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University of California Publications. [Geology 



peaks, near the Nevada line, and running through Leach's Point 

 and Burnt Rock Mountains to El Paso Peak, north of the mining 

 camps of Eandsburg and Johannesburg. That the eastern and 

 northern boundaries as thus outlined are given not without a 

 measure of reason is shown by the fact that they define the 

 limits between the northern Great Basin region of markedly 

 parallel mountain ranges and the southern Mohave Desert region 

 of lower ranges without notable parallel arrangement. 



METAMORPHIC, PLUTONIC, AND VOLCANIC ROCKS 

 OLDER THAN THE UPPER MIOCENE 



The oldest rocks encountered in the region of the Mohave 

 Desert were two series of metamorphosed sediments : the one 

 exposed in the sharp hills east of Oro Grande station on the 

 Atchison, Topeka, and Santa Fe Railway; the other outcrop- 

 ping in the northern portion of the San Bernardino Range, in 

 the northwest quadrant of the San Gorgonio Atlas Sheet of the 

 United States Geological Survey. Plutonic rocks of a general 

 granitic composition intruded these two series of metamorphics 

 and outcrop in many places in the Mohave Desert. The eroded 

 surfaces of the plutonics are covered by lava flows in the vicinity 

 of the town of Barstow. A much altered schist and gneiss, 

 which has been referred to the Archean by Hershey, 3 outcrops 

 just north of the Mohave River northwest of Barstow, flanking 

 on the south a granitic range. 



THE OKO GKANDE SEEIES 

 A series of marbles, quartzites, and slates, which have already 

 been described by Hershey, 4 is given this name because of the 

 proximity of its outcrop to Oro Grande station, which is situated 

 less than a mile west of the western limit of the exposure. The 

 quartzite is well cemented and exhibits planes of cleavage. Two 

 varieties of marble were noted, one cream-colored with very 

 coarse calcite crystals and the other with finer crystals and 



s Some crystalline rocks of southern California, Am. Geol., vol. 29, pp. 

 286-287, 1902. 



* Op. ext., pp. 287-289. 



