328 REPORT UNITED STATES GEOLOGICAL SURVEY. 



parallelism denotes their intimate relation to the larger mountain corru- 

 gations on either hand. This basin area reaches far to the south, into 

 the district visited by Mr. Gannett and Dr. Peale, where it is described 

 as showing essentially the same surface features. To the west of the 

 Blackfoot Eange and within the lower or northern bend of Blackfoot 

 Elver, an area of between 300 and 400 square miles is occupied by low 

 hill-ridges forming the northern prolongation of the Mount Putnam Eange 

 and its outlying eastern highlands, which successively die out in the 

 edge of the plain south and north of the debouchure of the Blackfoot. 

 These low hill -ranges are intersected by parallel valleys and little basin 

 expansions, which pleasantly diversify the country and distribute valu- 

 able agricultural lands where otherwise the country would be useful 

 only for grazing purposes. 



MOUNT PUTNAM — PORTNEUF RANGE. 



One of the highest points of this range, that on which Station I was 

 made, is situated just over our southern line, in the district surveyed by 

 Messrs. Gannett and Peale, where the range, indeed, attains its principal 

 magnitude. It consists of a somewhat rugged axis of ancient quartzites, 

 alternating with slaty micaceous shales, and apparently resting upon 

 a heavy mass of the latter deposits, which are partially exposed in the 

 high shoulder overlooking Eoss Fork drainage, on the west side of the 

 range, facing the Snake plain. Just to the north of Station I, the range 

 is broken though by a branch of Eoss Fork, which opens out into a 

 pretty little hill-environed basin, excavated in the sedimentary deposits 

 which succeed the quartzites on the east; and where it crosses the range 

 on its way into the plain it has cut a short gorge across the tilted Silu- 

 rian quartzites and limestones. This rift marks the northern breaking 

 down of the higher crest of the range, which to the north, within the 

 present district, is continued in a much lower ridge, whose culminating 

 summits rise but a few hundred feet above the Snake plain. This low 

 prolongation of Mount Putnam sweeps round from a north to a north 

 l>y east direction, terminating in rather abrupt hills, which form the 

 western angle of the debouchure of Lincoln Creek drainage, a few miles 

 south of Fort Hall. The declivity facing the plains is hi places quite 

 abrupt, the opposite flank much more gently descending in broad undu- 

 lations into the shallow depression in which Lincoln Creek and branches 

 of Eoss Fork rise, its surface for the most part covered with herbage, 

 with here and there rocky points clothed with gnarled cedars, and copses 

 of undergrowth and aspen clinging to the sides of the ravines which 

 intersect the ridge. 



The quartzites which crown the high northern peak of the range pass 

 beyond the western border of the northern ridge prolongation, where 

 they have been denuded and concealed beneath late Tertiary deposits, 

 which, according to Professor Bradley, make up quite an extensive for- 

 mation outlying the ridge. In the craggy ridge north of the south gorge 

 of Eoss Fork the quartzites and limestones of the Lower Silurian reap- 

 pear, where they show a wavering strike, with a general bearing west of 

 north and dipping east. The strike of the rocks diverges a little west 

 of the trend of the ridge, which, as it bears more and more to the east 

 of north, is successively occupied by more recent deposits, until the 

 Jurassic caps the northeast end of the ridge, as observed by Professor 

 Bradley. 



A low spur dividing the Eoss Fork drainage from Lincoln Creek, and 

 spreading over a Hinited area to the north and west ; forming an undu- 



