476 REPORT UNITED STATES GEOLOGICAL SURVEY. 



agents, are clear and readily intelligible. The elevation of the range 

 appears to have been effected by a series of parallel folds, trending gen- 

 erally southeasterly and northwesterly, and culminating along the south- 

 ern border where the Archaean nucleus is upthrust in. a great wedge- 

 shaped mass, the degradation of which has produced a symmetrical cone 

 something like the sharp ridge peaks and aiguilles in the Tetons. The 

 eastern portion of the crest of the range, I believe, has never been ap- 

 proached, so that we are unable to say what the relations of the nucleal 

 rocks are in that quarter. So far as could be determined from the heights 

 at the western extremity to the eastward the Archaean core gradually 

 ginks, admitting a greater and greater depth of sedimentaries to rest 

 upon the axial region. 



The range forms a sort of transverse bar or truss connecting the Wind 

 Eiver Eange with the Tetons. Dr. Hayden in 1860 found the saddle at 

 the eastern end filled with Tertiary deposits, which rise high up on the 

 opposite or western flank of the Wind Eiver Mountains. In a preceding 

 page it was stated that to the west the Archaean ridges sink and are 

 plated with the sedimentaries, so that while this relationship to the 

 two great ranges is quite marked as a topographic feature, geologically 

 considered it probably forms an independent uplift, the exact date of 

 which I am unable to state, although it was at a comparatively modern 

 period, as evidenced by the tilted position of the Pliocene beds that dip 

 gently off the flank of the range in the borders of Jackson's Basin. This 

 fact alone would lead to the inference that the Gros Ventres have been 

 subjected to elevatory movements which were continued after the depo- 

 sition of the Pliocene lake-beds to a very recent date compared with 

 any records of similar movements in the range of the Tetons. 



To the north of the Gros Ventre Eange, and a few miles west of or 

 about midway in the western slope of the continental watershed, Buffalo 

 Fork of Snake Eiver has excavated a deep, flaring canon across a dome- 

 shaped uplift, the nucleal rocks of which consist of much contorted, fer- 

 ruginous-stained gneissose ledges, of which a height of several hundred 

 feet is exposed, forming the abrupter walls of the gorge. This is prob- 

 ably the locality referred to by Professor Comstock in his report to Capt. 

 W. A. Jones on the geology of the country traversed by the military 

 reconnaissance of Western Wyoming, 1873, p. 105 : " Between the Two- 

 Ocean Pass and Togwotee Pass there are exposures of a metamorphic 

 group which undoubtedly represent the ridge of some mountain range 

 or its spurs, though it is a difficult matter at present to define its rela- 

 tions to the main chains." The date of this uplift, however early its in- 

 ception, must have extended to a period subsequent to the deposition of 

 the Tertiaries, which latter both Professor Bradley and Professor Corn- 

 stock report as being much disturbed on the flank of the mountain ; and, 

 as elsewhere remarked, there are some grounds for the belief that even 

 the latest of the later volcanic accumulations, the great breccia conglom- 

 erate, may have to some slight degree at least partaken in this upward 

 movement. Once comprehending the isolated position of this bulged- 

 up mountain mass, the mind naturally seeks some explanation of the 

 origin of the gorge which separates the dome into halves, and whose 

 bed is at present threaded by the diminished channel of Buffalo Fork. 

 The apparent continuity of the Palaeozoic deposits which reach up into 

 the very crest of both the north and south side mountains, and Avhich 

 once doubtless spanned the chasm that now divides them, militates 

 against the supposition that the course of the present stream was marked 

 out by, or followed a fracture or crevice across the mountain ; and al- 

 though the results of upheaval are here manifested in an extraordinary 



