Belations of Trap-Bocks with Ores of Copper. 303 



six thousand inhabitants. The transportation of the ores to 

 the sea was very expensive before the construction of a 

 railroad, which follows the valley of Rio-del-Cobre, and 

 which gave an impulse to the subterranean works. 



The pitons and ridges are composed principally of green- 

 stone and serpentines, whose eruptions have elevated and 

 modified the argillaceous schists of an unknown age. The 

 rocks of contact present very strongly marked metamorphic 

 alterations, which resemble all the appearances of the gabbro, 

 whether green or streaked with red. The specimens brought 

 by M. Arrieta, enable us to trace the changes of the argil- 

 laceous strata into the greenstones and serpentines. The 

 cupriferous repositories, accumulated near Cobre, are en- 

 closed in green slates, the steatitic nature of which indi- 

 cates the vicinity of eruptive rocks, which are very profusely 

 penetrated with crystalline sulphuretted iron. 



The repositories appear to be irregular repositories of con- 

 tact parallel to the upraised strata. The manner in which 

 the pertinencias (concessions) are disposed, indicates from the 

 first this accumulation which often characterises the irregular 

 repositories, for all the principal mines are concentrated with- 

 in a rectangle of 1200 yards by 600, and not disposed in 

 lines, as when we have to do with true veins. 



Among these repositories, that of Isabelita presents forms 

 very definite and expressive. It is a kind of chimney, in short, 

 an " erect mass'^ as it would be called by German miners. It 

 has been mined to the depth of 180 yards. This repository, 

 according to the expression of the miners, sinks like a nail 

 into the ground, and seems to enlarge in depth. 



The vein-stones which filled the repository along with the 

 steatitic matters, which appear to be derived from the in- 

 closing rocks, are crystalline quartz, dolomite, and carbonated 

 lime. These vein-stones {jgangues)^ often pure, form crystal- 

 line magmas^ mingled with copper pyrites, which is the nor- 

 mal ore ; they are often eliminated by the argillaceous gan- 

 gues, which are penetrated with crystals of iron pyrites, 

 rather than with copper pyrites. Lastly, the hydroxides of 

 iron, penetrated with native and oxidised copper, everywhere 



