192 C. W. MAKSH. GEOLOGICAL NOTES 



This vein, like most other metallic formations on the Earrier, 

 has suffered superficial alteration, the intensity of which has 

 apparently been proportionate to the metallic and other salts 

 acted upon, the siderite having been changed to limonite, or 

 rarely to hematite, either being more or less manganic. The 

 greater portion of the lead appears to have been removed to 

 lower levels, while a part of its originally combined silver has 

 remained concentrated as chlorides in small cavities in the masses 

 of limonite, where it has evidently been protected from disturbing 

 agencies. Quartz, though largely represented along the course 

 of the surface outcrop, is rarely met with in the lower levels, 

 where it has apparently been replaced by siderite and probably 

 by galena and zinc-blende. This change has probably been effected 

 by the actions of carbonic and other acids, set free during the 

 superficial alteration of the iron, and the exchange of bases 

 between earthy and metallic salts in the presence of organic 

 matter from vegetable decay. The removal of metallic salts from 

 the upper rocks, by the solvent actions of descending solutions, 

 is evidently still in active progress. The lead and zinc ores, 

 affected by the periodical movements of the foot-wall rocks, have 

 their fractured surfaces frosted over by lead carbonate, native 

 silver, or a combination of silver with antimony or arsenic. At 

 other times fine irregular layers of antimonial galena, with 

 graphite, and sometimes siderite or zinc, often not thicker than 

 sheets of paper, and variously arranged, have been the cementing 

 medium. The most suitable conditions for this process would 

 probably exist at or above water level, where the concentration 

 of lixiviating solutions from the surrounding rocks to the fissure, 

 would be constantly taking place. Below the water level the free 

 circulation would not only be impeded, but the dissolved salts 

 might be diffused laterally through the rocks by their permeating 

 waters. Furthermore, the drainage of the rocks, by the deeper 

 workings of the mine, is lowering the water level from 50 to 100 

 feet per year, thus throwing open fresh ground for downward 

 lixiviation, while the atmospheric oxygen, admitted by such deeper 

 workings, may be playing an important part in the arrangement 

 and distribution of the various salts set free from solution. 



The carbonaceous matter occurring in these rocks appears to 

 have been deposited simultaneously with the sediments, periods 

 during its deposition being marked by strata richer or poorer in 

 carbon. These deposits may have taken place on the ocean's 

 bed, or in land-enclosed basins, most probably the latter ; in 

 either case the metals present in the waters at the time appear 

 to have been eliminated and fixed by the decomposition of the 

 organic matter in the sediments. 



