MUSEUM OF COMPAKATIVE ZOOLOGY. 



89 



various Copper Deposits of Lake Superior," * wrote concerning the copper 

 ores : ** 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 tlieit 

 way, they spread in smaller injections to 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 miuci'al substances 

 capable of being chemically combined with the co[)per. But where, 

 .at a distance, the mass was diffused in smaller proportions into innumer- 

 able minute fissures, and thus presented a comparatively large surface 

 of contact with the surrounding rocks, there the most diversified com- 

 binations could be formed, and thus the various ores appear in this char- 

 acteristic distribution. The relations which these ores bear to the rocks 

 in which they are contained, sustain fully this view, and even the cir- 

 cumstance that the black oxide is found in the vicinity of the mam 

 masses, when the sulphurets and carbonates occur at greater distances 

 from them, would show that this ore is the iTsuIt of the oxidation of 

 some portion of the large metallic masses exposed more directly to the 

 influence of oxygen in the process of cooling. Indeed, the phenomena 

 respecting the distribution of the copper about Lake Superior, in all 

 their natural relations, answer so fully to this view, that the whole pro- 

 cess might easily be reproduced artificially on a small scale ; and it ap- 

 pears strange to me that so many doubts can still be expressed respecting 

 the origin of the copper about Lake Superior, and that this great feature 

 of the distribution of its various ores should have been so totally over- 

 looked." 



Prof. J. D. Dana remarked : " The copper occurs in trap or sandstone, 

 near the junction of these two rocks, and has probably been produced 

 through the reduction of copper ores by the heat of the trap when first 

 thrown up." f This view is retained in his later editions of the same 

 M- o rl^ . t 



Dr. C. T. Jackson later advocated his former view that the sandstone 

 of Keweenaw Toint was of the same age as the New Red sandstone of 

 Europe, but in addition he claimed that this sandstone (New Eed) was 



*■ Lake Supciior : its Physical Character, Vegetation, 

 March, 1850,) pj). 427, ^28. 



t Syatcm of Mineralogy, 3d ed., May, 1850, p. 508. 

 t 4th ed,, 1854, Part II. p. 17 ; 6th ed., 1868, p. 15. 



and Animals, ( Boston, 



