rETEOGllAPHY— PHONOLITE. 503 



Plate I, Fig-. 2. A second crystal, of the hexagonal basal section, has 

 magnetite grains, not only as a border, but also filling up the central part 

 and leaving only a small amount of brown mica. This residue is not at 

 all dichroitic, thus agreeing with the plane of section determined from its 

 crystallogra])hic outline. In both cases, the magnetite grains are much 

 larger than those scattered through the groundmass, the latter particles 

 being very numerous and quite small. The groundmass is crypto-crystalline 

 as before. The amount of silica is 72.33 per cent. 



The rock [137], also from the top of Custer Peak, is again a rhyolite, 

 and very similar to [134] and [135], the sanidin in the colorless ground- 

 mass, together with the quartz and magnetite, presenting no essential differ- 

 ences. The example of biotite, with its border of magnetite grains, is much 

 finer than in [135] and is figured in Plate I, Fig. 2. The groundmass is a 

 little more microlitic in structure. In the rhyolites thus far examined, there 

 was found in the sections but little, if any, fluid-like structure, the ground- 

 mass being more granular and crystalline in character. 



The rock [139] from Black Butte is a phonolite, and very different in 

 appearance and behavior from the trachytes and rhyolites. The ground- 

 mass has a gray ish-bro (vn color and a greasy or waxy luster, Avith dark- 

 green, irregularly shaped streaks and masses through it. It is quite 

 compact or crypto-crystalline, and weathers to a white mass. An occasional 

 whitish crystal can be distinguished with the aid of the loupe. 



In the thin section under the microscope, it is seen to consist of a 

 groundmass composed mostly of nephelite, with sanidin, nosite and green 

 hornblende, in which latter mineral are imbedded transparent hexagons and 

 rectangles of nephelite (Plate I, Fig. 3). The rock gelatinizes very strongly 

 in hydrochloric acid, the soluble portion being 24.08 per cent. Boiled in 

 nitric acid and tested with ammonic molybdate, it gives but a very faint 

 reaction for phosphoric acid. The silica is quite low, only 56.32 per cent. 

 The crystals of nephelite, very conspicuous and abundant, are quite 

 uniformly distributed through the rock, the characteristic sections being 

 very distinct. The hexagons vary somewhat in size, a few quite large ones 

 being present, and all have similar inclusions, probably glass particles as 

 they are brownish in color. No needle-like microlites were found in any 



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