GEOLOGICAL SURVEY OF THE TERRITORIES. 63 



basin in tlie character of the bowlder-deposits. At the upper end of the 

 basin the entire surface is paved with rounded masses of granite and 

 basalt, some of great size; but as we descend thej' become smaller in size 

 and less abundant, so that the lower portion of the basin reveals a con- 

 siderable thickness of the light-gray marls and sands, which give evi- 

 dence of having been deposited in comparatively quiet waters. So thick 

 are the bowlders over the upper half of the basin that it can never be 

 used exce])t for grazing-purposes, while the lower half is already occu- 

 pied with fine farms. The rush of the waters through the Middle Caiion 

 must always have been very great, but their force almost entirely sub- 

 sided before reaching the Lower Caiion. 



Between the Madison Valley and that of Passamari Creek there is a 

 long somewhat irregular range of mountains, the geology of which I can 

 present in this report only in general terms. Although the structure is 

 comparatively simple, it would require long and faitMul labor to work 

 out all the details. The general elevation is not great, 7,000 to 8,000 

 feet, and only two or three peaks rise up so as to be prominent, Pyroxene 

 Peak and Old Baldy. As usual, the nucleus is composed of the various 

 kinds of metamorphic strata, with effusions of igneous rocks, while the 

 unchanged sedimentary strata, mostly the Paleozoic, are observed at 

 ail elevations resting on the granites. The evidences of erosion are 

 more striking in this range than in the mountains between the Madison 

 and the Gallatin, probably because their general elevation is at least 

 1,500 to 2,000 feet lower, and they were much longer exposed to the 

 wasting influences of the aqueous forces. Deep caSons are worn into 

 the mountains on either side, and the numbers of old dry ravines or 

 gorges are almost countless. Prior to the effusion of the igneous rocks 

 the greater part of the erosion took place, though comparatively little had 

 been done to produce the present configuration. The skeleton or frame- 

 work was formed of the metamorphic rocks with the remnants of the 

 Silurian and Carboniferous strata that were left after the vast work of 

 erosion, which occurred prior to the outflow of the igneous rocks. In 

 subsequent erosions the latter have protected the sedimentary beds over 

 a great area, so that they are exposed in little patches everywhere, some- 

 times on the summits of the mountains or at all elevations in the ravines 

 or cailons. Sometimes the igneous rocks have been worn away from 

 the surface for a considerable area, leaving a greater or less thickness 

 of the limestones. In some localities they present a great vertical 

 thickness and then again thin out or disappear entirely. There is an 

 interesting but obscure feature which is shown in the mountains on 

 both sides of the Madison. There is, in restricted localities, an enorm- 

 ous development of a very hard gray quartzitic sandstone, apparently 

 partially metamorphosed. It evidently forms the underlying rocks of 

 the sedimentary strata, resting on the strictly metamorphic gneiss. The 

 various members of the survey have examined it most carefully, but 

 have never been able to tind any trace of oi^ganic life, yet it undoubtedly 

 belongs to the oldest Silurian, It makes its appearance quite abruptly, 

 with a thickness of 1,000 to 2,000 feet, and as abruptly disappears or 

 thins out. It is well shown in the IMiddle Canon of the Madison and in 

 the lower part of the caiion of the Gallatin, and is thinly represented in 

 several other localities. We may say of all these different groups of 

 strata that they appear at times in grand pro])ortions, and are soon lost 

 or are only thinly sho\vn. Along the east side of the Gallatin River, 

 above the Three Forks, at least 1,000 feet of shales, clays, sandstones, 

 &c., which belong undoubtedly to the Potsdam group' of the Lower 



