30 REPORT UNITED STATES GEOLOGICAL SURVEY. 



of rhyolite which corresponds to the capping of Mount Evarts at m, as 

 also to the capi^ing of the hill n, and the rhyolitic substratum of the 

 plateau n. The basaltic and hornblendic trachyte conglomerates of 

 Sepulchre Mountain extend down to the point o. 



The West Fork of Gardiner River is a small creek that rises along 

 the south base of Sepulchre Mountain, and runs down an open, grassy 

 valley, upon a bed of drift. At g it turns abruptly to the northeast 

 and descends through a rhyolite gorge to the lower valley. The main 

 branch of the Middle Fork, which heads in the Gallatin Eange south 

 of Electric Peak, appears in the sketch at i, passing in a most sin- 

 uous course across the foreground to the right. After passing a 

 considerable distance to the south it is joined by Indian Creek, and 

 afterwards by Obsidian Creek. From the junction of the latter it turns 

 to the north at a sharp angle, and descends into the lower valley at /. 

 The low ridge (t), which may be seen extending down between the 

 West and Middle Forks, is a much-prolonged spur of Electric Peak, 

 that terminates near the mouth of Indian Creek. At the left it is 

 capi3ed with conglomerates and breccia, which are identical with the 

 conglomerates and breccias of Sepulchre Mountain. Much of the ridge 

 is covered by drift deposits, and the rounded, sparsely-tree-covered 

 slopes abound in rich upland pastures. In the lower part of this ridge 

 there are masses of basalt, the extent of which I am quite unable to de- 

 fine. It seems probably that they may underlie a large area of the drift- 

 covered flats in the vicinity of the junctions of the main branches of 

 the Upper Gardiner. The course of the main Upper Gardiner is much 

 obstructed by beaver dams, as indicated by the sinuosity of its course. 

 Before reaching the main descent into the lower valley at e, it passes 

 through a shallow canon, the walls of which are probably formed of 

 the rhyolites. However, this particular region remains quite unex- 

 plored. Here, also, the pine forest prevails, and beyond this, extending 

 to the valley of the East Fork c, there is but little open pasture land. 

 Of this region I must plead total ignorance, excepting as to its general 

 character, which is that of the other parts of the rhyolite plateau. The 

 E ast Fork has its sources along the western base of the Washburne Eange 

 and passes through a number of small lakes. My only opportunity of 

 observation in the western part of the Upper Gardiner Basin was en- 

 joyed during a hurried trip from the Hot Springs to the Gibbon Geyser 

 Basin, by way of Obsidian Creek. 



Leaving the springs, we passed up through the pass I, and followed 

 the ISiorris road down the valley of the West Gardiner, iiast the small 

 lake s, and thence across the main Gardiner, and up Obsidian Creek. 

 About the mouth of Indian Creek there are deposits of compact dark- 

 gray basalt. Beyond this there are only the rhyolites and obsidians. 



In ascending Obsidian Creek, by way of the wagon-road which con- 

 nects Mammoth Hot Springs with the Geyser Basins, we pass first 

 through broad meadows and parked forests. Farther on, the valley 

 narrows up and the timber becomes extremely dense. At a point about 

 12 miles above the junction of the creek with the main stream, there is 

 a narrow gateway, known as Obsidian Canon, through which the road 

 and creek pass. From the east side of the valley a low promontory ex- 

 tends forward to the creek and breaks oft" in an abrupt nearly vertical 

 wall, in which the obsidian rocks are exposed. The road appoaches 

 the canon along the west side of the valley, and crosses to the east side 

 at the lower end of the canon ; in order to avoid the swampy ground 

 that borders the stream, it has been carried across the steep debris 

 slopes of the obsidian clifis. For a half mile it is paved with glassy 



