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Show 292 GEOLOGY OF THE HIGH PLATEAUS. River. The country which thus concentrates its waters into Potato Valley may be regarded as a vast amphitheater, with a radius vector varying in length from 12 to 18 miles, and of which the ramparts of the Aquarius and Table Cliff form the upper rim. The amphitheater is the work of erosion, being a westward extension of that vast denudation which has removed thousands of feet.of strata from the whole region spread out before our gaze. As we study the panorama before us, the realization of the magnitude of this process gradually takes form and conviction in the mind. The strata which are cut off successively upon the slopes formerly reached out indefinitely and covered the entire country to the remotest boundary of vision. Their fading remnants are still discernible, forming buttes and mesas scattered over the vast expanse. The same process of reasoning by which the mind joins the edges of strata across the abyss of a narrow cafion enables us to join their edges across wider intervals. The restoration of the Trias to its Pre-Tertiary condition is made almost at a glance, since the vacant spaces are few. The restoration of the Jurassic and Cretaceous is precisely the same in nature and equally simple, though the spaces to be covered by it are much wider. The Tertiary is wholly wanting to the eastward. There remains only a single outlier to the southwardâ€" Kaiparowits Peak. But its former extension over the whole of the Plateau Country admits of no serious doubt after we have once mastered the plan of the drainage system and of the Post-Eocene displacements. The rivers alone might not be sufficient to demonstrate the conclusion, nor would a restoration of the displacements, but the two together admit of no other interpretation. How far eastward and southward the lava-cap extended cannot be determined. Remnants of alluvial conglomerates, with large fragments of trachytes and augitic andesites, are found more than 20 miles eastward, and they are indistinguishable from the rocks now forming the summit of the plateau. But how far they have been carried is a question which it is impossible to answer. The altitude of the eastern front of the Aquarius above the country which it overlooks is upon an average about 5,500 to 6,000 feet, and the thickness of the strata removed from its vicinity is probably about 4,000 to |
