i'-24 Rogers — The Occurrence of Cristobalite in California. 



spinel twin. The crystals have a peculiar enamel-like 

 appearance, which is probably due to the fact that they 

 consist of aggregates of a-cristobalite. 



'1 lie cristobalite is infusible before the mouth blowpipe 

 and is insoluble in hydrochloric acid and aqua regia. It 

 scratches »lass slightly. 



The identification of the mineral as cristobalite, how- 

 ever, rests largely upon the optical properties. The 

 index of refraction is 1483 ± -003, which was determined 

 by means of the immersion method. The liquids used 

 were determined accurately to the fourth decimal place 

 on the refractometer and were rechecked at the time of 

 the determination. They had changed less than 0-001 in 

 a year's time. 



A little tridymite was found adhering to one of the 

 cristobalite octahedrons. It was a typical penetration 

 twin group with sharp edges and was clear and glassy in 

 contrast with the subtranslucent cristobalite. It was 

 positively identified by its index of refraction. 



Hyalite opal and calcite are associated with the cristo- 

 balite in cavities and were formed later than the cristoba- 

 lite. Opal also occurs in filaments which are probably 

 pseudomorphous after acicular calcite crystals. 



2. Cristobalite from Jamestown, Tuolumne County, 

 California. 



Cristobalite has also been identified in a porphyritic 

 augite andesite found by the writer near Jamestown. 

 Tuolumne County. This rock is a bowlder from the 

 coarse gravel found in the buried river channel beneath 

 the Table Mountain latite flow described by Ransome.' 1 

 The bowlders had been exposed by drift mining for gold 

 and were for the most part hornblende andesites. The 

 andesite bowlders have probably been derived from the 

 andesite breccias to the northeast. 



The Cristobalite-bearing andesite. This is a reddish- 

 gray porphyritic rock containing andesine and augite as 

 the dominant minerals. The andesine has approximately 

 the composition Ab r , (! An 44 which was determined by the 

 maximum symmetrical extinction angles in albite twins. 

 Many of the andesine crystals have a narrow opaque 

 zone near the outer margin and beyond this a colorless 

 rim. The probable explanation is that the feldspar was 

 partially altered by magmatic gases and then at a later 



" Bull. U. S. Geol. Surv., 89, 1898. 



