PETROGEAPHY— ItHYOLlTE. 491) 



the microscope, the sanidin appears to constitute the greater mass of the 

 rock, being in some places quite clear and })ure. The crystals, however, 

 are very indistinct in outline, and are apparently aggregated to one mass. 

 There is hornblende in abundance in large, fine, green crystals, somewhat 

 decomposed and including grains of magnetite of good size, although they 

 are but slightly dichroitic. Nearly every crystal has magnetite either in it 

 or adhering to it. Occasionally, several hornblende crystals are grouped 

 about a large grain of magnetite. This is quite a usual characteristic of 

 trachyte. The hornblendes vary in size from quite large and clear, green 

 crystals to small blade-shaped or prismatic crystals with ragged termina- 

 tions. There is a little biotite present, and an occasional apatite crystal. 

 Magnetite, besides that mentioned as inclusions, is scattered abundantly 

 through the rock in large to very small grains, approaching a little to a 

 crystalhne form The groundmass is feldspathic, and has a fine-grained 

 structm-e resembling a fibrous microlitic aggregation. 



The rock contains 57.75 per cent, of silica, and the soluble portion in 

 hydrochloric acid amounts to 13.48 per cent. 



The rhyolite [125] from near Deer Mountains has a porous appear- 

 ance, caused by numerous empty cavities, the mass of the rock itself being 

 quite compact. Its color is pale-brownish to pink, having an indistinct 

 banding of two colors, light and darker brown. Upon close examination, 

 quite a number of quartz grains of good size can be seen, while many of 

 the empty cavities have a very plain imprint of the quartz crystals that 

 formerly filled them. With the loupe, a granular structure is recognized, 

 but no crystals of any mineral except the quartz already mentioned. In 

 the section under the microscope, it consists apparently of whitish, opaque, 

 rounded bodies, with large and very clear quartz masses in a partly granu- 

 lar and isotropic groundmass, in which are also numerous small quartz 

 grains. The rounded bodies are somewhat fibrous in structure, and gen- 

 erally have a narrow^, transparent rim, and are probably imperfectly formed 

 spherulites. They are aggregated in large masses, leaving only small 

 interstices of the groundmass between them, so that the greater part of the 

 rock is made up of these masses. They resemble spherulites a little, and 

 a fainth' hexagonal shape is also noticeable. The large quartzes ]iol;irize 



