TEALE.l DESCEIPTIVE GEOLOGY SNAKE EIVER AREA. 563 



will be more fully described by him i)i bis report. Mr. St. John considers 

 them to be of the same age as the lake beds exposed ou Bear Eiver 

 which coutaiu similar fresh-water forms. 



PORTNEUF RIVER. 



The Portiieuf is a large branch of the Snake, and has a rather curious 

 course before it reaches that river. It rises north of our district and 

 tiows south for about fifteen miles from its head into the broad upper 

 valley. It then turns southwest across the valley and gradually enters 

 the caiion. In the latter it soon takes a southern course again, which it 

 keeps for about teu miles, when it makes a right angle and tiows west- 

 ward across the range, and after a course of almost teu miles in that 

 direction makes another right-angled turn out into the valley of Marsh 

 Creek, in which it flows northward, separated from Marsh Creek by a 

 narrow strip of basalt. At the north end of this valley it again turns 

 to the west to cross Bannock Eange, after which it leaves our district 

 with a northwest course to flow out into the Snake Eiver Plains. 



Upper Portneuf Valley. — The upper portion of the Portneuf is in a 

 broad grassy valley, which is the northern extension of Basalt Valley, 

 which reaches to Bear Eiver west of the Soda Springs Hills. Opposite 

 the point where the Portneuf enters the canon the valley is about eleven 

 miles in width. The hills on the east side are mainly Carboniferous, 

 especially at the south end of the valley. Jurassic rocks may outcrop 

 toward the north, but they were not recognized. On Station 82 lime- 

 stones outcrop, dipping northeast, and containing 



Froductus Costatus, 

 Spirifer Eoclcymontamis, 

 Crinoidal stems. 



In 1871 the following fossils were obtained from the foot of the same 

 hills at Twin Springs, 6 miles farther south : 



Zaplirentis ? iStanshuryi. 



lAthostrotion ? 



Rhoinbopora "? 



Polypora ? 



Ghonetes 1 



Spirifirina ? 



Euomplialus ? 



Crinoidal fragments. 



The latter are probably from a lower layer than those first given. The 

 hills on the west side were not visited, but they are probably composed 

 of Carboniferous rocks, with perhaps Jurassic toward the north. At the 

 entrance of the Portneuf to the canon, quartzites outcrop, dipping to the 

 westward at an angle of 10° to 15°. These quartzites are underlaid by 

 limestones, and form a synclinal depression along the eastern side of the 

 Portneuf Eange. 



It appears, therefore, that the Upper Portneuf Yalley is in part located 

 1 )y an antichnal fold. No basalt occurs in the northern part of the valley. 

 It is not met with until the Portneuf is crossed. An accumulation of 

 Quaternary drift covers the beds, but judging from the outcrops in 

 Crentile Valley this entire basin was covered by a lake which extcBded 

 from Pliocene into comparatively recent times. At the end of Pliocene 



