70 



University of California Publications. 



[Geology 



enclosed in a fine grained ground mass composed mostly of glass. 

 No ferromagnesian mineral is present, the only iron bearing min- 

 eral being occasional cubes of magnetite and flakes of hematite, 

 the feldspars frequently being stained with the latter. The most 

 abundant phenocryst is sanidine. Its frequent straight extinc- 

 tion and absence of repeated twinning served to distinguish it 

 from the plagioclase present. It is very abundant, and some- 

 times occurs in well terminated crystals, but usually in broken 

 fragments, frequently badly kaolinized. A relatively small 

 amount of plagioclase occurs, of which the highest extinction 

 angle observed on (010) was 10.5°. This would indicate either 

 albite or oligoclase. The ground mass is very fine grained, and 

 under the high power is seen to be composed of minute frag- 

 ments of feldspar-, apparently sanidine, intimately mixed with 

 unindividualized glass. 



Determined solely by its optical properties, this rock would 

 be called a trachyte, since no quartz phenocrysts were observed. 

 A silicia determination, however, showed it to contain 72.13 Si0 2 . 

 Hence it is classed as a Bhyolite. A similar rock from above the 

 Sonoma Tuff on the west side of Wooden Valley, Napa County, 

 yielded 72.36^ Si0 2 . 



A specimen from a quarry in Sarco canon, east of Napa City 

 shows the following features : 



Maeroseopically it is a nearly pure white rock, with a rough, 

 sugary fracture and conspicuous absence of ferromagnesian min- 

 erals. The only colored constituents are small specks of hematite. 

 Large phenocrysts of feldspar are visible to the eye, but no 

 quart/,. It has a roughly laminated appearance. 



Microscopically it shows large phenocrysts of sanidine em- 

 bedded in a micro-crystalline ground mass of a very acid plagio- 

 clase in well shaped micro! ites in simple twins. These laths have 

 a well defined parallel arrangement, due to flowage of the magma. 

 Scattered octahedra of magnetite are present, and a large amount 

 of hematite dust, which by its arrangement serves to bring out 

 the flow structure more clearly. No primary quartz was observed, 

 but a small amount of secondary quartz was seen. This rock 

 is very similar to that at Mt. St. Helena, and the writer has 

 classed it as rhyolite. 



