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Show RHYOLITESâ€"SOUTHERN PORTION OF THE TUSHAR. 177 extending far to the westward, expanding as they extend. Of this rock, a dark purplish porphyritic rhyolite, 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 uplifting 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 wrere 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 portion in many respects. It exhibits a totally different 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 uplift 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 wTell-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 |
