O. C. Marsh—Lake-basins of the Rocky Mountains. 49 
Art. [X.—Ancient Lake basins of the Rocky Mountain Region ; 
by Prof. O. C. MarsH. 
THE existence of several large fresh water lakes in the Rocky 
Mountain region, during Tertiary time, is now well established, 
mainly through the researches of explorers whom the striking 
scenery of the ‘“ Bad Lands,” or the extinct animals entombed 
in them, have attracted thither. The geological age of some of 
these lakes, however, seems to be still in doubt, at least widely 
different opinions on this point are freely expressed. The ex- 
tent of these various lake-basins, and their relations to each 
other, are likewise a subject on which information is especially 
desirable, and hence the results of some recent observations 
are here presented. 
The deposits left in these old lakes show them to be of 
Eocene, Miocene, or Pliocene age, the fauna of each formation 
being entirely distinct, as well as quite different from existing 
species. 
I. Hocene Lake-basiis. 
The oldest of the great Tertiary lake basins of the West are 
of Eocene age. The one first discovered and best known, 
which has been called the Green River basin, lies between the 
ky Mountains and the Wasatch range, in the depression 
now drained by the Green River. It has the Uintah Mountains 
for its southern border, and extends north at least as far as the 
ind River range. This basin was visited by the writer in 
1868, but first explored in 1870, when he traced its deposits for 
several hundred miles, and from the rich vertebrate fauna fully 
determined its Eocene age.* These same beds have since been 
pronounced Miocene by Prof. Hayden and others, but the 150 
Species of extinct vertebrates now known from them prove 
been determined in this country. A comparison of almost any 
group of these fossils with the corresponding one from the 
Paris basin will afford sufficient evidence on this point. Some 
of the extinct mammals, indeed, indicate a still lower horizon, 
The latter are in part brackish water beds, containing, with 
Some characteristic Cretaceous fossils, many remains of plants, 
Which have led Hayden, Lesquereux, and others to regard them 
as Tertiary. The evidence from the fossil plants is far from 
* This Journal, vol. i, p. 191, March, 1871. 
Am. Jour. Sct.—Turrp Serres, Vou. IX, No. 49.—Jan., 1875. 
