110 
IOWA ACADEMY OP SCIENCES. 
for western loess (one-tenth to one-fourth of an inch) 
would be sufficient to form most of these deposits respec- 
tively in the 8,000 years, usually computed, since the reces- 
sion of the glaciers. 
The objection made by Doctor Chamberlin* that “ the eolian 
deposits are measured, not by the quantity of silt borne by the 
winds and lodged on the surface, but by the difference between 
such lodgment and the erosion of the surface, ’ ’ is met, at least 
in part, by the theory offered, for it is a well-known fact that 
timbered areas, even when very rough and with abundant 
slopes, are scarcely eroded by even the most violent precipi- 
tation of moisture. Professor Udden’s recent admirable 
report! also bears on this question, and should not be over- 
looked by the student of loess-problems. 
• Sixth . — No distinction can be made between the origin of 
eastern and western loess. The finer quality and lesser thick- 
ness of the former rather suggest that there had been more 
moisture {i. e., a shorter dry period during each year) and, 
hence, less dust; that the winds were less violent, and that 
there were greater areas completely covered with vegetation, 
this resulting in the necessity of transporting dust much 
greater distances, which would therefore be finer. J 
It should be borne in mind that the above noted differences 
between the regions in question actually exist to-day. There 
is more rain, — there are larger areas closely covered with 
vegetation, and less violent winds prevail in eastern Iowa and 
eastward, — and considering the position of mountain chains and 
seas, the same differences must have existed for a long time. 
That they did exist, during’ the deposition of the loess, is also 
indicated by the proportionately somewhat larger number of 
species in the eastern loess which prefer or require moist 
habitats. But the fauna of the eastern or Mississippi river 
loess is essentially a terrestrial fauna. The great fluviatile 
groups, now everywhere common in the streams of eastern 
Iowa, are wanting in the loess, and .the few fossil aquatic 
species are such as to-day prefer ponds, and are often found 
even in those which dry up during the summer. 
It may again be emphasized that the fossils show no greater 
difference, between the surface conditions which existed during 
the deposition of the loess of the eastern and western parts of 
*.Jour. of Geol., Vol. V, p. 801. 
tThe Mechanical Composition of Wind Deposits. 1898. 
•4:See Udden, 1. c. pp. 56, 57 and 67. 
