bt.johx.] KEGION OF JOHN GRAY'S LAKE. 357 



forms the watershed between the sources of John Gray's Creek and 

 the Little Blackfoot to the west. This ridge has an extent of some eight 

 miles in our district, attaining a pretty uniform altitude of 7,200 feet, its 

 dominating summit rising about 1,000 feet above the level of John Day's 

 Lake basin on the east. Near the southern line of the district it is sud- 

 denly contracted and broken down in a broad, deep sag, through which 

 flows a pretty little tributary which rises in the low basaltic rim on the 

 east flank of the ridge in the immediate vicinity of the gentle slopes sur- 

 rounding John Gray's Lake. A well-beaten highway from the country 

 to the southeast crosses the ridge at this pass, and following up the valley 

 of Little Blackfoot it crosses over the low basaltic plateau at its head, 

 and thence descends the west side of Willow Creek out into the Snake 

 plains at Taylor's Bridge; a branch road crosses into the upper basin 

 of the Blackfoot, and thence westerly to Fort Hall and Boss Fork Agency, 

 where it again joins the great highway leading into Montana. 



This ridge, which may be appropriately termed Gray's Lake ridge, 

 south of the above-mentioned pass and lying within our limits, was 

 found to present a simple monoclinal structure, the result of erosion, by 

 which the east flank of an anticlinal fold was destroyed prior to the 

 effusion of volcanic matter, which latter gently rises on either slope of 

 the present ridge as heavy beds of more or less scoriaceous and compact 

 dark basaltic lavas, which strongly resemble the material found in the 

 summit of the Willow Creek ridge at Station XIV. The nucleus of the 

 ridge is here seen to consist of the ordinary gray and drab Carboniferous 

 limestones, interbedded with hard buff and reddish-buff sandstones or 

 sihceous beds, and dipping southerly at an angle of about 30°. The 

 ridge is here quite low, but it gradually rises to the southeast, where its 

 bulk is composed of the same formation, the beds seeming to curve round 

 the southern extremity of the ridge in a sharp quaquaversal, with out- 

 flanking foot-bills made up of red arenaceous deposits and sandstones and 

 drab beds. 



North of the pass these same deposits are seen in force in the north- 

 east flank of the ridge, and after crossing to the southwest side, the same 

 deposits also flank that side, where they incline southwestwardly, and 

 probably here compose the entire bulk of the exposed strata in the main 

 ridge. A mile or so southwest of the pass, at the crossing of the Little 

 Blackfoot, in the extremity of a low upland spur which puts out into 

 the valley at this point, the Carboniferous again appears, showing a 

 thickness a hundred feet, more or less, of heavy bedded, gray, cherty 

 and spar-seamed limestone, dipping 55°, S. 5° W. A few fossils were 

 seen here, Productus, Spirifer, &c. The relation of the latter exposure 

 to the ledges that form the crest of the ridge south of the pass was not 

 clearly shown; but it is presumed that it belongs to the same uplift, 

 although the trend of the strata has rather sharply changed more to an 

 east- west course, if this inference prove correct. 



The Little Blackfoot here occupies a wide trough-like valley, which 

 heads in the low basaltic plateau of the Willow Creek divide a few miles 

 to the northwest, and is bounded on the southwest by a similar narrow 

 mountain ridge, the distance from crest to crest along our southern 

 boundary being about four miles. The middle of the valley is occupied 

 by a level tract, through which the little stream flows with many wind- 

 ings between low terraced borders which gently rise to the foot of the 

 adjacent ridges. These border slopes in places conform to gently in- 

 clined basaltic benches, as is the case in the lake ridge to the south of 

 the pass, where they were found extending quite to the crest in a low 

 place in the ridge; but in the mid- valley the level surface is covered with 



