"V 



Dr. BiGSBY on the Geography and Geology of Lake Huron. 195 



porting beds of horizontal limestone, are found to be the exclusive and predo- 

 minating rocks. 



The remaining- secondary beds, which I have to describe, are calcareous. 

 I shall divide them into two species, — the limestone of St. Joseph and the 

 isles on the northern shore, and the limestone of the Manitoulines; not in- 

 tending thereby to decide that they belong to formations essentially different, 

 but only to distinguish the one from the other, by reason of their difference in 

 character, in organic remains, and in geographical position. 



The limestone of St. Joseph and the northern isles, is, wherever I liave 

 examined it, horizontal. It occurs usually in weathered crumbling ledges, 

 seldom exceeding six feet in height ; or it floors the beaches in broken pave- 

 ments, advancing far into the shallow waters. 



This species assumes various aspects in distant parts of the same level ; but 

 it is most commonly of different shades of brown and green. It is earthy, 

 rather soft, knotty and slaty. 



In the Channel of Pelletau it is granular, passing into compact, and then 

 differs little in hand specimens from the Dudley Rock in Staffordshire; having, 

 like it, much disseminated calcspar. 



In this channel, and in the isles near St. Joseph, it is frequently an aggre- 

 gate of microscopic globules, opaque, and of a Dutch-green colour, mixing in 

 spots with limestone of the granular or crystalline texture. In the Narrows, 

 this oolitic species presents translucent nodules, less than a pin's head, which 

 I believe to be quartz ; especially as a quartzose conglomerate is immediately 

 at hand. On Green Island the green colour is often very strong; and the 

 stratum contains a number of brown oblong nodules, as large as millet-seeds. 

 The rock rests upon a blackish-brown limestone-slate, seen only under water, 

 and as truly schistose as that of Niagara. 



The limestone of Thessalon Isle, in which the new species of orthoceratite 

 (PI. XXV. fig. 1, 2, 3) is found, is decidedly magnesian ; and in the compact 

 parts of it has the saccharoid texture belonging to dolomite. Its cavities, and 

 those of the organic remains, are lined with primitive rhomboids of the triple 

 carbonate of lime and magnesia. 



There is little doubt but that the secondary belts of the primitive rocks in 

 the islets about La Cloche and the French River, are of the St. Joseph species 

 of limestone. The limestone here lies in horizontal beds ; it is much darker 

 than that of St. Joseph. Its texture internally is firm ; but it crumbles into 

 knotty flakes. I only observed bivalves in it, but others have seen orthocera- 

 tites in some of the westernmost of the isles*. 



* The occurrence of similar belts of secondary strata, crowded with organic remains, and rest. 



