Rogers — The Occurrence of Cristobalite in California. 223 



thick tabular parallel to (010). They show zonal struc- 

 ture, the exterior zone being more sodic than the interior 

 as is generally the case. 



The augite occurs in euhedral to subhedral phenocrysts 

 of stout prismatic habit and square cross-section with 

 (100), (010), and (110), the latter form being subordi- 

 nate. Sections parallel to (100), which are recognized by 

 the parallel extinction, are somewhat pleochroic from 

 greenish to reddish while sections parallel to (010) are 

 non-pleochroic. The maximum extinction-angle is about 

 42° and the maximum birefringence about 0-022. 



Magnetite occurs in small euhedral to subhedral crys- 

 tals. There are also a few large, almost opaque skeleton 

 crystals which could not be identified. 



Some glass is present in the groundmass and the rock 

 as a whole is docrystalline and dopatic with seriate por- 

 phyritic texture or fabric. 4 



Cristobalite in the rock. The cristobalite is found for 

 the most part in cavities of the auganite but it is also 

 identified in thin sections as anhedra which, however, may 

 be entire cavity fillings. These anhedra are weakly bire- 

 fringent with a kind of mosaic structure, which is due to 

 minute, roughly rectangular crystals. The parts of the 

 mosaics usually extinguish together as a whole but vary 

 in their birefringence. One imperfect euhedral .crystal 

 was observed in the thin section. It is about half of an 

 octagon. A part of it extinguishes parallel (and perpen- 

 dicular) to one edge of the octagon, while the other por- 

 tion is almost dark between crossed nicols. These tests 

 prove the existence of twinning in the original high- 

 temperature cristobalite. The maximum double refrac- 

 tion of the cristobalite is about 0-0023. The value de- 

 termined by Fenner 5 is 0-003. 



Euhedral cristobalite in cavities. The euhedral cris- 

 tobalite occurring in cavities of the auganite are opaque 

 white to subtranslucent octahedra, usually y 2 mm. to 

 % mm. in diameter with a maximum size of 1 mm. The 

 crystal edges are straight and some of the faces are 

 smooth and plane but many of them are slightly concave, 

 sometimes with symmetrical triangular depressions. A 

 few contact spinel twins without reentrant angles were 

 noted and also a peculiar double contact and penetration 



4 These terms were introduced by Cross, Iddiiigs, Pirsson, and Washington. 

 (Jour. Geol., 14, 692-707, 1906.) 



5 This Journal (4) 36, 354, 1913. 



