446 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



indeed, more agreeable temperature was experienced at our mountain 

 bivouacs than in tlie valley. The pasturage, however, is excellent; and 

 at seasons the woods and plains are stocked with game ; elk, deer, ante- 

 lope, and bears abound, while in the forests two or three kinds of grouse 

 are found, and the streams afford abundance of large, delicious trout. 



Just within the mouth of the Gros Ventre Valley several small rivulets 

 of warm, slightly saline water were discovered flowing into the stream 

 on either bank. They are fed by springs which appear in marshes in 

 the bottom land half a mile or so above the i^oint where the river enters 

 Jackson's Basin. The inflowing spring waters seem to perceptibly 

 raise the temperature of the river water below the point where they 

 issue into the Gros Ventre, from which it may be inferred their volume 

 is considerable. These springs probably have their sources in, or issue 

 from, the Carboniferous limestone, bluffs of which line either side of the 

 narrow valley at this point. 



Sedimentary and volcanic roclcs. — ^We now come to certain small and 

 isolated areas of older sedimentary formations properly belonging to the 

 basin, and in the brief subjoined notice will also be included mention of 

 such comparatively inconspicuous volcanic deposits as are met with in 

 patches disconnected from the great flows which hem in the head of the 

 valley to the north. 



In the somewhat narrowed southern half of the basin two groups of 

 low, grassy hills rise from the plain to the height of a few hundred feet, 

 and which are chiefly conspicuous for their isolation. These are known 

 as the Upper and Lower Gros Ventre Buttes. The lower and larger 

 group lies between the Snake and Little Gros Ventre Creek, just below 

 the confluence of Gros Ventre Eiver, and consists of a series of low hills 

 penetrated by narrow, shallow, parallel depressions, in a northerly and 

 southerly direction, or corresponding approximately to the direction of 

 strike of the sedimentary deposits which enter largely into their comi)o- 

 sition. At the northwest the hills culminate in a bolder eminence in 

 which dark, rusty volcanic ledges appear, extending southward in a low 

 spur. These Professor Bradley found to consist of " red, gray, black, 

 brown, and variegated por^jhyritic breccias, including much jasper, but 

 partly porous, loose-textured, and even ashy. The beds are much dis- 

 torted, but have a general northwesterly dip." Similar deposits were 

 noticed rising gently southeasterly in the eastern portion of the grouj), 

 where they are underlaid by heavy deposits of dark- weathered limestone, 

 probably of Quebec age. The latter beds appear in the steep bluff" along 

 the west side of the Little Gros Ventre, facing a prominent spur of the 

 Gros Ventre Mountains, which juts out into the valley at this point, the 

 beds diiDping at a moderate angle northwestward. At the south end 

 of this group Professor Bradley observed in a low sharp butte horizontal 

 beds of gray limestone, which were compared with the Carboniferous. 

 The interval between the east and west portions of this group comprises 

 a shallow undulating depression, which may be partially filled with soft 

 Tertiary deposits, over which is spread a mantle of Post-Tertiary mate- 

 rials. 



About eight miles northeastward of the above buttes, a similar but 

 smaller cluster, called the Upper or I^orth Gros Ventre Buttes, occu- 

 pies the interval between the Snake and Gros Ventre Elvers. In the 

 northern and higher portion of this group Professor Bradley saw ex- 

 posed a fair representation of the Palaeozoic series of the region, con- 

 sisting of " gray quartzitic sandstones, which are i)robably of Potsdam 

 age," overlaid by conglomeritic limestones so characteristic of the Que- 

 bec Group, as developed in the T6ton Mountains, and these in turn fol- 



