Microscopic Structure of Canadian Limestones. 167 



valley of the Black river, re-enters Canada at the lower end of 

 Lake Ontario, along which these limestones extend in a broad band, 

 and crossing to Lake Huron stretch along the chain of the Mani- 

 toulin Islands, and then run again to the southward along the 

 west side of Lake Michigan. There are also in Canada outlying 

 patches on the Ottawa and Lake St. John. Throughout all these 

 regions the gray crystalline beds are more or less represented ; 

 though in the State of New York they appear to be in the upper 

 part of the formation, and to thin out and disappear toward the 

 South.* Specimens from Chateau Richer below Quebec, from 

 Ottawa, from the La Cloclie mountains, and from great Manitoulin 

 Island, exhibit very nearly the same microscopic characters with 

 those of the Montreal quarries. In the stone of Chateau Richer 

 crinoids predominate. In that of Ottawa there is a greater pre- 

 valence of fragments of shells. In that of La Cloche aud Mani- 

 toulin the materials are much the same as at Montreal. 



The conditions of the accumulation of this great and extended 

 mass of animal fragments, it is not difficult to understand. An 

 ocean area, probably not of great depth, the growth of multitudes 

 of branching corals and crinoids, the destruction of these by the 

 waves and by the death of successive generations, the drifting of 

 their remains by currents over the bottom, the occasional invasion 

 of the clear water by muddy sediment — these are the conditions 

 which must have prevailed when the gray Trenton limestones 

 were formed. Professor Hall and Mr. Billings have remarked that 

 the Brachiopod shell-fish of the Chazy and Trenton are usually of 

 smaller size than that which they attain in overlying formations* 

 This may have been due to the conditions so favorable to the 

 spreading of organic fragments over the sea bottom. 



In the Island of Montreal the Black river and Chazy limestones 

 crop out from beneath the Trenton. The quarries at Pointe Claire* 

 worked for the Victoria Bridge, are believed by Sir W. E. Logan 

 to represent principally the former. The western or back quarries 

 on the Mile End road and those of Isle Jesus belong to the latter. 

 The stone worked for the piers of the Victoria Bridge presents 

 several varieties in alternate layers. One of these has the coarse 

 crystalline aspect of the gray Trenton, but it consists principally 

 of fragments of Brachiopodous shells ; masses of coral however oc- 

 curring in some layers. A finer variety which constitutes a large 

 proportion of the stone, is made up of rounded and comminuted 



* See Geological Surveys of Canada and New York. 



