412 REPORT UNITED STATES GEOLOGICAL SURVEY. 



wliere it is flanked by the sedimentaries the central portion of the range 

 is fashioned into scries of extensive amphitheatres and Alpine basins, 

 in which the streams gather from their sources in the snow-flats to force 

 their way through the canons which score the great foreland descending 

 into Pierre's Basin. The basis or foundation of the range is Archfean, 

 which constitutes the northern three-fourths of the main crest, expand- 

 ing in the northern half, where these rocks occupy the whole width of 

 the range between Jackson's Lake and the volcanic foreland south of 

 the North Fork of Pierre's Eiver. 



This Archsean expansion culminates in a lofty spur, which terminates 

 in a pyramidal peak overlooking the densely Avooded volcanic foreland, 

 which here reaches high up on the mountain border, or a relative height- 

 of 2,500 feet above the plain north of Pierre's Basin. To the north the 

 Archaean nucleus is prolonged in a narrow belt, which finally sinks be- 

 neath the upraised sedimentaries and volcanics which lap up around the 

 northern extremity of the range, the latter merging into the great vol- 

 canic lilateau which extends thence far to the north beyond our district. 

 In the vicinity of the debouchure of Leigh's Creek the sedimentaries 

 again j)eer out from beneath the volcanic flow, and thence southward 

 they hold a prominent position in the western flank of the range, and 

 toward its southern extremity completely bridge over the Archaean core, 

 which descends in a rapidly narrowing belt in the eastern front, and 

 finally disappears in the vicinity of East Pass Creek, beneath the level 

 of Jackson's Basin. 



As a whole, the topographic characteristics of the range may be likened 

 to a great block tilted along its eastern border, the drainage of the west- 

 ern slope confined to long narrow canons separated by broad interven- 

 ing tables of sedimentary origin, while that of the east side lies in i^ro- 

 found short gulches mainly excavated out of granitic or metamorphic 

 rocks. 



Although our own examinations afforded a pretty comprehensive view 

 of the general geognostic features of the range. Professor Bradley had 

 opportunity for iDrosecuting more detailed examinations in certain local- 

 ities, which, for lack of time, we were prevented reahzing to the same 

 extent. In his report of 1872, he describes the Archaean area of the cen- 

 tral mass to consist of micaceous, hornblendic, talcose, and chloritic gran- 

 ites, gneisses, and schists, the former white or flesh-colored, and occur- 

 ring in massive beds. The gneisses and schists are much distorted, the 

 prevailing strike holding an east- west direction, with variable and gen- 

 erally steep dips. These rocks are intersected by numerous large and 

 small quartz veins, with a few of granite. On the North Fork of Pierre's 

 Eiver our young men succeeded in obtaining a single "color," indicating 

 the auriferous character of some of these veins in that portion of the 

 range at least. Besides these, there are a few dike-like exhibitions of 

 dark trap-rock, which at some points Professor Bradley found conform- 

 able to the bedding of the granites, though it was difficult to decide 

 whether their occurrence in this i:)Osition should be taken as indicating 

 their intrusive character, or were deposited as flows on the ancient sur- 

 face upon which the inclosing sediments were accumulated. One of these 

 trap-dikes forms a well-marked feature in the southern base of the great 

 cone of Mount Hayden, which may be traced as a dark ribbon descend- 

 ing the west face and crossmg intervening spur-ridges to the west, where 

 it is lost beneath the quartzite of the Potsdam, which crowns the ridges 

 in the neighborhood of the Pulpit. 



A very similar dike, doubtless also trap, occurs in the northern half 

 of Mount Moran, and which may be seen from either side, appearing like 



