OF THE UPPER MISSOURI. 
127 
tween the North Platte and Wind river they reach a thickness of four hundred feet. From 
this divide, throughout the Wind river valley, they occupy the greater portion of the 
country, and though inclining in the same direction with the older strata, the beds do not 
dip more than 1° to 5°. They differ from the other deposits in the great predominance 
of arenaceous sediments, and in the absence of vegetable remains, but they contain frag¬ 
ments of turtles and numerous freshwater and land shells, of the genera Helix , Planorbis, 
Vivipara , &c. The entire thickness of these deposits may be estimated at from fifteen 
hundred to two thousand feet. From the fact that these deposits do not conform to the 
true lignite beds, and that detached portions are seen lying upon the sides of the moun¬ 
tains but slightly inclined, while the corresponding beds are shown in the valley below, 
we infer that they were accumulated long before the mountains were raised to their 
present height, or perhaps during the gradual process of elevation. This is especially 
shown at the upper end of the Wind river valley. Passing over the Wind Itiver moun¬ 
tains, we again see them holding the same position on the western slope, and possessing 
the same lithological characters. While the lignite beds on the west side of the divide 
incline at a large angle, the more recent beds, although in some places occupying the very 
crest of the mountains, seldom incline more than three to five degrees. 
The most interesting additional facts which we have obtained in regard to the White 
river Tertiary beds, are their geographical extension, and the evidence of their age in rela¬ 
tion to the Lignite Tertiary deposits. We can now show beyond a doubt that the former 
must have been accumulated long since the latter. We have ascertained that they extend 
southward along the Laramie mountains to Willow springs, within ten miles of Cache la 
Poudre, that they also extend up the North Platte to the Box Elder creek, and even beyond 
are small outliers, showing that much has been removed by erosion. Passing over into the 
Laramie plains, we find at the source of the Box Elder, and extending over to the head of 
Bates’s Fork, a large development of this Tertiary, and it also reaches far westward to the 
Medicine Bow mountains. We also know from the observations of Dr. Hines, that it oc¬ 
cupies a considerable area among the Sweet-water mountains, extending over into the 
Green river valley. We have, along the North Platte, the overlapping of the White river 
beds upon the Lignite strata, thus affording the evidence of superposition for their relative 
age. The same fact was noticed between the north fork of the Shyenne and the head of 
Cherry creek, where beds of marl and limestone, containing Planorbis , Limnea , &c., the 
same as are seen in the Bad Lands proper, repose upon true Lignite Tertiary strata. 
Again, while the White river beds hold for the most part a horizontal position, those of 
the Lignite Tertiary are often much disturbed. Near the Black hills, the former seem to 
have been elevated to a considerable height by the upheaval of the mountains, but they 
do not, in any case, incline more than one degree, while north of the Black hills the lig- 
