246 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



ALTERATION OF LATER ANDESITE FROM NORTH STAR SHAFT. 



To compare the completely altered rock No. 3 with No. 2 (which appears to 

 be the freshest of the rocks analyzed, and may he taken as representing nearly 

 the original composition of No. 3, except for the partial oxidation of the iron), the 

 two analyses are given together in the following table: 



Analyses of later andesite. 



It is noticeable that both analyses show the same percentages of titanium, 

 another highly refractory substance, as well as of phosphoric acid. The phosphoric 

 acid is contained in the apatite, which resists decomposition very strongly. This 

 strengthens the belief that these percentages afford a measure of the change of 

 the other constituents. 



Nearly all of the soda has been extracted, and the silica has been somewhat 

 attacked and removed. On the other hand, the magnesia is unchanged, as are 

 probably the lime and potash" and the iron. The loss of bulk of the rock occa- 

 sioned by the removal of the soda and silica is compensated by the addition of large 

 quantities of carbonic acid and sulphur, producing carbonates of lime and iron and 

 sulphide of iron. It will be noticed that most of the remaining iron oxide is in 

 the ferrous condition; this probably is present as siderite. No dark iron or mag- 

 nesian silicates were noted among the decomposition products. The amount of 

 lime present is in excess of the amount required to form calcite with all the 

 carbonic acid in the rock; indeed, a small portion of this carbonic acid is required 



For these conclusions compare not only the foregoing Uible, but also the table on page 219, showing variations ot 

 frth rock of this kind. 



