EHYOLITIC I'UMICE. 885 



tliroujiliout the whole mass, except in the case of the k'ss fusible jiieces. The Ihiid 

 inclusions have wholly disappeared from the <jlass of thin section JL'G, which is both 

 colorless and brijiht yellow, and is full of opa([ue red particles, without doubt red oxiile 

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

 yellow. 



From the foregoing it appears that the richly quartzose, rliyolitic pmnice in the 

 vicinity of Eichmoud Mountain, containing, as it does, a large i)er(entage of tricliuic 

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

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

 an intermediate position miueralogically 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. Tlie glassy groundmass 

 is nearly colorless, and contains only a small ann)nnt of black particles and starlike 

 groups of trichites. Another altered i)umice, 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 tufTof fine grain occurring over 

 a small area on the east slope of Horuitos Cone, where it appears as a bedded dei)Osit 

 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 piu'plish brown glass crowded 

 with fragments of feldspar, hyi)erstliene, 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 nuiltitude of colorless glass inclusions and a few large 

 ones of brown glass. The hypersthene has the pleot-hroism common to that of the 

 neighboring andesite, and the greenish brown hornblende tragments 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 inore or less feldspar microlites and black 

 grains and trichites in very beautiful aggregations. 

 MON XX 25 



