424 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



tions greatly perplexes and confuses the study and identification of syn- 

 chronous strata in the various parts of the field in \i&w. 



About midway between the southern end of the range and Station 

 XXXVIII, or some five miles north of Station XLIII, the Carbonifer- 

 ous first reaches the eastern front of the range. It here caps the cliffs 

 of Magara limestone, in the heights about northwest of the Lower Gros 

 Ventre Buttes, where the latter is in turn underlaid by the low escarped 

 Quebec ledges, from beneath which the rusty and yellow deposits of the 

 quartzite horizon also api)ear, gradually sinking to the south, until all 

 disappear, save the Carboniferous which forms the foot-hUls in the im- 

 mediate vicinity of East Pass Creek. 



In further explanation of the geological structure of the T^ton Eange, 

 attention is directed to the longitudinal and transverse sections given in 

 an accompanying plate, which convey a tolerably accurate view of the 

 profile of the range along the lines of the sections. The drainage map, 

 reduced from the topographic sheets, exhibits the general distribution of 

 the geological formations so far as it was possible to make them out in the 

 course of a necessarily brief visit. 



SNAKE EIVEK KANGE. 



Under the general term Snake Eiver Eange is included the belt of 

 highlands to the west and south of the Teton Eange, which has a gen- 

 eral northwest and southeast course of about sixty miles within this 

 district, ranging from twelve to twenty miles in breadth. Towards the 

 southern border of the district the Snake Eiver has forced a narrow 

 passage through the range, which is known as the Grand Caiion, where 

 for a distance of about twenty-five miles the stream is confined between 

 mountain walls, with httle or no intervale room. In the vicinity of the 

 Grand Caiion the highland belt occupies the whole area between the upper 

 and lower course of the Snake, and Avhere it reaches its gTeatest breadth. 

 On the northeast it impinges against the south extremity of the Teton 

 Eange, and thence northwesterly it gradually narrows and finally mer- 

 ges into the volcanic upland south of the western course of Pierre's Eiver. 



The Snake Eiver Expedition of 1872 further distinguished this moun- 

 tain belt by local appellations, which, if not absolutely of topographical 

 importance, maybe accepted for the convenience of descriptive purposes. 

 To the north the range culminates in a series of peaks and high ridges, 

 the highest of which attain an absolute altitude of 10,000 to 10,420 feet, 

 to which the name Pierre's Mountains has been given. About fifteen 

 miles from the northern extremity of the range, it is traversed by a low 

 sag, which opens an easy passage across from Pierre's Basin to the lower 

 valley of the Snake, the southwestern flank opening into an extensive 

 rugged basin area which drains out into the Snake but a few miles above 

 its debouchure into the great plains. To the south of this low pass lies 

 an equally if not more rugged low mountain region, which fills the great 

 bend of the Snake and passes southward into the adjacent district, finally 

 dying out in the watershed between the Green and Bear Eiver Basins. 

 That portion of the latter section of the range nearest the southern end 

 of the Teton Eange has received the name Teton Pass Mountains, and 

 which may be understood to embrace a narrow mountain belt extending 

 from the head of Pierre's Basin in the vicinity of the Low Pass^ south- 

 westerly to the Upper Snake, in the region of the foot of Jackson's Basin, 

 or above twenty miles in length. The highest eminences in this latter 

 mountain-belt do no exceed 9,n500 feet, but to the south the highlands 

 culminate in more lofty summits, whose isolation rather than actual alti- 



