478 NOTES ON SOME OXIDISED COPPER ORBS. 



all the outcrops are distinctly associated on the one hand with 

 the garnet intrusions, and on the other with the channels of the 

 springs and with the tufaceous and sinter-like deposits already 

 referred to, it will probably be found that the copper has its 

 origin in the same deep-seated locus as the erupted garnet-rock 

 — that in fact the garnet intrusions opened the way for the out- 

 flow of the cupreous solutions, and at the same time originated 

 the cavities for the after-reception of the existing deposits. At 

 first the metals were present exclusively as sulphides, but the 

 sulphides have been gradually oxidized, the oxides partly con- 

 verted into carbonates — and still later, part of the carbonates 

 have been converted into silicates, or at least, highly silicified. 

 There may have been other operations in the cycle which I have 

 not been able so far to detect, but those mentioned seem to me 

 to be quite distinctly proved. 



List of oxidized copper-ores from the Torreon Mines, 

 Mexico, deposited in the Museum of the Koyal Institution of 

 Cornwall, in illustration of the foregoing paper. 



No. 1. Unchanged chalcopyrite from El Promontorio. 



No. 2. Red oxide of copper still enclosing kernels of chal- 

 copyrite and coated with crystallized malachite, from La Prieta. 



No. 3. Red oxide of copper enclosing kernels of grey 

 copper-ore (chalcocite) and coated with malachite, from El 

 Promontorio. 



No. 4. Red oxide of copper in form of and replacing chal- 

 copyrite, mingled with malachite and blue chrysocoUa — the 

 whole a good deal silicified, from San Francisco. 



No. 5. Oxydized iron pyrites in garnet-rock with a little 

 cuprite and malachite, from El Promontorio. 



No. 6. Crystallized malachite, enclosing oxidized crystals 

 of iron pyrites, from San Francisco. 



No. 7. Mixture of oxides of iron and copper with garnet- 

 rock and calcite, with a little carbonate of copper, from El 

 Promontorio. 



