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University of California Publications. 



[Geology 



somewhat granitic habit. Under the microscope, the clastic 

 origin of the rock is readily apparent. Numerous quartz grains 

 show their detrital origin in the subangular character of their 

 outlines. They have received a common orientation and show 

 strain shadows, and some have been crushed. Rounded grains 

 of altered plagioclase are quite abundant. A portion of the 

 feldspar is unstriated, and may be monoelinic. A not inconsid- 

 erable amount of decomposed reedy hornblende is distributed 

 throughout the slide. With it is a little biotite and chloritic 

 material. A fine grained, feebly polarizing paste, whose exact 

 nature is indeterminate, constitutes a sort of cement for the 

 elastic particles. Some accessory magnetite, pyrite, titanite, and 

 abraded zircon occur scattered through it. This rock seems, 

 therefore, to represent an indurated arkose sandstone. 



Sheared Quartz Porphyries from the Little Kern. — Associ- 

 ated in conformable sequence with the sedimentary series on the 

 Little Kern in both strike and dip, are several hundred feet of 

 sheared quartz porphyries and tuffs. They are of pearl gray 

 color, of waxy, unctuous lustre upon their foliation planes, and 

 show numerous easily recognized clihexahedra of quartz, which 

 lie embedded in an aphanitie base. They are macroscopically 

 identical with the quartz porphyries described by Turner from 

 the Downieville Quadrangle, and placed by him as very probably 

 of Jura-Trias age. One facies here, however, shows also por- 

 phyritic glassy feldspars. 



Under the microscope the numerous quartz phenocrysts show 

 severe strain shadows. Some have been scattered and granu- 

 lated. The outlines of the quartzes are usually irregular, but 

 occasionally bipyramidal forms occur showing corrosion and em- 

 bayment. A slight conchoidal fracturing of the crystal edges is 

 sometimes apparent. Feldspars are not numerous, and are 

 poorly preserved, usually drawn out and kaolinized. The ground- 

 mass is extremely fine grained, presenting in ordinary light a 

 sort of shagreened appearance due to the interweaving of mica- 

 ceous material. The development of this material often serves 

 to bring out the former flow structure. Magnetite and some dis- 

 jointed apatite needles were noted as accessories. Small irreg- 



