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



Rhomboidal pearlspar and dog-tooth calcspar are the only accidental mine- 

 rals I have met with in the limestone of St. Joseph. 



This bed is rich in organic remains^ and some of them are new. Their sub- 

 stance is not silicifiedj but remains calcareous. 



Wherever this limestone occurs^ orthocerae are very abundant^ ^ying heaped 

 thickly together in small patches through the rock ; so that when the surface 

 is exposed^ innumerable sections of them are displayed. They are accompa- 

 nied by milleporae, madreporae^ encrini, shells^ &c. The orthocerae are never 

 entire upwards ; but it is by no means uncommon to find the lower termina- 

 tion complete, the chambers and the siphuncle extending regularly to the 

 bottom. They vary much, not only in their absolute size, but in their pro- 

 portions. They are seldom less than 2 inches broad at the larger end, and are 

 usually from 10 to 18 inches long. Some taper so imperceptibly, as in short 

 pieces to appear almost cylindrical ; and I have seen specimens only 3 inches 

 broadvat the top, that were 4 feet long : generally, however, they are broader 

 in proportion to their length. Major Delafield and others have seen them 6 

 feet long. 



f Their lower termination is either acute, as in the specimen, Plate XXVI. 

 fig. 5, from St. Joseph's; or round and obtuse, as in the specimens, PI. XXVI. 

 fig. 3 and 6, from an island off the northern shore, near Thessalon. 



ing in undisturbed horizontality upon the older rocks, is common in both the Canadas. Thus, at 

 the Falls of Montmorenci, 7 miles below Quebec, gneiss dipping south-east, almost vertically, is 

 covered by horizontal layers of limestone breccia and limestone, the latter containing the Conu- 

 laria quadrisulcata of Sowerby, and, among other species of trilobite, that figured in Plate IV. 

 fig. 3, 6, 7, 8, of the Memoire sur les Crustacees Fossiles, by MM. Brongniart and Desmarest, 

 from specimens found at Llandeilo, and preserved in the Ashmole Museum at Oxford. 



Limestone breccia reposes horizontally on highly inclined mica-slate at Malbay. Repeated in- 

 stances of the same kind are met with in the St. Lawrence among the Thousand Isles, the great 

 breach of Lake Ontario. Here both sandstone and limestone in horizontal beds overlie gneiss, 

 the limestone being without organic remains, but having imbedded in it splendid masses of sulphate 

 of strontia. I have seen each rock in contact with the gneiss, in places 40 miles apart, but at the 

 same level. 



A decidedly crystalline texture often prevails in the limestones of the Canadas, but rather in 

 the higher layers than in the inferior ; and this crystalline limestone is universally crowded with 

 organic remains. Thus at the Falls of Montmorenci, and at Point aux Trembles, 30 miles above 

 Quebec, it is seen loaded with organic remains, and particularly with large pointed orthoceratites, 

 300 feet above the St. Lawrence, capping a succession of horizontal beds of black and brown gra- 

 nular limestone, with few organic remains. At the Bridge of Jacques Cartier, and at the Falls of 

 the Chaudiere on the Ottawa, crystalline limestone is found in a similar position : it abounds at 

 the east end of Lake Ontario. The crystallization in these cases is genuine, and does not depend 

 on the presence of encrinitic remains : the facets are rhomboidal. The limestone is fetid. 



f Excepting figures 3, 6, and 1 1, ail the drawings are of the natural size. 



