248 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



In No. 2 and No. 4 again the close correspondence of the alumina, titanium, 

 and phosphoric acid the last two representing probably, respectively, the resist- 

 ant rutile needles (sagenite) in the biotite, and the apatite indicates that the 

 relative bulk of the rock has not been greatly changed by decomposition. The 

 fact, however, that the percentages of each of these constituents in No. 4 is 

 slightly in excess of those in No. 2 may be taken as indicating that a slight 

 reduction of density has taken place. 



Like rock No. 3, rock No. 4 shows an almost complete loss of soda, and a 

 similar loss of silica, both these processes being carried further than in No. 3. 

 Like No. 3, the lime has not been noticeably affected. Unlike No. 3, most of the 

 potash has been removed, while the iron, which in No. 3 had not been noticeably 

 affected, is here present in quantity certainly largely exceeding the original 

 amount. The writer has computed the totul metallic iron present in the different 

 rocks as follows: No. 1, 3.82 per cent; No. 2, 4.81 per cent; No. 3, 4.24 per cent; 

 No. 4, 8.04 per cent. The magnesia, not noticeably affected in No. 3, is here 

 doubled. Therefore the waters removed soda, potash, and silica, and brought iron 

 and magnesia in partial compensation, the rest of the loss being compensated for 

 by the addition of large amounts of water, carbonic acid, and sulphur. 



Judging from the microscopic analysis, the iron of this rock is chiefly 

 contained in pyrite, siderite, uralite, and chlorite; the magnesia in uralite, chlorite, 

 and talc. The alteration of augite to chlorite or uralite involves a relative increase 

 of magnesia and a decrease of lime. Dana, speaking of uralite pseudomorphs 

 after pyroxene, remarks:" 



"The most prominent change of composition in passing from the original 

 pyroxene is that corresponding to the difference existing between the two species 

 in general; that is, an increase in the magnesium and a decrease in the calcium. 

 The change, therefore, is not strictly a case of paramorphism, though usually so 

 designated." 



Discussing the alteration of feldspar the same writer remarks:* 



"When the waters contain traces of a magnesian salt a bicarbonate or silicate 

 the magnesia may replace the lime or soda, and so lead to a steatitic change or to a 

 talc when the alumina is excluded." 



Dana indexes this "steatitic mineral" as "magnesia aluminate." 

 SIDERITE AS AN ALTERATION PRODUCT. 



The abundance of siderite in the altered later andesite is of some interest, as it 

 has not been often detected among the minerals resulting from hot-spring action/ 

 It is almost always present as a decomposition product of the biotite, pyroxene, 



aSyrtem of Mineralogy, 6th ed., p. 890. l>Op. cit., p. 820. cLlndgren, W., Trans. Am. Inst. Min. Eng., vol. SO, p. 607. 



