1 148 A TREATISE ON METAMORPHISM. 



In the Upper Mississippi Valley chert is not abundant and in that district 

 the chief oxidized zinc ore is smithsonite. In the southwestern district of 

 Missouri chert is very abundant, and the dominant oxidized ore is calamine, 

 although smithsonite is also there present in important amounts. 



The oxidized ores are often very rich and frequently they are concen- 

 trated in large irregular masses. These features, exceptional as compared 

 with the sulphides of the first concentration, are due to two processes : First, 

 through downward transportation by the solutions there may be segregated 

 in a small vertical distance a large part of the materials which had a wider 

 vertical distribution as sulphides; and second, residual concentration takes 

 place — that is, the carbonates and sulphates of lead and zinc are much 

 more insoluble and heavier than the country rock with which they are 

 associatated — the limestone. As the limestone is eroded the oxidized deposits 

 sink down and thus accumulate. 



SULPHIDE OEES. 



Galena. — If it be premised that the ascending waters evenly distributed 

 the sulphides, at least so far as the vertical element is concerned, although 

 across the vein these sulphides may or may not be arranged in a definite 

 order, it is certain that downward-moving waters, combined with denuda- 

 tion, may concentrate the galena at high levels and the sphalerite at lower 

 levels. (See pp. 1144-1145.) 



Galena is the most difficultly oxidizible of the sulphides of lead, zinc, 

 and iron. (See p. 1140-1141.) Moreover, this compound is immeasureably 

 more insoluble than the limestone. By the solution and mechanical erosion 

 of the limestone and by the oxidation and solution of the sphalerite and 

 iron sulphide above the level of ground water, the galena is concentrated. 

 Whitney estimates that to make one-third meter of residual clay in the lead 

 and zinc district of Wisconsin requires 10 to 12 meters of limestone and 

 shale." Since in this district the residual material is often several meters 

 thick, it follows that at and near the surface there may be concentrated as 

 a residual product an amount of galena which was originally distributed 

 through several or many meters of limestone. That these processes of 

 erosion of the limestone and the removal of the sphalerite and marcasite 

 by oxidation have taken place upon an extensive scale is shown by the 



o Hall, James, and Whitney, J. D., Geology of Wisconsin; vol. 1, 1862, pp. 121-125. 



