228 GEOLOGICAL SURVEY OF THE TERRITORIES. 



as having been examined by Dr. Hayden. From the southwest angle 

 of the valley, Red Eock Pass affords a broad, flat, grassy opening to 

 Eed Eock Lake, one of the ultimate sources of Beaver-Head Fork of 

 the Jefferson. This was not visited by us, though seen from a distance. 

 It was afterward examined by Messrs. Peale and Holmes, of Dr. Hay- 

 den's branch of the expedition. The levels, as decided by the observa- 

 tions of both parties, are as follows : Henry's Lake, 6,492 feet ; Tyghee 

 Pass, 7,063 feet ; Eaynolds's Pass, 6,911 feet ; Eed Eock Pass, 7,271 feet. 

 The most favorable route for a railroad from Montana to Oorinue, which 

 is now talked of, is apparently via Eaynolds's Pass and the valley of 

 Henry's Fork. Down this valley the grades would be very easy, averag- 

 ing only about 25 feet per mile from the pass to Taylor's Bridge, on 

 Snake Eiver, and the cost of grading very slight, while the large valleys 

 which open on the east, containing large areas of valuable grazing and 

 farming land, would be opened to settlement, and would very soon fur- 

 nish considerable local business. 



The short range on the west side of Henry's Lake shows at base, near 

 Sawtelle's ranch, a considerable body of dark hornblendic, slaty schist j 

 but the mass of the ridge consists of much-folded metamorphic lime- 

 stone, which, in its lower portions, shows a considerable thickness of 

 light drab, almost white, rock, which would make very fine building- 

 material. Its higher portions, however, contain very numerous 

 thick and thin bands of white, often transparent, quartz, corresponding, 

 in general appearance and relations, with the bands of chert which 

 accompany certain portions of the Carboniferous limestones in this re- 

 gion. From this character, in the absence of any opportunity fco deter- 

 mine its age by tracing the bed to its unmetamorphosed portion, I have 

 referred it, with very little doubt, to the Carboniferous. Upon the crest 

 of the ridge, we found the outcrop of a dike of trap, about 60 feet wide, 

 standing conformably between the layers of the limestone, and therefore 

 supposed to have been deposited in that relative position before the beds 

 were upheaved into their present nearly vertical position. The subse- 

 quent metamorphism of the whole series has obliterated any marks of 

 alteration of the adjoining beds of limestone, which we might other- 

 wise have looked for. From the crest of the ridge it was evident that 

 the back spurs were of the same general structure and composition at 

 the main ridge, at least for two or three miles. 



On the east side of Eaynolds's Pass, metamorphic rocks form the base 

 and lower slopes of the mountain, while quartzites and limestones ap- 

 pear near its summit. This structure continues nearly to Tyghee Pass, 

 where the upper rocks come down to the level of the plain, for two or 

 three miles, and then rise again, exposing the granites, gneisses, horn- 

 blende •schists, &c., until we reach the outcrop of volcanic sandstone 

 before described. Just west of our camp, at the mouth of Tyghee Pass, 

 the lowest of the quartzites is exposed on the bank of a small creek. 

 The bedding is nearly vertical, with a strike trending about JS^. 53° E. 

 Unconformably upon the edges of this bed lie about 200 feet of 

 a light-drab, impure limestone, of Quebec Group age, from which we 

 obtained with difficulty three fragments of characteristic trilobites. 

 The dip is about 45° N. 54° E. As we have elsewhere found 

 quartzites of supposable Potsdam age lying conformably beneath the 

 Quebec Group, we are led to question whether this quartzite may not 

 possibly be of still earlier age ; but, in the absence of fossils, there are 

 no means of deciding the question. But this may very well be Pots- 

 dam, as I presume it really is, without making it necessary that it 

 should everywhere else lie, as here, unconformably beneath the Quebec. 



