696 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



uniform elevation, and the streams liave very little fall tliroiighoiit. 

 The height of the basin ranges from 6,000 to 6,300 feet. 



There is no timber in the basin, but sage and grass are abundant 

 everywhere. The buttes and ridges that diversify the surface have some 

 small quaking aspens on their sheltered slopes. 



THE YALLETS OF THE POKTNEUF AND MARSH CREEK. 



Still travelling westward, we cross a great field of basalt, with here 

 and there an extinct crater with its characteristic shape admirably pre- 

 served, and passing through a gap in a -range of low, broken hills, the 

 passage-way for a stream of lava, we enter another broad, flat valley — that 

 on the upper course of the Portneuf. This stream, a large branch of 

 the Snake, also pursues a tortuous course to effect a junction Avith the 

 main river. Heading near the north line of my district, it flows nearly 

 south for 36 miles, in the last 14 of which it gradually works its way 

 into the Portneuf Eange. Then it boldly turns west and carves a caiion 

 for itself through the range and flows out into its lower valley. Here, 

 in former times, it was joined by Marsh Creek, a long stream which 

 drains the broad southward extension of this valley ; but a stream of 

 basalt which flowed through the cafion of the Portneuf entered the A^^al- 

 ley on the left-hand side of the river and separated the streams, and by 

 filling up the bottom of the valley crowded them over on either side. 

 For 10 miles below their former junction, these streams now flow apart, 

 joining at the foot of the valley, where the Portneuf turns again to the 

 west to efiect a passage through the Bannack Eange. This tongue of 

 basalt which separates the streams is nowhere two miles in width, while 

 its average breadth is only a mile. Judging from some of the older 

 maps of this region, one might suppose that the flow of basalt which 

 separated these streams was of very recent occurrence, indeed, within 

 the present century. 



The Portneuf x)asses the Bannack Eange in a narrow caiion-like val- 

 ley, then turns northwest, while the mountains recede on the right and 

 left, and finally it reaches the Snake in the great basalt plain near the 

 site of Old Fort Hall. 



The upper valley of the Portneuf is one with that of the Bear, known 

 as " Gentile Yalley," the divide between the two streams being a broad, 

 flat field of basalt. Along the Portneuf the valley is very fine for agri- 

 cultural purposes, although the supply of water is very limited, the Port- 

 neuf here being scarcely worthy of being dignified with the name of river. 

 Pasturage is excellent, both in the valley and in the hills on either side. 

 The elevation is about 5,500 feet. Below the cafion there is little land 

 valuable for agriculture, but on the benches and lower slopes of the 

 mountains there is much fine grazing. Passing down the river past the 

 Bannack Eange, we enter a bay of the Snake Eiver x)lains, and here we 

 again find some arable land with excellent grazing over large areas of 

 plains and kills. 



The southern and broader part of the lower valley of the Portneuf is 

 occui)ied by its main tributary. Marsh Creek, a sluggish, swampy stream, 

 as its name indicates. This valley has a maximum width of 5 miles by 

 a length of 12 miles from the head to the point where the Portneuf en- 

 ters it. Its surface is floored with old lake deposits of gravel and finer 

 material from the jyicient Lake Bonneville, the progenitor of the Great 

 Salt Lake of the present day. At its southern end this valley connects 

 with pache Valley by Eed Kock Gap, a notch in the.hills, whose level is 

 not higher than that of the valleys on either hand. The gap is \ery nar- 



