ALTERATION OF THK EARLIER ANDESITE. 



221 



the average type. This change is probably due to the alteration of the horn- 

 blende to chlorite, the lime being in part carried out of the specimen instead of 

 being entirely precipitated in place as carbonate, its place being taken by 

 magnesia. These changes are, however, mainly compensating, and probably 

 indicate a local rather than a widespread interchange. Apart from this the 

 correspondence of the bases is close. In the average fresh type, however, the 

 percentage of silica is 3.69 greater than in the Tonopah rock, and that of 

 water is 3.80 less, while the Tonopah rock contains 0.52 per cent carbonic acid. 

 The conclusion is the same as in comparing the first Tonopah rock, that in this 

 second specimen there is an increase of over 4 in the percentage of water and 

 carbonic acid, which has entered into the composition of chlorite and calcite, 

 while this gain has been compensated by a decrease of 3.69 in the percentage 

 of silica. The process of alteration, while not quite so far advanced, is similar 

 to that in rock No. 1, except that the lime has been abstracted and com- 

 pensated for by an increase in magnesia. The presence of sulphur in the 

 waters is indicated by the relatively small amount of iron oxide which has been 

 changed to pyrite, a change which did not take place in rock No. 1. The carbonic 

 acid present is only a fifth of that in rock No. 1, showing that in the case of rock 

 No. 2 the conditions were favorable to the acid acting as a solvent and trans- 

 porting the lime from the rock, rather than as a precipitant and entering into 

 the rock's composition. A poverty in lime in the circulating waters is the 

 apparent explanation. 



ALTERATION OF EARLIER ANUKSITE FROM FRACTION NO. 2 SHAFT. 



The comparison between rock No. 3 and the average fresh type may be 



made as follows: 



Comparison of Tonapah rock No. 3 with average type. 



