PETEOGEAPHY— EHYOLITE. 501 



turning- from quite pale-brown to dark greenish-brown, almost black, upon 

 revolution of the analyzer alone. When cut parallel to OP it is more olive- 

 brown and is not dichroitic. 



The groundmass is pale-brown in color, half-glassy in character and 

 inclines a little to a microlitic texture, with traces of a fluid-like structure. 

 In it are numerous small, black specks, but no magnetite was observed. 



The rhyolite [131] from southeast of Terry Peak has alight yellowish- 

 brown color, with broad reddish, parallel bands across it. It is quite com- 

 pact and fine-grained, and of a peculiar appearance like burnt cla}^. The 

 outside surface is weathered to a very dark-brown, the light shade showing 

 on a fresh fracture. 



In the section, the groundmass appears quite finely crystalline, being 

 made up of small, rather dusty crystals of sanidin, with some magnetite 

 and an abundance of quartz in small rounded grains The silica in the 

 rock is 72.22 per cent. 



The sanidin through the groundmass is easily seen with crossed nicols, 

 as the indistinctly rectangular crystals are then light on a dark ground. 

 They are, however, quite cloudy and the outlines are not at all sharp. The 

 quartz polarizes brilliantly in colors, blue and yellow, and is present mostly 

 in irregularly shaped grains of small size, many of which have inclusions 

 of the groundmass with microlites and magnetite. They seldom have any 

 crystalline form, and are in considerable abundance. Magnetite is in small 

 submetallic g-rains; also a little biotite, none of which, however, is dichroitic. 

 Nothing very peculiar was noted in the groundmass, except that the reddish 

 bands, colored by iron, were plainly seen in the section. 



The rhyolite [134] from the top of Custer Peak is very homogenous 

 in appearance and structure. It has a light-gray color and a fine-grained 

 texture, with an occasional small, black needle of biotite visible. In the 

 section, is seen a colorless groundmass, in which are numerous quartz grains, 

 abundant magnetite, a little biotite and sanidin as in the preceding rock, 

 seen as faintly defined crystals on a dark ground. The silica present 

 amounts to 68.62 per cent. 



The sanidin apparently constitutes the greater part of the groundmass, 

 but the outlines of the crystals are very irregular and in general scarcely 



