236 W. LINDGHEN — CALIFORNIA GOLD-QUARTZ VEINS. 



mass can then sometimes be profitably mined and milled, though it is 

 of very low grade. Veins and seams of quartz are often entirely absent 

 in these impregnated zones. In the cases which have come under my 

 observation the action on the rock is much the same as in the decom- 

 posed wall-rocks of the veins — that is, there is an abundance of carbon- 

 ates and iron-pyrites in sharp edged, little crystals. While there is 

 abundant evidence of replacement by carbonates, I have not yet seen 

 anything proving a replacement by quartz, though the possibility of such 

 a process cannot be denied. However, in these deposits the action of 

 the solution on the rock-forming minerals must have produced much 

 free silica in solution and probably also much sodic silicate ; in fact, there 

 are in these deposits occasional masses of granular, grayish quartz very 

 different from the ordinary vein-quartz and probably partly chalcedonic. 

 This quartz often contains iron-pyrites in small scattered crystals, and 

 appears to represent in part residual masses from leaching, in part depo- 

 sition from the supersaturated silicious waters. H. W. Fairbanks has 

 recently described two deposits in El Dorado county, the Big Canyon 

 and the Shaw mines,* as showing in marked degree a replacement of the 

 rocks by silica. Though the latter mine was not worked during my ex- 

 amination of the Placerville sheet, I have, through the kindness of Mr 

 H. W. Turner, had occasion to examine an excellent suite of specimens, 

 lately collected. The vein is partly in black slate, partly in a feldspathic 

 dike. Both rocks contain an abundance of stringers and seams of quartz 

 and calcite, but I fail to see any evidence of replacement of the wall- 

 rock by the former mineral. On the contrary, the porphyritic dike is to 

 a very marked degree converted into carbonates in the vicinity of the 

 veins. Regarding the Big Canyon mine, I have seen only two specimens 

 of greenstone impregnated by pyrite from this mine, and collected by 

 Mr H. W. Turner. These specimens show carbonatization to a consid- 

 erable extent, but no evidence of replacement by silica. It is not in- 

 tended to deny that such a process may take place, but only to point 

 out that it is something requiring more and more detailed investigation. 

 Calcite is found pseudomorphic after an enormously large number of 

 minerals, while pseudomorphs of quartz after other minerals are much 

 less common. 



Genetic Conclusions. 



The country-rock altered to carbonates, standing in strong contrast to 

 the vein filled nearly exclusively by quartz, affords a much-needed key 

 to the genetic processes of the deposits. It shows, first, that besides 

 silica, the water circulating in the fissures contained large amounts of 



* Twelfth Ann. Rep. State Mineralogist, 1894, pp. 103 and 114. 



