peale.] DESCEIPTIVE GEOLOGY — BLACKFOOT BASIN. 55 T 



valley is a inonoclinal valley. The ridge on the west side is the north- 

 ern continuation of the Preuss Range. The caSon of the Blackfoot 

 marks a point of curving in the strike. The canon is cut in the western 

 portion of the ridge, and the limestones through which it is cut dip to 

 the southwest. Following the ridge to the south, the strike is seen 

 curving to the south. A station was made on the ridge nearly opposite 

 the head of the East Pork. Light-colored limestones formed the sum- 

 mit, with underlying quartzites. In the limestones I obtained — 



ffemipronites crenistria. 

 Elwm bopora lepidodendroides. 

 Productus, like P. costatus. 



These proves their Carboniferous age. The beds dip steeply to the 

 northeast. The hills just east were too high for us to see definitely the 

 geological structure in that direction, and it was storming too hard to 

 allow us to follow the ridge northward from the station. As seen from 

 the valley on the west, the northern portion of the ridge has the strata 

 dipping west. This dip also is seen in the canon of the Blackfoot, as 

 already noted. Another station (b) was made six miles farther south, 

 on coarse, arenaceous limestones, the strike being a few degrees east of 

 north, and the dip a few degrees north of west. Between the two sta- 

 tions (a and b) there must, therefore, be. a curve in the strike. 



It is evident, from what was observed, that this ridge is a sharp anti- 

 clinal, with a slip in the beds on the west side, as indicated in the accom- 

 panying illustration of the section. The western side of the ridge is a 

 bluff face, rising 1,200 feet to 2,000 feet above the valley. It increases 

 in elevation southward. In the southern end of the range the western 

 members of the anticlinal form the principal mass of mountains. On 

 Station 61 the limestones are nearly vertical, dipping a little north and 

 west. Station 60 is a short distance south and west of Station 61, and 

 there the limestones dip steeply to the westward. Prom the latter sta- 

 tion Mr. Gannett brought in an indistinct fragment of a fenestellaf. 

 South of Station 60 the topographical features indicate that there may 

 be a curving of the beds to the eastward, implying some connection 

 between the Preuss Range and the Sublette Range. The region between 

 the two was, however, not visited, and this opinion is therefore merely 

 conjectural. I have considered the Preuss Range at this place because 

 it has seemed the most natural, as the ridge west of the East Fork of the 

 Blackfoot is its northern continuation. I shall speak of it again when 

 speaking of the drainage of Bear River. 



Middle Creel:. — This name I have applied to the stream coming into 

 the river between the East Fork and the South Fork. The stream is 

 small and unimportant, but the valley or depression is a large one, and 

 continues southward beyond the head of the creek parallel to the high 

 ridge which rises above its eastern side. The valley is parallel to those 

 of the East and South Forks. Its length is about 14 miles altogether, 

 while Middle Creek has a length of about 6£ miles. In the upper por- 

 tion of the valley is a creek having approximately the direction of Mid- 

 dle Creek. Seen from any of the surrounding points it appears to be 

 the same stream, but when followed is seen to cut across the western 

 ridge to the South Fork of the Blackfoot. 



This upper valley was perfectly dry when we traveled up it about the 

 middle of August. Xorth of it is another creek which rises against the 

 bluffy wall on the east side of the valley and flows directly across the 

 ridge parallel to the creek just described. From this stream a range of 

 low hills extends northwestward to the Blackfoot. This range is a syn- 



