422 REPORT UNITED STATES GEOLOGICAL SURVEY. 



valley here shows much limestone debris, among which occur heavy mass- 

 es of gray siliceous rock which have tumbled from ledges above. The 

 latter also i)robably pertains to the Carboniferous, representing the uj)- 

 per siliceous horizon. 



Just below the last-mentioned locality the valley widens, giving room 

 for the narrow intervale which continues thence to the debouchure into 

 Pierre's Basin, its surface willow-grown and the little brook converted 

 into a chain of pools by beaver-dams. Again the "red beds" set in, 

 overlaid by drab shales and shaly and compact drab, spar-seamed lime- 

 stone, with obscure exposures of brown- weathered limestone, which bear 

 tmmistakable resemblance, lithologically, to the Jurassic deposits else- 

 where encountered in the district. These are in turn succeeded by softish 

 gray thin-bedded sandstones, exposed in low (Zeftm-covered slopes two or 

 three miles within the mouth of the valley, below which the vesicular dark- 

 brown jiorphyritic and drab and pink trachyte volcanics reappear, ex- 

 tending thence out into the borders of Pierre's Basin. The water-worn 

 debris so plentifidly occurring in the valley is composed of red sand- 

 stone and gray siliceous and drab limestone bowlders, the absence of 

 Archaean erratics being a notable fact and showing that none of the 

 water-courses at the southwest extremity of the range have eroded their 

 "beds to the crystalline basis upon which the sedimentary formations 

 rest. This is an extremely interesting Mttle valley, abounding in pictur- 

 esque scenery 5 on the one hand, the steep rocky acclivities which rise 

 into the dominating peak at the extreme south end of the range, deep 

 gorges oi)ening short vistas into the mountain, their beds a chaos of 

 tumbled rocks; on the other, lower and more gentle ridges, densely 

 wooded with beautiful forests of pine and spruce, rise, rank upon rank, 

 into the broken mountain region included in the great bend of Snake 

 Elver. 



It is quite apparent that for at least a few miles north of West Pass 

 Creek, the foot-hills along the west flank of the Teton Eange are made 

 up of the Triassic "red beds" and Jurassic deposits overlaid by a rem- 

 nant of the peculiar soft gray sandstones which have elsewhere been 

 provisionally referred to the Laramie Group. But these are soon re- 

 placed by the siliceous beds and limestones of the Carboniferous period, 

 which rise uj) into the Twin Mountains at the extreme south end of the 

 range, on the easternmost of which Station XLIII was located at an 

 actual altitude of 10,193 ieet, or 1,063 feet above the summit of the pass, 

 a mile to the south. The southern declivity of the latter eminence is 

 heavily mailed with Carboniferous limestone more or less cherty and 

 spar-seamed, which dips 47° S., 40^ W. The foot of the slope in the im- 

 mediate vicinity of the saddle is made up of the buff siliceous deposits 

 whose debris strews the surface, while on the summit of Station XLIII 

 lower limestone horizons are reached, charged with Zaphrentis and large 

 masses of LitJiostrotion, Sjpirifer, several species of Froductus, and a 

 smaU Atliyris. These summit strata show a variable degTce and direc- 

 tion of inchnation, ranging from 5° to 25°, W. 25° to 60° N., the beds 

 actually folding partially around the mountain from a northwest to a 

 southerly direction. The same is also observed in the higher mountain 

 two and a half miles to the northwest, whose escarped southeast face shows 

 the strata nearly level along the northeast and southwest line of strike, 

 while on the southern flank they pitch down rapidly into West Pass 

 Creek Canon. It seems highly i^robable, also, that the saddle between 

 these mountains occupies the position of a synclinal of one of the sec- 

 ondary folds with which this portion of the range was wrinkled. 



A section in the ridge of Station XLIII, presents very nearly the 



