142 PROCEEDINGS OF THE ACADEMY OF 
The following section at this point may serve to render more clear the order of 
succession of the beds. 
c. — Yellow magnesian limestone, ...... 50 feet. 
h. — Yellowish gray arenaceus limestone, ..... 40 feet. 
a, — Potsdam sandstone, as described, . . . . 30 to 50 feet. 
Azoic rocks standing vertical. 
Beds h and c are Carboniferous and conformable to bed a. We continue to 
see this formation whenever we approach the central portion of the Black Hills, 
and in some localities the greater part assumes the character of a coarse con- 
glomerate composed of worn fragments from all the varieties of rocks beneath. 
Sometimes the lines of lamination are very irregular, as if the materials had 
been deposited by ocean currents. I observed this bed on the northeastern 
side of the Black Hills near Bear Peak, dipping at an angle of 20° to 30*^. 
Inasmuch as we have no paleontological evidence of the existence of this 
formation in the Rocky Mountain range, we must depend upon the somewhat 
uncertain data of lithological resemblance and position, for its geographical 
distribution. Prof. Hall, in Stansbury's Report, often describes a bed of sand- 
stone corresponding in its lithological characters and geological position to the 
Potsdam sandstone in the Black Hills. Stansbury's Island, (Grreat Salt Lake) 
the summit of which is three thousand feet in height, is capped with carbonif- 
erous limestone, which also rests upon a coarse sandstone and conglomerate. 
Again, north of Great Salt Lake City, the "limestone overlies a coarse sand- 
stone or conglomerate, which almost invariably accompanies it." "In several 
localities, as at Promontory Point, and near Mud Island, the metamorphic 
strata appear to be overlaid by a coarse conglomerate or coarse sandstone, 
which is partially altered and assumes the character of a quartz rock. ' ' Marcou, 
in the third volume of Pacific R. R. Reports, page 156, speaks of a sandstone 
occurring near the Aztec Mountains. He says : ' ' We travelled seven miles upon 
the granite, and on our right we found a cliff twelve hundred feet high. From 
the base to the middle we found the granite, then a band of red sandstone, 
(Devonian or Old Red.) Above this, the beds of limestone and gray sandstone, 
belonging to the mountain limestone." The following day "we travelled three 
miles on the granite, the remainder on the Old Red Sandstone." 
The diagram given, showing the order of the superposition of the different 
rocks, would apply equally well to the similar beds in the Black Hills. Many 
other less evident indications along the base of the Rocky Mountains might 
be cited from published Reports, but what has been said will be sufB.cient to 
show what we may hereafter expect with regard to its geographical distribution 
in the far west. 
III. Devonian? Formation. 
The evidence of the existence of the Devonian formation near the eastern 
slope of the Rocky Mountains, is, as yet, quite obscure. Owing to the meta- 
morphosed condition of the rocks, the fossils have been wholly obliterated, or 
only indistinct traces of them remain. About twelve miles west of Fort Lara- 
mie, the Platte river cuts through a series of strata three to four hundred feet 
in thickness, resting unconformably upon metamorphosed azoic rocks. The 
upper members of the series contain undoubted carboniferous fossils, which are 
sufficient to fix, with a good degree of precision, their age ; but resting upon 
the azoic rocks, is a very hard, compact quartzose limestone, evidently meta- 
morphosed to some extent, which, from its position and lithological character, 
I am disposed to refer to Devonian, though it maybe of Silurian age. On the 
Platte river, it holds a horizontal position for the most part, but in a few local- 
ities the underlying azoic rocks are thrust up through it, distorting it at every 
angle. About ten miles north of Fort Laramie, near Raw Hide Peak, it is again 
exposed, the strata being vertical, alternating with soft, dark, blue clay slate. 
I did not see any indications of it in the vicinity of Laramie Peak, or in the 
Black Hills. 
[June, 
