of the Rocky Mountains. 71 
into a friable sandstone arranged in thin layers, with the lamine 
quite oblique, overlaid by a considerable thickness of conglom- 
erate. The dip is about 20° east. Resting upon these supposed 
Potsdam rocks at this point and inclining at about the same an- 
gle are layers of limestone, containing numerous fossils which 
prove them to belong to the Carboniferous age. 
Again, farther southward along the same range, near the source 
of the Chugwater river we find the same limestones, well devel- 
oped, containing some Carboniferous fossils, and underneath them 
and inclining in the same direction is a group of strata of a brick 
red color, more or less changed by heat, holding the position of 
the Potsdam sandstone in other localities. In some places these 
mountains, washed down by the stream sa e 
closely resembling the rock under consideration. Near 
Sources of Powder river we penetr to the nucleus of the 
Ose and clay slates of the Azoic series, in very nearly the same 
Manner as in the Black Hills before described. The Potsdam 
sandstone in this region is quite well developed, attaining a 
thickness of 200 feet, and exhibiting its usual variable lithologi- 
