feale.] DESCRIPTIVE GEOLOGY SNAKE RIVER AREA. 567 



The descent of the Portneuf to the mouth of Marsh Creek is much more 

 rapid than the descent of Marsh Creek. The bed of the former was prob- 

 ably raised by the basalt, and it was obliged to cut a new bed on the 

 east side of the valley to join Marsh Creek at the narrower portion of 

 the valley. 



As to the age of the beds, all that can be said at present is little. Fos- 

 sils were obtained from them on the Portneuf in 1871, but they were forms 

 rhat might be of very late Tertiary age or of Quaternary age, being un- 

 distinguishable from existing forms. We have seen that the beds have 

 not been disturbed since their deposition. They are older than the depos- 

 its found in the centre of Cache Valley on Bear Elver, and they differ in 

 character from the disturbed Pliocene beds that are found in Cache 

 Valley and in Malade Valley, and are evidently more modern. I believe 

 they were deposited in the same lake that occupied this valley, Cache 

 Valley, Salt Lake Valley, and the valley of the Upper Portneuf, and of 

 Bear Lake Valley. When the Marsh Creek portion of the lake was 

 drained the deposition of sediments continued in Cache Valley, covering 

 the deposits of the same age as those in Marsh Creek Valley wherever 

 the lake extended. 



PORTNEUF RANGE. 



The Portneuf Eange extends southward from Eoss Fork between the 

 Upper Portneuf Valley and the Lower Portneuf Valley, to the canon of 

 the Portneuf, which is at right angles to the trend of the range. This 

 constitutes the northern portion of the range, which is naturally divided 

 from the southern. The latter extends south from the Portneuf Caiion 

 to the northern end of Cache Valley, with which it really ends as a 

 distinct range, although geologically it is connected with the Bear Eiver 

 Eange by a mass of low hills, in which rocks of Silurian age outcrop, and 

 through which Bear Eiver cuts its canon from Gentile Valley to Cache 

 Valley. The northern portion of the range culminates in Mount Putnam. 

 On Mount Putnam white quartzites outcrop at the south end, and at the 

 north end shaly limestones. On Station 76, at the south end of this 

 northern portion ot the range, gray and blue limestones outcrop dipping 

 to south 75° east at an angle of 30°. This angle increases towards the 

 north, and on Mount Putnam it is 80° to 85°. 



Below the limestones of Station 76 are quartzites, according to Pro- 

 fessor Bradley (Eeport for 1872, p. 204), and below them limestones of the 

 Quebec Group. Below the latter occur the quartzites, in which the anti- 

 clinal fold west of the Portneuf is marked as shown in the accompanying 

 section. The continuation of the section westward will be considered 

 under the Bannack Eange. East of Station 76 dark-blue limestones 

 with red and yellow bands rest upon the lighter-colored limestones of 

 the station, and above them a white quartzite. The latter appears to 

 form the floor of a synclinal depression ; for as we approach the Portneuf 

 we find it rising and dipping gently to westward, the angle of dip being 

 10° near the Portneuf. In the synclinal depression resting unconform- 

 ably on the quartzites is a conglomerate, made up of angular fragments 

 of limestone. The outcrop was seen only at one point and appeared to 

 be horizontal in position, and is probably of the same age as the con- 

 glomerate at the top of the lake deposits in Gentile Valley. 



The Portneuf Eange here is, therefore, seen to be monoclinal, with an 

 anticlinal axis a short distance to the westward and a synclinal depres- 

 sion to the east, which has been occupied by an arm of the lake that 

 once filled the Upper Portneuf Valley. 



