180 GEOLOGY OF THE HIGH PLATEAUS. 



degradation, and the altitudes of the eruptive centers increased. Now and 

 then came a long interval of repose, indicated by the quiet accumulation of 

 considerable, though very local, masses of stratified conglomerate here and 

 there. Again the energy was renewed and fresh outbreaks occurred, fol- 

 lowed by a long rest. After a protracted series of alternating eruptions and 

 unequal intervals of rest there came a very long period of repose to be 

 reckoned by a geological standard of time, during which these massive 

 conglomerates accumulated and the huge volcanic piles were razeed — a 

 period in which there may have been eruptions, but in which, on the whole, 

 the ceaseless erosion leveled down the highlands and leveled up the low- 

 lands. 



But the building of the conglomerate beds did not close the volcanic 

 cycle. After they had acquired their enormous bulk there came another 

 period of outbreaks, some of them in the old localities, others in new ones, 

 pouring fresh sheets over the wasted centers and over their scattered and 

 stratified debris, piling up fresh mountains of lava and generating a new 

 topography. This second series of eruptions differed strikingly in litho- 

 logical character from the first. The earliest series in the Tushar, so far as 

 known, is andesitic and trachytic ; the second is rhyolitic and basaltic. In. 

 the northern part of the range the dominant rock of the second series is 

 rhyolite, with a limited occurrence of basalt. In the southern part of the 

 range the relative abundance of the two groups is reversed, rhyolite being 

 uncommon, and in most areas being replaced by true trachyte. These 

 beds cover both the central part of the Tushar and the conglomerates at 

 the southern end. They lie upon the eroded surface, filling old ravines and 

 spread out in broad sheets over the tabular summit, obliterating upon the 

 surface the definition between the conglomerate and the degraded mass 

 which furnished its materials, though the junction is exposed in the eastern 

 front of the range by the great fault which at a later epoch was formed by 

 the general uplifting of the whole mass. 



The southern termination of the Tushar is marked by a group of lofty 

 summits a few hundred feet lower than Belknap at the northern end and 

 Delano near the center, but full 1,600 feet higher than the wall and tabular 

 summit which connects them with the central part of the table. They are 



