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Show EPOCHS OF ERUPTIONâ€"TRACHYTES. 59 considerable as they were, have been chiefly buried by trachytes, but the conglomerates derived from them are still conspicuously displayed. These fragmental masses lie around the eruptive centers in beds often more than a thousand feet thick, and cover areas of which the aggregate extent must considerably exceed 500 square miles. The third epoch of activity was by far the grandest of all. It was marked by the extravasation of trachytic masses, alternating with augitic andesites and dolerites. A long interval of time separated these eruptions from the andesitic outbreaks just described, for the andesitic rocks were extensively degraded by erosion and their fragments gathered into conglomeratic masses before the earliest outpours of true trachyte. The area of activity was greatly extended in the trachytic age, new places opened and poured forth immense floods, which at length became so vast that they overwhelmed and buried the greater part of the district, generating a new topography. The northern part of the Sevier Plateau, which had given vent to the propylites and andesites, became a focus of still more extensive trachytic eruptions. From this center they spread in all directions. Those which rolled eastward are most conspicuously displayed, and the first impression is that the larger portion of the trachytes flowed in that direction. Some of the grander sheets extended more than 20 miles to the southeast of their origin, and die out near the base of Thousand Lake Mountain. To the southward they make up the greater part of the bulk of the Sevier Plateau, reaching nearly 25 miles from the vents, and commingling with floods poured from median vents in the plateau. To the northward they stretched beyond the locus of Salina Canon, where they have been much wasted by erosion, but heavy masses are still left to indicate their former magnitude. To the westward the sheets are abruptly cut off in the face of the escarpment of the west front of the Sevier Plateau, which reveals more than 3,000 feet of their mass resting upon the andesites and propylites. Beyond this a great fault throws down Sevier Valley, in which they are seen in a few places beneath later rhyolites. It is by no means certain that all the foci of eruption have been ascertained. So great have been the changes produced by erosion, that the superficial features have been thoroughly remodeled by it. No lofty, |
