342 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



The rock whose chemical composition is given in the third and fourth 

 analyses occurs as a massive sheet beneath another of very similar character 

 on the southeast fork of Beaverdam Creek. It is dark purplish gray, with 

 numerous phenocrysts of labradorite, augite, and serpentinized olivine 

 (1647, 1648, and 1651). A lighter-colored modification (1649) carries por- 

 phyritical biotite and white amygdules of zeolite. In thin section it is 

 holocrystalline, and resembles closely the rock from the south base of Bison 

 Peak (1131), except that the olivine is serpentinized. The groundmass is 

 similar and there is considerable orthoclase. In the variety with biotite 

 there are cloudy isotropic patches, which may be analcite. The massive 

 lava immediately overlying it is similar in general appearance, but contains 

 leucite, and will be described later in connection with the banakites. 



The rock whose chemical composition is given by analysis 5 is a 

 massive surficial flow on the top of the mountain east of Pyramid Peak, 

 resembling in general appearance the lava flow from the same place already 

 cited as similar to that from the base of Bison Peak, but the phenocrysts 

 are fewer and smaller (1475, 1476). The two specimens collected differ 

 slightly in grain. They are holocrystalline and very fine grained, with an 

 abundance of augite and magnetite microlites in the groundmass. The 

 feldspars are minute lath-shaped crystals and allotriomorphic grains, with 

 low double refraction. There are spots where minute grains of augite and 

 magnetite are clustered together and are inclosed in a yellowish substance 

 which is almost isotropic and has the outline of leucite. These impure 

 leucites are scattered through the rock and are not very numerous. They 

 are very small and can not be more definitely identified. Their outline 

 and inclusions are characteristic. The groundmass carries irregular patches 

 of light-brown mica, small phenocrysts of augite and serpentinized olivine, 

 and still fewer larger crystals of augite and olivine. It is a leucite-bearing 

 modification of this magma without any chemical differences in composition. 



The rock of analysis 6 (1316) is from an 18-inch dike on the ridge 

 northeast of Indian Peak. It is gray and aphanitic, with abundant small 

 phenocrysts of augite and lime-soda feldspar, with rather low extinction 

 angles. In thin section it is seen to be holocrystalline and very fine grained, 

 consisting of indistinctly outlined feldspar microlites, in part alkaline, with 

 low double refraction and no polysynthetic twinning. There is a subordinate 

 amount of idiomorphic microlites of biotite, prisms of augite, and grains of 



