180 PHYSICAL GEOGRAPHY. 



the Silurian rocks. Mr. Hall remarks that, had the eroding 

 agency removed this bed, and penetrated to the soft strata 

 of the Onondago salt-group, Erie would probably have been 

 the deepest of the lakes.* The southern brim of its basin 

 is so low, that an easy canal communication has been opened 

 to the Ohio, a tributary of the Mississippi ; and other water 

 connections might be made with facility. Lake Ontario 

 occupies a hollow in the silurian rocks inferior to those of 

 the Erie basin f ; and the country which lies to the south of 

 it has in general a level character, though the Clinton and 

 Niagara groups of rocks rise in places in high escarpments. 



For a full account of the heights of the southern border, 

 the whole valley of the great lakes and the St. Lawrence, 

 and for detailed descriptions of the rock formations, the 

 reader may consult the several geological surveys published 

 under the authority of the legislatures of New York, 

 Michigan, and Ohio: the preceding brief notices have 

 been extracted therefrom for the purpose of showing the 

 general character of the country, and the lowness of the 

 barrier which separates the valleys of the Mississippi and 

 St. Lawrence. I may add, as a further exemplification 

 of the passes on the south side of the St. Lawrence, that a 

 subsidence of 400 feet would cause the waters of Lake 

 Ontario to flow through the valleys of the Mohawk and 

 Hudson into the Atlantic, and at the same time con- 

 vert Lake Champlain into a maritime strait, thereby 

 forming islands of the states of New York, New England, 

 and Maine, and of the British colonies of New Brunswick 

 and Nova Scotia. 



Before we proceed to trace the northern bank of the 

 valley of the St. Lawrence, it will be convenient to notice 



* Hall, p. 408. 



f In the Medina sandstone, grey sandstone, the Hudson River 

 group of shales, and, towards its eastern extremity, in the Trenton 

 limestone. — Hall. 



