

XII. 



I 



GEOLOGICAL RELATIONS OF THE VARIOUS COPPER 

 DEPOSITS OF LAKE SUPERIOR. 



THE general distribution of the different copper ores in the region 

 of Lake Superior, presents some facts which seem to me to have a 

 direct bearing upon the theory of their origin. It is a very remark- 

 able circumstance that the largest masses of native copper should 

 occur upon Point Keewenaw, and that the non-metallic ores should 

 be diffused at various distances from the central region where the 

 largest masses of native metallic copper occur. The various sul- 

 phurets and carbonates are found on the northern shores and about 

 Lake Huron, in far greater proportion, and over a wider extent, than 

 anyAvhcre nearer the metallic centre. The black oxide itself is 

 found beyond the limits of the large metallic masses, and nearer to 

 them than the other ores. I cannot help thinking that this particu- 

 lar distribution has direct reference to the manner in which these 

 various copper ores were diffused in the country where they occur. 

 They seem to me clearly to indicate that the native copper is all 

 plutonic ; that its larger masses were thrown up in a melted state ; 

 and that from the main fissure through which they have found their 

 way, they spread in smaller injections at considerable distances ; but 

 upon the larger masses in the central focus, the surrounding rocks 

 could have little influence. New chemical combinations could hardly 

 be formed between so compact masses, presenting, in comparison 

 with their bulk, a small surface for contact with other mineral sub- 

 stances capable of being chemically combined with the copper. But 

 where, at a distance, the mass was diffused in smaller proportions into 



