EPOOnS OF ERUPTION— 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 con- 

 glomeritic 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 dii'ections. Those 

 which rolled eastward are most conspicuously displayed, and the first 

 impression is that the larger portion of the trachytes flowed in that direc- 

 tion. 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 commin- 

 gling with floods poured from median vents in the plateau. To the north- 

 ward they stretched beyond the locus of Salina Cailon, where they have 

 been much wasted b}^ erosion, but heavy masses are still left to indicate 

 their foraier 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 ascer- 

 tained. So great have been the changes produced by erosion, that the 

 superficial features have been thoroughly remodeled by it. No lofty, 



