292 - GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the zones alternate in the values of their extinction angles, so that it would 

 seem as though the composition of the feldspar oscillated between narrow 

 limits from the center outward. In other cases the center exhibits a higher 

 angle of extinction than the marginal zones, from which it may be inferred 

 that the composition changed interruptedly from the center outward. The 

 feldspars are sometimes free from inclusions, sometimes crowded with glass 

 inclusions, or these may be arranged in zones. Microscopic crystals of 

 pyroxene, apatite, and magnetite are also included in some feldspars. 



The hornblende is like that in the varieties of andesite just described; 

 not often idiomorphic, except in the prism zone ; frequently with dark border, 

 which is sometimes broad and compact. Often the border consists of minute 

 pyroxenes. Sometimes the hornblende is surrounded by comparatively 

 large crystals of pyroxene, magnetite, and feldspar, the aggregate having 

 an irregular outline in some instances, and in others having the outward 

 form of hornblende. The colors are red-brown, brown, and, less often, 

 green. Occasionally the color varies in strength zonally. Hornblende 

 incloses both augite and hj^persthene in comparatively large crystals, and 

 also in smaller grains, and exhibits the same relations of intercrystallization 

 with the pyroxenes which exist between these_ three minerals in the diorites 

 of Electric Peak. In one rock (1652) the brown hornblende is intimately 

 intergrown with plagioclase, as in the case of hypersthene in the andesite 

 lava from Mount Hood, Oregon. 1 The pyroxenes are augite and hyper- 

 sthene, generally in almost identical small crystals, with like forms in the 

 prismatic zone — unit prism and both pinacoids, together with fiat terminal 

 faces. The cleavage and twinning are normal. They have nearly the same 

 color in thin section — pale green. But the hypersthene has reddish tones 

 for rays vibrating transversely to the prismatic axis of the crystal. They 

 are distinguished only by the differences in their double refraction and 

 their extinction angles, measured from the prism axis. Their most frequent 

 inclusions are crystals of magnetite. Rarely they inclose red hornblende. 



The PTOundmass of most of the hornblende-pyroxene-andesites studied 

 is glassy and crowded with minute microlites. Often these are so close 

 together that it is not possible to say whether there is any amorphous 

 glassy substance between them. In some cases it is evident that the whole 



'Iddiiigs, J. P., The eruptive rooks of Electric Peak and Sepulchre Mountain, etc.: Twelfth 

 Ann. Rept. U. S. Geol. Survey, 1892, p. 612. 



