GEOLOGY OF THE SAN JOSE DISTRICT 283 



than the andesite which surrounds it that it is always left in 

 relief by the differential effects of weathering. It is of a light 

 green color in the field. Its texture is porphyritic, with glassy 

 sanidine crystals in a dense homogeneous ground-mass, as the 

 chief phenocrysts. Small specks of biotite may be seen in the 

 hand specimen. The rock has a peculiar greasy luster. 



Microscopic Characters. — Under the microscope this tinguaite 

 is found to have the pilitic texture where minute a^girite needles, 

 amounting to 50 per cent, of the whole, make up a dense felty 

 aggregate, in which sanidine, analcite, and, rarely, porphyritic 

 crystals of pyroxene and biotite patches are found. 



The analcite crystals are in small squares less than .1 mm. 

 in diameter, and in polygonal and rectangular sections. They 

 are colorless and appear so abundantly as to cause darkness by 

 crossed nicols over most of the field. 



The sanidine phenocrysts are clear and colorless. They 

 give blue-gray interference colors. Carlsbad twins may often 

 be noted. The sanidines have no good cleavage cracks, but 

 they exhibit the usual crude parting parallel to (lOo). 



A few aegirite needles of an older generation occur. This 

 mineral in the ground-mass is faint green or almost colorless. 

 The individuals are very small, usually only .1 mm. in length. 

 They are sharply bounded, and appear packed in together with 

 tiny feldspars to form a very dense aggregate. The small crys- 

 tals of sanidine associated with them are all automorphic, and 

 many of them show twinnmg. Traces of magnetite, rare patches 

 of biotite and areas of secondary limonite are to be noted. 



(b) Corona Type. — The type here described could not be 

 found in place. The dike from which the bowlder, collected 

 in the bed of Bretaiia Creek, came, was doubtless up near the. 

 waters of that stream against the Baril Range. 



Megascopic Description. — The hand specimen shows a slate.- 

 colored rock, having a felsitic texture. Phenocrysts of greasy- 

 looking nephelite and crystals of sanidine are visible to the un- 

 aided eye, and with a pocket lens the carlsbad twinning may 

 be noted on the feldspar. A dark silicate sparingly dissemi- 

 nated through the rock proves to be aegirine-augite. 



