CENTEAL VENTS OF THE SEVIER PLATEAU. 



235 



completely buried in the seas of lavas which were poured out around it. 

 At a later date it has been excavated by the erosion of Grass Valley and 

 one side of it exposed. This is a large tufa cone, which must once have 

 been nearly 1,800 feet high, and was formed by showers of small frag- 

 ments blown from the orifice. They are seen dipping to the southeastward 

 in a large ravine recently excavated in the side of the plateau, and the 

 angle of dip is from 28 to 30 degrees near the summit, but decreases towards 

 the base. The fragments are mostly augitic andesite and are closely com- 

 pacted with very little cementing material. They are very sharp and 

 angular, showing no evidence at all of attrition The stratification is quite 

 perfect and the entire mass is thoroughly consolidated into a coherent body 

 of stratiform layers. It is noticeable that the fragments are seldom of largo 

 size, rarely exceeding in weight ten or fifteen pounds. Only a small seg- 

 ment of this cone is now exposed, and such portions as have been excavated 

 have been ruthlessly attacked by the waters, which have incised deep 

 ravines, which are destroying the cone almost as fast as they are unearthing 

 it. Far above it rise the massy sheets of trachyte and the pediments 

 formed in the projecting sheets lap around it on both sides. Probably it is 

 a very common thing in the history of a volcanic pile for its earlier cones 

 and monticules to be overwhelmed and buried by later outpours. But it 

 may give some notion of the magnitude and grandeur of the eruptions of 

 the Sevier Plateau to §ee a cone of this magnitude inclosed in rock, as if it 

 were a mere trifle. 



The conglomerate forms the principal mass of the plateau south of the 

 central vents for a distance of nearly 20 miles, where it becomes confluent 

 with similar beds derived from the volcanic masses disgorged from the 

 southern vents. It is frequently intercalated with enormous sheets of horn- 

 blendic trachyte, erupted during the long period occupied by the accumula- 

 tion. The conglomerate forms the intervening summit of the plateau 

 between the eruptive localities, and has a thickness never less than a 

 thousand feet and several exposures show more than 1,600 feet of it. Into 

 its composition enter all the varieties of the andesitic and trachytic rocks 

 forming the series of eruptive masses to the northward, which are cemented 

 together by volcanic sand and decomposed fine detrital matter. The 



