404 REPORT UNITED STATES GEOLOGICAL SURVEY. 



intrusive ledges similar in i)osition and mode of occnrrence to the expo- 

 sures in the before-mentioned amphitheatre walls. About a mile south- 

 east of Station XXVIII, the mountain rises into a second prominent 

 summit. Passing the saddle between these points, the whole southwest 

 face of the mountain is covered with the angular slabs and dehris of 

 dark -bluish to greenish-gray, hornblendic trachyte, weathering to a pale- 

 reddish hue. In the latter mountain-peak the eruptive mass seems to 

 rest ui)on metamorx)hosed bluish slates, the line of junction iiursuing a 

 northwest and southeast course, the slates dipping southwestwardly. 

 The eruptive material here has more the appearance of a wedge-shaped 

 mass, though it was undoubtedly connected with the dike-like effusion in 

 the west spur of the main summit ; and so, also, with the intrusive sheets 

 which were noticed in the northeast face of the mountain. Hence, it 

 appears that the phenomena observed at this locahty suggest one of 

 those local outbursts or a concentration of volcanic activity, by which the 

 sedimentary deposits were fractured and split apart, the molten matter 

 in the one case forming wedge-shax)ed dikes, and in the other intrusive 

 sheets following the plane of bedding. It may be that this locality 

 marks a centre of erux)tion from which were derived much of the earlier 

 volcanic products which were spread over the mountain summits, rem- 

 nants of which are stiU to be found on high eminences. But the imper- 

 fect study we were able to make rather leads to the conclusion that at the 

 time of this eruption, which was doubtless subsequent to the disturb- 

 ances by which the sedimentaries were upraised and folded, the volcanic 

 activity was wholly subterranean in its manifestation, and that the pres- 

 ent appearances at the surface are due to the excessive denudation which 

 the region has been subjected to within a comparatively modern date. 



This locality is one also of unusual economic interest and importance, 

 on account of its placer mines. These are situate along Iowa Gulch, a 

 main tributary of McCoy Creek, and on the upi^er course of Tin-Cup 

 Creek, which flows south and east into Salt Eiver, a few miles above its 

 confluence with the Snake. At the former locality the mines are oper- 

 ated by the usual various processes, and at the time of our visit there 

 were about twenty-five whites and three or four times as many Chinamen 

 engaged in the works. The mines were discovered in 1870, since when 

 the district has had a fluctuating i^opulation, and though, perhaps, never 

 remarkable for extraordinary yields in gold, the present occupants of the 

 mines claim to be reaping fair returns. The auriferous gravels are j)rin- 

 cipally accumulated in what is known as Bilk's Gulch, which rises imme- 

 diately under the summit of Mount Caribou, and consists of abraded 

 volcanic and sedimentary materials largely mixed with the red earth 

 derived from the softer shales. The mines are distributed along Bilk's 

 Gulch and Iowa Gulch to its confluence with McCoy Creek, a distance 

 of about three miles. The chief difficulty, as usual, is the limited supply 

 of water, which is said to last about three months, the " cleaning uj;) " 

 requiring a month or so more, for which a limited quantity of water suf- 

 fices. As to the productiveness in gold of these placers, nothing definite 

 was ascertained. The placers on Tin-Cup Creek are also said to be mod- 

 erately productive ; but the single day we had at our disposal for the 

 examination of this section did not i)ermit visiting this locality. Gold 

 has been obtained in the sands all the way down the valley of McCoy 

 Creek, and, I believe, in the bars along the Snake, but not in remunera- 

 tive quantity. Indeed, the whole region has been pretty thoroughly 

 ransacked, and jirospect excavations were encountered in the most ex- 

 traordinary and unexpected places. 



From what has been said, it is evident that the source whence the 



