2i6 AUSTIN F. ROGERS 



lites. The orthoclase and cristobalite are largely products of 

 recrystallization of the spherulites. The cristobalite probably was 

 formed at a comparatively high temperature and as the temperature 

 became lower, tridymite was formed, and at about the same time 

 fayalite and magnetite appeared. The opal is the only mineral 

 that was formed by ordinary solutions. The oxidation of the faya- 

 lite, a sHght staining of the lithophysae, and whitening of the 

 orthoclase rods are the result of weathering. 



CRISTOBALITE IN SPHERULITIC OBSIDIAN 



The Yellowstone Park occurrence, the San Bernardino County 

 occurrence of the present paper, and two undescribed CaHfornian 

 occurrences, all taken together, point to the conclusion that cristo- 

 balite is one of the characteristic minerals of spheruHtic obsidian. 

 In fact it seems to be more common in such rocks than tridymite. 

 It is probable that in some cases cristobalite has been identified 

 incorrectly as tridymite. For example the minute pellets referred 

 to and figured by Iddings' in the spherulitic obsidian of Yellowstone 

 Park are probably cristobalite instead of tridymite. 



' Seventh Ann. Rept. U.S. Geol. Survey (1888), p. 264, PI. 12, Fig. 3. 



