220 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



the same; that is, the transformation of pyroxene, hornblende, and mica in the 

 Tonopah rock largely to calcite, chlorite, muscovite, and hematite was effected 

 without appreciable gain or loss of the bases, but some of the silica was abstracted, 

 its place being taken by water and carbonic acid, which entered into the decompo- 

 sition products mentioned." Therefore, since these waters abstracted instead of 

 precipitating silica, they were characterized by relative poverty in silica. They 

 were also carbonated. The lack of sulphur and sulphides in rock No. 1 also shows 

 the absence of sulphur combinations in the altering waters. 



ALTERATION OF EARLIER ANDESITE FROM CALIFORNIA AND TONOPAH SHAFT. 



Tonopah rock No. 2 and the average type may also be compared as to their 

 chief constituents: 



Comparison of Tonapah rock No. 2 with average type. 



The difference between the totals of these two analyses is again accounted 

 for mostly by the difference in titanium, the percentage of which in the Tonopah 

 rock is 0.77 and in the average rock 0.23. When these are added the total of the 

 Tonopah analysis is 99.85 and that of the average analysis 99.82. 



The bases present correspond very closely, the only noticeable difference being 

 in the proportions of lime and magnesia. In the average type the percentage of 

 lime is three times as much, or 2.78 more, and that of magnesia is somewhat 

 more than half as much, or 1.64 less. If the lime and magnesia in each rock 

 are added together the percentage of these constituents is only 1.12 greater in 



"It 1 only the water given off above 100 C. (H z O-(- in the analyses, p. 27) which can be considered chemically 

 combined. The rest(HaO-) is probably mainly hygroscopic, mechanically contained. In the average analysis com- 

 pared, aa well as the Washoe und Eureka analyses, however, this distinction is not made. Therefore all the water in 

 the Tonopah rock* Is considered together in comparing with these analyses, and the hygroscopic water in one is -up 

 posed to be offset by that in the other. Most of the water in the Tonopah analyses, it will be seen, is chemically 

 combined. 



