106 RECONNAISSANCE FROM CARROLL, MONTANA, 
fused mass. They show very little color in polarized light. They may be zeolitic; but a chemical 
analysis, which the circumstances do not now admit of, would be required to settle the point. 
In the ravine spoken of, 355 feet below the summit of Cone Butte, we were surprised to find a 
series of slates and sandstones. The cut (Fig. 7) will give some idea of the relations of the rocks, 
it being a sketch taken from a point below to the west. The total width of the gap is about 70 
yards; the trachyte rising abruptly on both sides. The trachyte of the hill to the south is quite 
similar to that of Cone Butte. The section in the gap is as follows: 
Coarse ferruginous sandstone, vertical ........-0.00ce eee eee eens igi eugene Y 3 feet. 
Hine blue shale. vertical: ¢c.02 vucupun we sa wanda lew nate dan denne ale ews eked 20 feet: 
Slate, sometimes shaly, sometimes a good slate, and very sandy, in layers; color 
whitish and yellowish; dip 70° south .... 2.2... 02.222 :0eeeeeee ea ie ae eee aw 180 to 200 feet. 
The strike of these slates is east and west. 
The age of these rocksis uncertain, as the only fossils found in them were some cycloidal fish- 
scales, with occasional impressions of fish-vertebrae and spines, which were quite numerous in some 
layers in the slate. It is hardly to be doubted, however, that they are Cretaceous; and the position 
of some rocks, also containing fish-scales and probably identical with these, observed at another 
point, as noted below, suggests that they are probably Upper Cretaceous, perhaps No. 5. 
The present position of these slates is very remarkable: they lie far above (about 600 feet) 
the rocks visible in the hills below, and doubtless owe their elevation to the eruption of the 
trachyte, having been squeezed up between the two great masses of igneous rock. They show 
little trace of the influence of heat upon them. 
The hills to the south and east, forming the eastern extremity of the Judith Mountains, are, as 
far as observed, trachyte. Black Butte, or Buffalo Heart Mountain, was not visited; but its posi- 
tion and similar appearance show that it is also eruptive, probably exclusively so. The sand- 
stones dip away from it even more distinctly than from Cone Butte. It may be remarked here 
that the trachytic hills are very distinctly marked in appearance, and may be recognized with 
certainty even ata considerable distance. Their sides are covered with the loose blocks of the 
rock, and have a distinct reddish color, due to the oxidation of the iron which exists in considerable 
quantities in the trachyte (see description), which is quite conspicuous and characteristic. 
West of Cone Butte (see e, fig. 5), there is a break in the hills, and alow pass called “ Ross’s 
Cut-off” gives passage to frequent Indian parties. It is free from timber, and of gradual slope, so 
that upon one occasion wagons were brought through without serious trouble. It is from this low 
pass that Box Elder takes its rise. 
In this gap, the observations made were unimportant; the rocks being mostly covered up with 
soil and grass. It was interesting, however, to note that the even, gradual slope of the terrace 
before mentioned extends quite into the pass, with the same character well preserved. At e, (Fig. 
5,) just on the edge of the hills which rise on the west side of the gap, a series of black shales were ob- 
served, vertical, and with a strike nearly north and south. These hills at g and to the south are all 
trachyte. We crossed. them at one point, dragging our horses over the loose talus, much to their 
and our own discomfort, and found the height a little less than that of Cone Butte. On their west- 
ern side, the trachyte shows itself in a series of columns, which are very regular and well formed; 
much more so than is common in this rock. This trachyte differs somewhat from the others 
described in the larger proportion of hornblende present. As before, the orthoclase appears in 
distinct crystals of large size, and the triclinic feldspar in thin-bladed fragments. The whole has 
a pasty base. A little valley, in which rises a small stream of cool water, lies just to the west, and. 
on the other side is a high limestone bill (at k), the only exposure of the older sedimentary rocks 
which we met with in this part of the hills. 
This limestone rises in a series of sharp ridges, very distinct, and seen from a distance as a 
number of white lines running up the sides of the hills. It dips 50° northeast; the strike being 
northwest. The upper layers are white, semi-crystalline, and very profuse in flinty fragments. 
These are exposed by the weathering, and, on the surface, the rock has quite a coralline aspect. 
Lower layers are firmer, blue, and also cherty, though not to the same extent as those above. A 
very careful search showed that fossils were very rare, though a few were found, enough to deter- 
