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A, Guyot on the Appalachian Mountain System. 169 
in the plateaus in the neighborhood of Lake Erie, where the mean 
altitude of the plateau reaches 2000 feet, the valleys preserving 
a height of 1500 feet while the hills reach 2600 feet. ' 
_ This tableland forms a remarkable water-shed from which the 
waters descend by the Susquehanna into the valley of the Chesa- 
peake and the Atlantic ocean, by the Genesee and St. Lawrence 
to the same ocean, and by the Alleghany and Ohio to the Gulf of 
Mexico, The Susquehanna thus starts from Lake Erie at the 
extreme western border of the plateau, and runs across all the 
Appalachian system and its mountain ranges to its eastern base. 
More to the southward the eastern escarpment of the plateau 
divides, as far as the sources of the Potomac, the waters of the’ 
Atlantic coast from those of the Gulf of Mexico. It is the same 
escarpment which bears the local name of Alleghany Mountain, 
a name which continues to be applied, south of the waters o 
the Potomac, to the dividing ridge along the sources of the va- 
rious branches of James {iver, and even to the irregular hills 
which form a water-shed between the waters of the upper Ro- 
anoke and New River, across the Great Valley, near Christians: 
burg. Through all this middle region the name of Blue Ridge 
is applied to the main eastern chain which separates the Great 
valley from the Atlantic slope, and which is cut by all the 
rivers which flow out of it. ; 
he southern division, from New River to the extremity of 
the system, is much the most remarkable for the diversity of its 
physical structure and its general altitude. Even the base upon 
which the mountains repose is considerably elevated. Although 
the elevation of the Atlantic plain at the eastern base of the 
mountains is only 100 to 800 feet in Pennsylvania, and 500 in 
_ Virginia near James river, it is 1000 to 1200 feet in the region 
of the sources of the Catawba. In the interior of the mountain 
