564 REPORT UNITED STATES GE0L0C4ICAL SURVEY. 



time, however, there were disturbances wliicli isolated i)ortions of the 

 lake. Considerable disturbance was noted in the Pliocene beds north 

 and south of this locality, and will be noted in other places. 



IPpper Fortneuf Caiion. — As has already been stated, the Poitneuf 

 enters the caiion flowing south. It is parallel to the strike of the 

 quartzites that outcrop here, and j)robably is on the line of an anticlinal 

 axis forming' the high part of the range south of the canon, as the hills 

 west of Basalt Valley show eastward dips. At the head of the canon 

 the basaltic flow comes to the river, and continues through to Marsh 

 Creek Yalley. It is, however, frequently covered by the local drifts 

 As the river enters the caiion it is llowing soiith, but it curves a little 

 eastward as it approaches the bend. Above the bend, on the west side^ 

 white sandstones show, probably of Pliocene age. In the angle stands 

 a high butte of limestone, probably the remnant of an anticlinal ridge. 

 Below the bend the basalt shows more jflainly, and on the south side a 

 tunnel has been cut into the side of the hill which shows two layers of 

 basalt separated by a sandy layer containing rounded bowlders. All 

 along this portion of the river there is a great abundance of calcareous 

 tufa which extends out into the river, often forming dams which cause 

 numerous waterfalls from 5 to 20 feet in height. In this part of the 

 canon also, as we proceed down it, the basalt is seen outcropping now 

 on one side and again on the other, but it probably underlies the whole 

 valley. As the river emerges from the caiion it is seen on both sides^ 

 but a short distance below it is found only on the west side extending 

 westward to Marsh Creek. The descent of the river from the mouth of 

 the canon to the bend at the mouth of Marsh Creek is quite rapid, and 

 the basalt forms a rapidly increasing bluff on the west side. On the 

 east side, a few miles below the bend, an outcrop of lake beds containing 

 fresh-water forms (Flanorbis, &c.), allied to existing forms, was noted. 

 There are also isolated outcroppings of limestones and shales. It is 

 probable that the lake beds just noted once covered the entire surface, 

 but have been eroded away. The drift from the hills has covered the 

 beds so that difficulty is experienced in trying to determine their exact 

 relations. The basalt ridge breaks off before the mouth of Marsh Creek 

 is reached, and the angle between the mouth of Marsh Creek and the Port- 

 neuf has an isolated hill 400 or 500 feet high, in which limestones outcrop. 

 Xorth of the bend yellow, purplish, and gray quartzites and shales out- 

 crop, inclining at high angles, indicating the presence of an anticlinal 

 axis. This is probably the same axis noted just south of the bend of 

 the Portneuf, which will be described under the head of Marsh Creek. 

 The centre of the anticlinal appears to be composed of dark drab slates, 

 either Cambrian (Lower Silurian) or Huronian. Above the slates are 

 purple and gray quartzites. A short distance beyond, the dip has 

 changed and is again to the northeast ; and we cross a great thickness 

 of quartzites and slates. These beds are exposed in the section of Sta- 

 tion 79, and will be described under the head of the Bannack Eange. 

 They form an anticlinal ridge, across which the river cuts to the Snake 

 Eiver plains. The basalt ends in several tables before this anticlinal 

 is crossed, but it appears again below at a lower level. Whether the 

 source is the same for both it is impossible to say. The lower out- 

 crop ai3i)ears to be connected with the flow of the basalt in the Snake 

 Eiver j)lains. In this portion of the valley, also, white sandstones make 

 their appearance on the flanks of the hills outside of the basalt. They 

 resemble those seen at the upper part of the Marsh Creels Yalley, but 

 whether or not they are the same beds I am unable to say. I am inclined 



