OF THE UPPER MISSOURI. 123 
the aggregate thickness of the gypsum strata is about one hundred feet, while near the 
source of Snake river there is a thickness from fifty to eighty feet. It also occurs to a 
considerable extent at the foot of the mountains, on La Bonte creek, a branch of the 
North Platte. 
At the time of writing the preceding report, no division had been made of the red beds 
separate from the Jurassic, because at that time there was no proof that they were distinct, 
no organic remains having been obtained from them. We only knew that they held a 
position below the well-established Jurassic rocks, and that, so far as had been observed, 
there was no physical break in the sequence of the strata. These red beds, however, when 
compared with similar red rocks in the Old World, had been often referred to the age of 
the Triassic or New Red Sandstone. The explorations of Dr. Newberry in New Mexico 
revealed quite a number of species of plants and the bones of a large saurian animal which 
seem to direct his mind toward the Triassic epoch. The plants discovered by Dr. New- 
berry in New Mexico were referred by him to the genera Zamites, Pterophyllum, &c., and 
regarded as similar to those of the Keuper (Upper Trias) of Europe. Though the evidence 
so far obtained points quite strongly toward the Triassic epoch, it is not yet considered 
sufficiently clear to warrant a positive opinion in regard to their age. If these red arenaceous 
deposits really represent a distinct geological epoch, it seems quite strange that they have 
as yet yielded so few organic remains. They have already been examined with considerable 
care over an area, in the vicinity of the Rocky mountains, extending from latitude 49° far 
southward into New Mexico. On the west side of the Wind River mountains we have 
discovered fossils beneath the red beds, which may include those in the Jurassic. 
The Jurassic rocks are everywhere revealed overlying the red deposits just mentioned, 
and possess an equal geographical extension. 
Their fullest development and most fossiliferous etinditic seems to be along the margins 
of the Black hills, where they have furnished the most satisfactory evidence of their age. 
Along the northeastern slope of the Big Horn mountains, this group of rocks presents its 
usual appearance of gray and whitish calcareous and arenaceous layers, with indurated 
somewhat variegated beds of more or less laminated marls, containing in great abundance 
Belemnites densus, Pentacrinus asteriscus, a new species of Ostrea, Pecten, &e. 
At Red buttes we find a fair development of these beds with the same fossils, but as we 
proceed southward toward Long’s Peak, the intercalated laminated marls disappear, and 
the whole formation seems to be reduced to a thickness of fifty to one hundred feet, with 
very few fossils. Along the southwest side of the Big Horn mountains and the northeast 
side of the Wind River mountains we have a thickness of Jurassic rocks from eight hundred 
to one thousand feet, containing organic remains in the greatest abundance. Crossing the 
Wind River mountains we observed the strata corresponding to those upon the eastern 
side with Belemnites densus, Ostrea, &c. Returning to the eastern slope at the sources of 
