ALTERATION OF THE EARLIER ANDESITE. 287 



With this last conclusion the writer is in accord, for the Tonopah district seems to 

 show clearly that the distinctions between the two classes of veins are artificial, the 

 predominating alteration of the wall rock, whether to sericite and quartz, or to 

 chlorite, calcite, etc., depending not so much upon the original character of the 

 wall rock or the waters, as upon the abundance and intensity of the latter, and on the 

 size of the circulation channels; and in each case the vein materials may be the same. 

 The writer has already pointed out the close analogy of the Comstock and some 

 other districts to the Tonopah district; in some of the districts the one phase of 

 alteration is especially represented, in others the opposite extreme. 



FINAL, COMPOSITION OF MINERALIZING WATERS. 



The waters which accomplished the "propylitic" alteration of Nos. 1 and 2, 

 therefore, were capable by virtue of their carbonic acid, etc., of decomposing the 

 original minerals and forming new carbonated and hydrated minerals which were 

 more stable under the new conditions. They were not able to remove any large 

 quantities of the bases, with the exception of a slight amount of lime, magnesia, 

 and silica, and of the alkalies. The character of such waters would then be very 

 different from what it was when they were fresh from their channels of active 

 circulation. They were at first, if the reasoning is correct, highly charged with 

 silica and potash, with some carbonic acid and sulphur, and with silver and 

 gold and relatively small quantities of other metals. They would finally, as a 

 result of their interchange with the rocks which they have so profoundly altered, 

 be less highly charged with mineral substances and would contain soda largely in 

 excess of potash, important amounts of lime and magnesia, some iron, a little silica, 

 and a very little alumina; and at the best only traces of the rarer metals. The wall 

 rock in fact has, by its reactions with the mineralizing solutions, acted as a screen, 

 and has separated successively the different constituents from the waters. Similar 

 phenomena have been previously observed, and a chemico-physical explanation (the 

 hypothesis of osmotic action) has been offered." Dr. G. F. Becker remarks: 



"On this hypothesis the concentration of ores in deposits would be largely due 

 to the fact of the lack of action between their solutions and the wall rocks; and the 

 decomposition of the country rock, so often observed near veins, would be due to 

 the absorption of solutions of gangue minerals by the walls. In short, there would 

 be a species of concentration by dialysis." 6 



The writer's explanation, however, as indicated above, is of a purely chemical 

 character. He assumes that the ores of the veins did not penetrate far into 

 the wall rocks because they were all immediately precipitated in the main cir- 



oBecker, G. P., Mineral Resources U. S. for 1892, D. S. Geol. Survey, p. 166; Eighteenth Ann. Kept. U. S. Geol. Survey, 

 pt. 3, p. 68. Lindgren, W., Trans. Am. Inst. Mln. Eng., vol. 30, p. 691. 

 i> Mineral Resources U. S. for 1892, U. S. Geol. Survey, p. 157. 



