36 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



growth of hypersthene, the augite occupying a nearly cen- 

 tral position. The vertical axes of the two minerals are 

 similarly oriented. The augite is simply twinned, the two 

 halves of the section showing a very slight difference of 

 extinction. The hypersthene is much fractured and pre- 

 sents the appearance of a number of rods placed in parallel 

 position, side by side and end to end. The interstices 

 between the rod-like parts are filled with a brownish yellow 

 isotropic substance closely resembling the opaline silica in 

 the slides already described. This mineral also forms an 

 intricate network in most of the feldspars, and constitutes a 

 considerable proportion of many of the yellowish patches 

 which are so numerous in the rock. These yellowish areas 

 are largely aggregates of hypersthene and feldspar. 



The hypersthene is prismatic in habit and occurs, in gen- 

 eral, either alone or in aggregations together with occa- 

 sional feldspars. The feldspar and hypersthene are the 

 most abundant minerals of the slide, the former being some- 

 what in excess of the latter. The hypersthene has a pro- 

 nounced pleochroism, c being light green in color, 16 reddish 

 brown, and a very pale reddish. The absorption formula 

 is c>6>a. 



In one slide two small sections of free quartz were 

 seen occurring quite close together and showing rounded 

 and somewhat corroded boundaries. The sections are both 

 crossed by numerous cracks. 



The feldspars are all much resorbed, and seldom show 

 crystal boundaries. The brownish network which most of 

 them contain is frequently central, leaving a narrow border 

 free from inclusions. 



The ground-mass, with a high power, is seen to consist of 

 glass filled with crystallites and microlites, with here and 

 there perlitic cracks. With a low power the crystallites 

 and microlites appear only as dusty particles. The perlitic 

 cracks are distributed very irregularly, some portions of the 

 slides being entirely free from them. They are usually 

 quite numerous near the larger aggregates of the pheno- 

 crysts, and frequently those bordering these areas are stained 



