ALTERATION OF THE EARLIER ANDESITE. 



219 



A similar conclusion is reached by comparing the analysis of the Tonopah 

 rock with analyses of Eureka and Washoe andesites. For the purpose of com- 

 parison, the following table is presented: 



Comparison of Tonopah unth Waihoe and Eureka rocks. 



oTheae are the designations (riven by Hague, Mon. U. S. Geol. Survey, vol. 20, p. 282. The designations previously 

 given by Becker, Mon. U. S. Geol. Survey, vol. 3, p. 152, are 3 and 4, later hornblende-andesite; 5, mica-diorite. 

 6 Mon. U. S. Geol. Survey, vol. 20, p. 264. 



The difference between the sums of the first two analyses is largely accounted 

 for by the difference in titanium, of which the Tonopah rock contains 0.72 per 

 cent and the average rock 0.23 per cent. When these are added the sums are 

 99.58 and 99.82 respectively. 



It will be seen that there is a remarkable similarity in the amounts of the 

 bases present in the first two analyses. In the Tonopah rock more of the iron 

 is in the ferric condition, but the amounts of iron in the two rocks are almost 

 identical. 



In the altered Tonopah rock the percentage of silica is about 6 less than in the 

 average type, while that of water is 2f greater. The Tonopah rock contains 2f per 

 cent of carbonic acid, which is lacking in the average type. Thus the increase of 

 6 in the percentage of water and carbonic acid in the Tonopah rock offsets the 

 increase of 64 in the percentage of silica in the average type. Since free primary 

 quartz is apparently rare in all these rocks," the silica is combined with the bases to 

 form the silicates, feldspar, hornblende, and mica; and since the amounts of the 

 bases are equal in both analyses, the original amount of silica was probably nearly 



a For the Eureka type see Arnold Hague, Mon. U. S. Geol. Survey, vol. 20, p. 234. 



