8S6 THE GEOLOGY OF MINNESOTA. 



[Copper. 



depth of 265 feet below the surface of the ground, from one-fourth to one-half inch 

 thick. The principal vein of copper was accompanied by several thinner ones which 

 extended in crevices in the ore and also in the enclosing rock. Malachite, azurite and 

 cuprite also existed in small quantities, thought to be secondary results by alteration 

 of the pure copper, indicated by the discovery of an octahedron, which consisted at the 

 centre of copper, surrounded first by cuprite and outwardly by malachite. According 

 to Mr. Eby some copper specimens were as large as six by ten inches, and some 

 were remarkably well crystallized and free from alteration products. Alteration 

 products from the metallic copper extend downward into the ore for several feet, but 

 above it the ore is free from such products. Dr. C. P. Berkey has discussed these 

 minerals and their paragenetic relations in the paper above referred to. 



Remark. This occurrence of metallic copper is in the upper part of the Lower 

 Keewatin and is hence in the oldest rocks known to contain that substance. Its full 



I 



extent is unknown. It may develop into a much larger body as the iron mining 

 proceeds. The position of the metallic mass transverse to the ore body and to the 

 main structures of the country rock indicates that its origin is not coeval with the 

 ore nor with the country rock. In that respect it differs remarkably from the iron 

 ore. Its manner of occurrence is very similar to that of the copper sheets in a heavy 

 diabase layer of the Keweenawan at Fall river near Grand Marais already described 

 (No. 200). Whether its date of origin is as late as that of the Keweenawan copper 

 deposits is an interesting question which there is at present no means of answering 

 satisfactorily. It can only be said that so far as its alliances and structures can be 

 interpreted, they favor the idea of its dating from the same great convulsive epoch. 

 If that be admitted there result some interesting corollaries. 



1. The metallic copper of the Keweenawan was not the result of fusion or 

 reduction of previously existing post-Archean ores. 



2. It did not come to the surface of the earth by reason of igneous eruption. 



3. The causes that gave it origin and location were not primarily seated in the 

 Keweenawan, unless, as suggested by Dr. Berkey, the Keweenawan. formerly existed 

 in the region of Tower. There are some extensive diabase dikes in the region of 

 Vermilion lake supposed to have the Keweenawan age, and these may ultimately be 

 thought to prove the former existence of the Keweenawan overlying the Archean in 

 an extensive deposit in that region. Aside from that consideration this metallic 

 copper is far isolated geographically and structurally from the Keweenawan as now 

 known. 



4. If dating from the Keweenawan, but not of the Keweenawan, this occur- 

 rence still points to some relation or alliance of the rock (greenstone) in which the 

 copper occurs, with the diabase of the Keweenawan. 



