120 ON THE GEOLOGY AND NATURAL HISTORY 
terial in the form of surface beds or in layers thrust between the fossiliferous strata. 
These igneous beds thin out rapidly as we recede from the point of effusion. A large 
number of these centres of protrusion may be seen along the slope of the mountains west 
of the Judith range. ‘The erupted material sometimes presents a vertical wall three hun- 
dred feet high, then suddenly thins out and disappears. The Judith, Bear’s Paw, and 
Little Rocky mountains seem to be composed for the most part of granite and other rocks, 
with igneous protrusions here and there. I have in a former paper expressed the opinion 
that the central portions of our mountain ranges are composed of feldspathic granite, and 
to a certain extent this is true in regard to the more eastern outliers, but more recent 
observations have convinced me that these rocks, which I have defined by the term erup- 
tive, compose by far the greater portion of the mountain masses of the West. 
We have already alluded to the fact that the Potsdam sandstone in its western exten- 
sion, was first made known as occurring in the Black hills. It here rests upon the up- 
turned or nearly vertical edges of the schists, clay slates, and granitoid rocks, and the 
inference was drawn that the same rock would be found forming an outcropping belt all 
along the eastern slope of the Rocky mountains. Subsequent explorations have shown 
that it also occurs along the margins of the Big Horn range, near the summit, holding the 
same relative position, and exhibiting the same lithological characters. A few thin layers 
of fine.calecareous sandstone were observed, filled with fossils characteristic of this period. 
At the head of La Bonte creek in the Laramie range, I noticed a bed resting discordantly 
upon azoic slates, fifty to one hundred feet in thickness, holding the same position and 
possessing the same lithological characters which it reveals at other localities. I could 
discover no fossils in it at this point, but I am confident that this bed represents the Pots- 
dam sandstone. The same bed seems to occur all along the mountains from Laramie peak 
to Cache la Poudre creek, underlying the well-known Carboniferous strata, and resting 
upon the decomposing granitoid rocks, which form the nucleus of the first or lower ridge. 
This rock (the Potsdam) is more or less changed by heat from beneath, but I was able to 
trace it continuously from the source of the Chugwater creek to the source of Cache la 
Poudre, a distance of over one hundred miles. It was also seen along the eastern slope 
of the Wind River mountains, but did not contain any organic remains. 
The above facts show very clearly that in its western extension, the primordial zone of 
Barrande is represented only by a thin bed of sandstone, never exceeding one hundred and 
fifty feet in thickness, and that is seen only in a very narrow outcropping belt near the 
margins of the mountain crests. ‘The stratified azoic rocks upon which it rests discord- 
antly, so far as my observations have extended, never reach a very great thickness in the 
West. i 
On both sides of the divide of the Rocky mountains, so far as our explorations have ex- 
