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Show 174 In the San Bernardino Mountains the main mass is granite, accompanied by syenite, gneiss, mica- schist, talcose- schist, and primitive clay- slate. Occasionally, as for instance between Martin's ranch and Cajon Pass, the granite gave rise to the formation of beds of arcose, a rock in which granite debris has been recemented, forming a sort of granitic sandstone resembling to some extent granite; but the uniform grain, friability, and rusty surface of the fragments elucidate its true nature. The western slopes of the San Bernardino Mountains are covered by extensive beds of detritus and dtbrU, which in some portions are auriferous, as in Ly tile Creek Canon, where gold- washing is carried on on a large scale. In the vicinity of the Cajon Pass the Azoic rocks are covered by broken strata of a conglomerate exhibiting a changing dip, being at first about 30° to the south, and finally, near the Pass, 20c to the north. These inclined strata are overlaid by horizontal beds of bowlder and dibrU drift 4 to 5 feet in thickness. While dikes of dyorite and trachyte occasionally appear in the northern portion of the mountain- range, the spurs south of the Gray and San Bernardino Peaks consist largely of volcanic materials. One of the isolated peaks east of the Jaciuto and southeast of the Sau Bernardino Mountains is Chukawalla Peak, with about 4,700 feet altitude, towering conspicuously above the plain, and whose shape resembles a trapezoid surmounted by a pyramid. The primitive rocks of this mountain are traversed by volcanic dikes, ( chiefly trachyte and basalt,} while at the base beds of bowlder and conglomerate have accumulated, the latter showing a dip of from 20° to 25° to the south. Among the primitive rocks a mica- schist of porphyritic structure deserves mention; it consists of a tine- grained mixture of quartz and biotite, containing muscovite in plates of one- sixteenth square inch imbedded. There are said to occur also lead and copper ores at Chukawalla Mountain, but they are not worked. The primitive rocks of the River- Side and Half- Way Mountains consist of granite and gneiss, the latter garnetiferous. While here we fiud the feldspar of a green color, it is pink with that of the Monument Mountains north of the former ranges. At the Mohave range, 20 miles farther north, a series of Azoic rocks is met with, viz, a fine- grained granite, containingsimultaneously biotite and muscovite, a syenite with veins of a coarse aplite, then hornblende- schist and quartzite. As this range consists largely of volcanic material, it will be again mentioned hereafter. Among the mineral occurrences in the primitive rocks of the Cerbat range, muscovite in large plates may be mentioned. In this range are found extensive lodes of metalliferous quartz ; also the neighboring Black Cafion range, Payute range, Providence Mountains, and Opal range, contain metalliferous lodes in the primitive rooks. At thePanamint range we find primitive limestone and slate- clay as accompaniments of the granite. THE PALAEOZOIC FORMATION. This formation, chiefly represented by limestone and quartzite, becomes conspicuous at the Riverside Mountains, Opal range, the saline flats of the Mohave, and the Inyo range. On the eastern slopes of the Riverside Mountains, on the Lower Colorado, are exposed, for a distauee of over 5 miles, layers of a gray siliceous highly- crystalline limestone, that must be referred to this formation ; a view first expressed by Dr. Newberry.* The rock- surface is very uneven, full of little cavities, caused by the sand-winds that attacked the calcareous particles of the rock soouer thau th © - siliceous ones and carried them oft*. In the rock itself paleontological evidence is iu vaiu searched for, but the accompanying quartzite exhibits crinoidal forms. Farther north, near Fort Mohave, is another Paheozoic region. The post stands upon a terrace 40 feet in height, consisting of rounded, water- worn bowlders washed down from the mountain- range* on either side. These large traces, testifying of the activity of the river in former ages, when its bed was far above the present level, contaiu, among bowlders of trachyte, basalt, and granite, such of a peculiar quartzite resembling seuii-opal, aud with organic forms ( chiefly of erinoids) in a state of astonishing perfection. Treatment with hydrochloric acid removes the last traces of adhering carbonate of Hnie and brings the forms still better to light. Proceeding farther up the river, Paheozoio limestone is again found at Bowlder Cation. In the Opal Mountains, at the boundary of Nevada anil California, this nick plays au important part, occupying portions of the very crest, exhibiting there frequently incliued strata; thus 3 miles southwest of Ivan-pah dipping 60° to the west. In this'limestone fossil remains are scarce, a single athy-ri. « and few crinoidal stems haviug been the poor result of a long search. The flanks of the range are covered by a conglomerate consisting of pebbles of carboniferous limestone, granite, and quartzite, through which numerous gullies aud ar-royos have beeu washed. It is a striking fact that while the Opal Mountains are covered largely by Palaeozoic strata, these were not met in the opposite Payute range, hardly 30 miles east and of about equal height. Farther north this formation is met with in the Argus and Inyo ranges, where it * See Lieutenant Ives's reconnaissance upon the navigability of the Colorado River. |