46 



University of California Publications. 



[Geology 



occur in smaller flakes than in the mica-diorite above described, 

 and the rock accordingly presents a harder surface to the weather. 

 Its specific gravity is 2.731. 



Microscopically the essential minerals are seen to be a soda- 

 lime feldspar and biotite, a small amount of quartz and green 

 hornblende and a little orthoclase. The accessory minerals are 

 apatite and titanite, and the decomposition products kaolin para- 

 gonite and chlorite. 



The feldspars are not very fresh, but are much clouded with 

 kaolin and paragonite. Favorable striated sections gave by M. 

 Levy's statistical method a maximum angle of 19°, indicating a 

 medium basic andesine of a composition of about Ab.. An 2 . 



One section was found twinned on both the albite and the 

 Carlsbad laws upon which M. Levy's method of concurrent angles 

 was used. It gave the extinction angles 8° and 18°, indicating 

 a section of andesine (Ab. An.,). 



Some of the feldspar sections are free from albite twinning, 

 and look like orthoclase. Some of these are rhombic in shape, 

 and three gave the angles — 12°, — 24°, and — 15°. These sec- 

 tions were probably cut parallel to (010). The negative sign 

 precludes the possibility of orthoclase, as also the large angle 

 24°, orthoclase 's maximum extinction angle being 21° against c. 



Orthoclase is, however, probably present in small amount. 

 Certain sections show inclusions of plagioclase with poikilitie 

 stmcture, all the plagioclase inclusions extinguishing at the same 

 time. 



The biotite is identical with that of the mica-diorite above 

 described. It contains little magnetite, its principal decompo- 

 sition product being chlorite. 



A small amount of green hornblende is present, which seems 

 to have crystallized later than the biotite. 



Chemical Analyses. — Two analyses of these rocks were made 

 by the writer. No. I. is the biotite-diorite, and No. II is the 

 quartz-biotite-diorite. 



