98 GEOLOGY. 
At some distance east of Bent’s Fort, a hard, compact, gray limestone has an outcropping 
on the road. It does not seem to include any organic remains; but a yellow red sandstone 
lying over it contains numerous impressions of shells. It is so soft that in transporting them 
the specimens of it were crushed to powder in the first hour, and I am left unable to determine 
to what species they belong. As much as I can recollect of them, they may belong to some 
species of pecten. 
The chain of high and steep bluffs which begins some miles above Bent’s Fort, is chiefly 
made up of two kinds of limestone; the upper one white, pure, hard, and fine-grained nearly 
to compactness, includes no organic remains, and would, if located in a less remote part of the 
country, offer a very valuable material to the practical arts, or for building purposes; the 
lower, a brownish rock, is interspersed with thin layers of crystals of carbonate of lime, and 
bears some indications of petrifactions, but so undistinguishable that it is hardly possible to 
make out what they are. The stratification of both is horizontal. To judge partly from what 
I saw myself, partly from specimens brought to me, a blackish, hard limestone and a soft shale 
support them ; the latter cropping out some twenty miles west of the fort in a kind of bottom 
land. 
Having crossed the Arkansas near the mouth of the Apishpa, (presumed Huerfano,) our road 
leads over a gravelly and, in general, sterile soil—we are in the great American desert—and 
over sandstone, through which a number of creeks and rivers have cut their channels. The 
underlayers of this sandstone are a yellowish, hard, fine-grained rock, the upper part soft, and 
eaten through by the atmospheric agents. 
A few miles east of the cafion through which we entered the mountains proper, a hard, 
compact, dark-gray limestone, with some slight traces of petrifactions and sulphuret of iron 
interspersed, and above it a limestone which is apparently identical with the lower brownish 
limestone near Bent’s Fort, crop out with a dip of 8° to 9? to the northeast. They rest on a 
hard silicious shale, through which igneous rocks have made their eruption near the mountains, 
forming buttes of sometimes remarkable shape. The most remarkable of them is the Huerfano 
butte, a little mountain of conical shape, consisting of black granite (quartz and black mica) 
and the just-mentioned hard silicious shale, the latter lying on the top of the butte. 
In concluding the geology of the Plains, I will here mention a kind of geological riddle 
which I have not been able to solve: About fifteen yards distant from the above-mentioned 
limestone hills of the cretaceous period, there is a little butte almost as high as the hill to 
which it stands nearest—about thirty-five feet—and of a shape which will be best seen from the 
accompanying drawing of it. It stands quite isolated, and consists not of limestone, but of a 
Isolated Shale Butte, standing adjacent to the limestone hills in the valley of the Upper Arkansas, 
soft foliating shale. Even should that shale be found to support the limestone of these hills—. 
_ which I could not decide, not having any instruments to dig into the ground—it would still - 
| 
