510 MORRIS M. LEIGHTON 
supply. The distinctness between the two in color remains to be 
explained. Three working hypotheses may be tested. (1) The 
difference in color is original, the lower having been derived from 
a reddish silt, the upper from a buff silt. (2) The difference in 
color was brought about by weathering of the lower loess before 
_ the upper was deposited. (3) The lower was deposited under 
climatic conditions which favored oxidation to a reddish tinge 
during accumulation and then with a change in climate the suc- 
ceeding deposits were not so highly oxidized, only a buff color 
being produced. The first and third hypotheses do not neces- 
sarily imply a distinct interval between the two stages of deposi- 
tion, while the second, of course, does. A solution was sought in 
the nature of the contact, the evidences of weathering, and the 
fossil content. 
1. Character of the contact.—In perhaps most places the contact 
between the two loesses is gradational, the gradation, however, 
taking place within a few inches. At first glance, this might be 
taken to record continuous deposition. But such an interpreta- 
tion must not be held too rigidly in the case of wind deposits, since 
materials of an old surface may be mixed with new sediments, 
either by the wind itself or by organic agencies, thereby obscuring 
the record of an interval. 
2. Difference in oxidation.—The reddish tinge of the lower 
loess is somewhat greater at the top than lower down in this 
deposit, although the reddish tinge characterizes the whole deposit. 
The stronger color at the top of the lower loess suggests an interval 
of weathering before the deposition’ of the overlying, but the 
reddish tinge throughout must be otherwise explained. 
3. Differences in content of lime carbonate.—li the lower loess 
was subjected to weathering, it should show evidence of leaching of 
lime carbonate. The acid test revealed the presence of some lime 
carbonate throughout the reddish loess and into the overlying buff 
loess, but the reaction was notably feebler in the top of the reddish 
loess than in the base of the buff. While not strong, this evidence 
is suggestive of at least a brief interval of weathering between the 
two loess epochs although it is realized that there may have been 
an original difference in the lime content of the two loesses. Some 
of the larger shells, such as the Polygyras which extend from bottom 
