INTERMINGLED RHYOLITE AND BASALT. 431 



augitic character is unquestionable. These augite needles often project 

 from the basalt into the glass. In one case the colorless glass contact zone 

 contains moss-like aggregates of red and opaque iron oxide attached to 

 augite needles. 



Spherulitic crystallization has taken on a variety of forms. In several 

 instances it produces a border against the basalt, radiating into the glass, 

 in which there are also isolated spherulites of the same character — that is, 

 distinctly fibrous and faintly doubly refracting, with no well-marked dark 

 arms. Where megascopic spherulites occur they are distinctly fibrous and 

 porous, with spherules of tridymite. The fibers or microprisms are mostly 

 optically positive; occasionally negative. These spherulites surround pheno- 

 crysts and also fragments of basalt. Some smaller spherulites are brown 

 in transmitted light and have brown curved rays or fibers, distinct from 

 those of feldspar. In the rhyolite-basalt fusion on Gardiner River the augite 

 needles associated with the spherulites have been beautifully developed, 

 and will be described in that connection. 



The phenocrysts of quartz are sometimes rounded; those of sanidine 

 in certain cases possess a marginal zone which is clouded, and appears 

 to contain another transparent mineral. This exhibits uniform orienta- 

 tion throughout. Porphyritical plagioclase is less common. Fragments of 

 basalt and detached crystals from it are scattered through the rhyolitic rock. 

 In one instance the rock mass had more of the appearance of basalt than of 

 rhyolite, and yet contained rounded phenocrysts of quartz with numerous 

 olivines, occasionally in skeleton forms, and plagioclase and augite, with 

 one individual of sanidine. The augites in the basalt are pale green in 

 thin section; those from the rhyolite are stronger green. 



With the pellets of tridymite which are attached to the sides of cavities 

 in the basalt are occasional projecting* crystals of brownish-green horn- 

 blende. Parts of the rhyolite are noncrystalline, with an almost micro- 

 cryptocrystalline texture and hypidiomorphic structure, in which the shapes 

 of feldspar crystals are recognizable. 



In the intermingling of rhyolite and basalt on the south side of Gardiner 

 River, south of Bunsen Peak, the resulting product is quite the same as 

 that just described. Most of the rhyolitic portion, however, is noncrys- 

 talline, in part microspherulitic. A smaller part is glassy. The brown 

 fibrous spherulites exhibit numerous dark arms, and are optically negative. 



