468 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



BTJPPALO FORK MOUNTAINS. 



Tlie general stratigrapliical features of the Buffalo Fork uplift are 

 brought out most clearly from Station XL VIII, and the observations 

 made from the latter point, supplemented by actual examinations in the 

 southern half of the great dome and the views presented therefrom, 

 afford a comprehensive understanding of its geological structure. Our 

 route of approach led over the slopes between Buffalo Fork and Black 

 Eock Creek, in which, as before mentioned, the Tertiary deposits con- 

 stitute the subjacent rock strata, the surface, however, being pretty gen- 

 erally enveloped in a coating of drift-like materials, intermingled with 

 which are pebbles of chalcedony and fragments of fossil wood. 



On reaching the elevated open valley-basin on the upper course of 

 Black Eock Creek, craggy outliers of volcanic breccia-conglom>srate are 

 met with, which a Little higher up form a low barrier across which the 

 stream has cut a miniature canon. These ledges consist of a dark paste 

 holding angular fragments of dark basaltic and drab trachytic lavas, 

 showing in the mass what appears to be a bedded structure, dipping at 

 an angle of 40° about west-southwest. The thickness of the deposit at 

 this locality was estimated at above 500 feet. Overlying the breccia in 

 the crests of the low ridges occurs a peculiar deposit formed of water- worn 

 pebbles and bowlders of quartz and various volcanic rocks held in a 

 fine light drab paste, possibly of volcanic origin. The latter was found 

 in irregular masses, but not revealing their relations to the bedded 

 breccias. The bed of the stream and the adjacent morain-hke undulat- 

 ing open slopes below are strewn with drift materials, amongst which 

 occur water-worn fragments of rusty-buff and red sandstone, Quebec 

 and Carboniferous limestone debris, which was derived from the adjacent 

 mountains. Just below these open ridges, in the west side of a little 

 stream flowing iato Buffalo Fork, the surface is broken by the outcrop 

 of a heavy ledge of hard, reddish-buff and gray, laminated sandstone, 

 standing nearly vertical, the strike about E". ]!^. E. and S. S. W. Less 

 than a mile below the breccia barrier, in the south bank of Black Eock 

 Creek, deep-red shales and sandstones appear, showing a thickness of 

 100 feet, more or less, dipping southwestward. These deposits doubt- 

 less hold the position of the "red beds" of the Trias, and will form the 

 initial i)oint of the section reaching up into the summit of the mountain 

 to the north, on which Station XLIX was established. 



Station XLIX occui)ies a low but commanding peak about two miles 

 west of the main summit of Buffalo Fork Peak, overlooking the caiion 

 of Buffalo Fork and a large extent of its mountain valley to the north- 

 east. The southwest and southern slopes of the mountain are very 

 abruiDt and plated with heavy deposits of hard reddish-buff and gTay 

 laminated sandstones, almost a quartzite, which occurs in immense slides 

 of debris on the lower slopes. Higher uj) the mountain-side these are 

 seen to be underlaid by heavy beds of gray, drab, and buff, cherty lime- 

 stones and brownish-drab magnesian limestones, which reach up into 

 the crest at the station. The limestones are charged with characteristic 

 Carboniferous fossils, amongst them a medium-sized OrtJwceras. The 

 angle and direction of their inclination is very variable, gradually steep- 

 ening lower on the southwest flank of the mountain, where they dip at 

 one point 65° to 75°, W. 25° S., while in the summit the dip is 35° to 

 40°, W. to W. 25° S. The brownish-drab magnesian limestone near the 

 summit is very like similar ledges in the debouchure of the Gross Yentre 

 Eiver. On the southwest flank of the mountain, within 300 feet of the 

 summit, basaltic and trachytic erratics were met with. 



