ALTERATION OF THE LATER ANDESITE. 



247 



to form siderite with the ferrous oxide. There remains a small amount of lime 

 (about 1.35 per cent) which it is difficult to assign to any of the recognized minerals 

 except the zeolites, which therefore may be supposed to be chiefly lime zeolites. 



As there are not present any recognizable colored minerals into which the 

 magnesia has been transferred from its original combination in the pyroxene and 

 the biotite, the magnesia is probably contained in one of the colorless minerals, 

 and the presence of talc in the sericitic aggregate which forms a large part of the 

 feldspar pseudomorphs is indicated, in accordance with the conclusions reached for 

 specimen No. 4 (see p. 240). At the same time the analysis indicates that in this 

 aggregate all or a large part of the original potash in the feldspar is now contained 

 in the form of sericite. 



The sulphur trioxide shown in the analysis of No. 3 is probably contained in 

 gypsum, a mineral abundantly found elsewhere in this altered rock. It appears 

 to result from the action of waters containing sulphuric acid (derived from oxidation 

 of the pyrites) on the calcite. This is a recent process and one distinct from that 

 by which the main alteration was produced. 



The waters which produced this main alteration were, therefore, highly charged 

 with carbonic acid and sulphur; they left these materials, with some water, in 

 exchange for soda and silica, which they carried away. 



ALTERATION OF LATER ANDESITE FROM MONTANA TONOPAH SHAFT. 



The relation which the altered later andesite from the Montana Tonopah shaft 

 (No. 4) bears to the fresh rock (No. 2) may be seen by comparing their respective 



analyses, which follow: 



Anali/ses of later andesite. 





