ISLE ROYALE. 331 
south. These ridges are, of course, made of the heavier, more resistant 
layers. They seldom reach 500 feet above the lake level. The inter- 
mediate linear valleys are worn in the softer amygdaloids and other less 
resistant rocks. At the ends of the island these valleys are occupied by 
long, narrow extensions of the lake, and between them the ridges continue, 
constituting the so-called “fingers” of Isle Royale. Ridges and valleys 
both change in trend as the island is followed to the northeast, as does the 
island itself as a whole, and this is evidently due to a similar curving in the 
trend of the underlying rocks.’ 
It is evident, from the descriptions of Messrs. Foster and Whitney, 
that we have here merely a repetition of what is seen everywhere else in 
the course of the Keweenawan rocks. They describe the beds as exceed- 
ingly well marked and mostly thin, and as provided with strongly-devel- 
oped vesicular portions, and lower portions which are often columnar. 
The more coarsely crystalline kinds of which Foster and Whitney speak— 
such as that of the ridge along the northwest shore of the island, and that 
of Blake’s Point, at the northeast—evidently belong with the coarse gabbros 
of this memoir; while the porphyritic kinds which they mention as occur- 
ring at several points would appear to belong with my diabase-porphyrites. 
They say nothing, however, of the occurrence of red rocks, which 
might be quartziferous or granitic porphyries, although one would expect 
such to occur, especially on the north side of the island. One occurrence 
which they describe is of interest, namely, the sandstone veins running 
down from an overlying sandstone into the cracks of an amygdaloid at the 
mouth of Chippewa Harbor, on the south side of the island. The same 
thing may be seen at many places on the Minnesota coast, as already indi- 
cated. 
THUNDER BAY TO NIPIGON BAY. 
As shown in the following chapter, the slates of the west and north- 
west sides of Thunder Bay, and again those of Pie Island and Thunder 
Cape as far around as Silver Islet, belong with the iron-bearing rocks of 
the South Shore. The east shore of Thunder Bay, however, and the whole 
of the peninsula between Thunder and Black bays, are occupied by a 
1See, also, Chapter IX. 
