368 WRIGHT! LITHOPHYSAE IN A SPECIMEN OF OBSIDIAN 



The sphericities and lithophysae. All the larger spherulites are 

 hollow and are lined with minerals similar to those contained in 

 the lithophysae of obsidian from other localities. The predomi- 

 nating mineral crystallizes in fibers arranged radially, is weakly 

 birefracting, extinguishes parallel with positive or negative elon- 

 gation, and has refractive indices: a > 1.520, y < 1.530. Some of 

 the sections extinguish in a manner indicative of submicroscopic 

 twinning. These properties agree with those of a potash-soda 

 feldspar. The size of the fibers is not uniform; near the outer 

 wall of the spherulite the individual fibers are exceedingly fine 

 but toward the center they increase in size, become clearer and 

 are there associated with a second, more transparent mineral, 

 tridymite, which appears in characteristic, small nodules and 

 clusters, studding the cavity walls. Under the microscope the 

 tridymite aggregates are weakly birefracting, and have an aver- 

 age refractive index slightly less than 1.480. Minute octahedra 

 and irregular grains of magnetite are disseminated throughout 

 the crystalline mass of each spherulite. These grains increase 

 also noticeably in size from the wall toward the center of the 

 cavity. 



In addition to the above predominating minerals well devel- 

 oped, tabular crystals of brown transparent fayalite were observed 

 in some of the cavities. They are identical in optical properties, 

 so far as determined, with the Icelandic and Yellowstone Park 

 lithophysal fayalite. In several of the cavities single, small 

 crystals of a jet black mica of uncommonly high refractive in- 

 dex, apparently slightly above 1.70, were noted. The optic 

 axial angle is small (2E less than 50°) ; the dispersion of the optic 

 axes is noticeable. This mica is interesting because its pres- 

 ence, as a water-bearing mineral, proves that water vapor was 

 among the gases in the lithophysal cavity. 



This mineral association and the pronounced increase in granu- 

 larity from the margin to the center of a cavity prove that dur- 

 ing the crystallization of the spherulite chemically active volatile 

 components were present and attacked part of the material of 



