ROCK ALTERATION. 



225 



ALTERATION OF TYPICAL EARLIER ANDESITE FROM MIZPAH HILL. 



The relations of No. 6 are as follows: 



Comparison of Tonopah rock No. 6 with arerage type. 



This shows the characteristic alteration of No. 5. with some further advances. 

 As in Nos. 4 and 5, the magnesia, lime, and soda are almost entirely eliminated. 

 The alumina is further reduced than in No. 5, 14 per cent of it having apparently 

 been abstracted, while the iron is slightly stronger. The decrease of the 

 excessive potash to make room for the increasing silica noted in No. 5 is here 

 carried further, No. 6 containing 0.92 per cent less potash than No. 5, and 0.52 per 

 cent more silica (in proportion of the whole rock composition). 



ALTERATION OF EARLIER ANDESITE FROM WALL OF MIZPAH VEIN. 



No. 7 may be compared as follows: 



Comparison of Tonopah rock No. 7 with average type. 



16843 No. 4205 15 



