50 0. C. Marsh—Lake-basins of the Rocky Mountains. 
beds has been clearly made out at several different localities. 
At one of these, in 1870, the writer found above a seam of 
coal Ostrea congesta Conrad, a typical Cretaceous fossil, and a 
crinoid allied to Marsupites of the English Chalk. Below the 
coal, but in the same series, were remains of fishes and _ turtles, 
both of Cretaceous types, and teeth of a Dinosaur.* Near! 
conclusive evidence has since been found at other localities. 
In considering a question of this kind, where the evidence 
from fossils appears conflicting, it should especially be borne in 
mind that vertebrates afford a much more accurate guide to — 
climatic and other geological changes than invertebrates, and — 
vastly more so than plants. 
This Eocene lake basin remained dry land during all of Mio- 
cene time, and perhaps much longer. It was then again sub- — 
merged for a short period, and its eroded surface covered with 
water-worn debris from the surrounding mountains. The evi — 
dence of this is seen in the coarse conglomerate crowning the — 
highest buttes, which have thus escaped in part the enormous — 
denudation most of the deposits in this basin have suffered. 
South of the Uintah Mountains, a second and larger lake ex- — 
isted in the Eocene. It was 2,000 feet or more lower than the — 
northern lake, and received part of its waters from that source. — 
It had the Rocky Mountains for its eastern border, the — 
New Mexico. This basin was first explored, and its Eocene — 
age established, in 1870, by the writer, who finding it distinct 
The fauna entombed in both these Eocene lakes is essen- 
tially the same, and indicates a tropical climate. is 18 
gon west of the Blue Mountains, but as only a few plants have 
itherto been found in its deposits, its relations in time to the 
great central Eocene lakes cannot as yet be determined. 
* This Journal, vol. i, p. 195, March, 1871. + Loc. cit., p. 196. 
