Rogers — The Occurrence of Cristobalite in California. 225 



stage conditions were again favorable for the formation 

 of the feldspar. The colorless rims in this case are more 

 calcic than the interior. 



The augite is much like that in the Tehama county 

 auganite with the (100) section pleochroic from reddish 

 to greenish. The maximum extinction -angle is about 43° 

 and the maximum birefringence about 0-025. Some of 

 the augite is altered to an aggregate of fibrous crystals 

 with a maximum extinction angle of about 23°. This is 

 probably a mineral of the amphibole group. 



Magnetite occurs in small crystals and a small amount 

 of apatite is also present. Elongate opaque crystals are 

 red by reflected light and doubtless represent hematite 

 pseudomorphs after some ferro-magnesian mineral, the 

 most likely of which is hornblende. Red scales through 

 the rock are probably specularite (crystalline Fe 2 3 ) 

 crystals. 



The rock is docrystalline and dopatic, and has a seriate 

 porphyritic texture or fabric, some glass being present 

 in the groundmass. 



Crist obalite in the rock. A weakly birefringent mineral 

 with rather high relief occurs as cavity fillings and also in 

 patches in the thin section. "With the sensitive-tint plate 

 it exhibits a mosaic of rough squares. Some spots show 

 a radial excentric structure. This is characteristic of 

 tridymite but the mineral is identified as cristobalite for 

 the index of refraction is greater than 1480 (tridymite is 

 less than 1480) 7 . The explanation, as I shall presently 

 show, is that the mineral is cristobalite with the form 

 of tridymite. 



Cristobalite paramorphs after tridymite. Cavities of 

 the augite-andesite just described contain minute (y 2 mm.) 

 confused aggregates of translucent crystals. Many of 

 these are well-formed six-sided tabular crystals but they 

 are not clear and glassy like typical tridymite. Several 

 distinct penetration twins of the form so characteristic of 

 tridymite were noted. The form is that of tridymite but 

 the index of refraction is 1483 ± 0-003, which agrees well 

 with that of cristobalite. The explanation is that the 

 mineral is a paramorph of cristobalite after tridymite and 

 so far as I can learn this is the first recorded instance of 



7 The work of Fenner (this Journal (4), 36, 351-356, 1913) of the Geo- 

 physical Laboratory of the Carnegie Institution of Washington enables us to 

 positively distinguish cristobalite and tridymite by means of the refrac- 

 tive index. 



