ORE DEPOSITS 5 



down liy meteoric witters to be precipitated by the chemical 

 action of the underlying sulphides, or the constituents of the 

 wall rock, or the effect of ascending solutions. 



The surface illustrations are represented in the phenomena, 

 known as gossan, or the eisener hut of the German, or the chapeau 

 defer of the French. This is well illustrated in the case of copper 

 mines where the sulphide of copper has been dissolved and oxi- 

 di/ed to a sulphate and reprecipitated at lower altitudes enriching 

 the zone. The contact between the oxidized and the unoxi- 

 di/ed portions of the ore is the richest part of the entire vein. 

 The iron sulphide becomes converted into an oxide or hydrous 

 oxide, the reddish or yellowish-brown mineral so often seen at the 



FIG. 1. Superficial alteration of a contact deposit. A, limestone; G, 



granite; 0, ore body. 



surface. The result of this oxidation and the downward trans- 

 ference of the copper is the enrichment of the mineral vein 

 for some distance. This enrichment varies with the depth and 

 with the lowering of the water level, as erosion brings the upper 

 portion of the vein within the zone of weathering. The down- 

 ward transportation of metals or minerals already in lodes or 

 pockets is the most potent factor determining the upper por- 

 tion of the ore deposit, whose peculiar features are due to the 

 nature of descending currents. It is not infrequent that as- 

 cending and descending waters work simultaneously. The 

 result of this combined effort will give the richest deposits. In 

 such cases a uniform law may be stated; first, an increase in 

 value with depth of the zone of greatest enrichment; second, a 



