ON THE NORTHWEST SHORE OP 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', u', a', a'. Slate entangled in trap. b. Greenstone, c. 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 



