EHYOLITIC PUMICE. 



throughout the whole mass, except in the case of the less fusible pieces. The fluid 

 inclusions have wholly disappeared from the glass of thin section L'L'G, which is both 

 colorless ;i nd bright yellow, and is full of opaque red particles, without doubt red oxide 

 of iron. It is rich in trichites and microlites of feldspar, some of which are colored 

 yellow. 



Prom the foregoing it appears that the richly quartzose, rhyolitic pumice in tin- 

 vicinity of Richmond Mountain, containing, as it does, a large percentage of triclinic 

 feldspar, which is, however, subordinate in amount to the monoclinic, and at the same 

 time carrying a varying amount of biotite, pyroxene, and green hornblende, holds 

 an intermediate position mineralogically between the dacite and the rhyolite of Res- 

 cue Canyon. 



A thin section of pumice, 241, altered to a compact glass by the rhyolite of Pinto 

 Peak, is interesting as containing only a little mica in addition to the quartz and 

 feldspar, and therefore closely resembling in composition the surrounding rhyolite. 

 In addition to these phenocrysts, which are few, is garnet. The glassy groundmass 

 is nearly colorless, and contains only a small amount of black particles and starlike 

 groups of trichites. Another altered pumice, 242, from the basin west of Secret 

 Canyon road, is like the last in composition, the light brown glass being in places 

 filled with rectangular microlites of feldspar. 



Differing greatly from the foregoing pumices is a tuff of fine grain occurring over 

 a small area on the east slope of Hornitos Cone, where it appears as a bedded deposit 

 of dark gray volcanic sand, altered by an outflow of basalt to a blue black, basalt- 

 looking mass. Thin section 223 shows it to consist of a purplish brown glass crowded 

 with fragments of feldspar, hypersthene, and augite, with some black bordered horn- 

 blende and large grains of magnetite. The feldspar is wholly tricliuic, the angle of 

 extinction in several instances exceeding that of labradorite and corresponding to 

 anorthite. It also contains a multitude of colorless glass inclusions and a few large 

 ones of brown glass. The hypersthene has the pleochroism common to that of the 

 neighboring andesite, and the greenish brown hornblende fragments are all sur- 

 rounded by a black border. There is no doubt that this tuff belongs to pyroxene- 

 andesite. though it is the only occurrence of the kind met with in the district. The 

 brown glass is in places globulitic, with more or less feldspar microlites and black 

 grains and trichites in very beautiful aggregations. 

 MON xx 25 



