36 
ON THE GEOLOGY AND NATURAL HISTORY 
a group of elevations with a granite nucleus also and surrounded by a series of azoic strata 
occasionally penetrated by dikes or outbursts of trappean rocks. 
“ The Black bills, or more properly mountains, lying between the forks of the Shyenne 
on the 44th parallel, between the 103d and 105th meridians, cover an area of 6000 square 
miles. Their bases are elevated from 2500 feet to 3500 feet, and the highest peaks are 
about 6700 feet above the ocean level.”* Juya-Kara peak is formed of vertical columns 
of basalt surrounded with trachytic rocks, which seem to have been thrown off from the 
summit when the columnar mass was protruded. Near Bear peak on the northeastern 
side of the Black hills is another example of the protrusion of these basaltic columns, which 
are five-sided, the sides varying from eight to twenty inches in width. At this locality 
some of the columns lie in nearly a horizontal position, the greater portion, however, in¬ 
clining at an angle of 20° to 40°. 
From our examinations we thus find that the important outliers on the eastern slope of 
the Ilocky mountains are formed of a granite nucleus surrounded by a series of azoic strata, 
composed of argillaceous and talcose slates, gneiss, syenite, quartzose, and calcareous rocks. 
From these facts and from published reports of numerous explorers we think we are war¬ 
ranted in the conclusion that the great central Ilocky mountain range possesses a similar 
geological and mineralogical character. The investigations of Sir John Ilichardson and 
Mr. Isbister indicate a striking similarity in the rocks all along the eastern slope from the 
Saskachewan to the Arctic sea, and the results of the other explorers show a like resem¬ 
blance far south to Mexico. 
The azoic strata which I have described as resting upon granite on the eastern slope of 
the Ilocky mountains appear to be similar in lithological characters and to hold the same 
geological position as the azoic rocks so largely developed around Lake Superior and in 
Canada. 
CHAPTEK VIII. 
II. Potsdam Sandstone (Lower Silurian). 
The evidence of the existence of this formation in the vicinity of the Rocky mountains 
was ascertained for the first time in the summer of 1857, during Lieut. Warren’s explo¬ 
ration of the Black hills, and first published in a paper read before the Academy of Natu- 
* Preliminary Report of Explorations in Nebraska and Dacota, in the years 1S55, ’G, and ’7. By Lieutenant 
Warren, Topographical Engineer. Page 67. 
