, 
266 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
general stratigraphical succession displayed on the Minnesota coast. Cer- 
tain groups of beds are plainly to be made out, and in many cases minute 
stratigraphical measurements could be made in detail through thicknesses 
of thousands of feet. Of course, the greater the detail attempted, the greater 
would be the obstacles met with, in the way of faults—which are numerous 
along the Minnesota coast—thinning out of beds, corrugations of beds, and 
similarity of lithological composition between different layers. 
All of the Keweenawan beds of the Minnesota coast belong to the 
Lower Division of the series. The same statement applies to all of the Ke- 
weenawan rocks of the North Shore, except a small area at the southeast 
corner of Isle Royale. 
The following are the subordinate groups of beds into which I have 
subdivided the Keweenawan rocks of the Minnesota coast, with a total thick- 
ness of upwards of 20,000 feet. The thickness of the first group is so un- 
certain, and indeed irregular, that it is difficult to give an approximately cor- 
rect estimate of the total thickness. Above the lowest group, as already 
said, the thickness appears to lie between 17,000 and 18,000 feet. In all 
probability, 22,000 to 24,000 feet would not be very far from the truth as 
an estimate of the total thickness. ; 
I. Tue Saint Lours RIvER GABBRO AND ASSOCIATED RED PORPHYRIES.— 
These rocks are chiefly coarse orthoclase-gabbro, but include also ortho- 
clase-free gabbro, and a very few beds of fine-grained diabase. Red 
augite-syenite and granitic porphyry occur in large areas, constituting at 
times the entire mass of hills. Felsitie porphyries occur, but more rarely. 
Similar rocks, similarly associated, occur at the same horizon about the 
headwaters of Poplar, Cascade, and Brulé rivers, but are not found where 
they should appear at the Grand Portage end of the coast, though it is 
quite possible, and even probable, that some of the overflows of coarse 
gabbro found capping the slates of the Thunder Bay country belong here. 
The thickness of this group is difficult to estimate, but is probably not 
overstated at 6,000 feet. 
Il. Tae Dutura Grour.—This group is a succession of heavy but 
sharply-defined beds of very fine-grained but aphanitic rocks, belonging to 
the ashbed type of diabases, and to the diabase-porphyrites. A very few 
