OF THE UPPER MISSOURI. 37 
ral Sciences, in March, 1858. Its largest development 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 nucleus of the eleva- 
tion. I observed no positive indications of this formation in the Laramie mountains or at 
Rawhide peak, but in most places the Carboniferous strata rested unconformably upon the 
metamorphic rocks, except in a few localities where a quartzose limestone which is of 
doubtful age is interposed. From lithological characters and position, I infer that a series 
of beds resting upon metamorphic rocks at the head of Niobrara river, are of the age of 
the Potsdam sandstone. Proceeding northward from Fort Laramie, we do not meet again 
with this formation until we reach the southeastern side of the axis of elevation, when we 
find it resting unconformably upon gneiss, hornblende, argillaceous and talcose slates, &c., 
and composed of a variegated gray and reddish-gray quartzose sandstone, filled with small 
plates of mica. Some parts of it are very compact and silicious, others a coarse friable 
grit containing seams almost wholly composed of broken fragments of shells cemented 
with a fine calcareous grit. The more compact masses contain fossils which are quite 
well preserved, among which can be recognized species of Lingula, Obolus, and Trilobites, 
similar to or identical with those found in the Potsdam sandstone in other well-known lo- 
calities. The existence of this formation in some of the important outliers of the Rocky 
mountains being established beyond a doubt its geographical distribution in the far West 
then becomes a question of the highest interest. We are now prepared to believe that it 
is exposed by upheaval all along the eastern slope of the mountains to a greater or less 
extent, and probably coextensive with the Carboniferous, Permian, Jurassic, and Creta- 
ceous rocks. I will here cite some examples from published reports of beds of sandstone 
which correspond both in their lithological characters and position to the Potsdam sand- 
stone of the Black hills. Hall, in Stansbury’s Report, says that Stansbury’s island (Great 
Salt lake) is three thousand feet high, capped with Carboniferous limestone, which also 
rests upon a coarse sandstone and conglomerate. Again, north of Great Salt Lake city 
the limestone overlies a coarse sandstone and conglomerate, which almost invariably ac- 
companies 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 Railroad Reports, page 156, speaks of a sandstone occurring in the 
Aztec mountains. He says: “ We travelled seven miles upon the granite, and on our 
right we found a cliff twelve hundred feet in height. 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. 
