142 KEMPS ORE DEPOSITS. 



the copper was thought by the earlier investigators to be in the 

 eruptive rocks themselves, and that with them it had come in some 

 form to the surface and had been subsequently concentrated in the 

 cavities. Pumpelly has referred it to copper sulphides distributed 

 through the sedimentary, as well as the massive rocks from which 

 the circulating waters have leached it out as carbonate, silicate, 

 and sulphate. Although the traps are said by Irving to be devoid 

 of copper, except as a secondary introduction, it would be interest- 

 ing to test their basic minerals for the metal in a large way, as has 

 been so successfully done by Sandberger on other rocks. It is 

 probable that these may be its source. 



Irving states that the coarse basic gabbros of the system con- 

 tain chalcopyrite, but they do not occur near the productive mines. 

 The electro-chemical hypothesis of deposition was earliest advo- 

 cated (Foster and Whitney), and on account of the electrolytic 

 properties of the two metals copper and silver, at first thought it 

 has strong claims to probability. Still the unsatisfactory charac- 

 ter of all experiments made in other regions to detect such action 

 militates against it. Pumpelly, however, has worked out an ex- 

 planation much more likely to be the true one. He found, on 

 studying the mineralogical changes which have taken place in the 

 rocks, that the alteration had been very extensive ; that it had 

 proceeded through a series of minerals involving at one stage a 

 change in the iron present from protoxide to sesquioxide (which 

 would occasion a reducing action), and that at this stage the cop- 

 per was deposited. In two fissure veins near Portage Lake sul- 

 phides and arsenides of copper occur, and in a vein near Lac la 

 Belle, on the Mendota property, a little chalcocite has been found. 

 These are the only instances, yet recorded, of sulphides or related 

 compounds of copper in the district. A pocket of melaconite, the 

 black oxide, was opened in the early days at Copper Harbor. 



The discovery of copper dates back to the explorations of the 

 French, who in the seventeenth century left the settlements on the 

 lower St. Lawrence and penetrated the Great Lakes. The country 

 was the scene of a great mining excitement in the forties. After 

 many vicissitudes and exploded schemes the district settled down 

 to the largest production of any American region. Within the 

 last few years, however, Butte, Mont., has temporarily exceeded it. 

 Many interesting traces of prehistoric mining were found by the 

 early explorers, for the copper was a much prized commodity 

 among the aborigines. 



