3i8 THE CANADIAN NATURALIST. [Vol. vii. 



THE NATIVE COPPER MINES OF LAKE SUPERIOR. 



By James, Douglas, Quebec. 



The Jesuit fathers who, in extending the domain of Christianity 

 two centuries ago, explored and described parts of the American 

 continent, which are still almost as wild as then, likened Lake 

 Superior to a relaxed bow on whose string rests an arrow, the 

 north or Canadian shore being the bow, the south or L^nited 

 States shore the bow-string, and the arrow the promontory of 

 Keweenah, which, protruding from the south shore far across 

 the lake, divides its waters almost into halves. This promontory, 

 while one of the most salient geographical features of the lake, 

 is moreover geologically and mineralogically the most remarkable, 

 for on it, running from end to end, exist in their greatest de- 

 velopment those cupriferous beds of trap and conglomerate in 

 which native copper occurs under conditions most puzzling to 

 the mineralogist, and from which it is being extracted in quan- 

 tities sufficient to supply the growing wants of the United States 

 and to threaten the stability of the copper market elsewhere. 



In the present article, it is not my object to discuss the cos- 

 mical bearing of the subject, but to describe two of the most 

 noted mines near Portage Lake and the means adopted to extract 

 the mineral from their ores. Nevertheless, a sketch of the geol- 

 ogy of the region and of the mining elsewhere in it is necessary 

 as a preface. Lake Superior is framed in primitive rocks. The 

 gneisses and granites of the Laurentian formation at places rise 

 in bold cliffs out of the waters along the east and north shores, 

 and where the shore line in its trend to the south-west leaves the 

 Laurentides, the intervening space is occupied by a narrow belt 

 of Huronian slates and conglomerates, on which seem to rest un- 

 conformably, judging from the scanty evidence afforded by the 

 survey of this part of the north shore, but conformably, according 

 to Brookes and Pompelly,^ who have examined the lines of con- 

 tact on the south shore, a series of beds of bluish shale, sand- 

 stone, indurated marls and conglomerates, interstratified with 

 trap, which is sometimes amygdaloidal. 



Sir William Logan subdivides this great mass of rock, whose 



* American Journal of Science, June, 1872. 



