of Lake Superior. ■ 261 



geology and its geography, and so inadequate is the attention 

 hitherto bestowed upon it, that almost every thing toward its elu- 

 cidation on these points is yet to be done. 



The rocks of the main of Nipigon Bay are granite, greenstone, 

 and red sandstone, as far as is yet known ; — those on the north 

 side of the insular groups at its mouth, are red and white sand- 

 stone, and conglomerates, reposing, in some instances, on amyg- 

 daloid, while, on their south side, porphyries and amygdaloid ex- 

 clusively are met with. The rocks of the Mammelles, and of the 

 skirts of the Black Bay, except a small portion on the west of 

 Gravel Point, are amygdaloidal and simple trap, with red and 

 white sandstone. This sandstone, with its occasional concomitant, 

 puddingstone, is now first again seen from the vicinity of Marmoaze 

 (except at Cape Maurepas), and extends from the east end of the 

 Pay Plat to the Grand Portage, at least, a distance of 140 miles. 

 At the latter place, slabs of this rock occur, whose uninjured state 

 denotes its proximity in situ, in which form it has been observed 

 on the main by Mr. Thompson, twenty-two miles west of Fort 

 William. It is quite the predominating rock on the inner side 

 of the Nipigon Belt of islands, and exists at all levels, inter- 

 spersed with small, unconnected, and unfrequent, patches of 

 amygdaloid, westwards; it is then lost until it re-appears in the 

 cliffs in the isles at the mouth of Black Bay ; there also contiguous 

 to trap, simple, and amygdaloidal. It lines the south side of the 

 base of Thunder Mountain, and is occasional, as debris to the 

 Grand Portage. Isle Royale, and the islands between it and the 

 north main, have not been examined. 



The colour of this sandstone is uniform throughout considerable 

 spaces, and is white, reddish yellow, brownish, and brick red. 

 Fifteen or sixteen miles, however, from Cape Verd, this rock is 

 spotted and clouded like that of Batchewine Bay. It is generally 

 coarse granular, consisting, as far as the eye can discover, of 

 quartz nodules — with a scanty cement of calcareous, ferruginous, 

 or clayey matter ; but a specimen which I found in aciuaL super- 

 position on amygdaloid, near the N.W. corner of St. Ignatius, 

 is fine grained, ferruginous brown, and having a faint lustre, when 

 S2 



