CRISTOBALITE IN CALIFORNIA 215 



benzol. (It sank in the liquid in which gypsum [sp. gr. = 2.32] 

 remained suspended and floated in the liquid in which brucite 

 [sp. gr. = 2.39] remained suspended.) 



In still another respect did the mineral agree with cristobahte. 

 It is infusible and on heating to a high temperature by means of 

 a blowpipe it became semi-transparent and on cooling it suddenly 

 became translucent again. This sudden change in appearance, 

 which may serve as a blowpipe test for cristobahte, is due to the 

 conversion of the high-temperature /3-cristobalite to the low- 

 temperature a-cristobalite, which, according toFenner, takes place in 

 artificial cristobalite at a temperature varying from 198 to 240° C. 



Tridymite. — The exterior of most of the cristobalite spherulites 

 when examined with a high-power binocular microscope are seen 

 to be studded with very minute six-sided crystals of tabular habit. 

 These are identified as tridymite by their index of refraction, n = 

 1.475=^.005. The tridymite is transparent in contrast to the 

 translucent cristobalite. Some of minute cristobalite spherulites 

 apparently are converted into tridymite, but careful microscopic 

 examination shows that the cristobalite is simply covered with the 

 tridymite. 



Opal. — Several of the lithophysal cavities are coated with a thin 

 layer of hyalite opal {n slightly less than 1.45), which was evidently 

 the last mineral formed. No chalcedony or quartz could be 

 detected in any of the specimens, 



Fayalite. — A few, small (i mm.), brown, transparent crystals 

 of fayalite (Fe2Si04) are found on the free surfaces of the hthophysae. 

 Three crystals (dark areas) are shown in the photograph. They 

 are tabular in habit and resemble the fayalite from the spherulitic 

 obsidian of Yellowstone National Park. The fayaHte has an index 

 of refraction greater than 1.74 and gives an iron borax bead test. 

 Some of the crystals have been altered to a black opaque substance. 



Magnetite. — Very minute octahedral crystals of magnetite are 

 visible with the high-power microscope. 



FORMATION OF THE MINERALS 



The minerals lining the walls of the Hthophysae evidently were 

 formed by hot gases hberated by the crystallization of the spheru- 



