No. 389. ] THE WHITE RIVER TERTIARY. 407 
The occurrence of mammal bones on the surface may be 
taken as indicating the way they occur within the loess. Skel- 
etons are more or less complete according to the extent of 
disturbance by carrion feeders. Skulls, limbs, or individual 
bones are commonly found separated — carried off by other 
animals. Limbs are very apt to be bitten off across the 
upper limb bone, leaving the head of the humerus or femur 
in its socket. Bones are often more or less gnawed or 
bitten, sometimes weathered, but never water-worn. In and 
around “salt licks,” or lagoons, a mingled heap of bones 
of many animals is often found, individuals usually hopelessly 
mixed. 
The upper part of the Prairie Loess contains bones of Bison 
and other modern prairie inhabitants in various early stages of 
fossilization, The lower part contains Equus bones. 
THE ASoLIAN HYPOTHESIS. 
I believe that the White River clays, in Colorado at least, 
are chiefly zolian deposits, similar in origin to the Prairie Loess. 
Most of the sandstones are probably fluviatile, especially those 
occurring in lenticular masses. The sandstones with the s¢razz- 
fied clays parallel the modern fluviatile deposits of the valleys. 
Some sandstones may be zolian. 
1. The fauna is what we should expect to find in an open, 
grassy region. The animals are all land forms, and the most 
abundant genera, Oreodon, Mesohippus, and Hyracodon, have 
distinctively cropping teeth. The abundant and varied fauna 
has been supposed to indicate a warm, moist climate; it is paral- 
leled, however, by that of the dry region of South Africa, and 
the modern fauna of the plains is by no means small. The 
occurrence of land tortoises cannot be explained by the lake 
theory, and its explanation of the other land fauna appears an 
exceedingly improbable one, involving several practical impos- 
sibilities, as noted above. 
2. The occurrence of specimens is precisely, in every detail, 
the same as above stated in the modern prairie deposits, and 
their abundance in Colorado is about the same. Gnawed bones, 
