226 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



This is an intensification of the alteration shown in the immediately preceding 

 analyses. The lime, magnesia, and soda are reduced to trifling quantities. The 

 refractory alumina and iron are further reduced than before, 22 per cent of the 

 alumina and 85 per cent of the iron having been removed. The substitution of 

 silica for potash (as well as the other elements) has made marked progress, the 

 percentage of potash being 1.91 less than in No. 6, and that of silica being 2.75 

 per cent more. In this way the excessive potash, caused in some of the preceding 

 cases by introduction of this element by the circulating waters, is here again 

 brought down to the original quantity. 



ALTERATION OF EARLIER ANDESITE TO VEIN MATERIAL. 



Rock No. 8 may be compared as follows: 



Comparison of Tonopah rock No. 8 with average type. 



This shows the further continuation of the changes indicated in the preceding 

 analyses, the alumina and potash being gradually removed to make way for the 

 increasing silica. 



MAXIMUM ALTERATION LOCATED ALONG THE VEIN ZONES. 



The specimens thus examined, selected as being fairly well representative, 

 show an increasing intensity of alteration, beginning with only a slight modifi- 

 cation of the constituents of the decomposed rock and terminating with the 

 intense silicification which reaches its maximum in the quartz mineral-bearing 

 vein of the district. Considering the alteration from the standpoint of the altering 

 waters rather than the altered rocks, the order in this transition series is reversed, 

 for these changes have been brought about by solutions which circulated along 

 the fracture zones which are now largely transformed into veins, and penetrated 

 the adjoining rock so thoroughly that no even moderately fresh representative 



