178 
IOWA ACADEMY OP SCIENCE 
Now the question is pertinent : If Clidastes and Favosites and these 
other things represent pre-Aftonian faunas, why may not all the fossils 
he older than the gray els in which they are imbedded? The fossils 
themselves furnish the answer. All the pre-Aftonian types, even those 
that are completely silicified, are very much battered and worn; they 
are mere weathered and abraded fragments retaining nothing of the 
beanty and freshness of corresponding fossils taken from their original 
matrix, and they occur in very limited numbers. On the other hand 
the fossils referred to the Aftonian occur in far greater numbers than 
could be expected in accordance with ,any reasonable probability if 
they have been washed out of the pre-Kansan drift. If they are pre- 
Aftonian they must have come out of the drift, for the Boyer, the 
Soldier, the Maple and the other streams along whose valleys the fos- 
sil-bearing gravels occur, have no access to any preglacial surface any- 
where. Their valleys are excavated in drift. The bones and teeth 
V'Ould have to be very plentiful in the drift if the drift has furnished 
all we find in the gravels, but there is no record of the finding of any 
mammalian remains in the pre-Kansan. 
A very large proportion of the Aftonian fossils are not abraded 
or worn in even the slightest degree. They are fresh and perfect as a Pla- 
centiceras taken directly from concretions in the Pierre shales around 
the Black Hills. Some are broken, as would be expected, but many 
that vmuld be liable to breakage under hard usage are perfect. The 
imperial jaw represented in figure 1, plate 25, of the Bulletin paper, 
would not travel very far as part of a ground moraine. The tibia, 
figure 5 of the same plate, is not marred or scratched ; the same is true 
of the great tooth and the cervical vertebra, figures 7 and 8. The 
specimens shown in figures 2, 4 and 6 are broken, but the fractures are 
fresh and there are no signs of abrasion. The fine mastodon tooth 
above 7 is absolutely perfect so far as signs of wear or iveathering are 
concerned. A few small chips are broken from some of the cups, but 
the fracture surfaces are as recent in appearance as if the breaking 
had been done in taking the tooth from the pit. There is practically 
no wear showing on any of the material represented in the plates of 
the published paper, and there is much more equally as perfect. The 
contrast between this material and that which is known certainly to be 
pre-Aftonian is very striking. 
Jt is but just to say that there is in the collection a considerable 
amount of fragmental material that shows the effects of weathering 
and abrasion, but it is exactly v^hat would be expected of animal re- 
