IOWA ACADEMY OP SCIENCES. 
85 
indicating conditions not 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 presence 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 out. 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 trutn of the second proposition that the loess was 
deposited slowly is supported by the following facts; 
9 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, taut he has since found recent forms of 
several of the species common in the loess which exhibit great variation under diiferent 
conditions even in the same locality. For example, shells of living Mesodon multiUn- 
eata Say, from diiferent 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. 
loSee Le Conte's Geology pp. 136, 137 
