PETROGRAPinCAL NOTES. 13 



Under the microscope appear among the porphyritic 

 crystals a few sharply defined plagioclastic feldspars which, 

 as well as those of the groundmass, probably belong to the 

 oligoclase or andesine series. Inclusion of glassy ground- 

 mass are frequent, and occasionally such of augite, also 

 appear. 



Of the bisilicates there are in porphyritic form a few, more 

 or less w^ll-defined pale green augite crystals, and also 

 some more plainly idiomorphic prisms of hypersthene with 

 its usual characteristics, the latter being the older. Olivine 

 is conspicuously absent. 



The groundmass consists of striated plagioclase micro- 

 lites, grains and prisms of augite (the hyperstheue is evi- 

 dently not represented), and finally grains of magnetite, 

 usually clinging to or included in the small augites. 



Cementing these, there is an abundant light-brown globu- 

 litic glass. The round discs or balls of the globulites are 

 of a darker coffee -brown color, and usually about 0.00'2 

 mm. in diameter; often the}^ are included together with the 

 glass in the porphyritic feldspars or augite. Using the 

 terminology of Rosen busch, we may thus characterize the 

 rock as an olivine free hypersthene basalt, hypocrystalline- 

 porphyritic, and with an approximation to hyalopilitic 

 structure. 



Basalts of this unusual type have already been described 

 from several places along the Pacific Coast; from Oregon, 

 by J. S. Diller"^ and A. B. Emmons, t from Lake county, 

 California, by G. F. Becker,:j: and from San Salvador (C. A.), 

 by Hague andlddings.^ 



The locality here describe^d is about 500 miles south of 

 the occurrence in Lake county (Central California, north of 

 San Francisco). 



*Am. J. Sc. XXVIII, 252. 

 tBull. Calif. Acad., No. 4. 1884. 



JGeology of the Quicksilver deposit of the Pacific Coast, monograph XIII, U. S. Geol. 

 Survey. 

 gAm. J. Sc. XXXII, 187. 1886. 



