37(i (iEOLOGY OF THE EUiiEKA DISTEKJT. 



glass witli gas, the otlier gas without glass, which gas appears iu the larger cavities 

 to be associated with water, suggesting that its condition at the time of its indosure 

 was that of highly expanded steam. This crystal is further interesting as a sporadic 

 development of mici'opegmatitic structure. 



The plagioclase is in crystals very similar to those of sanidine, but is not nearly 

 so abundant, being almost entirely wanting in all of the thin sections from the rhyolit« 

 dikes (Nos. 138, 140, 141, 142, 144, 143, 146, 153, 155, 148, 150, 134, 137). The twinned 

 lamelhe vary considerably in length, breadth and frequency, and in most all the indi- 

 viduals are twinned both after the law of albite and that of pericline, besides which 

 the composite crystals are also twinned in a manner corresponding to the Carlsbad 

 twins of orthoclase, which can be seen from the outline of the section and inequality 

 of the sets of angles of extinction in the two halves, Fig. 7, PI. iii. The investi- 

 gation of the extinction angles was not very satisfactory, owing to the scarcity of 

 favorable sections; the majority of readings were low, the highest being 17°, leading 

 to the conclusion that the triclinic feldspar is, for the most part, oligoclase. It is also 

 of very pure substance, with few gas cavities and more rarely small glass inclusions ; it 

 is without zonal structure and has poorly marked cleavage. The feldspar is extremely 

 fre.sh in the thin sections from the region of Pinto Peak and in those from most of the 

 dikes, but is partially replaced by calcite and kaolin in thin section 146. In 138 it is 

 entirely altered to calcite and kaolin, the latter appearing in the thin section as a 

 colorless aggregate of fibrous, faintly polarizing particles. 



The most abundant and constant of all the ingredients is quartz, the pheno- 

 crysts of which are well developed dihexahedrons and angular fragments, less fre- 

 quently rounded grains. It is irregularly cracked and of very pure substance, fi'ee 

 from inclusions, except an occasional "bay" of groundmass and a few colorless glass 

 inclusions with single gas bubble, around which, in some cases, is seen in polarized 

 light the phenomena of strain or unequal tension, the effects of which are still further 

 shown by small cracks that pass through the center of the dihexahedral glass inclu- 

 sions and extend a short distance into the quartz crystal, constituting three planes 

 corresponding to three of the planes of symmetry parallel to the vertical axis. These 

 appear in longitudinal section as a straight line or an inclined fi'acture, and in cross- 

 section as a six rayed star. A fine illustration is found in thin section 111, Figs. 1 

 and 2, PI. IV, where a cross-section and longitudinal section occur within 1""" of 

 one another. In the cross-section of quartz is a minute duid inclusion with moving 

 bubble, a very rare occurrence, though quite numerous fluid inclusions are found in 

 the fine dihexahedrons of (luartz iu thin section 127. Quartz in irregular grains forms 

 a large part of the groundmass. Small phenocrysts of biotite are found sparingly in 

 some of the thin sections, but are wholly wanting in others. The biotite is for the 

 most part free from magnetite grains or other inclusions when fresh. It is altered in 

 some cases to a colorless, brilliantly polarizing mica, crowded with yellow, opaque 



