344 GEOLOGY OF THE EUREKA DISTRICT. 



(Irons fairly flooded witli fluid inclusions of water and liquid carbon dioxide; the 

 groundinass is a coiuparatively coarse grained aggregation of quartz and feldspar, 

 the latter more highly developed but entirely decomposed, there is a little completely 

 altered biotite, much colorless mica and some epidote; the second thin section, from 

 the side of the dike, is a dense gray mass, poor in quartz i)henocrysts, but rich in 

 small crystals of faldspa-r and biotite, the latter partially altered to chlorite; the 

 groundmass is finer grained and shows in places an incipient micropegmatitic struc- 

 ture, which is noticeable around the quartz crystals and also in pseudospherulites ; 

 there is a little titanite, but no hornblende. 



Thin section 28, fi'om a narrow dike farther up the Spring Valley road, has 

 been already alluded to as presenting a most beautiful example of micropegmatitic 

 structure. It is a very flue grained rock, having a few small quartz dihexahedrons 

 containing fluid inclusions of both kinds and portions of groundmass, besides a 

 few crystals of feldspar, but no biotite or hornblende. Around the quartz and 

 smallest feldspars are frames of feather or fernlike aggregates of intercrystallized 

 feldspar and quartz, producing the effect of a flowered pattern on the microgranitic 

 groundmass of quartz, feldspar and colorless mica. Thin section 29 from a small dike 

 west of Castle Mountain is somewliat similar; the extremely fine grained groundmass 

 bears numerous quartz crystals, with bays of groundmass, some inclusions of color- 

 less glass and a few of water with moving bubbles, also crystals of feldspar com- 

 pletely altered to a cryptocrystalline aggregate, probably kaolin, besides calcite and 

 hydrous oxide of iron ; and a little decomposed biotite. The groundmass is micro- 

 grauitic with an incipient, micropegmatitic structure developed around the quartzes, 

 (larnet occurs in well formed rhombic dodecahedrons, having long slender needles 

 radiating from their centers, which exert no influence on polarized light and are of 

 an indeterminable nature. 



Another variety of granite-porphyry is finind near the summit of the Fish 

 Creek Mountains, thin section 30. It is a fine grained rock, rich in biotite and horn- 

 blende. TL e sections show it to be composed of long rectangular feldspar crystals, 

 six-sided mica plates and rather stout hornblende crystals cemented together by 

 quartz and feldspar, with well developed micropegmatitic structure. Except in this 

 last respect the rock closely resembles the flue grained micaceous modification of the 

 large granite porphyry dike near Wood Cone, No. 11. The feldspar is much altered, 

 chiefly at the center of each individual, the product being partly potash-mica, partly 

 calcite. Of the fresher crystals many are triclinic. An estimate of the relative 

 abuudance of the two feldspars, however, is impossible under the cu'cumstances. 

 Quartz does not occur in large phenocrysts, but forms the greater part of the ground- 

 mass, where, with a little feldspar, it assumes the peculiar structure already alluded 

 to. It carries a small number of minute fluid inclusions with moving bubbles. The 

 biotite, in quite sharply outlined six-sided crystals, is reddish brown in its fresher per- 



