CHANGES IN MINERALIZING WATERS. 235 



process is continued with diminishing strength. Potash here is thrown down by 

 the waters, and its amount is greater than in the original rock. It might be 

 argued that in these rocks it may be a concentration, and that its percentage 

 increase is only apparent, and is due to its remaining constant while the volume 

 of the rock increases; but the decrease in the similar rocks 7 and 8 shows that 

 this can hardly apply. Again, it may appear that the increased potash in the 

 zone represented by 4, 5, and 6 was extracted from the zone represented by 

 7 and 8, and that the original waters did not necessarily contain much potash; 

 but the formation in the main vein zones of often large proportions of potash 

 minerals bespeaks an original large amount of this element, as noted on a 

 preceding page. 



CHANGES IN WATERS AS A CONSEQUENCE OF ROCK ALTERATION. 



The waters that traversed and altered this broad belt of rock," by the depo- 

 sition of silica and of potash, were themselves affected by the interchange and 

 emerged into the outer zones quite transformed. Rock No. 3 indicates that they 

 had no longer any excess of silica and that their solvent power was much 

 weaker. Still they dissolved part of the lime and magnesia in the rock, as well 

 as the alkalies, particularly potash. The fact that they dissolved potash shows 

 that they no longer contained an excess of this element. Rock No. 2, still 

 farther removed from the center of circulation, shows less change in the bases, 

 the alkalies being practically unaltered. The lime and magnesia have been 

 disturbed, but not to so great an extent as in No. 3. As in No. 3. much of 

 the lime has been extracted (though not so much as in No. 3), but while in No. 

 3 the magnesia also has been extracted, this constituent is relatively increased 

 in No. 4, and largely compensates for the loss of lime. Here, then, the 

 waters replaced some lime by magnesia and abstracted another part. The analysis 

 also indicates that some silica was abstracted. By this time, therefore, the waters 

 had so effectually precipitated the great excess of silica indicated by their first 

 effects (as, for example, in No. 8) that they were now able to take up fresh 

 silica from the rocks which they traversed instead of precipitating it. The 

 presence of carbonic acid and of sulphur is indicated by the pyrite and by the 

 analysis. The carbonic acid, though undoubtedly active as an agent in the altering 

 processes, was in the more highly altered types so hard pressed by the more 

 urgent silicification that it was free to form very little carbonate; but on the 



aOn account of the small area of outcropping earlier andesite at Tonopah, the dimensions of these zones, such as 

 the zone ol siliciflcation, can not be given They are probably variable. The earlier andesite outcrops within the 

 limit of the map only on Mizpah Hill and Gold Hill, covering a maximum east-west extent of over 2.000 feet. Several 

 veins ouicrop in this distance, principally on Mizpah Hill. Nearly all of this andesite is siliciried in varying degrees, 

 the less altered specimens coming principally from underground workings in areas where the andesite does not 

 outcrop. 



