340 
POOR ON MAP OF THE UNITED STATES [June 14, 1858. 
of it occupied by the Great Lakes is necessarily embraced. The 
dividing line between the waters running into the lake, and those 
running into the Mississippi, is for a long distance imperceptible — 
the country drained by each presenting similar aspects and structure. 
The highest point on the line of the Illinois Canal, between Lake 
Michigan and the Illinois River, a tributary of the Mississippi, is only 
eight feet above the level of the lake : in other words, a cut of ten 
feet in depth, for not a great distance, would turn a portion of the 
waters of Lake Michigan into the Mississippi — so nicely poised in 
the centre of the continent are these great Inland Seas. It seems 
not unlikely that the ocean once flowed through the valleys of the 
Mississippi and St. Lawrence, forming an island of that portion of 
the United States occupied by the Alleghany, and its connecting 
ranges of mountains. 
The third grand division of the continent shown on the map is 
that drained by the St. Lawrence and its tributaries. Although 
upon the south shores of Lakes Michigan and Erie there is nothing to 
mark the dividing line between the great division already described 
and the one now under discussion, as we go north and east the 
boundaries between the two become well defined. The summits 
between Lake Superior and the Mississippi are elevated all the way 
from 500 to 800 feet above the lake, except in one instance, at the 
head of the St. Croix River, where there is a break, the lowest point 
of which is only 366 feet above the lake. On leaving the south 
shore of Lake Michigan and going east, the surface of the country 
gradually rises, till it attains, in the State of New York, an eleva- 
tion of nearly 1700 feet above the sea. This plateau, both in the 
States of New York and Pennsylvania, falls off abruptly into the 
basin of Lake Erie, in a distance, in many cases, of six or eight 
miles. The head waters of the Ohio, the great eastern tributary 
of the Mississippi, rise within a few miles of this lake. In fact, all 
the great lakes have only a very limited area of drainage on 
their southerly shores. It is not till Lake Ontario is reached that 
the St. Lawrence basin becomes well marked. As the waters of 
Lake Michigan could, without great expense, be turned into the 
Gulf of Mexico, so could the waters of Lake Erie be conducted into 
the harbour of New York. The great plateau of the Alleghanies, 
a short distance from the outlet of Lake Erie, suddenly falls 
off into the basin of Lake Ontario. At the dividing line be- 
tween the waters flowing into this lake and the Hudson River, it 
is depressed 145 feet Mow the surface of Lake Erie. It is through 
this great defile or depression in the continent, that the Erie Canal 
is constructed. The long level on this canal, which corresponds to 
