494 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



of deep-red arenaceous shales and harder sandstone beds, "with a single 

 limestone stratum observed, approximating a total thickness of 2,500 

 feet. The deep-red shaly deposits also include layers of drab-colored 

 shales. The sandstones are gray or rusty buff to brown weathered and 

 light red in color, in one of the lower layers of which occur very in- 

 distinct bodies which resemble medium -sized ovoid lamellibranchiate 

 shells, but too imperfect for determination. These deposits are included 

 between a limestone holding fossils of a Permo-Carboniferous facies 

 below and a ledge of grayish-drab limestone with Jurassic forms above. 

 Elsewhere in this mountain range, as in the vicinity of Teton Pass, 

 horizons show deep-red sandstones and shales. The structural accom- 

 paniments in this region, as also the probable distribution of the same 

 deposits in belts occurring to the southwest, have already been alluded 

 to in the notice of the observations made in the Snake Eiver Eange. 



The last above-mentioned series of " red-bed " exposures, according 

 to Professor Bradley, reappear to the east of the Snake in the moun- 

 tainous belts outflanking the Gros Ventre Range on the southwest ; and 

 along the northern flank of the latter range and in the Gros Ventre Val- 

 ley they constitute a heavy series, including a bed of gypsum. Their 

 occurrence in the vicinity of the Buffalo Fork uplift is, so far as could be 

 made out, accompanied by a typical representation of the series. 



JURASSIC. 



It is diiflcult to assign to this member of the Mesozoic age a set of 

 stratigraphical characters typical of its occurrence in all portions of the 

 district. In the southwestern section, where the Jurassic strata consti- 

 tute an important series in the low mountain folds either side of the 

 Blackfoot, showing a thickness of a couple of thousand feet, more or less, 

 the lower portion is made up of gray and drab limestones, sometimes 

 magnesian and in places cherty, interbedded with chocolate-colored 

 partially indurated shales and occasional layers of reddish sandstone, 

 the upper portion showing several layers of gray chocolate or rusty 

 weathered, sometimes arenaceous limestones, with drab and chocolate- 

 colored shales and indurated argiilo-arenaceous beds. This is essentially 

 the stratigraphical structm^e of the formation or that portion of it exposed 

 in the low divide between the Blackfoot and Eoss Fork-Lincoln drainages. 

 At the latter locality, the upper limestones are quite fossiliferous, being 

 charged with Aviculopecten, PseudomonoUs, Camptonectes, Myacites, Pho- 

 lodomyaj and other Lamellibranchiates ; the lower beds preserving only 

 comminuted fragments of fossils, although elsewhere these beds afford 

 fragments of Pentacrinites and a small (xryph(Ea{^:)^ &c. However, in a 

 single stratum low in the series a few obscure fossils, in the condition of 

 casts, were met with including Aviculopecten{1) and tAvo or three other 

 small Lamellibranchiate shells, Dentalium(1) and a minute Gasteropod 

 like Yiviparus. 



The same series of upper beds also appears in the low ridges east of 

 the Blackfoot, at Stations XII and Xlll. At the latter locality these 

 well-marked Jurassic limestones are succeeded on the north by a series 

 of gray limestones, reddish quartzitic sandstones, and red shales, occu- 

 pying a belt about a mile across, reaching over to the Carboniferous 

 ridge, the complicated structural features of which are very obscure and 

 were not satisfactorily made out ; but these limestones were found to 

 contain at one x>oint an obscure Lamellibranchiate resembling Aviculo- 

 pectbiiy and at another place the fragmentary remains of what appear 

 to be Cemtites. Of the latter, during the past season, Dr. Peale dis- 



