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University of California Publications. [Geology 



either side. Of the 6,090 feet of strata, 3,175 feet, or very ap- 

 proximately one-half of the series, are of volcanic and pyroclastic 

 origin. 



Petrography. — Under the quartz porphyries included under 

 subdivision 1 is a light colored, strongly sheared rock which 

 presents the appearance of a fine grained gneiss upon its cross- 

 fracture, and that of a very fine grained biotite schist upon its 

 foliation planes. The mica, however, is not evenly distributed, 

 bu1 is localized in patches. Across the schistosity minute augen 

 of quartz and feldspar are visible. These features are somewhal 

 variable, depending upon the degree of dynamic metamorphism. 

 Favorable sections show under the microscope an abundance of 

 quartz dihexahedra, which are often better preserved than ap- 

 pears consistent with the macroscopic habit of the rock. The 

 phenocrysts are cracked, and polarize in various colors, the dif- 

 ferent tints of which, however, persist over areas relatively 

 large. Inclusions of the groundmass do not occur. The quartzes 

 possess sharp and linear boundaries. More frequently they are 

 shattered and drawn out, and show strong strain shadows. In 

 somewhat less abundance than the quartz are the feldspar pheno- 

 crysts, chiefly orthoclase in a variable state of preservation. It 

 sometimes retains its idiomorphism, but is more commonly broken 

 up, and the cracked fragments are embraced in large muscovite 

 plates. Occasionally a nearly obliterated plagioclase feldspar, 

 surrounded by white mica, can be detected. The groundmass 

 is a line mosaic of quartz and orthoclase, and contains much 

 flaky biotite scattered throughout it. The constituents are all 

 oriented with their major axes parallel, and the biotite flakelets 

 are arranged in concentric fashion around the quartz and feld- 

 spar phenocrysts. The accessories magnetite, apatite, zircon, 

 and titanite were found present in small proportions. A minor 

 faeies is a much darker colored schist, very fine grained and 

 showing only small feldspar augen. It consists of a large number 

 of kaolinized feldspars, shattered, and their corners abraded off, 

 embedded in a finer feldspar mosaic, in which but a small amount 

 of quartz is scattered. Disseminated throughout the matrix is a 

 very large quantity of finely divided biotite, which serves to 



