THE AGE OF THE MAMMALIAN FOSSIL FAUNA 511 
to top in the lower loess, are more fragile in the upper part, thus 
strengthening the evidence of leaching. 
4. Bearing of the fossil content—A small collection of fossils 
from each of the two loesses was made, and these have been exam- 
ined by Curator Frank C. Baker of the Museum of Natural History, 
University of Illinois. The fact that the collection was not 
exhaustive limits the conclusions that may be drawn as to the fossil 
differences between the two deposits. However, two points seem 
to be clear: first, the lower loess has notably more fossils than the 
upper; and secondly, the large Polygyras, so characteristic of the 
lower deposit, do not seem to pass into the upper. ‘Two of these 
Polygyras have been designated new varieties by Professor Baker; 
one, Polygyra profunda pleistocenica, is smaller than the normal 
species of today, and the other, Polygyra multiineata altonensis, is 
larger than its living representative. These and some of the other 
forms are described ina paper by Professor Baker.’ If these should 
be found to be limited to the lower loess, the fact might indicate 
some climatic condition of unusual character. 
From the foregoing evidence it would seem that a brief epoch 
intervened between the deposition of the reddish loess and the 
buff loess, but that the difference in color is not wholly due to 
this epoch of weathering. ‘There is the possibility that the reddish 
tinge was given the lower loess by oxidation during the period of 
accumulation, but this would imply a climate of rather moist 
and dry extremes. The helices of the lower loess, such as Polygyra 
profunda and multilineata, according to Baker, are fond of rela- 
tively damp places in forest litter, beneath old logs and rubbish, 
and hence seem to be negative evidence. However, the Polygyras 
of this deposit have variational differences from these types, and 
these differences may be significant in this respect, although this 
is conjecture. The alternative view that the reddish tinge is due 
to the original color of the silts which were then present on the 
Mississippi flood plain, is somewhat favored. Reddish silts beneath 
loess are known upstream at least on the Iowa side where they are 
described as “‘red loam.”? The writer has also observed ‘‘slack- 
x 
t Nautilus, Vol. XXXIV (1920), pp. 61-66. 
2;W. H. Norton, ‘Geology of Scott County,” Jowa Geol. Survey, Vol. TX (1899), 
pp. 486-88. 
