240 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



are seen arranged in zones parallel to the rays of the pressure figure." These have 

 often the characteristic crystal form of siderite. They are translucent under high 

 powers, but under lower powers show in aggregate the white, cloudy appearance 

 characteristic of siderite. Between these siderite zones is quartz. 



Pseudomorphs of calcite after pyroxene, with a few tin}' zeolite needles and 

 some siderite, were noted. 



Pseudomorphs after feldspar consist of calcite and an aggregate of fibers 

 resembling in large part sericite, with some zeolite needles. 



Pyrite and siderite are abundant, disseminated or in clusters. The siderite 

 frequently forms alteration rims around the pyrite. Aggregates of siderite some- 

 times show characteristic cleavage and even crystal outline. 



Small smoky apatites occur in the pseudomorphs after biotite. 



4- Entirely altered later andesite (219) from Montana Tonopah shaft. Type 

 for first 278 feet. Green pyritiferous rock, mottled with white feldspar pheno- 

 crysts, and with apparent kaolin coatings on joints. 



Under the microscope the rock is seen to be entirely decomposed. The ground- 

 mass is a white, opaque aggregate containing quartz, some siderite, and much 

 cloudy material (which is very likely kaolin), with some chloritic material. 



The feldspars are completely altered to pseudomorphs, made up of calcite 

 and a clear, colorless aggregate showing sometimes rather low interference colors, 

 while many fibers reach yellow, red, and even blue of the first order. The 

 individual grains are fine, and are often in the shape of vermicular strips, made of 

 fibers perpendicular to the long direction of the strips. Along these strips the 

 extinction is wavy, traveling from one end to the other, similar to the behavior 

 of spherulites. Also occasionally similar clear areas are nearly isotropic, low, 

 doubly refracting and faintly spherulitic, like the pseudomorphs after feldspar 

 described in specimen 53 (p. 214), where the material seems to be a kaolinic mixture. 

 Other areas are of low-refracting spherulitic material, resembling chalcedonic 

 silica. 



Portions of this white pseudomorphous mixture, showing still the feldspar 

 cleavage, were separated from the rock, and were tested chemically by Mr. George 

 Steiger, of the United States Geological Survey. The calcite was leached out of 

 these pseudomorphs and the remainder was examined and found to contain, besides 

 considerable combined water, principally silica and alumina, with a small proportion 

 of magnesia, roughly estimated at about 4 or 5 per cent. The material therefore 

 appears to be a mixture of an aluminous mineral with some magnesian mineral, 

 probably talc, and with free silica. 



aSee Rosenbusch-Iddlngs, Microscopical Physiography of the Rock-making Minerals, 2d. ed., p. 257. 



