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Show 182 GEOLOGY OF THE HIGH PLATEAUS. summit of the crest are still discernible, though now they lie at least 3,000 feet below them. Immediately south of Midget's Crest lies Dog Valleyâ€"a pleasant moderately diversified platformâ€"with an absolute altitude of about 7,500 feet or 1,500 feet above the Sevier at Circle Valley. It is a part of the last-mentioned focus of eruptions of the second or middle epoch, but erosion has leveled down most of the ancient irregularities, and left it a field of rolling hills, well covered with soil, loam, and sharp gravels. Its real history might not have been suspected, were it not for the vast floods of lava which spread out from it in all directions for many miles, growing thinner and broader as they recede. Southwest of Midget's Crest the altitude of the plateau gradually diminishes until its summit at last is lost in the next region. The fault which originated the escarpment of the plateau suddenly becomes a mono-clinal which dies out in the space of about 6 miles. This monoclinal is composed of conglomerate of unknown thickness, but not less than 1,500 feet in the vicinity of the flexure. It turns up at an angle of 28° to 30° against the diminishing wall of the plateau, but soon straightens out towards the south and decreases rapidly in thickness. It is composed of basaltic (doleritic) materials chiefly, quite similar to, and perhaps identical in part with, the remnants of that kind of rock forming the extreme summits of the salients on Midget's Crest. The western base of the Tushar I have seen in part only, and have given that part merely a cursory examination. It is possible that there exists a fault of about 1,200 feet along this base with a throw to the west; a continuation of the Hurricane fault, which appears in great force about 15 miles south of the southwest slope of the Tushar. But I have not verified the existence of such a fault in this locality, and such an occurrence may not be necessary to explain the features presented, so far as observed-The summit of the table, after maintaining for about 10 miles an easy slope to the west, suddenly increases the descent of the profile to the broad plain below. The surface contour here cuts across the ends of the lava sheets, which are seen to be considerably attenuated when compared with the huge masses exposed upon the upturned eastern flank of the range. |
