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236 THE CANADIAN NATURALIST. [Vol. X. 



All this presupposes the coDtinent at a higher level (at least 

 600 feet). During some portion of the tertiary times, at least the 

 •eastern portion of the continent must have stood a thousand or 

 twelve hundred feet higher than at present, as indicated by the 

 soundings in the St. Lawrence river (near the mouth of the 

 Saguenaj), in the New Y^ork Harbour and off the mouth of the 

 Chesapeake Bay. 



The rate at which the upper lakes was excavated would depend 

 partly upon the rate of the excavation of the Dundas valley and 

 its extensions through the limestone, at first by a slow abrasion, 

 and the solution of the carbonate of lime by the carbonic acid 

 held in the water, and afterwards by the undermining of the 

 hard rocks on the removal of the Medina shales. 



(T'o he continued in our nexl.) 



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