peale.I DESCRIPTIVE GEOLOGY — SNAKE RIVER AREA. 563 



will be more fully described by Mm in bis report. Mr. St. Jobn cousiders 

 tii em to be of tbe same age as the lake beds exposed ou Bear Biver 

 which contain similar fresh-water forms. 



PORTNEUF 'RIVER. 



The Bortneuf 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 

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

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

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

 keeps for about ten miles, when it makes a right angle and flows west- 

 ward across tbe range, and after a course of almost ten mfles 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 Bange, after which it leaves our district 

 with a northwest course to flow out into tbe Snake Biver Plains. 



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

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

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

 the point where the Portneuf enters the caDon 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 



JProductus Costatus, 

 Spirifer Boclcym ontamis, 

 Crinoidal stems. 



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

 hills at Twin Springs, 6 miles farther south : 



Zaplirentis ? Stansburyi. 



Lithostrotion ? 



Bhombopora "? 



Polypora ? 



Chonetes % 



Bpirifirina ? 



Euomplialus ? 



Crinoidal fragments. 



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

 bills 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 Bange. 



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

 by an anticlinal fold. ISTo basalt occurs in the northern part of the valley. 

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

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

 Gentile Valley this entire basin was covered by a lake which extended 

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



