1866.] BAUERMAN COPPER-MINES OF MICHIGAN. 461 



of the rock and the presence of organic matter, which prevents oxi- 

 dation. In coarser and more easily permeable rocks, however, not 

 containing organic matter, it is easy to see how the finely divided 

 sulphide would be readily oxidized by infiltrated atmospheric water, 

 giving rise to sulphates, carbonates, and other oxidized minerals — a 

 condition which is exemplified by the cupriferous Triassic Sandstones 

 of Cheshire, which contain only oxidized compounds of copper. 



The size of the accumulated masses of metal appears to be mainly 

 dependent upon the size of the cavities in which they are deposited, 

 whether in the amygdaloids or in the main fissures ; and their absence 

 in the compact traps is probably only due to the non- occurrence of 

 such cavities. In almost all cases the introduction of the metal has 

 been preceded by the deposit of minerals produced from the decom- 

 position of the rock, such as quartz, calcite, chlorite, and zeolite ; and 

 in the larger cavities it is often followed by transparent crystals of 

 calcite, which are formed over branching masses of copper, or even 

 show signs of simultaneous deposition, being filled with fire-spangles 

 of metal arranged parallel to the diagonal striations or lines of growth 

 on the rhombohedra. Similar alternations in the formation of 

 zeolites, more particularly analcime, have been described by Whitney. 



Bischoff* has shown that hydrated silicates of copper, both 

 artificially prepared and the natural mineral dioptase, are sensibly 

 soluble in pure water, but much more readily so when carbonic acid 

 is present, the solution in the latter case being attended with a par- 

 tial decomposition and separation of silica. A reaction of this kind 

 is suggested by the occurrence of chalcedonic and quartzose kernels 

 in the amygdaloids of the higher portion of the trappean series, while 

 copper is found lower down. As regards the reduction of the metal 

 from solution, it is probable that the chief agents have been sub- 

 stances containing protoxide of iron derived from the decomposition 

 of the trap itself. Professor Andrews, of Belfastf , has suggested a 

 more potent reducing-medium in the presence of metallic iron in 

 certain varieties of basalt in Ireland, and other crystalline rocks 

 from other localities ; but it does not appear to be very likely that 

 such an agency can have been at work on Lake Superior, as it is 

 difficult to suppose that an eminently oxidizable substance like finely 

 divided iron could have remained unaltered after the changes pro- 

 duced by infiltration of water had once been set up ; and these 

 changes appear to have preceded the deposition of the copper. 



A point of considerable interest, but which we have at present no 

 means of determining precisely, is, whether the copper in the fissure- 

 veins is of the same age, newer, or older than that of the amygda- 

 loids of Portage Lake. I am inclined to think that the latter are 

 the older class of deposits, and that they have served as feeders for 

 the fissure-veins, for the following reasons: — 



1. The lodes carry more copper between amygdaloid walls than 

 they do when encased in compact trap. 



2. The transverse joints and fissures of the amygdaloids on 



* Vol. ii. pt. o. p. 1887. t Brit. Assoc. Report. 



