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Show 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 fragments 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 compacted 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 large size, rarely exceeding in weight ten or fifteen pounds. Only a small segment 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 see 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 accumulation. 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 |
