BEAR VALLEY. 



193 



Plateau and Lower Sevier Valley. As these last-mentioned formations 

 have been inferred provisionally to be of Green River age, the beds of 

 volcanic sand, &c., may form an upward continuation of the same group, 

 or may even be considerably more recent, though many circumstances 

 seem to indicate that they were deposited in immediate succession to the 

 definite Green River beds without any protracted interval to separate them. 

 Their significance is purely local. They indicate that the eruptive activity 

 had commenced and had given vent to large masses of lava before the 

 extravasation of the older volcanic masses now remaining, and that these 

 most ancient ejections had been wasted and either utterly swept away or 

 buried where they have not up to the present time been laid bare. These 

 beds are seen in considerable mass on both sides of Upper Bear Valley, 

 and on the southeast side they constitute the lower courses of the two 

 mountains which tower above it and the long curtain wall which connects 

 them. Resting upon them is a sheet of lava of very interesting character. 

 It is identical in constitution with a sheet exposed in East Fork Canon, and 

 which will be described in detail in the chapter on the Sevier Plateau. 

 Upon this lava rests a layer of coarse rhyolite, which is evidently much 

 more recent in age, and forms the summit wall of the west side of Upper 

 Bear Valley. This layer is not seen on the eastern side, but in place of it 

 numerous trachytic beds are found alternating with conglomerate. 



At the bases of the two mountains these same beds of volcanic sand 

 are seen and the succession of trachytes and conglomerates. The upper 

 masses of the mountains are mostly trachytic, though between the flows 

 there is one prominent conglomeritic mass. The stratification is remarka- 

 bly even throughout, considering the volcanic nature of the components, 

 but it is not horizontal. In both mountains there is an east or east-south- 

 east dip, and they present the general aspect of great buttes left by the 

 denudation of the surrounding country, though the similitude is not exact. 

 A portion of their eminence, however, is due to a fault of about 800 feet 

 displacement which runs along their western bases, and the remainder of 

 their relative altitude is probably due to the denudation of the general 

 platform to the east of them and to the dip of the beds. These eruptions 

 are all very ancient (Miocene?), and since their extravasation they have 

 13 II p 



