PEALE.] DESCRIPTIVE GEOLOGY BLACKFOOT BASIN. 559 



f 

 This increases to 80°, and we cross a synclinal axis, the west side of 

 which dips south of east 50° to 60°. The beds in which these strikes 

 and dips were observed were massive limestones, probably Carbonif- 

 erous. The strikes continued southward would fall on the hills west of 

 Bear Eiver. It seems probable that a short distance north of this caiion 

 the beginning of a secondary anticlinal might be found. 



On Station 89, sandstones and limestones, like those of Station 90, 

 outcrop, and contain similar indistinct fossils. The dip, however, is 

 north 420 east, or at right angles to the trend of the range from this 

 point to the Blackfoot. West of the station it is difficult to determine 

 the dip, but it appears to be toward the southwest. If this be so, the 

 station is almost on the line of an anticlinal fold, and Station 90 is also 

 l^robably on the same line. East of Station 89 a westward di]3 was 

 noted. 



South of Station 90 the ridge presents a somewhat bluff face toward 

 the valley of the South Fork of the Blackfoot, and the dip is probably 

 to the southwest, but of this I cannot be certain, although the bluff face 

 would seem to indicate it. At the northern end of the ridge Mr. Gan- 

 nett found a capping of basalt 500 or 600 feet above the level of the 

 valley. Limestones outcropped beneath the basalt. 



Eeviewing this region of the southern branches of the Blackfoot, we 

 find it to consist topographically of four wide valleys and four ridges or 

 spurs from the Preuss Eange. The crests of the most eastern and the 

 most western are separated by an interval of about eleven miles. The 

 ridge on the west is 8,448 feet high in the highest part, and that on the 

 east 8,879 feet. The two ridges between are low and present rounded 

 outlines, and the valleys are broad and well grassed. 



Geologically we iind a series of anticlinal and synclinal folds. The 

 streams sometimes occupy the axes of the folds, and sometimes are in 

 monoclinals. There are at least three anticlinal axes that are parallel, 

 having the general direction northwest and southeast. These are sepa- 

 rated by corresponding synclinals. In the eastern anticlinal there is a 

 slip in the beds, as shown in the accompanying plate, which gives a bet- 

 ter idea of the structure of this region than can, i^erhaps, be given in 

 words. 



Another thing that is noticed is the gradual curving in the strata. 

 Strikes which farther south are approximately north and south now 

 curve to the westward, until by the time our north line is reached the 

 strike is northwest and southeast. The cause of this change may, per- 

 haps, be looked for in the uplift of the Teton Eange in Mr. St. John's 

 district. 



With exception of the northern end of the Preuss Eange, the rocks 

 are mainly those of the Jura-Trias, as determined by the few and indistinct 

 specimens secured. On the west side of Aspen Eidge a limited outcrop 

 of Carboniferous appears, the massive limestones occurring there having 

 been so referred on lithological grounds. 



Toward the south the low rolling ridges culminate in the rugged peaks 

 of the Preuss Eange, much narrower, but attaining a greater height. 

 Northward we find them dying out, and forming isolated hiUs and short 

 ranges in a basin-like region, the lower places being filled with basaltic 

 flows and drift deposits. 



This region would form a most excellent summer range for cattle. It 

 is a little high for general agricultural louri^oses. 



Northern brandies of the Blaclcfoot. — There are two good-sized creeks 

 having their origin in the hiUs south of John Gray's Lake. The most 

 eastern of these has a southeasterly course. It heads in rocks of Jura- 



