OPHITIC STEUGTURE. 437 



tals, with not so high double refraction as in the previous rocks. There is 

 serpentine, resulting from the partial alteration of olivine. The iron oxide 

 is mostly in rod-like crystals. The ophitic form of the basalt beneath 

 rhyolite on Mount Everts (622) is like the first ophitic basalts in composi- 

 tion. The augite is about the same size, but the labradorite prisms are 

 larger, and there is considerable globulitic and microlitic glass base scattered 

 through the mass. Olivine is abundant in microscopic crystals. A few 

 labradorites form phenocrysts. The rock is full of irregular cavities. The 

 top sheet of basalt near the mouth of Bear Gulch, east of Gardiner, and 

 another basalt flow in this vicinity, are like the last one, but in these there 

 is an approach to a parallel arrangement of the feldspar prisms inclosed in 

 the augite crystals (PI. LIX, fig. 2). 



BASALTS RELATED TO THOSE WITH OPHITIC STRUCTURE. 



These basalts resemble those just described in mineral composition and 

 in the character of the minerals, except that augite occurs in smaller crystals 

 and grains, and not in relatively large micropoikilitic ones. Augite occupies 

 about the same spaces as when it takes part in ophitic structure, but is in 

 aggregates of small crystals. Olivine and magnetite are the same as in the 

 ophitic rocks. There is a small amount of microlitic glass base. 



Another modification of the basalt beneath the rhyolite on Mount 

 Everts (623), and a vesicular basalt from the northern base of Prospect 

 Peak (634), are finer-grained rocks, like the ophitic form of the first- 

 mentioned basalt in composition, but with only a part of the augite inclosing 

 labradorite, and thus being partly ophitic, the remainder of the augite being 

 in small allotriomorphic grains between idiomorphic feldspars. The ferro- 

 magnesian minerals are in excess of feldspar, and magnetite is abundant. 

 The chemical composition of the basalt from the north base of Prospect 

 Peak (634) is shown by the following analysis. In this rock lime is con- 

 siderably higher than magnesia, and soda is greatly in excess of potash. 



