44 REPOET UNITED STATES GEOLOGICAL SURVEY. 



The steep slopes that reach up to the plateau on the south side of 

 the valley are composed of Tertiary conglomerates and rhyolites, the 

 former j^revailing towards Elk Creek and the latter towards Tower 

 Creek. The trachytes are probably younger than the conglomerates, 

 although in many places they seem to pass beneath the conglomerate 

 walls. This appearance is doubtles the result of their having been 

 tilled into sharply-cut depressions in the Tertiary rocks. 



The brecciated trachytes are found again in Junction Butte, where 

 they may be seen on the north side, beneath the capping of black ba- 

 salt. Here the color is reddish yellow. The weathered surfaces and 

 debris slides are almost as dark as the basalt. There is not a large ex- 

 posure, although it is i^robable that the layer forms the base of the 

 butte and rests upon the granites. These trachytes are nowhere more 

 than 200 feet in thickness and occur at an elevation above the river 

 of from 100 to 600 feet. It is not improbable that they all belong to 

 the same flow and that the old valley was filled up to a pretty gen- 

 eral level by that flow. The sheets of trachj^te had undergone a large 

 amount of erosion prior to the flow of the basalts. The rhyolites, 

 which belong chiefly to the plateau flows, descend to the valley in two 

 or three places. As mentioned before, the low hill that lies between 

 Garnet Hill and the Elk Creek Meadows is capped with a mass of 

 light-gray rhyolite that closely resembles the plateau rhyolites. Be- 

 neath the rhyolite we have the brecciated trachyte. Between the 

 bridge and Tower Creek the rhyolites outcrop along the brink of the 

 shallow canon, beneath sheets of basalt. At the bend of the river, 

 near the mouth of Lost Creek, there is an outcrop of rock that resem- 

 bles a rhyolite but which has the appearance also of a sandstone. It 

 is probably disintegrated rhyolite recomiDacted. West of this, toward 

 Elk Creek, the rhyolites occur in the plateau face and form the upper 

 part of the cliffs. They extend back toward the south and pass beneath 

 the conglomerates of the Washburn Mountains. The small stream which 

 I have called Lost Creek, because it apparently sinks from sight in the 

 lower part of its course, falls over a high wallof x)inkish-gray rhyolite 

 at the point of its entrance into the valley. 



I have already described the manner in which the rhyolites occur at 

 the mouth of Tower Creek. 



The basalts of this valley form an important geologic feature. Dark 

 masses of this rock may be seen on all sides. These are remnants of a 

 few flows or sheets, however, as attested by the uniformity of levels. In 

 age they are evidently the youngest of the volcanic products, and have 

 been poured into the valley at so late a period that it had reached al- 

 most its present conformation. 



The old river channel has been obliterated, and the irregularities in 

 the granites, tertiary sedimentaries, trachytes, and rhyolites filled in. 

 The highest occurrence of the basalts is about 600 feet above the 

 river bed at the forks, and the lowest is within less than 100 feet 

 above the river. Since the flow of the basalts, the river has therefore 

 cut through the 500 feet of basalt, and nearly 100 feet into the under- 

 lying formations. How nearly the river runs in its old channel it is 

 difficult to say. There seems to have been a tendency on the part of 

 the river to shift toward the base of the granite range in this part of its 

 course, if we are to judge by the distribution of the river drifts. The 

 great body of the igneous llows are at present on the south side of the 

 river. On the north side, below the junction, there are a few remnants 

 representing at least two distinct flows, which are set into depressions or 

 recesses in the granites. Fronting the river these remnants break down 



