504 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



The laminated porpliyritic trachyte reaches a short distance up the 

 valley of West Teton Pass Greek ; but it is seen here only in remnants, 

 and takes no conspicuous part in the formation of the topographic fea- 

 tures of this part of the basin. But, passing over the ridge at the south 

 end of the Teton Eange, on gaining Jackson's Basin, almost opposite the 

 mouth of East Pass Creek, Professor Bradley found a heavy mass of 

 porphyritic breccias, " much distorted, but having a general northwest- 

 erly dip," around which the sedimentaries are clustered, forming the 

 Lower Gros Ventre Buttes. 



It appears that the tilting of these volcanic ledges agrees with the 

 dip of the loose deposits of the late Tertiary in their immediate vicinity, 

 and was probably determined by the same disturbances. To the north, 

 at the southern end of Jackson's Lake, Professor Bradley observed 

 " high knobs of porphyries and trachytes," and opposite, at the extreme 

 western foot of the highlands outflanking Mount Leidy, a detached 

 mass of dark-green porphyritic lava caps a low ridge on the south side 

 of Elkhorn Creek, dipping 15° to 20° northwestward. It is not improbr 

 able other remnants of a similar character will be found to the north^ 

 connecting those above noticed with the great volcanic plateau of the 

 lake region, which Professor Bradley observed to be largely composed 

 of " porphyries and trachytes, with porphyritic obsidian" ; and through 

 the plateau deposits we may reasonably infer thence unbroken continu- 

 ity with the laminated porphyritic deposits which reach up on the west 

 flank of the Teton Eange. 



Professor Bradley has already noted the occurrence of porphyritic 

 trachytes on the northern or eastern slope of the divide descending to 

 the upper valley of the main Snake, where they cap the ridge, which is 

 based upon Mesozoic and Palaeozoic deposits, the volcanics dipping 30° 

 southwest, or a little less than the inclination of the sedimentary beds 

 which outcrop in the western flank of the ridge, opposite and below the 

 mouth of Lake Fork. But to the east the volcanics appear to change, 

 being made up of "rapidly-disintegrating volcanic rocks, mostly con- 

 glomerates of trachytic porphyry, obsidian," &c., with upthrusts of 

 " gray trachj^tic lavas and red basalt, partly vesicular, though mostly 

 compact," which Professor Bradley observed in high ridges about the 

 sources of the Snake, at an altitude of about 9,600 feet. The south- 

 ern side of this divide, descending to the lower course of Buffalo Fork, 

 shows no volcanics, at any rate none were observed during the present 

 season's work, in that neighborhood, unless the remarkable bowlder- 

 deposit that extends in the summits northeast from Station XL VIII a35e 

 partly ascribable to volcanic origin. 



Basaltic lava flows. — Next in order of chronological sequence we have 

 briefly to notice the great basaltic flows Avhich occupy so immense an 

 area, in this region, indeed nearly all the upland basins in the country 

 west of the Caribou Eange, and spreading out over nearly the entire 

 area of the great plains of the Snake. The physical characters of these 

 deposits in the plains region have already been so much discussed, that 

 only sach facts are here presented as were observed in the upland region, 

 which may contribute a moiety to our knowledge of their distribution 

 and something on the associated phenomena here encountered. 



Along the eastern border of the Snake plain the basalts have been 

 uplifted to a position several hundred feet above their level in the plain, 

 and throughout the lower vaUey of the Blackfoot they exhibit marked 

 disturbance, being elevated into low folds, one of which appears to have 

 great persistency, following nearly the course of the Blackfoot between 

 its northern bend where it cuts act-oss the western declivity and the 



