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Show 248 GEOLOGY OF THE HIGH PLATEAUS. still be discovered upon weathered surfaces, though in fresh fractures they appear to have gained an aspect very nearly homogeneous with the general mass. This phenomenon of the gradual vanishment of pebbles is not confined to the tufas, but is frequently seen in the conglomerates, some of which have been greatly altered and converted into a hard semi-crystalline rock strongly resembling andesite and hornblendic trachyte. Moreover, the inferior boundary of the larger sheet is indefinite in many places, and near the fault it appears to have passed lower down and involved beds which are not so much affected farther up the canon. The lines of bedding near the fault are nearly obliterated, and the thickness of the lava-like mass has greatly increased. I entertain very little doubt that the sheet is not a lava, either contemporaneous or intrusive, but is a metamorphosed tufaceous deposit. Farther up the East Fork Caiion, upon the north side, stands an isolated mass, consisting of phonolite, represented in Heliotype No. XI. It is a hill about 1,400 feet high, with steep flanks, covered with talus. Near the summit the cleavage of the rock in vertical planes is exhibited with clearness. Upon closer inspection a secondary cleavage, perpendicular to the foregoing, is also disclosed, and the viscous vitreous character of the lava is very conspicuous. Under the microscope it discloses very few crystals, and these are very small, consisting of nephelin. No feldspar was detected. The specimens brought home, though fair in appearance, proved to be much weathered and hardly suitable for microscopic or chemical investigation. The plateau mass around this hill was much eroded, and the eruption of the phonolite appears to have occurred after the erosion had far advanced, for it is an isolated mass, and its lavas flow over rugged ridges and ravines upon its northern side. GRASS VALLEY. Separating the second and third ranges of tabular uplifts is a broad depression, named Grass Valley; a name which has done great service in the West, for it may be found in every State and Territory. It is properly an appendage of the Sevier Plateau, from the platform of which it has been |
