298 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
eastern point of the island this rock was seen weathering out into little 
spheroids usually under half an inch in diameter. These black rocks are 
somewhat peculiar, and although allied to the ashbed-diabases of the Du- 
luth Group, they differ in the very high content of rounded augite particles, 
in which respect this rock differs, indeed, from any other as yet examined 
from the entire extent of the Keweenaw Series. The uppermost rocks of 
the island are coarser grained, and include rather coarse olivine-diabases, 
and at least one bed of an orthoclase-gabbro which, in the thin section, looks 
just like that of Lester River. 
It is difficult to decide just how to correlate these eastern rocks with 
those near Duluth. It is evident that in a general way they resemble the 
rocks of the Duluth and Lester River groups. The resemblance to these 
groups of the brown and gray diabases of the coast above Portage Bay 
is very strong. On the whole, since the peculiarly characteristic stratiform 
olivine-bearing beds and associated shaly amygdaloids of the Agate Bay 
Group are not seen here at all, I am disposed to regard that group as 
having thinned out to nothing, and to divide the rocks that are seen here 
between the Lester River and Duluth groups, which must also, of course, 
have greatly thinned, if this reference is a correct one. 
Beaver Bay Group—The Beaver Bay Group was made out from the 
coast cliffs between Split Rock River and a point about two miles below 
Baptism River; from numerous inland exposures for five to eight miles back 
from the mouths of Beaver, Baptism, and Temperance rivers, and from the 
coast cliffs between Grand Marais and a point in the Indian reservation, 
about four and a half miles above the western point of Grand Portage Bay. 
This group is characterized by the great predominance of coarse-grained 
rocks, next in abundance to which are felsitic and quartziferous porphyries 
and granite-like rocks. Dense brown and gray diabases of conchoidal 
fracture occur to some extent, while amygdaloids and fine-grained diabases 
of the ordinary type, though rare, are not excluded. 
While the prevalent coarse-grained rocks of the group are plainly 
enough flows, exhibiting the usual flat lakeward dip, and while the same is 
clearly true of much of the porphyry, there are other places where there is 
