ANDESITE OF COULTEE PEAK. 319 



prisms of pyroxene, for the most part hypersthene, and little magnetite. 

 The few small phenocrysts are labradorite, brownish-green hornblende with 

 little or no border, hypersthene, and angite. The mass is distinctly col- 

 umnar, in groups standing at various angles and sometimes curved. Near 

 planes of contact the columns are normal to the contact plane. On the 

 southeastern spur the massive andesite overlies dark-colored breccia, and 

 the surface of bottom contact, though quite irregular, slopes toward the 

 north. At the northern end of the mass the plane of bottom contact dips 

 steeply south, its trend being east and west. On the west spur of the 

 mountain a body of similar massive rock caps breccia, the contact dipping 

 north. The breccias beneath the body at Coulter Peak lie horizontal, and 

 the indications point to the massive andesite being a surficial flow in a 

 drainage channel trending west. 



A large body of similar rock forms the upper portion of the mountain 

 U miles northwest of Coulter Peak. It has the same petrographical char- 

 acters, but is denser and more crystalline in its main mass, approaching a 

 porphyry or felsite (1596, 1597, 1601, 1602). Near its contact with the 

 breccia it is dark colored and glassy (1598), becoming lighter colored and 

 lithoidal at a distance of 6 inches (1599), and still more so at a distance 

 of 10 feet (1600), where it is vesicular, with tridymite in good crystals. 



The rock representing the body of this mass is similar to that of 

 Coulter Peak in general characters, but is more highly crystallized. It 

 consists of prisms of labradorite about as large as in the other rocks, 0.15 

 mm. long, but few in number. The tabular or granular feldspar is more 

 abundant, and much of it is intergrown with quartz in micrographic 

 and micropoikilitic structure, which is shown in PL XXXVIII, fig. 4. 

 Magnetite occurs in minute grains, and there is some serpentine or chlorite. 

 The few small phenocrysts of hornblende and biotite have narrow borders 

 of magnetite in some cases and none in others. At the contact with breccia 

 it is a brown glass, as at Coulter Peak, in this case faintly polarizing, as 

 though devitrified. The beautiful microlites are like those in the other 

 brown glass just described, except that there is more green hornblende, 

 without dark border. At a distance of 6 inches from the contact the general 

 character of the rock is the same, but the brown glass base is replaced by 

 a gray microcryptocrystalline aggregate, which at 10 feet distance is micro- 

 crystalline, many of the parts being idiomorphic crystals of feldspar. The 



