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University of California Publications. 



| Geology 



monest variety of the serieitic quartz porphyries is a strongly 

 schistose variety presenting an irregularly mottled appearance 

 on its foliation planes. This mottled effect is due to the alter- 

 nation of areas black with biotite scales and areas of pale green- 

 ish gray color which shine with a distinctly oleaginous lustre. 

 On its cross-fracture the rock is exceedingly dense, and only 

 veiy sporadic, minute eyes of vitreous quartz can with difficulty 

 be distinguished. Under the microscope a few badly damaged 

 feldspar phenocrysts stand out in sharp contrast to the exceed- 

 ingly fine grained ground. They are unstriated, apparently all 

 orthoelase, kaolinized, and strongly mashed. Biotite occurs spar- 

 ingly in small flakes, occasionally localized in aggregates. The 

 groundmass is holocrystalline, but irresolvable. Some white mica 

 can, however, be identified with certainty. A little accessory 

 magnetite and zircon occur. 



A subordinate facies of the series of' quartz porphyries is a 

 strongly foliated, very white variety which shimmers with an 

 unctuous lustre upon its schistosity planes, due to the abundance 

 of small mica plates. Microscopically, the rock can be charac- 

 terized as a mosaic of quartz interwoven with white mica. The 

 mica, which is distinctly pleochroic in a light greenish tint, is 

 extremely abundant and possesses a definite linear arrange- 

 ment. At certain points, however, it flows around larger areas 

 of granulated quartz. A very little biotite is present. Some 

 few large aggregates occur which consist entirely of scaly white 

 mica. A little accessory magnetite and zircon are also found. 



The sheared feldspar porphyry is a rather remarkable ap- 

 pearing rock. It is light bluish-gray in color, and is closely 

 crowded with feldspar phenocrysts, which in consquence of the 

 shearing that the rock has undergone, cause it to split along 

 the foliation planes as if composed of small pebbles. In thin 

 section the feldspars are seen to be in poor preservation: they 

 are kaolinized, their crystallographic outlines are usually de- 

 stroyed, and the corners have been abraded off; they have some- 

 times been cracked and the groundmass has worked in between 

 the fragments. They have frequently been converted to seri- 

 eitic aggregates, especially upon their peripheries: a band of 



