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University of California Publications. [Geology 



A continuation of the same tuff is found exposed near the 

 lakelet south of Vandever Mt. It is a gray-blue rock liberally 

 sprinkled with glassy quartzes, and contains abundant inclusions 

 of sharp angular slate fragments up to three-quarters of an inch 

 in length. The rock weathers to an ash-white, and the small 

 dark fragments lend it a strong porphyritic aspect. The schist- 

 osity is but feebly developed. Under the microscope the numer- 

 ous glassy quartzes are immediately noteworthy. They are ang- 

 ular and jagged, often assuming fantastic forms. Powerful 

 strain shadows are apparent, and the longer axes of the crystals 

 have received a common orientation. Occasionally the jagged 

 points are shattered, and sometimes the quartzes are surrounded 

 by a very faint granulation aureole, but usually the demarcation 

 between phenocryst and ground is absolutely precise. Some small 

 sericitic aggregates were found, as were a few inclusions of car- 

 bonaceous shale. The groundmass is extremely fine grained, and 

 contains a great deal of flaked chloritic matter disseminated 

 through it. 



On Monarch Creek at 8,500 feet is a strongly sheared seri- 

 citic quartz porphyry less than 100 feet thick. It is of light 

 pearl gray color and shows an oily lustre upon its foliation 

 planes. The porphyritic quartzes are quite abundant, and their 

 frequent idiomorphic character is often apparent to the unaided 

 eye. Under the microscope the outlines of the quartzes are seen 

 to be usually of geometric perfection. The dihexahedral form 

 is often modified by corrosion and embayment phenomena, and 

 sometimes by a slight conchoidal fracturing. Severe strain 

 shadows are frequent, and occasionally the phenocrysts have 

 been cracked, allowing the matrix to work in between the sepa- 

 rated portions. The feldspars, of which a little is triclinic, are 

 very sparingly represented, and are poorly preserved. A little 

 biotite is present in flaky aggregations. The groundmass is a 

 fine grained aggregate of quartz and sericite. In it occur some 

 disjointed and faulted apatite needles ; cataclastic zircons show- 

 ing both splintery and idiomorphic terminations ; and some criss- 

 cross aggregates of minute tourmaline prisms. 



The More Basic Volcanics. — In the creek descending from 

 the 9,883-foot Peak west of Timber Gap is a 50-foot thickness 



