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435 
the only outlet when the water was at this elevation ; all the valleys 
farther west, being higher, and therefore only partial outlets when the 
elevation of the water was greater. 
The opening of other channels, (probably the St. Lawrence and Mo- 
hawk,) at a lower level, caused the discharge by the Chemung to cease, 
and from this period we have only these two outlets for the sea on the 
east. The discharge in these directions, continued till the separation 
of the upper and lower Lakes, which took place when the inland sea 
was reduced to about 600 feet above tide water. 
After the discharge by the Mohawk had ceased, which it did when 
the waters subsided to the elevation of 400 feet, the course of the St. 
Lawrence was by the valley of Lake Champlain and tlie Hudson river. 
The observations of Prof. Emmons, show that when the waters of this 
sea were at an elevation of about 300 feet above the ocean, an exten- 
sive discharge took place from the south end of Lake Champlani, as is 
evidenced by the polished surfaces and the wearing away of the rocks. 
At this period, a large proportion of the waters of the inland sea were 
discharged by the St. Lawrence and Champlain, into the valley of the 
Hudson. The partial obstruction at the Highlands, was the cause of 
the immense accumulation above ; and the flow by the Mohawk having 
nearly ceased, the eddying currents deposited the immense body of clay 
and sand, forming the plains between Albany and Schenectady, and 
which extend still farther north and south.* 
These conclusions seem incontrovertible, so far as regards the east- 
ern portion of the great basin once occupied by the inland sea. The 
Allegany could never have formed an outlet, but as before observed, 
must have discharged into this sea. There are no valleys communica- 
ting between the Allegany and Lakes Erie or Ontario, at a lower ele- 
vation than 1,200 feet. This river itself, within the State of New- York, 
has an elevation of nearly 1,200 feet above tide water. 
A barrier across the Mississippi of less than 700 feet above tide wa- 
ter, would turn all the waters of that immense valley, by way of the 
great lakes, and down the St. Lawrence. Of the existence of such a 
* The sand of these plains, as well as the great numbers of sandstone, boulders and 
pebbles, are clearly referable to the Potsdam sandstone, which once formed the bar- 
rier at the south end of Lake Champlain. See the reports of Dr. Emmons and Mr. 
Mather. 
