ON THE NORTHWEST SHORE OF LAKE SUPERIOR. 401 
The summit of the ridge nearest the bottom of Grand Portage Bay is composed 
of greenstone, which has been erupted through argillaceous slate. At the bottom 
of the hill the slate is unaltered, and dips 12° to 15° to the southeast. The green- 
stone has carried some large masses of slate with it, and completely enveloped 
many of them. Some of the enclosed masses are but little altered, while others are 
almost completely metamorphosed. The metamorphosis begins about twenty feet 
above the level of the first lake-terrace, and extends to the top of the slate, which 
mounts to within sixty-one feet of the summit of the ridge, which is two hundred 
and seventy-eight feet high. The width of the dike varies from seventy to eighty 
yards. Where the slates have become entangled in the trap, and separated widely 
a’, a’, a’, a’, a’. Slat tangled in trap. b Greenstone. ec. Amygdaloidal slate. d. Unaltered slate. 
from the mass, the metamorphosis is most complete, the rock assuming the appear- 
ance of a compact hornblendic slate, but still separating in the cleavage planes. 
The colour is sometimes reddish, and at others bluish black, like slag. Many of 
EXPOSED DIKE OF COLUMNAR BASALT, LAKE SUPERIOR. 
the enclosed masses are amygdaloidal. The course of the dike is northeast and 
southwest. It cuts the long point on the lower side of Grand Portage Bay, and is 
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