144 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
upper flows. The immediately underlying series of slates known to the 
Canadian geologists as the Lower Copper-bearing Group, called by Hunt 
the Animikie Group, and regarded in this memoir as unquestionably the 
equivalent of the iron-bearing Huronian rocks of the South Shore, is 
intersected everywhere by a very much more powerful system of dikes. 
These are spoken of more especially on a subsequent page, and are cited 
here merely that I may express my belief that in them, and in the smaller 
dikes of the Keweenaw Series itself, we see the source of the volcanic 
strata of the lake basin, all around whose rim I conceive the eruptions 
to have taken place, rather than from any one vent, as some have supposed,’ 
or from within any restricted portion of the basin. 
The great structureless masses of coarse gabbro, which in the Bad 
River region of Wisconsin, and again in the Duluth region of Minnesota, 
and thence northeastward to the Brulé Lake country, constitute so marked 
a feature in the geology of those regions, have been above alluded to as 
possibly owing their lack of structure to the enormous thickness of the out- 
flow. There are some things about them, however, that suggest another 
origin. The great coarseness of grain, the perfection of the crystalliza- 
tion, the abrupt terminations of the belts, the complete want of structure, 
and the presence of intersecting areas of crystalline granitoid rocks—all 
suggest the possibility that we have here to do with masses which have 
solidified at great depths. They certainly can not, however, be regarded 
as intrusive in the ordinary sense of the word; so that, unless we regard 
them as great outflows, we should be forced to look upon them as the now 
solidified reservoirs from which the ordinary Keweenawan flows have come. 
The acid rocks cutting these coarse gabbros are clearly intrusive. 
Of the original acid rocks of the Keweenaw Series, true granite has 
been observed only in the Bad River region of Wisconsin, where it is seen 
intersecting the coarse gabbro of the base of the series, and also the under- 
lying slates. It is there a coarse, flesh-colored, completely developed 
granite, cutting the gabbro in irregular masses and in broad bands. In the 
same region a brick-red granitic porphyry or granitell occurs in the same 
1See A. R. C. Selwyn, in Report of the Geological Survey of Canada for 1877~78, p. 15 A. 
