478 REPORT UNITED STATES GEOLOGICAL SURVEY. 



patch of Carboniferous, "whicli is detached from the main ridge by a de- 

 pression floored with basalt. 



Along the northeastern border of the Caribou Eange occurs a narrow- 

 strip of Palaeozoic deposits, extending a distance of 20 miles in a north- 

 westerly and southeasterly direction below the narrows in the lower val- 

 ley of the Snake. The older deposits are mostly covered by volcanic 

 flows, but the Carboniferous is finely displayed, especially the ujjper 

 horizons, in the lower flanks of the range, inclined to the southwest. It 

 appears that these deposits once arched over the trough of the present 

 valley of the Snake, which was excavated along the crest of an anti- 

 clinal fold, or a series of sharp folds, in this part of its course. 



In the Snake Eiver Eange, on the opposite side of the valley, there 

 are apparently two parallel belts of Palaeozoic strata, of which the south- 

 western reaches probably the entire length of the range, forming a grad- 

 ually expanding belt along the southwest border of the range to the 

 southeast. On the opposite side of the range, what remains after the 

 erosion of Pierre's Basin, constitutes an irregular, narrow belt, which 

 shows its greatest breadth to the northwest, in the Pierre's Mountains. 

 Both these zones are Carboniferous in their constitution, which consti- 

 tute the most ancient of the sedimentary formations as yet detected in 

 this range. They appear to have been raised up in great folds, that to 

 the northeast, borderiag Pierre's Basin, being accompanied by abrupt 

 flexure or faulting, with downthrow on its northeast side. 



The Teton Eange exhibits by far the most considerable area of these 

 rocks. The vertical displacement accompanying the upheaval is meas- 

 ured by thousands of feet, or between three and four miles, and on the 

 slopes of the Archaean axis almost if not the entire Palaeozoic series is 

 laid open to view in the gorges which the streams have eroded out of 

 their substance. They constitute a heavy plating along almost the en- 

 tire western declivity of the range, their continuity being intermitted 

 over a space of only a few miles across, between Bear Creek and the 

 north fork of Pierre's Eiver, where they have been denuded, the subse- 

 quent volcanic flows impinging on the granites. But to the north and 

 south they fold quite round the extremities of the range, and it is in the 

 latter quarter they maintain their comptete development, preserving the 

 topmost siliceous horizons of the Carboniferous, immediately succeeded 

 by the superimposed Triassic " red beds." The series as here consti- 

 tuted embraces the quartzites of the Potsdam, the Quebec Group lime- 

 stones, the Magara dolomitic limestones, and the Carboniferous, a 

 generalized section of which will be given presently. 



Assuming the movement to have been gradual, it'seems almost certain 

 that the elevation of the range began to the north, where we find it has 

 been subjected to the greatest amount of erosion, which has bared the 

 Archaean nucleus over an extensive area, and swept the Palaeozoics from 

 the entire breadth of the range. The precise date of the inception of 

 the elevatory movement may be beyond determination ; but, relatively, 

 it belongs to a post- Jurassic and possibly even to Cenozoic time, although 

 it is beheved to have preceded the elevation of the Snake Eiver Eange, 

 which latter movement involved a set of early Tertiary deposits, Lara- 

 mie (?), of which there has been observed as yet not a vestige in the 

 T6ton Eange proper. But this movement has continued up to a late 

 date as evidenced by the inclined position of the volcanics on the flank 

 of the mountain. 



In the Gros Ventre Eange the Palaeozoics occupy an area only less 

 extensive than that in the Teton Mountains. Also there here occurs 

 the same series of formations, but they have not been subjected to the 



