COPPER MINES OF LAKE SUPERIOR. 
171 
COPPER MINES OF LAKE SUPERIOR. 
§ 103. The occurrence of native copper, in sufficient abund¬ 
ance to make it an object of commercial value, is one of the 
most recent of geological discoveries. The fact is another 
illustration of a remark, that each mineral district contains a 
certain type of mineral character peculiar to itself. That type 
does not seem to be entirely dependent upon the surface 
geology; for that of Polk county, Tennessee, is not ostensibly 
different from North Carolina. Many other illustrations of the 
fact might be given. 
The geological position of the copper has been determined 
by the late Dr. Houghton and Dr. Jackson, and confirmed by 
other American geologists. They have shown that the native 
copper lies in veins in the trap rock, and that this rock forms 
alternations with the potsdam sandstone; and save the excep¬ 
tion, the exclusive metallic condition of the copper, its reposi¬ 
tories do not differ from those where it is found in other 
states, as that of the sulphuret or the gray copper ore. 
As a general illustration of the copper lodes of lake Superior, 
I have copied the annexed diagram from Foster and Whitney’s 
Governmental Report (fig. 43). In this diagram the shaded 
Fig. 43. 
A A layers of sandstone, D mass of trap containing the copper. 
part shows where the copper has been removed by means of 
the vertical shafts and horizontal galleries. It will be observed 
