SPHERULITES AND LITHOPHYS^E 



267 



0.5 mm. in diameter. The supporting needles rarely measure over 0.03 

 mm. in thickness (see figure 3) . Their optical properties, so far as could be 

 determined, are: y about 1.535, a about 1.530; birefringence weak: ex- 

 tinction oblique with c:y' from 0° to 28°; elongation is usually y', but 

 occasionally a. The plane of optic axes is apparently normal to the 

 elongation. Some of the needles have the appearance, between crossed 

 nicols, of 23ossessing exceedingly fine polysynthetic twinning. It was 

 thought at first that this mineral was albitic plagioclase, but several of 

 the above optical properties do not agree with those of albite and it is 

 not certain that the mineral is a feldspar. The composition of the ob- 

 sidian itself would indicate a feldspar. The tridymite has the usual 



^f^'^^^y^*" 



\ ' • 



"iL.T"^ 



Fi'Jiiiu:' ;?. -Tridymite Crystals supported by Needles uf Feldspar (?) in recrystaUized 



Lithophysa 



Specimen 88433. Magnification, 50 X 



characteristics: Tabular plates and thick prisms hexagonal in outline; 

 weakly birefracting in irregular fine patches; refractive index slightly 

 less than 1.480; plates grouped in characteristic twinned aggregates. 



The aspect of these lithophysal minerals and the manner of their 

 grouping: are such as to render untenable the hypothesis that they were 

 crystallized directly from the cooling niagma. A comparison of the 

 lithophys83 of this specimen with the radial spherulites and the incipient 

 lithophysiv of the specimens described above shows that the lithophysse 

 were origijially spherulites with a gas cavity, but that they have been 

 partly, and in some instances entire!}', recrystaUized by the action of the 

 vulaliltj components of the cavity at relatively high temperatures, the 



