16 
ON THE GEOLOGY AND NATURAL HISTORY 
from beneath. The fossils are Productus, Choneies, Spirigera, probably S. subtilita , 
Spirifer, Retzia mormoni, Rhynconella uta, corals and crinoidal remains. Nine miles farther 
to the westward we find the Carboniferous rocks again exposed by the wearing away of 
the Tertiary beds in the valley of Cottonwood creek. Here we have a series of limestone 
eighty feet in thickness, with a central axis similar to the exposure previously noticed, the 
strata apparently dipping northeast and southwest at an angle of about 10°, while the 
Tertiary beds again rest unconformably upon their upturned edges. The lower strata 
which are exposed in the channel of the creek, have been subjected to considerable heat, 
so that their color is changed to a brick red. 
Along the base of the mountains between the Laramie fork and the North Platte no Car¬ 
boniferous rocks are exposed; indeed all the older fossiliferous beds are concealed by a recent 
heavy deposit, consisting of gravel and water-worn boulders, from all the geological forma¬ 
tions represented in this region. This superficial deposit extends along the Laramie fork 
for a considerable distance from the mountains, the Tertiary beds being revealed here and 
there in the channel of the river. About twelve miles above Fort Laramie, both on the 
Laramie river and on the Platte, a remarkable deposit was observed, composed of a coarse 
conglomerate, fifty to one hundred and fifty feet in thickness, of a recent date and evi¬ 
dently accumulated since the rivers occupied their present beds. Indeed the form of the 
deposit is that of a basin twelve or fifteen miles in length, and reaching its greatest thick¬ 
ness only in the valleys of the rivers, while the elevated ridge between the rivers reveals 
the true Tertiary beds. The rock seems to vary in structure from an aggregation of parti¬ 
cles of quartz or a quartzose sandstone to an exceedingly coarse conglomerate, made up 
of every variety of material, much of which I have not yet seen in place. It appears to 
have been formed during the drift epoch, possibly at a later period by the damming up of 
the two streams near this junction, and the materials were doubtless transported by strong 
currents from the mountains in the vicinity. 
North and northwest of Fort Laramie we observe quite extended areas occupied by 
Carboniferous limestones with metamorphic and granitoid rocks. These exposures lie 
along the line of connection between the Laramie range and the Black hills. Surrounding 
all these exposures and occupying the valleys we see the Upper Tertiary beds, but never 
conforming to the older rocks. Near ILorse-shoe creek along the base of the mountains, the 
Cretaceous, Jurassic, Carboniferous, and Potsdam formations are exposed, the strata some¬ 
times inclining high upon the sides of the ridges of elevation. The White river Tertiary 
beds continue up to La Pule, when they gradually give way to the Lignite Tertiary beds, 
though remnants are still seen farther up the river. Here the White river Tertiary strata 
rest upon the Lignite beds, showing most conclusively their relative ages. 
Southward of Laramie river along the foot of the mountains we have a continuation of 
