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Show 186 GEOLOGY OF THE HIGH PLATEAUS. they occupied at the close of the earliest volcanic period presents a very strong analogy to those of Central France, as described by Sir G. Poulett Scrope in his work upon that region.* In point of magnitude the earliest eruptions of the Tushar were probably comparable to those of the Cantal, covering perhaps a larger area but with a greater thickness. After a long period of comparative quiet, during which the greater portion of the mass of these earlier eruptions was broken up by erosion and scattered over the adjoining lowlands and intervening valleys, came the second period of eruption, upon a scale grander than the first. The foci of activity were in close proximity to those of the first period. The outpours at the northern portion still remain in great bulk and are chiefly rhyolitic. But the grandest floods of all are in tbe center of the range, where they are laid open by several deep gorges, the largest of which is Bullion Canon. The course of the streams was here to the westward chiefly, where they widened out and grew thin as they receded from their origin. The total thickness remaining of these rhyolitic masses probably exceeds 2,000 feet, and there is good evidence that a considerable amount has been lost by erosion. What floods may be hidden beneath the floor of the Sevier Valley at the eastern base it is impossible to say or even to conjecture. Thus for the second time the Tushar was built up by extravasated materials and to an altitude greater probably than at first. A second period of comparative calm now followed, during which erosion was at work cutting deep gorges, carving out pediments, and leaving a rugged series of peaks and domes along the eastern flank. But another agency in mountain structure also intervened. This was an extensive vertical movement of the whole mass. At what precise epoch the faults which now separate it from the platform of the Sevier Valley were started it is impossible to say with precision. It is clear, however, that the commencement of the displacement was subsequent to the deposition of the great conglomerates which were formed by the destruction of the older Tushar, and it is almost certain that the displacements had not attained any great magnitude or a magnitude comparable to the present during the second eruptive period. The principal part of the uplifting has apparently * The Geology and Extinct Volcanoes of Central France, by G. Poulett Scrope, 1858. |
