238 . GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



dilation channels, just as the excessive silica did not penetrate to the "pro- 

 pylite" belt of the andesite because it was precipitated before it arrived there. 

 The evidence, elsewhere offered, that the veins themselves have formed chiefly 

 by replacement is plainly in favor of the writer's explanation. 



If such changes take place within a space of a few hundred yards, more or 

 less, laterally from main circulation channels, they must take place also along 

 those channels upward (though they would require a much greater distance), for 

 such veins as those at Tonopah, where the channels were for the most part not 

 open fissures, but only zones of maximum fracturing in the rock, and the vein 

 formation involved intense replacement and interchange. When the waters which 

 accomplished this change emerged above they would be in the transformed condi- 

 tion described for the lateral moving waters emerging from the propylitic stage of 

 alteration that is, they would resemble the waters of many hot springs, or the hot 

 mine waters of the Comstock (see p. 212). It is not necessarily true that springs, 

 even hot springs, associated with mineral deposits have a composition similar to 

 that of the mineralizing waters. As the mineralized area is eroded the critical 

 area for mineralization will in many cases probably retreat lower down, and the 

 same interchange between water and rock will be effected at a lower level. When 

 such water reaches the surface, after flowing through and being again to some 

 degree affected by the ores and the altered rock (which were stable under the 

 conditions of original deposition, but now under different conditions are subject 

 to solution and redeposition), it will still contain the solutions resulting from the 

 mineralizing reactions, rather than those which accomplished the mineralization. 

 This may perhaps explain in part why, although the formation of veins by hot 

 springs has in man}' cases been pretty satisfactorily demonstrated, and many such 

 springs emerge at the surface at the boiling point or over, no satisfactory observa- 

 tion has as yet been made of such a spring depositing near its exit a definite and 



typical vein. 



AT/TERATION OF THE LATER ANDESITE. 



The later andesite is not altered as much as the earlier andesite; it outcrops 

 over a much greater area, and is often found nearly fresh, save for the processes of 

 surface weathering, under which it disintegrates and decomposes easily. At many 

 places, both at the surface or underground, it is greatly decomposed. This alteration 

 is extremely irregular. 



STUDY OF TYPICAL SPECIMENS. 



Four analyses have been made to show the composition and alteration of the 

 later andesite. The rocks analyzed are described as follows: 



1. Nearly fresh later andesite (225) from Mizpah Extension xhaft, 2^5 feet 

 down. Rock nearly black, dense, and basaltic looking. A very dark green dense 



