1094 A TREATISE ON METAMORPHISM. 



Although Lindgren argues to the contrary with regard to the Sierra 

 Nevada," the suggestion that a part of the gold there has been reduced by 

 ferrous sulphate has . extreme plausibility. The gold associated with the 

 pyrite is native. From that district are given two analyses of the waters of 

 feeding streams (the only analyses reported) entering the lodes at a depth 

 of 125 meters. Both of these analyses show that sulphates and iron are 

 present. 6 According to the analyses the iron is reported as ferric; but 

 apparently no precautions were taken, when the waters were collected, to 

 prevent the oxidation of ferrous to ferric iron. Indeed, the precipitates of 

 yellow material, which is partly ferric oxide, made by underground springs 

 renders it highly probable that ferrous salts were contained in them before 

 oxygen came into contact with the solutions. The clean vein quartz, itself, 

 which holds a large number of fluid inclusions, contains sulphates," showing 

 that sulphate-bearing waters were present at the time the lodes formed. 



The ore shoots have great irregularities in richness, for which Lindgren 

 offers no explanation. The suggestion above made that the gold is precip- 

 itated in the metallic form by the reducing action of ferrous sulphate 

 explains all of these facts. The deposits are rich where the side springs 

 issued from cross fissures and furnished the ascending waters with ferrous 

 sulphate. The gold is in the metallic form because reduced by the ferrous 

 sulphate. 



Organic material is capable of precipitating gold from its solutions. 

 At many places the precipitation of the gold has been ascribed, in part 

 at least, to the influence of the organic matter. Richard d calls atten- 

 tion to the frequent association of metallic gold with sedimentary 

 rocks bearing organic matter in California, New Zealand, Australia, 

 and Tasmania. The most remarkable case is the concentration of gold 

 in veins where they cross strata of carbonaceous shale, called indicators. 

 Says Don/ "Away from the indicator, the greater part of the vein quartz 

 is absolutely barren; but at the intersection with the indicator large 



« Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven- 

 teenth Ann. Kept. U. S. Geol. Survey, pt. 2, 1896, p. 181. See also PI. V, p. 134. 



'; Lindgren, op. cit., pp. 121-123. 



'Lindgren, cit, pp. 130-131, 260,261. 



dRickard, T. A., The origin of the gold-bearing quartz of Bendigo Reefs: Trans. Am. Inst. Min. 

 Eng., vol. 22, 1894, pp. 314-315. Rickard, T. A., The indicator veins of Ballarat, Australia: Eng. 

 and Min. Jour., vol. 60, 1895, pp. 561-562. 



''Don, J. R., The genesis of certain auriferous lodes: Trans. Am. Inst. Min. Eng., vol. 27, 1898, 

 p. 569. 



