0. C. Marsh—Lake-basins of the Rocky Mountains. 51 
Il. Miocene Lake-basins. 
lakes. The best exposures of these Miocene beds are seen 
near the White River, and this name has very appropriately 
been used by Prof. Hayden to distinguish the lake basin in 
which they were deposited. In Northeast Colorado the same 
formation is well developed. The “Bad Lands” there were 
discovered and first explored by the writer in 1870.* These 
Miocene strata rest, with more or less unconformity, on an ex- 
tensive series of lignite-bearing beds, which in many respects 
resemble those beneath the Eocene basins. The age of these 
beds, also, is in dispute, but the remains of Dinosaurs and some 
other typical Mesozoic vertebrates, which have now been found 
sand were deposited over the same area. s both series of 
the dividing line in many places can be made out only by 
means of the vertebrate fossils they contain. In this way, the 
writer has recently ascertained, by personal observation, that 
most of the upper beds (D and E), 500 feet at least in thick- 
hess, which were called Miocene by Prof. Hayden,t are deposits 
of the more recent Pliocene lake. This would leave for the 
true Miocene beds a thickness of about 300 feet. The upper 
strata will be again referred to in considering the Pliocene lake. 
The fauna of the White River lake-basin is now well known 
to naturalists. It indicates a climate much less tropical than 
that of the Eocene lakes, as is seen in the absence of monkeys 
and scarcity of reptilian life. The Brontotheride, the largest 
known Miocene mammals, are peculiar to the lower strata of 
this basin. They fully equalled the Eocene Dinocerata in size. 
*This J .1, p. 292, Sept., 1870. 
{ trencoctoa American Phil Soe vol. xii, p. 105, 1862. 
