52 
IOWA ACADEMY OF SCIENCE Voi.. XXVIII, 1921 
Fig. 3. Appearance of eolian sand under the microscope. 
stratum are fine eolian sand grains, which are characterized by 
their minute size, remarkable uniformity and perfectly rounded 
form (figure 3). In the illustration the hair-lines are one-tenth 
of a millemeter apart. It is noted that the grains fall between one- 
twenty-fifth and one-fiftieth of a millemeter in their size. They 
are further distinguished from the glass of the pumaceous ash by 
being perfectly crystal. 
The Fifth Avenue street cutting is represented in cross-section 
in figure 4. Noteworthy features aside from the presence of 
the volcanic ash-bed are the great loess deposit, designated as the 
Peoria Clay, and the basal till which is thought to be possibly the 
Nebraskan drift-sheet. The loess section probably represents con- 
tinuous epirotic deposition during three interglacial and two gla- 
cial epochs, with great probability of its being deposited mainly 
during the last or Peoria epoch. Great stratigraphic significance 
is attached to this fact. 
It' is with some little doubt that the lowest drift deposit is re- 
ferred to the Nebraska Till. The bed is quite distinct from the 
local Kansas Till; and the dark red coloration of the latter is 
sharply set off from the dark blue of the former. It may be a till 
even older than the Nebraskan. This suggestion arises from the 
fancy that there may be in Iowa several drift sheets antedating 
the oldest of which we now know. 
