52 O. C. Marsh—Lake-basins of the Rocky Mountains. 
A still more ancient Miocene lake existed, in about the’same 
latitude, on the Pacific slope, near the central part of the present — 
State of Oregon. The Blue Mountains formed the eastern and 
southern shores of this lake, but its other limits are difficult to 
ascertain, as this whole country has since been deeply buried 
by successive outflows of volcanic rocks. It is only where the 
latter have been washed away that the lake deposits can be ex- 
amined, The discovery and first explorations in this basin 
were made by Rev. Thomas Condon, the present State gcolous 
of Oregon. The typical localities of this Miocene basin 
along the John Day River, and this name may very propel 
be used to designate the lake-basin. The strata in tie basin | 
are more or iene inclined, and of great thickness. One section, _ 
near the Jo ay River, examined by the writer in 1871, and — 
again in 187! 3, seems to indicate a thickness of not less ‘than 
5,000 feet. The upper beds alone of this series correspond to — 
the deposits in the White River basin. The lower portion also is 
clearly Miocene, as shown by its vertebrate fauna, which differs — 
in many respects from that above. Beneath these strata are ; 
seen, at a few localities, the Eocene beds containing fossil plan . 
mentioned above. They are more highly inclined than the — 
Miocene beds, and some of them show that they have been pe : 
jected to heat. The inferior strata elsewhere are Mesozoic, : 
apparently Cretaceous. Above the Miocene strata, Pliocene ‘bed : 
are seen in a few places, but baat covers nearly all. : 
IL. ae Lake- aster: 
the Rocky Mountains. A great Pl iocene lake was thus forme 4 
(=) 
cal strata for r more than 200 miles of its course. q 
The beds in this basin lie nearly horizontal. They are hate ‘ 
in color, and much more arenaceous than the Miocene below. — 
e€ upper strata consist of hard sandstones or calcareous | 
grits, which weather but slowly, and hence still form the great | 
table-lands over much of the area of the basin. The writer — 
has traced these high plateaux and the intervening isolated 
buttes from near the Black Hills south to the Arkansas River, — 
and found them all of Pliocene age. South of the Smoky Hill | 
River these strata rest directly on 1 the Cretaceous, a 
e fauna of this lake-basin indicates a warm temperate | 
climate. The more common mammals are a mastodon, rhinoce — 
roses, camels and horses, the latter being nagiectally abundant. — 
[To be continued. ] 
