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Show 176 GEOLOGY OF THE HIGH PLATEAUS. ing the northern summit of the Tushar mass, and occurs in several outlying knobs and small crests to the east and northeast of Belknap. But the northwestern slope of the range has been mantled by great floods of it, which have poured in massive sheets from summit to base, burying the antecedent topography of the mountain and generating a new one. The individual eruptions making up this rhyolitic mass appear to have been numerous, some very voluminous, others very small. The smaller ones are seen to fill up old ravines and to mold themselves upon uneven preexisting surfaces, while the grander floods pour over everything and spread out over great expanses of mountain side. Although this lava is, with the exception of a few minor basaltic streams around the western base of the Tushar, the most recent of all the outbreaks, yet absolutely it is of considerable antiquity. Since the extinction of the vents from which it was emitted there has been a long period of erosion. Belknap and Baldy, together with the eastern outliers, are mere remnants of piled-up sheets, which were perhaps once continuous, but are now separated by profound ravines, which have been excavated by erosion. The indications are abundant that the period separating the earliest from the latest eruptions was a very long one. The contact of the earliest liparites with the Jurassic quartzites shows heavy floods of lava pouring over a very uneven surface and piled up in layers by successive eruptions to a thickness of more than 2,000 feet. These, in their turn, show a subsequent degradation by erosion not only in the sculpturing and carving of the beds, producing an unconformity in some of the contacts, but also in the existence of local conglomerates composed of the water-worn fragments of the dilapidated rocks cemented by finer detritus derived from the decomposition of the feldspathic materials. These earlier eruptions appear to have been followed by a long period of calm, during which they were attacked by the degrading force and slowly wasted by decay. In many places the beds were cut through down to the quartzite and a fresh topography was carved out by erosion. Afterwards the activity was reopened with fresh eruptions of a different character. These second eruptions were grander than the first, some of the beds being many hundreds of feet in thickness, spreading over great areas, and |
