THE BOCK FORMATIONS. 31 



TERTIARY LAVAS. 

 ANDE8ITE8. 



EARLIER AJJDESITE (HORNBLENDE-BIOTITE-ANDESITE). 



Of the Tertiary volcanics, which occupy all of the Tonopah district proper, 

 andesite appears to be the oldest. The writer has called this andesite the earlier 

 andesite to distinguish it from a subsequently erupted rock of very similar composi- 

 tion. In the camp it is often called the "lode porphyry," since in it the most 

 valuable veins lie. 



Appearance. The earlier andesite has never been found in even an approxi- 

 mately fresh state, but is decomposed in varying degrees, sometimes only moderately, 

 often intensely. The freshest specimens are a light colored, dense, finely porphy- 

 ritic rock, with small glistening feldspar phenocrysts showing on a fresh fracture. 

 They have a greenish tinge, due to the presence of chlorite and similar secondary 

 minerals, if they are from the deeper unoxidized mine levels, and a yellow tinge 

 from iron oxide if they come from nearer the surface. On further alteration the 

 earlier andesite usually has become lighter colored and more siliceous, and at first 

 glance altogether resembles a rhyolite; by another process of alteration, especially 

 when there was a somewhat greater abundance of original ferromagnesian silicates, 

 the rock has become green of various shades. 



Original composition. From microscopic study it appears that the original 

 fresh rock was a hornblende-biotite-andesite, of medium composition. The struc- 

 ture is tine porphyritic, with relatively sparse phenocrysts in a glassy groundmass 

 containing many microlitic crystals and frequently showing original flow structure. 

 The phenocrysts were mostly feldspar, hornblende, and biotite, occasionallv quartz. 

 Hornblende and biotite were about equal in amount, sometimes one predominating, 

 sometimes another, and frequently one occurring in a given rock specimen almost 

 to the exclusion of the other. Pyroxene (probably augite) was apparently rela- 

 tively rare. The ferromagnesian minerals as a whole were not abundant, and the 

 rock had a rather siliceous character. The feldspar was typically andesine-oligoclase 

 (as determined in the fresher rock), though some of the feldspars ranged from ortho- 

 clase to labradorite, the basic varieties being more abundant. The feldspar crystals 

 are typically small, slim, and simple (i. e., not compound). Apatite in small crystals 

 is abundant, and zircon is frequent. 



Present altered condition.- In the ordinary altered condition these minerals are 

 often completely transformed. No actual biotite or hornblende has been found 

 in these rocks, although several hundred specimens have been studied micro- 

 scopically. These minerals are represented by their decomposition products 

 quartz, sericite, pyrite, siderite, and hematite, sometimes chlorite and calcite. 



