LEAD AND ZINC ORES. 1157 



(3) With the oxidized products of lead and zinc are associated barite 

 and gypsum. These are the by-products which should be formed by the 

 reactions of the sulphates of lead and zinc upon the alkaline earth carbonates. 



(4) At and near the level of ground water are very large crystals of 

 galena as compared with those deeper in the deposits. These crystals are 

 fully explained by secondary growths of galena deposited by descending 

 waters. 



(5) The products have a vertical order; the oxides, carbonates, and 

 silicates lie mainly above the level of ground water, the galena largely 

 above but extending below it, and the sulphide of zinc with the iron sul- 

 phides mainly below it. This vertical order is that which should be pro- 

 duced by the reactions of the descending oxidized salts of lead and zinc 

 on the sulphides of zinc and iron. 



(6) Superimposed upon the crustified sulphides of the first concentra- 

 tion are later crystals of galena and sphalerite, often different in character 

 from the earlier sulphides. For instance, in Wisconsin the later galena is 

 usually in octahedral crystals, and in Missouri the sphalerite is pure and 

 ruby colored. (See PL XII, B.) The exceptional appearances of these 

 products are fully explained by the reactions between the oxidized products 

 of lead and zinc and the sulphides deposited by the first concentration. 



While the precipitation at the second concentration of downward-moving 

 sulphates is partly accomplished, as has been explained, by the reaction 

 upon them of the low-grade sulphurets, thus producing- rich sulphides, the 

 reduction and precipitation of these oxidized products is partly accomplished 

 by the direct and indirect action of the organic matter, precisely as in the 

 case of the original precipitation. The power which organic matter has for 

 the reduction and precipitation of the oxidized products has been recognized 

 by several others who have written upon the lead and zinc of the Missis- 

 sippi Valley. Among these are Chamberlin," Blake, 6 Bain, c and Grant. d 



"Chamberlin, T. C, The ore deposits of southwestern Wisconsin: Geol. of Wisconsin, vol. 4, 1882 

 pp. 544, 546. 



>> Blake, Wm. P., Lead and zinc deposits of the Mississippi Valley: Trans. Am. Inst. Min. Eng., 

 vol. 22, 1894, pp. 630-631. Also, Wisconsin lead and zinc deposits: Bull. Geol. Soc. America, vol. 5. 

 1894, pp. 28-29. 



"Bain, H. F., with C. B. Van Hise and G. I. Adams, Preliminary report on the lead and zinc 

 deposits of the Ozark region: Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2, 1901, p. 213. 



'i Grant, IT. S., Preliminary report on the lead and zinc deposits of southwestern AVisconsin: Bull. 

 Wis. Geol. and Nat. Hist. Survey, No. 9, 1903, p. 83. 



