42 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



canon should follow the north side from the bridge to the mouth of Jasper 

 Creek, descending at three or four points to the base of the rhyolite cap- 

 ping; once at Jasper Creek, near the cave shown in Plate XX, again 

 midway between that point and the falls where there is some irregularity 

 in the flows, and fiually at the mouth of Tower Creek. On the south side 

 there are several points within the same distance, but the key tQ the 

 whole situation will probably be found at the last base of Mount Wash- 

 burn. 



juinCtion valley. 



This valtey is situated in one of the most interesting as well as one of 

 the most beautiful regions in the Park. The geologist might find a year's 

 profitable work within a radius of 10 miles about Baronett's Bridge, and 

 the tourist couhl find almost unlimited sources of pleasure. Geologi- 

 cally there are illustrations of many of the most interesting problems 

 of geological historj', and the very perplexing events of the "Volcanic 

 Tertiary" period may be studied to the best advantage. 



North of the valley rises the great range of Granitic Mountains that 

 must have looked down in succession upon the Eocene, Miocene, and 

 Pliocene lakes, and witnessed the extraordinary events of the volcanic 

 period. Here, during the latter part of the volcanic epoch, while the 

 subaerial deposits were in process of formation, the erosion seems to 

 have been more than usually deep. The trachytes and rhyolites were 

 poured out into valleys cut in the Tertiary and Paleozoic rocks. Subse- 

 quent flows tilled in the valleys eroded from the first flows, and so on 

 through the basaltic and conglomerate producing period, on account, 

 however, of the occurrence of so many products together their relations 

 are hard to make out. This may be remarked of the Tertiary strata es- 

 pecially, the several groups of which cannot easily be distinguished from 

 the conglomerates formed in more recent times tlirough alluvial and vol- 

 canic agencies, and positive identifications are very nearly impossible. 

 Of course prolonged study may make everything clear. 



Dr. Hayden has already given a very good general account of this 

 locality in his annual report for 1872, and others have given it casual 

 attention. I shall give as much additional detail as seems to me de- 

 sirable. The map (Plate XXII) will give a pretty correct idea of the 

 surface relations of the various formations. 



In following the general course of the Yellowstone Valley from below, 

 it will be seen that the East Fork is its direct continuation, occupy- 

 ing a line along the base of the granitic range, while the main river 

 crosses the volcanic x>lateau from the southeast, reaching the granite 

 in Junction Valley. The channel of the East Fork is in the granites 

 in the lower 10 miles of its course. The main river runs from the bridge 

 to the western base of Garnet Hill over a granite bed. Below this for 

 a short distance it has not yet i)enetrated the full thickness of the Ter- 

 tiary strata. Garnet Hill is the largest body of granite occurring any- 

 where on the south side of the river. The granitic rocks are generally 

 distinctly stratified, and have a northwest and southeast strike, with a 

 steep dip to the northeast. The more compact layers, which consist of 

 ieldspathic granite and mica schists, form pretty marked ridges that 

 run at right angles to the course of the river, and may be traced l^r up 

 the uneven slopes of the Yellowstone Mountains. On the south there 

 is an irregular and much broken ascent from the channel of the river 

 to the general level of the rhyolite plateau, an ascent of approximately 

 1,000 feet. Aside from the main Yellowstone, however, there is no im- 

 portant stream draining into Junction Valley from the south. Among 



