IOWA ACADEMY OF SCIENCES. 85 



indicating conditions \nou essentially different from those which 

 now prevail.' 



Additional weight attaches to the evidence of these molluscs 

 when we consider that they are in themselves witnesses to an 

 abundant flora of the period, for with scarcely an exception 

 they are purely herbivorous, and frequent places in which 

 shade, protection and food are furnished by abundant plants. 



The pr-esenca of a vigorous vegetation is further attested by 

 the leaching of peroxide of iron from the loess soil and its 

 deposition in tubules and concretions. ^^ 



That the amount of moisture was not excessive has already 

 been pointed oat. The great preponderance of terrestrial 

 molluscs, at least some of them, now capable of living and 

 multiplying in regions even drier than that under considera- 

 tion, and the majority of them living abundantly in our state 

 to-day, is certainly significant. 



But even if we grant that the average temperature was 

 somewhat lower than at present, and the amount of moisture 

 somewhat greater — conditions by no* means essential to the 

 phenomena of the loess — it cannot be questioned that the cli- 

 mate of the loess was sufficiently mild to support an abundant 

 fauna and flora from the very beginning of the formation of 

 these deposits. Glacial conditions certainly no longer existed, \ 

 for sufficient time must have elapsed after the recession of the ] 

 glaciers to clothe these prairies with verdure, for the mollusc 

 remains are found in the lowermost portions of the deposits 

 and the favorable conditions necessary for their development 

 must have existed from the very beginning. The prevailing 

 conditions being then essentially the same as now, and the 

 topography of the continent being essential as we find it to-day, / 

 it seems fair to assume that the prevailing strong winds were, 

 as now, northwesterly. This point will again be emphasized. 



The truth of the second proposition that the loess was 

 deposited slowly is supported by the following facts: 



^The writer formerly leaned toward the conclusion, drawn by McGee and Call in a 

 paper on the loess of Des Moines, that the occurrence of depauperate forms was proof 

 of a much colder climate than now prevails, but he has since found recent forms of 

 several of the species common in the loess which exhibit great variation under different 

 conditions even in the same locality. For example, shells of living Mcsodon multilin- 

 cata Say, from different points in the immediate vicinity of Iowa City, vary from 15 to 

 26 mm. in greater diameter, while fossils of the same species from the same region 

 now in the writer's possession vary from 13 to 23 mm. This variation seems to be 

 purely local and cannot be assigned to general climatic conditions. This was sug- 

 gested in the writer's paper to which reference has already been made, p. 93, foot- 

 note 2. 



lOSae Le Conte's Geology pp. 136, 137 



