GEOLOGY OF THE SAN JOSE DISTRICT 281 



angles to each other. It may extend out from the center as far 

 as the original boundaries of the feldspar or it may be surrounded 

 by a zone of feldspar crystallites packed in together like the 

 skeleton crystal needles in spherulitic bodies. These bundles of 

 needles are intergrown with definite initial forms of ^egirite. The 

 microscopic growths of this mineral resemble the occurrence of 

 pyroxene in the Arran pitchstone. Now and then lines of grains 

 of aegirite run out toward the center. Again a corroded space in 

 the original orthoclase has been filled by bundles of plagioclase 

 needles or by analcite and clear secondary feldspar, the latter 

 untwinned and surrounded by the zeolite. These circumstances 

 are favorable for deciding as to the relative indices of refraction 

 by the Becke method. The lines of light migrate outwardly 

 from the analcite into the feldspar on either side. When ob- 

 served with high powers the zeolitic substance is clear and 

 glassy while the secondary feldspar associated with it is dis- 

 tinctly granular. 



Two other generations of feldspars besides the large pheno- 

 crysts are well marked. Those belonging to the second period 

 are comparatively fresh. They appear as six-sided individuals 

 .3 mm. in length. Finally, feldspar appears as an aggregate of 

 orthoclase grains and minute lath-shaped crystals with soda 

 pyroxene in the ground-mass. Here the emerald green prisms, 

 arranged in felty aggregates, are noticeably shorter and stouter 

 than is usual in the ground-mass of other San Jose tinguaites. 



They are not frayed at the ends and their edges are firm and 

 sharp. They have not been noticeably crowded together by 

 the growth of the large feldspar phenocrysts, but have been 

 forced to grow with their long axes parallel to the latter, when 

 they lie near them. The aegirites at times cluster thickly about 

 a large grain of magnetite. 



Individuals of the pyroxene three times as large as those in 

 the ground-mass represent an older generation. Very rarely 

 phenocrysts of chestnut brown hornblende, pleochroic in shades 

 of brown .1 mm. in diameter, twinned parallel to (100) are 

 observed as nuclei for clusters of aegirite. The biotite of the 

 rock, in minute patches, is rare and of secondary origin. Mag- 

 netite grains do occur but not commonly. 



