236 GEOLOGY OF THE HIGH PLATEAUS. 



degree of consolidation is always considerable and is quite sufficient to 

 enable the edges to stand in great mural fronts many hundreds of feet in 

 height. In this respect it is as consistent as any of the calcareous sand- 

 stones of the region. It is, however', more easily attacked by the rains 

 and frost than the volcanics or even than the more massive kinds of sand- 

 stone. The included fragments exhibit all degrees of roundness by attrition; 

 are often quite sharp and angular; most frequently a little worn by current- 

 action; sometimes greatly so. Where the fragments are least worn they 

 are most abundant In many places the amount of cement is much less 

 than others, while in some places the fragments are relatively few. In size, 

 the fragments vary from a mere granule to two or three tons. The con- 

 glomerates are seen upon the slopes of Sevier Valley at the foot of the 

 western front of the plateau usually flexed upward a little and then cut off 

 by the great fault. On the east side of the plateau they slope down 

 towards Grass Valley (which is in great part a valley of erosion), and are 

 cut off in some places and dip beneath its floor in others, but reappear in 

 the western front of the Awapa Plateau. Whether these beds which are 

 seen in the Awapa are continuations of those in the Sevier Plateau is not 

 absolutely certain, but I think they are. 



About midway between the middle and southern eruptive centers the 

 Sevier Plateau is cut completely in twain by a mighty gorge called the 

 East Fork Canon. It is the old story — erosion. The plateau rose athwart 

 the course of the stream and was sawed in two. It is not a narrow chasm, 

 but a valley walled by ledge upon ledge. The dissevered beds above stand 

 a couple of miles or more apart facing each other across the depths; below, 

 the walls are from 1,000 to 2,000 feet assunder. The total depth varies in 

 different parts from 1,400 to 3,700 feet. The structure of the plateau is 

 thus clearly revealed. The upper rucks are volcanic conglomerate of 

 immense thickness, with intercalary sheets of coarse trachyte, the former 

 well stratified- The lower rocks are of a highly exceptional character, and 

 will be treated of at length in the latter part of this chapter. 



The third eruptive focus of the Sevier Plateau stands east of the head 

 of Panquitch Canon. It bears a strong resemblance in its features and the 

 character of its emanations to the northern vent (^Dlue Mountain). It is 



