214 AUSTIN F. ROGERS 



litic obsidians, but have not been recognized before on account of 

 the difficulty of distinguishing the cristobaHte. It was with great 

 difficulty that the writer succeeded in proving that the mineral 

 intergrown with the orthoclase was cristobalite. 



Imbedded in these larger spherulites are numerous, very minute, 

 spherical bodies, which consist largely of orthoclase with small 

 amounts of cristobalite. These probably represent earlier formed 

 spherulites of orthoclase which have been recrystallized during the 

 later main period of spherulite production. Such composite 

 spherulites have been described by Rutley"^ from another locality 

 in California. Rutley speaks of "isotropic matter" between the 

 rods or fibers of orthoclase, which in all probability is cristobalite. 



There is a little magnetite imbedded in the spherulitic material. 



MINERALS OF THE LITHOPHYSAL CAVITIES 



The following minerals were identified in the lithophysae : ortho- 

 clase, cristobalite, tridymite, opal, magnetite, and fayalite. These 

 minerals occur on the walls of the lithophysal cavities and the 

 specimens make beautiful microscopic mounts (Fig. i). 



Orthoclase. — On the free surface of the cavities the orthoclase 

 takes on the form of branching rods with weak birefringence and 

 positive elongation (parallel to 7), The index of refraction is 

 1.517^.003. 



Cristobalite. — The cristobalite occurs in minute (0.2-0.5 mm.) 

 translucent white spherulites, which are scattered over the surface 

 of the lithophysae as illustrated in the photograph. In fragments 

 the cristobaHte shows a well-defined spheruHtic structure with 

 elongation of the crystals parallel to the faster ray a. There is 

 intricate polysynthetic twinning and the extinction is oblique. The 

 double refraction is very weak. 



The index of refraction of the cristobalite is 1.483 ='=.003, 

 which was determined by means of immersion liquids. This deter- 

 mination positively distinguishes it from tridymite which has 

 indices of refraction of i. 469-1. 473. 



The specific gravity of the cristobalite is 2. 36^.04, which was 

 determined by means of a mixture of acetylene tetrabromid and 



' Qtiar. Jour. Geol. Soc, Vol. XL VI (1890), pp. 423-28. 



