204 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
doAvmward-inoving waters containing oxygen, the ores became oxidized 
to sulphates. The sulphate of lead (anglesite) underwent further 
alteration to the carbonate (cerussite). The sulphate of zinc, react- 
ing with the limestone or with the secondary chert which contained 
the ores, replaced the former with smithsonite and the latter, in part 
at least, with calamine, which also filled small cavities in the rocks. 
As the galena is less easily oxidized than the sphalerite, and its oxida- 
tion products are less soluble, the oxidized lead products are gener- 
ally found nearer the surface than those of zinc. The oxidized zinc 
ores extend generally from the surface, or close to it, downward to a 
short distance below ground-water level. In all observed cases the 
oxidized products have been deposited close to the place of original 
sulphide concentration, and are associated with secondary chert 
leached of its formerly contained sphalerite — the honeycomb rock of 
the miners. 
During the process of oxidation, carbonation, or silicification of the 
ores some of the products of oxidation of the sulphides were carried 
in solution below the level of underground water, where the} 7 were 
redeposited as sulphides. Gradually, also, the oxidation products 
deposited above ground-water level were taken into solution by sur- 
face waters and carried downward and redeposited in the same way. 
These sulphides, together with those of primary concentration, form 
an enriched /one, which is greatest not far below the level of under- 
ground water, and decreases downward. In the Joplin district the 
enrichment of the sulphide ores has resulted not in a better grade of 
the ore already existing, bu1 in an increase in the quantity of the ore 
of 1 he enriched /one. The lead sulphide is deposited at the highest 
levels, together with more or less sphalerite, and decreases in amount 
downward, while the sphalerite increases, so that in depth the latter 
dominates. Pyrite and marcasite are relatively unimportant in the 
Joplin district, and il can not be stated definitely that, they are more 
abundant, on the whole, at one level than at another. 
Where ores of secondary enrichment have been brought near the 
surface by the wearing down of the land through erosion, they would 
be acted on in the same way as ores of primary concentration, and 
would be again concentrated below ground-water level. As the 
Joplin district, however, has been so little eroded since the removal 
of the Coal Measure shales, it is believed that this concentration of 
enriched ores is of little importance. 
