EHYOLITES— SOUTHERlSr PORTION OF THE TUSHAR. 



177 



extending far to the westward, expanding as they extend. Of this rock, a 

 dark purplish porphyritic rhyoHte, the great central mass of the Tushar is 

 composed. 



The second period of activity was followed by another interval of 

 repose. During this interval the greater part of the uijlifting of the range 

 took place. The faults traverse and dislocate both the first and the second 

 series of eruptions. It was also a period of great erosion, during which the 

 turmoil of mountain peaks, domes, and spurs were carved on the eastern 

 flank, and that side of the range devastated in a striking manner by the 

 slow ravage of time. The third epoch of eruption was the least of all and 

 most local, being confined to the portion around Belknap and Baldy, and 

 furnishing the cream-colored rhyolite and a few small outbreaks of basalt. 



The southern portion of the Tushar contrasts with the northern por- 

 tion in many respects. It exhibits a totally difi"erent group of eruptive rocks. 

 In the northern part the extravasated rocks are rhyolites ; in the southern 

 part they are trachytes, augitic andesites, dolerites, and basalts. The form 

 of the southern part of the uphft is distinctly tabular or plateau-like, while 

 the northern part has the sierra aspect. 



About 3 miles south of Belknap, standing upon the brink of an old 



coulee, we look southward over a broad expanse of comparative calm lying 



at a slightly lower level. In this expanse the tabular form of the Tushar 



mass is no longer doubtful. A lofty plain diversified by ridges of erosion 



is spread out before the gaze, clad with spruce and aspen and opening in 



grassy parks. The abundant streams have carved gently-sloping ravines 



and pleasant knolls, where the dark lavas may occasionally be detected 



dipping very gently to the west, but near the eastern rim rising more 



boldly to the timber line (11,500 to 12,000 feet), where they are suddenly 



cut off and present their truncated edges to the eastward in the boldest of 



mountain slopes. This part of the plateau summit is about 22 miles in 



length, 8 to 10 miles in width, and the mean altitude about 10,000 feet. 



Erosion has given to this lofty watershed a surface very similar to that 



which may be observed in any well-watered country, and which is in 



strong contrast with the peculiar forms observable at lower levels where the 



precipitation is much smaller 

 12 H p 



