No. 6.] DOUGLAS — LAKE SUPERIOR COPPER MINES. 319 



total thickness can be but vaguely guessed at, into lower and 

 upper groups, and designates them as the upper copper-bearing 

 rocks of Lake Superior, in distinction to the Huronian or lower 

 copper -bearing series. 



The lower group occupies the north-western shore of the lake, 

 and sweeps rounds its extreme westerly end, but in the extension 

 eastward of it and the upper group they are divided from the 

 south shore by sandstones of a very different character to those 

 which are interstratified with their own traps and conglomerates. 

 These sandstones, which line the south shore, with but few in- 

 terruptions, from Sault St. Marie to Duluth, lie in horizontal or 

 very slightly inclined beds, and, being very friable, have been at 

 several spots fashioned by the water into the fantastic forms 

 known as the pictured rocks. Representatives of the same sand- 

 stone occur on some of the islands along the north shore, being 

 all that remains above water of the soft formation out of which 

 the bed of the lake was hollowed. But while they yielded to 

 the destructive agency of water, the harder beds of the copper- 

 bearing groups have withstood it, and these, therefore, as on 

 the point of the Keweenah promontory, rise abruptly out of the 

 lake, which has washed away entirely the sandstone from their 

 flanks, or, as towards the base of the promontory, spring at as 

 abrupt an angle out of the horizontal sandstone strata, or else 

 from islands, isolated or in groups, which, however, always bear 

 a marked relation to one another and to the lines of upheaval 

 distinguishable on the mainland. 



The lower group of these rocks which we have described as 

 confining the western end of the lake has not produced copper in 

 workable quantities, and differs also in lithological character 

 from the upper group. On the south side of the lake this upper 

 group forms distinct ranges, the more easterly of which are 

 traceable with remarkable parallelism from the base to the point 

 of the Keweenah promontory ; and they reappear on the north 

 shore, the more westerly in Isle E-oyale, in the Thunder Bay and 

 Neepigon promontories, and in the St. Ignace group of islands, 

 the more easterly in Michipicotin and in some of the headlands 

 of the coast. 



The age of these rocks is the subject of some difference of 

 opinion. They seem, from the observation of Brookes andPom- 

 pelly in Northern Michigan, to conform in strike and dip with 

 the Huronian schists, both uniformly dipping to the north at 



