378 GEOLOGY OF THE EUREKA DISTRICT. 



groundmass of 138 and 146 consists of somewhat larger grains of quartz in a crypto- 

 crystalline matrix, which is identical with the substance occupying the sections of 

 decomposed feldspars, already described as kaolin. Here, also, it is possibly the 

 alteration product of feldspar in the groundmass and not a devitrified glass. The 

 abundance of calcite is undoubtedly due to infiltration from the surrounding limestone. 

 Thin sections 142, 143, and 125 present less coarsely crystallized varieties, the first 

 two being similarly decomposed. 



A lower stage of crystallization, in which the groundmass is largely or entirely 

 glass, is found in thin sections 118, 155, 122, 117, 126, and 130, some of which are 

 partly crystalline. The glass is spherulitic. There are also narrow bands of fibers, 

 the fibers lying at right angles to the direction of the bands, which make the flow- 

 structure very pronounced, Fig. 1, PI. viu. Thin section 155 is interesting as containing 

 round and oval spherules of colorless glass, with a few concentric inclusions, remind- 

 ing one strongly of leucite crystals. They polarize faintly in radiating rays. An 

 entirely glassy modification, which occurs in a small chimney about ten feet wide, is 

 shown in thin sections 144, 145, and is a pale green glass rich in feldspar microlites, 

 some of which are striated. They are partly rectangular, with the four corners pro- 

 longed like a "skate's egg." The corners of others are fringed, but the majority 

 appear like bundles of colorless fibers, the larger of which are compact in the middle 

 and extinguish light as a single individual, Fig. 14, PI. in. One can thus trace 

 the connection from the single microscopic fiber to the dense, sharply crystallized 

 feldspar, that is large enough to be seen without the use of a lens. In the thin sec- 

 tion, from the buff-colored, porcelain- like portion of the same flow (145), the microlites 

 are more numerous and are accompanied by clouds of yellow spots with aggregate 

 polarization. 



Thin sections 129 and 132 are from rhyolitic pearlites, poor in microlites, with some 

 gas cavities and globulites of an indeterminable nature. The pearlitic structure, 

 consisting of spherical fractures which inclose one another like the imbricated scales 

 of an onion, is very well marked in thin section 129. The rhyolite at the head of 

 Yahoo Canyon (157) is similar to the eryptocrystalline forms of this variety and is 

 poor in phenocrysts. That from the saddle northeast of Combs Mountain (158) cor- 

 responds to the more coarsely microcrystalline kind and is very poor in macroscopic 

 crystals, which are quartz, feldspar, and biotite, the only inclusions noticed being 

 glass. 



A completely decomposed rhyolite, thin section 148, is worth mentioning. In the 

 hand specimen it is seen to be kaolin, with numerous quartz crystals. In thin section 

 it is colorless, the groundmass having no action whatever ou polarized light and 

 being filled with minute grains which are white in incident light, also larger yellow 

 and red grains of iron oxide, resulting apparently from the decomposition of magnet- 

 ite, besides other small transparent yellow globulites with high double refraction, 



