OF THE UPPER MISSOURI. 
121 
tended, a series of calcareous, areno-calcareous, and arenaceous beds are seen, which we 
have referred to the Carboniferous epoch. They vary in thickness at different points. 
Without specifying localities, it will be sufficient to remark, that all along the margins of 
any of the mountain elevations in the far West, these rocks are seen in a more or less in¬ 
clined position. 
Sometimes they are not visible for a short distance (as between the Laramie and Platte 
rivers, twenty or thirty miles), but it is plain that they have either been removed by ero¬ 
sion, or concealed by more recent deposits. Along the Big Horn mountains there are al¬ 
ternate layers of sandstone, arenaceous and magnesian limestones, many of which show 
oblique laminae and other indications that their deposition took place in shallow and per¬ 
haps turbulent waters. They are here developed to a thickness of one thousand to fifteen 
hundred feet, and incline high upon the sides of the mountains, at an angle of 50° to 70°. 
They contain few fossils, but these indicate rocks of the same age as those in the Black 
hills. Along the Laramie mountains, from the Red buttes to Pike’s peak, apparently the 
same limestones are seen inclining against the sides of the elevated ridges, at greater or 
less angles; and on the opposite side of the axis sloping down to the Laramie plains, the 
corresponding strata are seen, though leaning at much smaller angles, usually from 9° to 
15°. Along the Sweet-water and Wind River mountains, these rocks are highly deve¬ 
loped, and incline against the sides of the ridges of elevation as heretofore described. The 
corresponding portions are also seen on the west slope of the main range, at the sources 
of Green and Snake rivers, but not as conspicuously developed, the eruptive rocks pre¬ 
dominating. Crossing back over the dividing crest, near the sources of the Madison, Jef¬ 
ferson, and Gallatin forks of the Missouri, we find similar limestones largely developed, 
and covering a considerable area on the eastern slope. Near the junction of the three 
forks, and along Smith’s or Kamas river, we find them reaching a thickness of eight hun¬ 
dred to one thousand feet, often partially changed by contact with igneous rocks beneath. 
They were also observed around the Judith mountains, and also about the Bear’s paw and 
Little Rocky mountains. 
Nowhere in the Rocky mountain range, so far as my observations have extended, do the 
Carboniferous rocks seem to abound in oTganic remains, and the few usually seen are gene¬ 
rally found in a bad state of preservation, and comprise a limited number of species. The 
precise period to which these rocks belong, which are so persistent in all disturbed regions, 
is not positively known, the evidence from organic remains pointing to the age of the Coal 
measures, and sometimes to that of the Lower Carboniferous period; probably both mem¬ 
bers of the system occur there. 
At the foot of the Big Horn mountains, near the head of Powder river, I observed at 
one locality a series of beds which indicated the presence of Permian rocks. These beds, 
VOL. XII.—-16 
