
ROCKY MOUNTAINS—REVIEW OF SECTION. 73 
Potsdam.* Near the Snake River, on the western side of the water-shed, 
we again find as a conspicuous feature, a “blue, cherty Carboniferous 
limestone,” overlaid by siliceous rocks.f In the Black Hills of Dakota, 
yellowish Carboniferous limestones appear, and rest on grey and ferrugin- 
ous Potsdam sandstones, holding characteristic organic remains. The 
latter formation was here discovered for the first time, west of the 
Missouri River, by Dr. Hayden, in 1857.+ 
162. The rocks representing the Carboniferous formation, in the 
Rocky Mountain region, would therefore appear to be wide-spread, and 
to be represented in great part by massive limestones, which are 
remarkably constant in ‘their lithological character. | 
163. A section examined by Dr. Hayden, in the Uinta Range, i in 
Utah, is interesting as a standard of comparison with those on the forty- 
ninth parallel. Here, red beds, supposed to be Triassic, and from 150 to 
200 feet in thickness, are first met with, and pass downward into grey 
sandstones, quartzites and indurated arenaceous clays; then, alternate 
beds of thin grey limestone and sandstones; and finally, a massive 
limestone, about 1,000 feet in thickness, holding Carboniferous fossils, 
Next, dull purplish sandstones, with a series of thin layers of slate and 
clay, gradually passing down into brittle reddish and grey quartzites, and 
being more compact and massive. In this whole series, from the red 
beds above, to the lowest quartzites, no unconformity is apparent; the 
junction between the base of the limestone and the underlying sandstone 
becoming clear and regular. The total thickness of the beds exposed is 
estimated at 10,000 feet, of which 8,000 feet, is composed of the lower 
sandstones and quartzites. Of these last, Dr. Hayden believes the upper 
beds to be Silurian, and to pass gradually downward without any break 
in time, to rocks of Huronian age; the lower purplish quartzites being 
compared with those underlying the Cretaceous and Carboniferous rocks 
in Dakota, which are supposed to belong to that period. § 
164. The general similarity of this with the Boundary -section is 
apparent, and the resemblance of the rocks underlying the great lime- 
stone in the two localities is particularly interesting. The lowest impure 
dolomites of Series A. might very well be called purplish quartzites on a 
superficial examination, and would be such but for the presence of a pro- 
portion of calcareo-magnesian matter. The magnesian limestone B. 

* Geological Report Yellowstone and Missouri Pte ation 1869., p. 70. t Ibid., p. 88. 
¢ U. S. Geol. Sury. Territ. 1857-59., p. 112, § U.S, Geol. Surv, Territ. 1870., pp. 49-52. 

