214 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



entirely altered; pseudomorphs of chlorite after hornblende can be distinguished. 

 Numerous amygdule-like portions are lined with chlorite and filled with granular 

 quartz. 



3. Earlier andesite (293) from Fraction No. 2 shaft, at depth of 218 feet. 

 Purple rock with v T hite feldspar phenocrysts. Phenocrysts rather abundant, but 

 relatively small, the feldspars being the largest. The groundmass is glassy and 

 microlitic, with flow structure. There is abundant magnetite, frequent apatite, 

 and occasional zircon. 



The feldspars were determined as andesine; they are only partly altered 

 to fine muscovite. Pseudomorphs after original ferromagnesian minerals are 

 abundant, though small; biotite, pyroxene, and hornblende can be distinguished, 

 though no traces of the fresh minerals are left. The biotite has altered to 

 muscovite, with a small amount of siderite scattered through, and hematite 

 forming a zone around the edge. Rutile cr sagenite needles are included in the 

 biotite. Pseudomorphs after hornblende are of sericite or talc, with inclusions and 

 heavy rims of magnetite. Pseudomorphs after pyroxene or biotite are of quartz, 

 with a little calcite and hematite around the borders. Other pseudomorphs, which 

 are probabty after hornblende, but may be in part after pyroxene, consist of quartz 

 and sericite. 



4- Earlier andesite (53) from near Mizpah Hill. Pale pinkish-purple ground- 

 mass, with white phenocrysts. This shows what was originally a microlitic glass}' 

 groundmass, now containing abundant secondary quartz and sericite, with dissemi- 

 nated fine limonite, hematite, and siderite. Pseudomorphs after biotite phenocrysts 

 are of muscovite, with a very little siderite. Other phenocrysts, possibly of 

 hornblende, are represented by pseudomorphs of quartz, sericite, and a little 

 siderite. Abundant pseudomorphs after feldspar are of clear, translucent mate- 

 rial, which appears isotropic, but which high magnification often resolves into a 

 fine aggregate, the grains of which may sometimes be made out as spherulitic. 

 This substance has a very low double refraction and also a low single refraction, 

 but the latter is apparently higher than that of balsam." 



5. Earlier andexite (194) from Mispah mine, lease 86, 180-foot level, near Mizpah 

 vein. Rock of a light salmon-pink color. Shows several phenocrysts of feldspar, 

 whiter than the rest of the rock. No original mineral is left anywhere. The 

 groundmass, of which the fine microlitic glass} 7 composition and fluidal structure 



o Some of this material was Isolated and analyzed by Mr. George Steiger, showing 62.1 per c ent SiO, 19 per cent A1 S O 3 . 

 and 4.8 per cent K 2 O. Sodium was absent. These figures correspond to a composition of about 28. 4 percent adularia, 30 per 

 cent kaolin, and 27.5 per cent silica. Water, probably contained in the kaolin and the silica, was not determined, and 

 was disregarded In the computations. The substance is therefore probably to be regarded as a colloidal mixture of these 

 three alteration products of the original soda-lime feldspar, In nearly equal parts. As bearing upon the change which 

 this feldspar has undergone, the proportions of the different constituents in rather siliceous nndesinc, such as we may 

 believe, from examination of fresher rock, that this altered feldspar originally wns, are given: SlOj, 60.36; A1 Z O 3 , 25.45; 

 CaO, 5.14; NajO, 7.63; K S O, 1.21. The change evidently has consisted mainly in a removal of the soda and lime, and a 

 substitution in part of exogenetlc potash. 



