NATURAL SCIENCES OF PHILADELPHIA. 
141 
Peak, on the north-eastern side of the Black Hills, is another example of the 
protrusion of these basaltic columns which are also five-sided, the sides vary- 
ing from eight to twenty inches in width. Some of the columns lie in a nearly 
horizontal position, the greater portion, however, inclining at an angle of 20^ 
to 40*^. Bear Peak is an outburst of porphyritic trappean rocks, specimens of 
which closely resemble the Tertiary lavas of the Pacific coast. All the Creta- 
ceous beds and all the Jurassic to the blue limestone E of vertical section* 
are upheaved around Bear Peak. 
From the foregoing examinations I am led to the conclusion, that the main 
body of the Black Hills and the Laramie range is composed of granite of very 
ancient date. That it is older than the oldest fossiliferous rooks is obvious from 
the fact that in both localities several hundred feet of stratified azoic rocks are 
interposed between it and the Potsdam sandstone. I am also of the opinion 
that the azoic strata overlying the granite on the eastern slope of the Rocky 
Mountains are similar in lithological characters, and hold the same geological 
position as the azoic rocks so largely developed around Lake Superior and in 
Canada. 
n. Lower Silueian Rocks — Potsdam Sandstone. 
The evidence of the existence of this formation in the region of the Rocky 
Mountains was ascertained, for the first time, in the summer of 1857, during 
Lt. Warren's Exploration of the Black Hills, as has already been shown in a 
paper read before the Acad. Nat. Sci., in March 1858. Its largest developement 
and only fossiliferous condition is found in the Black Hills, where by upheaval 
it is exposed in the form of a narrow belt or zone engirdling the azoic and 
granitic rocks which form the central axis of elevation. I observed no positive 
indications of this formation in the Laramie Mountains or at Raw Hide Peak, 
but in most places the Carboniferous strata rested unconformably upon the 
metamorphic rocks, except in a few localities, where a quartzose limestone 
which may be of Devonian ? age was interposed. But at the head of the 
Niobrara river, a series of horizontal beds were exposed, resting upon the verti- 
cal edges of the metamorphosed rocks, which from their lithological characters 
I have considered as belonging to the Potsdam sandstone. 
The following section will show the descending order of the beds. 
c. — A yellow and reddish yellow, heavy bedded, friable sandstone, composed 
of an aggregation of quartz grains, cemented by calcareous matter, 
sometimes becoming a conglomerate, consisting of rounded quartz 
pebbles, .......... 22 feet. 
h. — Fine compact, reddish clay slate, 5 feet. 
a. — Very nearly like bed c, only more compact and heavy bedded, of a light 
gray color, sometimes yellow with a reddish tinge, . . 37 feet 
A series of metamorphic vertical strata, consisting of gneiss with silvery 
mica in large plates, micaceous and talcose slates, white quartz, &c. 
Proceeding on our route northward, this formation was not again seen untii 
we arrived at the main axis of elevation of the Black Hills. Near our camp, of 
September 24th, on the southeastern side, I observed resting unconformably 
upon gneiss, hornblende, mica slate, &c., a variegated, gray and reddish gray 
quartzose sandstone filled with small plates of mica. Some parts of it were 
compact and silicious, others a coarse friable grit, and at this locality seldoiri 
becoming a conglomerate, but containing seams almost entirely composed of 
comminuted fragments of shells, cemented with a fine grit. The more 
compact masses contained some fossils that were quite well preserved, among 
which were species of Lingula, Oholus and Trilobites, similar to, or identi- 
cal with, those found in the Potsdam sandstone in other well known localities. 
* See vertical section of rocks in the Black Hills, in a paper by F. B. Meek and F. V. 
Hayden, Proc. Acad. Nat. Sci., March, 1858. 
1858.] 
