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Show FISH LAKE PLATEAUâ€"THE GEAND GOUGE. 261 ing in steep slopes to the central line of depression, which declines to the westward until the gorge opens into Grass Valley, 4,300 feet below. Across the abyss rises the other wall, somewhat less lofty and abrupt than this, and we can look over it to the great irrigated farms of the Lower Sevier Valley, 40 miles away. In this gorge a grand section of volcanic rocks is exposed, of which the total thickness now visible will aggregate very" nearly 4,000 feet. The exposure, however, is not so advantageous for study as might be desired, since the upper third is inaccessible cliff and the lower two-thirds are heavily mantled with soil held in place by forests of spruce and aspen, or are hidden beneath huge banks of coarse talus. The disconnected exposures, however, are very many; and, so far as each one individually extends, it exhibits distinctly the local attitudes of the rocks. The first inquiry which arises is, whence came all these lavas ? The question is not easy to answer satisfactorily, for they were erupted far back in Tertiary time, and the changes which the country has undergone since their outpouring are very great. The nearest great centers of eruption which we are now able to identify with certainty are Blue Mountain, nearly 12 miles distant across Grass Valley, and Mount Hilgard, nearly as lar in the opposite direction. As for the Fish Lake table itself, it does not furnish very decisive indications of being an eruptive center. In the cliff wall which faces the great amphitheater the successive sheets are seen to lie nearly horizontal, parallel, and continuous over great distances. Although they cannot be reached from below, yet they can be distinguished by their colors, which are apparently identical with those in the great west wall of the Sevier table overlooking Monroe. The beds are very massive and are dark iron-gray (hornblendic trachyte), alternating with a number of shades of red (augitic andesite, argilloid trachyte, and dolerite). No distortion or confusion of the layers and no dikes were observed. None of those signs of a volcanic core or center which are seen in Blue Mountain or in portions of the Monroe amphitheater are here apparent. Nevertheless, it seemed to me that the source of these lavas could not be far distant. Since the face of the great cliff is parallel to the general course of the structure lines, it is not surprising that the evidences of an eruptive center should be few and inconspicuous, or even escape notice altogether. |