156 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
makagon, and eastward toward the Montreal River, lie directly against the 
older rocks, without any of the coarse gabbros intervening. The great ex- 
tent of coarse gabbros in Minnesota seems to sustain somewhat the same 
relations to the more regularly bedded portions of the series. The Minne- 
sota gabbro, with its accompanying syenite and granitic porphyry, occu- 
pies a belt from 5 to 20 or more miles in width, extending from the Saint 
Louis River at Duluth northward, to and beyond the Cloquet, and thence 
eastward to the region of Brulé Lake, in township 63; but where the base 
of the series comes out to the lake at Grand Portage the gabbro is absent, 
the regularly bedded diabases and amygdaloids resting directly upon the 
older slates. 
For these reasons I was at one time somewhat inclined to place these 
gabbros with the Huronian, and to regard them as possibly the equivalents 
of the great flows that crown the so-called Animikie slates in the region 
of Thunder Bay. In the Bad River region, however, where the rocks all 
dip at a high angle to the northward, the coarse gabbro appears to cut 
across the Huronian at a very small angle, and in such a manner as to come 
into contact with successively lower members of the Huronian,’ when the 
rocks are traced on the strike westward. This appears like an unconformity 
with the Huronian; and on this account, as also because of the close lith- 
ological relationship between the gabbros and the typical Keweenawan dia- 
bases, many of which approach gabbro in character, and because in a 
series made up chiefly of eruptive flows dovetailing into one another, there 
must often be sudden breaks, which in a sedimentary series would argue 
unconformity, I prefer to regard these coarse rocks as the earliest of the 
Keweenawan flows. I have already referred in the previous chapter to the 
possibility of their representing the slowly solidified and subsequently de- 
nuded reservoirs from which the later flows may have been in part derived. 
Ihave alluded above to an exception to the general fact of the thin- 
ness of the detrital beds at low horizons. The reference was to a set of 
dolomitic sandstones, running upward into red marly clays and limestones, 
which are largely developed in the peninsula between Thunder and Black 
1See Map of Plate XXII, Atlas of the Geology of Wisconsin. 
a 
