462 PROCEEDINGS or THE GEOLOGICAL SOCIETY. [-A-pr. Hj 



Portage Lake carry masses and sheets of copper analogous to, but 

 much smaller than, the masses of th€ northern mines. 



3. The amygdaloids adjoining the bearing portions of the veins are 

 often found to contain considerable quantities of copper for some 

 distance from either wall. 



The last of the above reasons may no doubt be made available in 

 support of the opposite hypothesis of the permeation of the cellular 

 rock by materials introduced from the lodes ; but in order to prove 

 this, it would be necessary to show that the rock was barren, except 

 within the distances explored. This is, however, by no means cer- 

 tain, as the miner stops at the limit of profitable working without 

 carrying on systematic researches along the strike of the bed, as is 

 done in the Portage district. 



Bischolf * showed, as far back as the year 1825, that copper may 

 be deposited from solution in a massive condition in a compara- 

 tively short time, by the action of organic matter upon solutions 

 containing sulphate of copper partly in the state of a salt of the sub- 

 oxide. This was observed at Linz, on the Ehine, where a solid mal- 

 leable mass of copper, weighing 2| lbs., was deposited in a wooden 

 vat, used in the concentration of blue vitriol leys obtained from the 

 lixiviation of poor copper ores after roasting. 



Where the amygdaloids are compact, and tolerably free from 

 cracks and joints, the metallic kernels have undergone no change, 

 and appear as clean brilliant masses on a freshly fractured surface. 

 It is different, however, at the outcrops, where the cavities are 

 usually empty, and their former contents, converted into malachite, 

 have been absorbed by the crystalline base of the rock, which is 

 stained green for a considerable distance round. Similarly in the 

 conglomerates atmospheric agencies have been largely at work, pro- 

 ducing malachite, red copper ores, ehrysocoUa, and similar secondary 

 products ; and another instance of the same kin-d may be adduced in 

 the old vein in the sandstones of Copper Harbour, which yielded 

 melaconite and chrysocolla in considerable quantities, but not 

 metallic copper. The mines, as a rule, are very dry, the deepest 

 requiring only a small amount of pumping-machinery of no great 

 power, and that only employed at intervals, in order to keep the 

 workings free from water. This comparative impermeability of the 

 rock is probably the cause that has preserved its metallic contents 

 with such a small amount of change, as it is well known that metallic 

 copper is rapidly oxidized when exposed to ordinary atmospheric 

 vicissitudes. This is well seen in the " floats," or small masses, 

 that have been removed from the rock by denudation, and are found 

 in the drift covering the backs of the lodes. These are often in- 

 crusted with a coating of earthy malachite nearly half an inch in 

 thickness. 



Perhaps the most interesting fact in connexion with the mineralogy 

 of the Lake Superior mines is the prominent occurrence of orthoclase 

 among the newest minerals in the lodes, and in succession to zeolites ; 



* Pogg. Ann. iii. p. 195. 



