Mr. Maclure on the Geology of part of N. America. 101 
the small distance that the falls of Genesee river have 
worn its bed from the lake, with the shallow beds of the 
Oswego and all the other rivers that run into the lake, as 
well as the general nature of all the Genesee country, op- 
poses the probability of the supposition or conjecture. 
above observations are equally applicable to the 
beds of the Hudson and Mohawk, before they fall over the 
ridge, from which it would appear that the most rational 
conjecture would be, to suppose the St. Lawrence wore 
down a passage through the high lands between Quebec and 
Montreal, as well as the Hudson, through the high lands 
above New-York, and until they had effected such a cut, 
the whole basin on the west side of the mountains, was the 
bottom of an immense lake. 
A similar mode of reasoning supports the conjecture that 
the basin of the Mississippi made part of the said lake, for 
the Tennessee river, while in the mountains under the name 
of the French Broad, has i 3 
to two hundred feet in solid primitive and i 
rocks, but when it comes into the basin, it is obstructed in 
its passage, at the Muscle Shoals, by a soft secondary sand- 
stone; the sources of the Ohio, under the name of New 
River, &c. &c. &c. have likewise cut deep beds in the 
mountains before they reach the great basin, but after their 
union into one great stream, the Ohio is obstructed at its 
falls near its mouth by a secondary limestone; from all 
‘which it would appear probable, that, had those rivers run 
as long through the secondary formation of the great basin, 
as their sources must have done to wear these beds so deep 
$n the primitive mountains, the accumulated waters of both 
the Ohio and Genesee would, long ere this, have worn away 
all the obstructing secondary rocks, and like all other great 
rivers that have run long in the same beds, would have been 
obstructed only by alluvion of their own formation. The 
Rappahannock, Potomac, James River, Roanoke, &c. &c. 
. that run into the Atlantic, have cut deep beds in their 
course through the mountains, through the level country 
their channels are shallow, and they all fall from twenty to 
thirty feet over the granite ridge into tide water, without hav- 
lng removed, the fall half a mile from where they begun, 
which could not have been the case had they run as long in 
e low country, as they had in the mountains. 
