FISH LAKE PLATEAU— THE GKAND GOKGE. 



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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 tlie 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 1 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 erup- 

 tion which we are now able to identify with certainty are Blue Mountain, 

 nearly 12 miles distant across Grass Valley, and Mount Ililgard, nearly as 

 tar 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 por- 

 tions 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 

 inconM])icuous, or even escape notice altogether. 



