OCR Text |
Show 60 GEOLOGY OF THE HIGH PLATEAUS. ^Etna-like summits or craters are visible, and it is doubtful whether the method of eruption was generally such as would generate mountains of that character; for the larger deluges appear to have emanated from fissures located within restricted areas. Yet apparently some piles of important magnitude were reared by the successive superposition of coulees around a central vent or pipe, and still bear evidences of their origin, though they have been reduced to mere remnants by the wear of ages. In the southern part of the district several foci of eruption are discernible. The most important was just east of the old andesitic center. From this one emanated the dark trachytic masses which have built up a great portion of the Aquarius. Another was situated at the southern base of the Tushar, and disgorged the masses which built the southern portion of that range. A line of vents stretched southwest from the Tushar along the western crest of the Markagunt, and sheeted over the greater part of that plateau. Still another occupied the position of Mount Hilgard, at the extreme eastern boundary of the High Plateaus, and a chain of vents stretched southward from it to Thousand Lake Mountain. Around the outskirts of the more compact inner district many minor eruptions occurred, overflowing numerous outlying patches. The rhyolitic eruptions occur chiefly in the Tushar, the Pavant, and Markaguntâ€"in a word, belong to the western margin of the district. Their grandest masses are displayed in the northern portion of the Tushar. They form the summits of this range, standing in high peaks, which are the loftiest in Utah, excepting two or three in the Uintas. Here no other eruptive rocks are associated with them, except a few small outbreaks of basalt which overlie them. The platform upon which they lie consists of meta-morphic Jurassic sandstone, upon the eroded surface of which they were outpoured. We find here evidence that the eruptions did not occur in rapid succession, but were separated by intervals of time sufficient to accomplish much erosion. Old valleys scored in the older lavas were filled up by later floods, which were, in turn, chasmed with ravines, revealing the contacts, and this process was repeated again and again. Two groups of rhyolitic rocks may be discerned in this locality, each presenting great variety in the texture, as is always the case with rhyolites, |