496 REPORT UNITED STATES GEOLOGICAL SURVEY. 



forms of Lamellibranchiates, and fragments of the columns of Pcntacri- 

 nites in widely-separated horizons in the series ; while there are layers, 

 as at Station XXYII ridge, characterized by peculiar faunal assem- 

 blages. At the latter locality, a bed 's thus charged with a beautiful 

 little Exogyra, Belent nites, &c. The latter fossil was also met with in the 

 vicinity of Station XX, in the northwestern part of the range. Along 

 the western border of the range the Jurassic is succeeded by the Ceno- 

 zoic(?) formations, which apparently constitute about a third of its 

 bulk, although the same deposits occur in belts of greater or less extent 

 farther east. 



Perhaps one of the most interesting sections of the Jurassic met with 

 during the season was that in the northeastern portion of the Snake 

 River Range, in the vicinity of Station XL, a detail description of which 

 is presented in a preceding page. In a thickness of about 1,500 feet 

 there are two or three heavy beds of limestone, one of which forms the 

 base of the formation, with heavy deposits of drab indurated calcareous 

 shales, and variegated chocolate-red clay shales, the upper 500 feet being 

 made up of grayish buff hard sandstones and sandy shales, capped by 

 a heavy ledge of rusty brown and reddish conglomeritic sandstone, 

 which has almost the compactness of quartzite. The latter deposit is 

 succeeded by light-drab fragmentary limestones and variegated shales, 

 which in turn are capped by a heavy ledge of reddish-buff, hard sand- 

 stone, the whole adding a thickness of 500 to 900 feet to the lower series, 

 or a total of 2,400 feet. 



No fossils were detected in the upper series of limestone strata, conse- 

 quently its relation to the lower series cannot be more definitely stated 

 than that it occupies a conformably superimposed position. But in the 

 lower series of limestones and indurated calcareous deposits, we find an 

 indubitable Jurassic fauna. The latter beds are crbwded with individ- 

 uals of a small Gryphcea, like G. calceola, and a small Ostrea(?) less 

 numerous. The basis limestone affords the remains of Pentacrinites, a 

 largish smooth Lamellibranchiate, resembling Gamptonectes, and a small 

 indeterminate Gasteropod. 



The conglomeritic sandstone bears some resemblance to the heavy 

 quartzitic sandstone capping the Jurassic limestone series in the vicinity 

 of Station XXIV in the Caribou Range, 15 to 20 miles in a direct hue 

 to the southwest; and it will be recollected at that locality this sandstone 

 horizon is also succeeded above by drab calcareous deposits. But at 

 the latter locality the determinate Jurassic limestone series, Avith its 

 associated shales and sandstones, attains a thickness more than a third 

 greater, while it presents, in the character of the associated strata, a 

 marked contrast to the lower series as it occurs immediately southwest 

 of Station XL at the present locality. This belt of Jurassic may be 

 traced to the southeast as far at least as Station XXXIX, and it will 

 probably be found to extend quite to the Snake, south of the Teton 

 Range. Remnants of Jurassic beds also occur in the debouchere of 

 West Teton Pass Creek, and probably once reached high up on the Teton 

 Range, lapping up on the southwest terminus, subsequent erosion, how- 

 ever, having removed them from the higher acclivities. 



Along the western end of the Gros Ventre Range, in the region front- 

 ing Jackson's Basin, the Mesozoic formations have been entirely denuded. 

 But on the north flank of the range Dr. Hayden noted the occur- 

 rence of Jurassic, constituting an outlying belt succeeding the Trias, 

 but which erosion has reduced to very variable width on the mountain 

 flank as compared with its appearance over less exposed situations. 

 Beyond their probable occurrence in the Mesozoic belt surrounding the 



