1154 A TREATISE ON METAMORPHISM. 



are subordinate amounts of sulphides of the second concentration. It is 

 believed also that the intimately intermingled sulphides at the lower hori- 

 zons in southwestern Missouri and the great disseminated deposits in the 

 massive limestone of southeastern Missouri are results of the first concen- 

 tration. It is thought that a second concentration by descending waters 

 explains through the reactions given the orderly distribution of the ores in 

 a vertical direction, i. e., the oxidized products and the enriched galena 

 above and near the ground water and the sphalerite deposits below the 

 level of ground water. The intermingled sphalerite and galena below the 

 level of ground water, while largely of the first concentration, are modified 

 to a variable extent by the second concentration. Further, it is believed 

 that the second concentration was a determinative factor in the production 

 of many of the rich deposits, especially their upper portions. The process 

 of concentration by descending waters is primarily chemical, but is also to 

 some extent mechanical. The latter is especially true of the galena loosened 

 by solution from the walls and of the oxidized products mixed with galena 

 which have accumulated as residual material in consequence of erosion of 

 the limestone. 



The above conclusions were reached from a study of the facts in the 

 field, without any knowledge of the experimental work in the laboratory 

 showing that iron sulphide precipitates zinc and lead as sulphides from 

 their oxidized salts, and that zinc sulphide precipitates lead as a sulphide 

 from its oxidized salts. It is thus seen that the facts in the field and the 

 experimental work of the laboratorj* supplement each other and strongly 

 confirm the correctness of the conclusions as to the manner in which the 

 vertical distribution of the ores is explained. 



Thus the above theory full}* explains and harmonizes the following: 



(1) The galena and sphalerite crystals are frequently corroded and 

 partly dissolved, and very numerous casts of them are found in the chert 

 and limestone. These facts show conclusively that the sulphides have been 

 dissolved above the level of ground water. 



(2) Upon the calcite, and frequently pseudomorphous after its crystals, 

 smithsonite is found, the calcite at these places showing corrosion and solu- 

 tion. This furnishes strong evidence that the oxidized salt of zinc, probably 

 zinc sulphate, has reacted upon the calcite. 



