Dr. BiGSBY on the Geography and Geology of Lake Huron. 193 



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 tau^ are of the same rock, with the same direction, but possessing more com- 

 pactness. At the foot of Lake George it is often crystalline, dense, slightly 

 translucent at the edges, conchoidal in fracture, but frequently also foliated ; 

 the fragments then becoming schistose, with a shining lustre. It is here very 

 commonly a conglomerate rock, of great beauty, studded with nodules of red 

 and brown jasper, averaging an inch in diameter, and usually arranging them- 

 selves in the form of belts or stripes, from one to five feet in breadth. Black 

 and brown hasmatite occur sparingly at this place. 



Two broad strata of greenstone occur in this rock, three miles apart ; the 

 lowest five miles from the Narrows, whose rock it resembles, though it is more 

 syenitic. 



The character of the sandstone, which I am now about to describe, appears 

 both in the position of the strata and in the texture of the rock itself, to be de- 

 cidedly different from the preceding. 



The greater part of Lake George, as well as of the Straits of St. Mary, 

 rests (I believe) on a horizontal red sandstone. I have observed in various 

 parts of this lake, large slabs of this rock, with sharp fresh edges, most of it 

 soft, and of dull lustre ; but frequently quite crystalline, and remarkably hard, 

 and white wi'h large ferruginous red spots. 



Coasting the northern shore of Lake George, towards dusk, in a canoe, 

 I fell in with a number of islets, with cliffs of brownish horizontal sandstone 

 strata, breaking into parallelopipeds. At the portage of the Falls of St, Mary, 

 this rock prevails in situ, especially in the half-inundated islets surrounding 

 the Rapids. 



It is in horizontal layers, eighteen inches thick. It is soft, splits readily, 

 and its principal colours are red, brown, or dull white, with frequent spots or 

 circlets of yellow. 



The sandstone is environed by morasses, but re-appears largely on the 

 south side of Lake Superior ; and an active search would perhaps discover it 

 in situ near Michilimackinac, where I have seen much of its debris, and 

 where there is gypsum. 



The shores of the main and islands near Michilimackinac present, for the 

 most part, only beaches of shingle, and rarely afford traces of rocks in situ. 

 At the Isles of St. Martin, however, we find a large deposit of gypsum. It is 

 an extensive bed, of the granular kind, white, gray, and brown, interspersed 

 with frequent masses of red, white, and brown selenite, occurring in shapeless 

 lumps, in veins, or in small and very thin tables, having three or more sides, 

 and sharp angles. 



The horizontal sandstone, above described^ in its general characters, and 



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