits 

 lithologically similar, that are doubtless formed by very differ- 

 ent agencies and under very different conditions. The infer- 

 ence to be drawn is that the eolic processes have been at work 

 upon certain deposits in the Mississippi Valley in a manner 

 and to an extent that has been, heretofore, little appreciated. 



