712 E. S. MOORE 



The rhyolite is quite fresh, brittle, and friable. Thin sections 

 show that it contains a very deep dark-brown biotite, some augite, 

 and, in one case, a grain of hypersthene, in addition to orthoclase, 

 albite, and quartz which is very glassy and brilliant. The ground- 

 mass is usually mostly glass. 



The obsidian consists of a brittle, dense, black glass, showing 

 flow structure. It is full of spherulites from o .05 mm. to 3 . 5 mm. 

 in diameter. The glass contains also phenocrysts of green horn- 

 blende, orthoclase, and zonally built crystals of orthoclase and 

 albite. The smaller spherulites consist of radiating needles of 

 feldspar, while the larger ones are nearly solid glass around the 

 center, with radiating dark lines and with concentric spheres 





Fig. 13. — Bombs of andesite and basalt from Mount Tarawera. In two of them 

 the light-gray cores of rhyolite may be seen (J natural size). 



becoming more distinct toward the exterior. These spheres are 

 alternately brown and gray. The outer thick zone is brown and 

 shows only a glass without crystal structure. The other zones show 

 radiating small crystals of feldspar under the high-power micro- 

 scope, but there is so little crystal structure that only a very slight 

 darkening and brightening can be observed on rotating the section 

 between crossed nicols, and there is almost no difference in bire- 

 fringence between the spherulite and the surrounding glass. A 

 distinct bending of the microlites in the glass around small spheru- 

 lites may often be observed. 



Bombs.— During the eruption of 1886 a considerable amount 

 of intermediate to basic rock was ejected from the crater. It has 

 been estimated that from 520,000,000 to 620,000,000 cubic yards 

 of material was blown out of the great fissure. This was largely 



