1158 A TREATISE ON METAMORPHISM. 



Chamberlin states that organic matter from the surface may have made its 

 way down through the openings of the limestone, and thus assisted in the 

 reduction of the descending oxidized products." As pointed out by Blake, 

 the effect of the organic matter is especially clear in the case of the large 

 sphalerite deposits of Wisconsin which rest upon the oil rock at the base of 

 the Galena. 



While the above statement concerning concentrations of the lead and 

 zinc ores by ascending and descending waters combined is made with 

 reference to the Mississippi Valle}~, it is believed that many of the lead aud 

 zinc districts of other parts of the world have had a substantially similar 

 history. 



ASSOCIATION OF COPPER AND IRON COMPOUNDS. 



Another very general association of metals is that of copper and iron. 

 Where this association occurs it is well known that above the level of 

 ground water metallic copper, cuprite (Cu 2 0), tenorite (CuO), malachite 

 (CuC0 3 .Cu(OH) 2 ), and azurite (2CuC0 3 .Cu(OH) 2 ) are very frequently 

 found. 



These compounds may be produced from any of the sulphides. It is 

 not worth while to attempt to write out the reactions showing their forma- 

 tion from all of the sulphides, but to make the principles clearer it may be 

 well to consider one illustrative case. For this purpose chalcocite is taken, 

 since this is the richest sulphide and the one which, as shown below, is 

 likely to be developed in the upper part of the belt of cementation and pass 

 in large quantity into the belt of weathering. By simple oxidation of the 

 sulphur of chalcocite, metallic copper is produced, according to the follow- 

 ing reaction: 



Cu 2 S+205=2Cu+S0 2 



Simultaneously, or later, the metallic copper thus formed may be oxidized 

 to cuprite, according to the equation: 



2Cu+0=Cu 2 



By further oxidation the cuprite may pass into tenorite, according to 

 the reaction: 



0u,O+O=2CuO 



"Chamberlin, cit., pp. 544-545. 



