Calkins:] 



Petrography of the Joint Day Basin 



137 



Microscopically examined, the groundmass is seen to be 

 similar in structure and composition to that of the quartz-basalt 

 just described. But the phenocrysts of olivine and of limpid 

 quartz are absent, and the rock is mineralogically more allied to 

 the pyroxene andesites. Feldspar is more abundant than in the 

 quartz-basalt, and seems to have crystallized in two generations, 

 though the distinction between phenocrysts and groundmass is 

 not sharp. The feldspars are labradorite, and the pyroxenes are 

 of the same character and from the same undergrowths as in 

 the quartz-basalts. Considerable glass is present. 



The quartz noted in the hand-specimen is without doubt an 

 inclusion. In polarized light it resolves itself into a granular 

 aggregate of many individuals. Liquid and gaseous inclusions 

 occur in great abundance, arranged in curving lines that pass 

 without interruption from one individual to another. Tin- 

 effects produced by resorption, namely the zones of glass and 

 augite, are similar to those observed in the quartz-basalt. 



An apparent inclusion of hornblende is represented in one of 

 the slides by an opaque mass, mainly of iron oxide, with a 

 hexagonal outline. 



Spherulitic Ehyolites of Currant Creek Hill. — The rhyolitic 

 lavas of this locality, while comprising a small amount of glassy 

 material resembling obsidian, are in the main eminently charac- 

 terized by tbe development of spherultic structures. In this field 

 a wide variation in the size of the spherulites is observed. In 

 certain places their diameters are near that of buckshot, while in 

 others they range from half an inch to nearly a foot. The 

 finer-grained rock is usually fairly coherent, but the coarser 

 material is generally thoroughly disintegrated, so that the 

 spherulites, more resistant to weathering, may be readily picked 

 from the crumbling matrix. 



Only from the fine-grained phase could hand specimens be 

 gathered, or slides prepared to illustrate the structure of the rock 

 as a whole. Macroscopically this rock is cream-white to yellow- 

 ish green in color, and composed of spherulites from one to 

 three millimetres in diameter, imbedded in a fine lithoidal 

 groundmass. The spherulites compose about half the bulk of 

 the rock. When broken, they distinctly show their radiate 



