OCR Text |
Show 182 manifold dislocations by later outbursts, and one of the instances is represented by the following section: FIG. 4.- Section from the cation at the Needles. G. Granite. a. Trachyte with pink matrix, containing imbedded quartz, orthoclaae, and biotite. b. Trachyte with brown matrix, imbedded sparingly oligoclase and hornblende. t. Trachytic tufa containing pebbles of a trachyte with a reddish matrix, in which is imbedded sparingly ougoolase. Here we have evidence of three distinct outbursts of trachytic masses; the trachyte that gave rise to the formation of the tufa, t, being the oldest, then following a, uplifting and dislocating the tufa strata, and finally the trachyte, 6, protruded and overflowed the tufa on the opposite side from a. In the Blue Ridge Mountains and Mohave range the trachytic eruptions appear to have reached their maximum, the masses measuring thousands of feet in height. At Union Pass, in the Blue Ridge Mountains, rhyolite, felsite, pitchstone, and globular porphyry, or pyromerid, form the accompaniments of the trachyte, while in the Mohave range basalt is associated with it. The volcanic tufas, conglomerates, and breccias of the latter range contain only material derived from trachyte and none of the neighboring basalt; thus giving evidence of the more recent age of the latter. The primitive rocks of the Mohave range are not so extensively covered by volcanic material as in the Blue Ridge Mountains. However, in the singular outlines of their peaks and crests, assuming fantastic shapes of towers, domes, and castles, the more sharply defined as they are devoid of forests and verdure, both rival each other. In the Riverside Mountains obsidian accompanies the trachyte; at Chuckawalla Peak, basalt and a phonolite rich in zeolite ; in the southern spurs of the San Bernardino Mountains, pumice ; and in the Mohave River Valley, rhyolite and recent lavas. In this valley a fine section of volcanic rocks is exposed at the Canon of the Caves, where, for a stretch of 5 miles, a series of trachytes of most vividly contrasting colors impress the traveler with the grandeur of inorganic nature. The spaces and fissures between the dikes are filled by Quaternary clays and conglomerate. Trachydolerite.- This rock forms dikes and hills in the Mohave, Payute, and Cerbat ranges, at Mountain Springs. It also is largely spread upon the island of Santa Cruz. Basalt- The most notable localities are the Virgin River Valley and vicinity, and Black Mountain range, in Southwestern Nevada; the eastern portion of Detrital Valley, in Northwestern Arizona; the Mohave range, and the Mohave River,* in Eastern California. The displacements due to its protrusion are on a grand scale in the mountains north of Callville, in Southern Nevada, from which region the accompanying section is taken. FIG. 5.- Section from vicinity of Callville, Nev. V, Basaltic protrusion; t, Triassio strata; q, Quaternary conglomerate. * The Mohave range has nothing to do with the Mohave River, being situated much farther east, and crossing the valley of the Colorado. |