62 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



After the observations made on the areas apparently pseudomorphous after 

 hornblende in No. 376, similar areas were looked for in this rock. They were 

 at first not evident, but some definite though irregular areas of a fine aggregate 

 similar to the pseudomorphs referred to were found. On close observation, 

 however, faint but distinct crystal forms shaped like those of No. 337 were dis- 

 tinguished. The area occupied by these forms is surrounded by a border of 

 similar fine aggregate, running irregularly off into the rest of the rock, which 

 so obscures the crystal-like outline that it would not have been detected save for 

 the observations made on No. 337. This aggregate is somewhat coarser than in 

 No. 337, and its nature can be determined. It is semispherulitic and semigranular, 

 and differs from the rest of the groundmass only in being slightly finer grained 

 and containing a little more biotite. It is a fine mixture of quartz, orthocluse 

 (sanidine), and biotite. Very small idiomorphic crystals, or phenocrysts, of sani- 

 dine form part of the aggregate. It thus appears that the original hornblende 

 (in part pyroxene?) substance has been replaced by rhyolitic material. 



Jfugmativ or'ujin of psevdomorphs. Since these pseudomorphs in No. 337 are 

 often in direct contact with perfectly glassy sanidine, they must be of magmatic 

 origin and must have been formed before or during the consolidation of the 

 rhyolite. It is probable that they represent hornblende, which was an earlv 

 mineral to crystallize and was afterwards decomposed by the siliceous magma 

 and pseudomorphosed to biotite and the fine aggregate. The process was plainly 

 a partial replacement of some material by others, for no mineral containing lime 

 in any quantity resulted. Indeed, it is somewhat difficult to determine where 

 the lime went to, for the analysis of the rock shows only so much lime as is 

 commonly contained in orthoclase. It seems difficult to explain .such a process 

 as this without supposing a chemical change in the magma. 



HOUXHLEXDE IX TOXOI'AH LAVAS. 



No hornblende or augite has been found in the white Tonopah rhyolites. 

 In the Tonopah rhyolite-dacite no fresh hornblende was seen, but there was 

 found in it one pseudomorph after hornblende, marked by crystals of specular 

 iron, the hornblende having been resorbed by the magma (p. 41). In the glassy 

 Tonopah rhyolite-dacite also only one small crystal of augite was seen out of very 

 many thin sections examined. In the Brougher dacite hornblende is rare, but has 

 been occasionally found. A specimen of dacite from Golden Mountain, at a point 

 south of the top of Mount Oddie, showed a single fresh hornblende crystal. This 

 Golden Mountain dacite is, as shown by the analyses (p. 58), closely related to the 

 near-by Oddie rhyolite, so that, as has already been mentioned, the two must be 

 considered as variations of a single magma. Augite is rare in the Hrougher dacite. 



