THE EOCENE PLATEAU. 
13 
The published notices of the paleontology of the Mesozoic formations 
explored by me are the following-: 
1875.—Annual Eeport of Chief of Engineers, 1875, II, 983. Cretaceous No. 3 of the 
Sangre de Cristo Pass. 
1875.—Proceedings of Academy of Philadelphia, 1875, p. 265. Trias of Galliuas 
Mountains. 
2.- THE EOCENE PLATEAU. 
The discovery of the Eocene deposit and its contained fossils was the 
realization of an anticipation of its existence, which I embodied in a report 
to Dr. Hayden the previous year (1873), and which was published* before 
I left for the field in 1874. Investigations into the stratigraphy of the 
southern Rocky Mountain region had demonstrated that the elevation of 
this part of the continent took place earlier than the corresponding regions 
in the north; in other words, that the elevation extended from the south to 
the north. This is shown by the greater exposure of the Mesozoic beds in 
the south, in connection with the abundance of lignite and other indications 
of extended land-surfaces during the Cretaceous period, and by the absence 
of lignite of the upper or Fort Union epoch, which is the coal-producing 
horizon of Colorado and the north. 
Supposing that the sudden appearance of faunae and florae is not due 
to creation, but to migration, I was led to look for the origin of the rich 
vertebrate fauna of the Bridger Eocene in connection with the abrupt dis¬ 
appearance of the Saurian fauna of the Fort Union Lignite Cretaceous, 
whose strata in Wyoming immediately underlie it. The change of fauna is 
strikingly a,brupt, passing from a Mesozoic to a Tertiary character without the 
intervention of extensive non-fossiliferous deposits. The succession of beds 
from the one to the other is in many places uninterrupted, and, according 
to Dr. Hayden, without non-conformity. There is no evidence whatsoever 
of terrestrial disturbance, such as would produce a great destruction and 
re-introduction of life. In fact, physical evidence, as well as biological law, 
is in favor of the view that the Tertiary fauna migrated from another region, 
and replaced the Mesozoic type of Saurians which had until then occupied 
the field. And in view of these facts I remarked {1. c., p. 16 ; Annual Report 
♦Bulletin of the United States Geological Survey of the Territories, No. 2,1874. 
