632 DESCRIPTIVE GEOLOGY. 



zon of the Weber Quartzite, has already been described in its main struct- 

 ural and lithological features, and the reasons assigned for regarding it as 

 of Carboniferous age. Within the limits of this map, the western slope, 

 which is traversed by numerous deep narrow canons, exposing the same 

 beds, all dipping at high angles to the eastward, forms for 10 or 12 miles 

 the eastern side of Eeese River Valley. To the southward, the ridge falls 

 away rapidly, causing a deep depression in the sedimentary strata, and in 

 about latitude 40° 10' passes under the great accumulation of rhyolite. 



At this point, the rhyolite, which has been traced along the eastern 

 foot-hills, occupies the depression, stretching westward across the entire 

 width of the range, and in Reese River Valley may be followed northward, 

 flanking the older rocks, to within 3 miles of Trout Creek. Here, on the 

 west side of the range, the rhyolites form a comparatively regular series of 

 hills, with steep slopes, inclined to the westward, and broken through at 

 intervals by drainage-courses. By reference to the geological map, it will 

 be seen at a glance that the large body of quartzite to the north is sepa- 

 rated from a smaller mass, which crops out through the general rhyolitic 

 accumulations to the south; the gap between the two bodies being only 

 about 3 or 4 miles in width. That this depression occupied by rhyolite is not 

 filled up by the mere flowing of rhyolitic material in a viscous state is easily 

 seen by the study of the topographical structure, but is rather a centre of 

 massive eruption, the volcanic product seeking the surface along lines of 

 greatest weakness in the older Palaeozoic series. 



Between the two quartzite bodies, the rhyolite forms a high prominent 

 peak, nearly 2,500 feet above the neighboring valleys, falling off with easy 

 slopes toward Reese River, but somewhat steeper on the eastern side. On 

 both the east and west slopes,, the peak is penetrated by long deep canons, 

 with the broad basin-shaped valleys near the top, so characteristic of rhyo- 

 lite mountains. The rock is similar to the rhyolites already described from 

 the range, a gray and reddish-gray felsitic groundmass, with large trans- 

 lucent fragments of quartz abundantly scattered through it. The smaller 

 body of quartzite to the south follows the main trend of the range, rising 

 above the rhyolite in a narrow ridge, about 7 or 8 miles in length. There 



