224 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 

 ALTERATION OF EARLIER ANDESITE FROM NEAR MIZPAH VEIN. 



The alteration of No. 5 may be compared as follows: 



Comparison of Tonopah rock No. 5 with arerage type. 



Here the same processes have been carried on as in rock No. 4, but more 

 thoroughly. As in No. 4, only tiny amounts of the magnesia and the lime are 

 left, while the soda has entirely disappeared. The removal of the more refractory 

 constituents alumina and iron has apparently proceeded farther than in No. 4. 

 Of the iron 80 per cent has been removed, against 70 per cent in No. 4; of the 

 alumina about 11 per cent, as compared with about 7 per cent in No. 4. On the 

 other hand, the silica has increased 17 per cent, as against 14 per cent in No. 4. 

 But the potash, while still showing an increase of 75 per cent over the normal 

 proportion in the type analysis, is somewhat less than in No. 4. It appears from 

 this (in conjunction with the succeeding analyses) that the increased activity of 

 the altering solutions, as indicated in the above figures, has begun to attack 

 some of the introduced potash and to replace it by silica, or, perhaps, rather 

 that the balance is more in favor of strong silicification than of the introduction 

 of potash. 



