1874.] Copper Mines of Lake Superior. 163 
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 development those cupriferous beds of trap and 
conglomerate in which native copper occurs under con- 
ditions most puzzling to the mineralogist, and from which 
it is being extracted in quantities 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 cosmical 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 geology 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 
unconformably, judging from the scanty evidence afforded 
by the survey of this part of the north shore, but con- 
formably, according to Brookes and Pompelly,* who have 
examined the lines of contact on the south shore, a series of 
beds of bluish shale, sandstone, indurated marls and con- 
glomerates, interstratified with trap, which is sometimes 
amygdaloidal. 
Sir William Logan subdivides this great mass of rock, 
whose 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 round 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 interruptions, 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 sandstone 
* American Journal of Science, June, 1872. 
