CRISTOBALITE IN CALIFORNIA 



213 



MINERALS OF THE SPHERULITES 



The solid portions of the spheruhtes consist of two minerals, 

 orthoclase and cristobaHte, and of these orthoclase predominates. 

 The orthoclase and cristobaHte occur in fibrous aggregates, which 

 on microscopic examination prove to be intergrowths. In spots, 

 which are probably cross-sections, they have an intricate interlock- 

 ing structure which suggests graphic texture. The orthoclase is 



Fig. I. — (X5) CristobaHte and Fayalite in a Lithophysa of the Obsidian from 

 San Bernardino County, California. 



cloudy, while the cristobaHte is clear. The cristobaHte is identified 

 by its index of refraction, which is slightly greater than 1.480. 

 Most of the cristobaHte areas are isotropic, but occasionally they 

 show very weak double refraction. The double refraction is uni- 

 form, which distinguishes the cristobaHte from the glass which has 

 practically the same index of refraction. 



It seems probable from the descriptions in the literature that 

 such orthoclase-cristobalite spheruHtes are characteristic of rhyo- 



