14 
THE EOCENE PLATEAU. 
of the United States Geological Survey of the Territories, 1873, 422), that 
“ there is good reason for believing that this incursion of Mammalia came 
from the south”. 
In Wyoming, the Bridger beds are immediately underlaid by the Wah- 
satch formation (which includes the Green River shales)', whose fauna pre¬ 
sents characteristic peculiarities when Compared with that of the Bridger 
epoch (see summary at end of Chapter XII). The result of my explora¬ 
tion in New Mexico was the discovery of an extensive area of Wahsatch 
beds, with their fossils. The latter represent the fauna which immediately 
succeeded the Fort Union Saurians and preceded the Mammalian life of 
the Bridger. According to Dr. PeaJe (Hayden’s Annual Report for 1874, 
published in 1876), the Wahsatch beds (“Green River beds”) become much 
heavier in Western Colorado, and I did not find the Bridger beds in New 
Mexico. New Mexico was then no doubt the source from wliich the fauna 
of Wyoming was derived, and the extension of the Wahsatch fauna prob¬ 
ably proved fatal to the latest representatives on the American continent of 
the Dinosaurian and other Reptilian forms of Mesozoic time. 
The relations which the Eocene bears to the Mesozoic formations in 
New Mexico are as follows: 
West of the hog-back of Cretaceous No. 3, with an interval of perhaps - 
two miles, at a point just north of the Gallinas Mountain, a sandstone bluff 
presents a bold escarpment to the northeast. This is the angle of a mass 
of rock whose eastern face extends southward parallel to the mountain- 
axis, and whose strata dip first 15°, then 10° south, and soon disappear 
beneath a similar mass. This series also presents an escarpment to the 
northeast, and its beds also dip 10° south, nearly opposite the cation of the 
Gallinas. This facade rises to from 600 to 900 feet elevation, and is cleft 
to the base by a deep gorge, the Catloncito de las Yeguas. I traversed this 
fissure, passing entirely through to the elevated country to the westward. 
Six miles from its mouth is a large pool, fed by a spring known as the Mare’s 
Spring. The canon is narrow, and the walls almost perpendicular. They 
are composed, at the “ puerta”, or entrance, of a moderately hard, reddish- 
brown sandstone. The canon is twenty miles in length; its bottom has a 
gentle rise ; and, as the sandstone has a gentle dip toward the west as well as 
