OF THE UPPER MISSOURI. 23 
I. Potsdam sandstone (oldest Silurian) containing Lingula, Obolus, and fragments of Trilobites. 30 to 
50 feet. 
Silurian. 
J. Highly metamorphosed strata standing vertical. 
K. Coarse feldspathic granite, forming mountain masses, 
As we approach the loftier ridges of the Black hills, we find them capped with an 
immense thickness of very variable sandstone, which doubtless belongs to Lower Creta- 
ceous formation No. 1, though it may pass down into Jurassic strata. No fossils were 
observed in it, excepting traces of uncharacteristic vegetable impressions with fragments 
of wood. One of these ridges by barometrical measurement was found to be thirteen 
hundred feet high, covered with pines and forming a portion of the Black hills proper. 
On the east side of the ridge the strata of sandstone slope gently down to the base; but 
the west side is abrupt, revealing the edges of the different beds, so that we see the sand- 
stone before mentioned as attaining so great a thickness, passing down into alternate layers 
of gray sandstone and clay ; containing fossils of the genera Ostrea, Ammonites, &c., with 
a freshwater deposit composed of a calcareous grit, with hard, dark-gray concretions con- 
taining fossils of the genera Unio, Planorbis, &c. Then comes a series of alternating beds 
of gray and red grits, and sandstones with numerous fossils of Jurassic types. In but one 
locality was any lignite observed in No. 1. Two seams were noted, one of them two to 
four feet in thickness, the other, six to eight feet, alternating with variable grits. 
Near the head of Beaver creek on the west side of our road, we observed a ridge about 
four hundred feet in height, running northwest and southeast, presenting a nearly verti- 
cal front, the different beds of which appear to be undisturbed; but looking upon the op- 
posite side, we find that the strata dip towards the prairie below at an angle of 20° to 30°. 
This ridge is composed of a great thickness of No. 1, passing down. into variegated clays 
and grits one hundred to a hundred and fifty feet in thickness, from which I obtained Am- 
monites, Belemnites, and other fossils of Jurassic types in great abundance. Pursuing our 
course eastward, we passed through a valley between two of the upheaved ridges which 
surround the Black hills. In this valley we observed a bed of bluish ash-colored lime- 
stone (E of vertical section), which presents many peculiarities from the disturbing influ- 
ences which have elevated this portion of the country. Sometimes it forms over large 
areas a sort of tessellated pavement from the peculiar fracture of its surface; again, it is 
puffed out, as it were, forming rounded protuberances thirty to sixty feet in height, the 
external surface yielding so as to adapt itself to any inequalities. It thus holds a great: 
variety of positions. All along the valleys and on the hills are large exposures of the 
brick-red grit bed D, with intercalated seams of gypsum varying in thickness from one 
inch to four or five feet. In one instance I saw a local bed of gypsum twenty feet in 
thickness. 
