506 KEPORT UNITED STATES GEOLOGICAL SURVEY. 



tributable to it diminish in either direction from the culminating point, 

 if the line of uplift be extended it exactly intersects the hill whose 

 northeast flank is plated by the trachytic flow above mentioned. This 

 naturally suggests the query, Was not the uplifting of the trachyte in 

 part, at least, due to elevatory forces accompanying the upthrust of this 

 dike-like mass of eruptive material? In the latter event, we would be 

 enabled to determine with precision the relative date of the eruption of 

 the hornblendic trachyte, which must have occurred at a period subse- 

 quent to the earlier trachytic flow^ if indeed it was not also antedated 

 by the basaltic flows. 



In the basin ridges west of John Gray's Lake basaltic and scoriaceous 

 lavas are gently upraised on their flanks. And so, also, around the 

 northern extremity of the Caribou Eange, where their position is ap- 

 parently the exact counterpart of their relation to the Blackfoot Range, 

 with which latter, indeed, they are continuous, forming the upland 

 slopes intervening between the low mountain ridges and the Snake 

 plain. This upland border region is deeply scored by the canon-courses 

 of the streams flowing out into the Snake, and the basalts of which it is 

 made up extend well up into the lower valley of the Snake, which has 

 cut a deep, narrow gorge, in places several hundred feet in depth, walled 

 by the sombre, igneous ledges. To the north and northeast this upland 

 gradually expands, and finally merges into the great volcanic plateau 

 which stretches north from the northern extremity of the Teton Eange, 

 forming the water-divide west of the lake basin at the sources of the 

 Snake. In the undulating slopes north of Pierre's Basin, and reaching 

 up on the great volcanic foreland on the west flank of the Teton Eange 

 a thousand feet higher, evidence of the former general distribution of 

 the basaltic flow over the whole extent of this upland region was not 

 wanting. As has already been noticed, the basalts in this region were 

 flowed over the denuded surfaces of the earlier trachytic flows. But 

 how to account for their gently uplifted position on the flanks of the 

 Teton and Snake Eiver Banges, no satisfactory data were gained, be- 

 yond the very natural supposition that their present position is the 

 result either of late-continued elevatory movements situated in the 

 mountain areas, or subsidence in the basin region ; for no evidence was 

 observed that would suggest -the mountains themselves as the sources 

 whence emanated the outflows which cover their lower declivities. 



East of the Teton Eange, in the region of Jackson's Basin, the basalts, 

 if ever they existed, have been entirely swept away ; indeed, erosion 

 has been carried to such an extent in this basin up to a very recent date 

 that only meagre remnants of the older volcanics are now to be met 

 with. But passing east to the summit of the continental watershed, its 

 level crest, southeast of Togwotee Pass, seems to be capped by a heavy 

 flow of late lavas in the midst of which rises a low dome of scoriaceous 

 and compacter lavas, which is, possibly, the remains of a crater. This 

 outburst occupies the very summit of this part of the watershed between 

 Togwotee and Warm Water Passes, at an altitude of 10,000 to 10,500 

 feet above the sea at Station LI. The dark lava and reddish to yellow 

 scoriaceous matter is precisely like that occurring at before-mentioned 

 localities supposed with some probability to mark the sites of crater- 

 vents j and if the wall of dark basalt-like rock, which extends along the 

 mountain crest several miles,' or as far as could be seen, from a point a 

 short distance northwest of Station LI, thence southwards, originated in 

 matter thrown out from the same vent, little disturbance or change has 

 taken place subsequent to the effusion, except that produced by the enor- 

 mous erosion which has acted upon all these materials up to the present 



