222 Rogers — The Occurrence of Cristobalite in California. 



Art. XV. — The Occurrence of Cristobalite in California; 

 by Austin F. Rogers. 



The high-temperature pseudo-isometric form of silica 

 known as cristobalite is a rare mineral found at Cerro 

 San Cristobal near Pachuca, Mexico (the original local- 

 ity) ; at Saint Vincent, Martinique (lava of 1902-3) ; at 

 Mont-Dore, Plateau Central, France ; at Olokele Canyon,. 

 Kauai (one of the Hawaiian Islands) ; J and at several 

 localities in Rhenish Prussia. 



While cristobalite has been reported from the Kendall 

 County, Texas meteorite, 2 it has not been recorded from 

 any of the terrestrial rocks of the United States. The 

 writer has been fortunate enough to find this unusual and 

 interesting mineral in specimens from two widely sep- 

 arated localities in California, viz. Tehama County and 

 Tuolumne County. At the Tehama County locality the 

 cristobalite occurs in definite, well-formed octahedral 

 crystals plainly visible to the naked eye while the Tuo- 

 lumne County occurrence is especially interesting in 

 that the cristobalite proves to be paramorphous after 

 tridymite. 



1. Cristobalite from Tuscan Springs, Tehama County, 



California. 



The cristobalite was found in a large bowlder of a gray 

 porphyritic igneous rock a few miles northeast of Tuscan 

 Springs, Tehama County, by Mr. R. M. Wilke of Palo 

 Alto, California, to whom the writer is indebted for the 

 opportunity of describing this interesting occurrence. 

 Mr. Wilke identified the mineral as cristobalite and the 

 writer has confirmed his sight determination by physical 

 and chemical tests. 



Cristobalite-bearing auganite. The cristobalite-bear- 

 ing rock is a porphyritic rock with labradorite and augite 

 as the dominant minerals and since it contains no olivine, 

 Winchell's useful name auganite' may be used for it. 

 The plagioclase is a labradorite with approximately the 

 composition Ab 3 An 2 , which was determined by finding 

 the maximum symmetrical extinction-angle in albite 

 twins. The euhedral phenocrysts of labradorite are 



1 Cross, U. S. Geol. Surv., Prof. Paper 88, p. 11, 1915. 



-Cohen, Meteoritenknnde, p. 260, 1903. 



* Mining and Sci. Press, 105, 656, 1912. Auganite differs from basalt in 

 the absence of olivine and from augite-andesite in having the plagioclase 

 more calcic than Ab, An,. 



