164 Copper Mines of Lake Superior. (April, 
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 them, and 
these, therefore, as on the point of the Keweenah promon- 
tory, 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 rela- 
tion to one another and to the lines of upheaval distinguish- 
able 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 distin¢ét 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 Royale, 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 
and Pompelly in Northern Michigan, to conform in strike 
and dip with the Huronian schists, both uniformly dipping 
to the north at angles of 50-—7o°, and where the Huronian 
come in contact with the sandstones above mentioned, there 
is the same sudden alteration in dip as between these same 
sandstones and the copper-bearing rocks on the Keweenah 
promontory. Hence, one would infer that the traps and 
conglomerates of the upper bearing series come next in age 
to the underlying Huronian schists, and that subsequently 
to their upheaval were deposited the sandstones whose 
horizontality has not been broken by any disturbing force. 
The sandstones are generally attributed to the lower 
Silurian system. 
Copper exploration on the Keweenah promontory have 
been made at innumerable spots over a distance of 100 miles 
along the strike of the beds, between the Point and Lake 
Agogibic, but the mines which have proved productive are 
confined to three districts, viz., Keweenah Point or Eagle 
River, Portage Lake, and Ontonagon. 
On the Point, the copper-bearing rocks attain their 
