OF THE UPPER MISSOURI. 
15 
from one inch to four inches in diameter, cemented together with a silicions paste. Large 
masses of this conglomerate have fallen to the base of the hills or are scattered over the 
plains below. We also pass through a large area covered with sandhills after leaving the 
Niobrara. These hills all have a dull reddish tinge, evidently from the eroded materials 
of bed 1). One of these hills was one hundred and eighty feet above the surrounding 
prairie with very steep sides, its present conformation being preserved by the roots of vast 
numbers of a species of Yucca ( Y. angustifolia), which cover the hill and seem to attain 
their maximum growth in the sand. The sandhills are composed of the eroded materials 
of the different Tertiary beds, and from the loose incoherent nature of the sand, they 
suffer continual change of form and position by the action of winds. 
On Rawhide Butte creek bed I) approximates more closely in its character to the 
Oreodcnbed B of the general section, at Ash Grove spring and Bear creek. In the valley 
of the creek, on an exposed or denuded area not more than eight or ten yards square, I 
observed fragments of a species of turtle (Tehtudo Ntbrascensis ) belonging to at least eight 
individuals, with a few mammalian remains (Oreodon Culbertsonii). The Upper Miocene 
beds occupy the country in the vicinity of Tort Laramie exclusively, and extend to the 
base of the Laramie mountains. Bed E attains the greatest thickness, having been eroded 
away to a great extent, while bed D becomes one hundred and eighty to two hundred feet 
in thickness. The channel of the Platte river cuts through Pliocene and Miocene strata 
alone from Fort Laramie to longitude 98°, a considerable distance below Fort Kearney. 
CHAPTER IV. 
GEOLOGY IN 'IIIE VICINITY OF FORT LARAMIE. 
The plain country in the vicinity of Fort Laramie is underlaid for the most part by the 
upper members of the White river Tertiary beds. By referring to the Geological map, it 
will be observed that west of Fort Laramie, between the two main branches, Laramie and 
North Platte, but two small exposures of the Carboniferous rocks occur. As we proceed 
westward toward Laramie peak the first exposure is seen near the head of Warm Spring 
creek, occupying an area of not more than five or six hundred square yards. Its upheaval 
is probably local, the limestone being revealed by the erosion and removal of Tertiary beds, 
which are in this immediate region apparently undisturbed, and lie unconformably against 
the upheaved mass. The limestone strata dip in every direction from a central axis. The 
fossils are quite abundant, but the hard and brittle character of the rock renders it next 
to impossible to secure perfect specimens. At the base of the exposure are two or three feet 
of ferruginous shale, the lower layers of which seem to have been slightly affected by heat 
