282 ANNUAL OF SCIENTIFIC DISCOVERY. 



York ; in the latter State it becomes the Highlands ; in Pennsylvania, 

 the South Mountains ; in Virginia, the Blue Ridge ; in North Carolina 

 and Tennessee, the Iron, Smoky, and Unaka Mountains. On the 

 north-west of the, great valley, between the latter and the borders of 

 the plateau parallel, there extends a middle zone of chains separated 

 by narrow valleys, the more continuous of which is the range which 

 bounds the central valley. This zone has a variable breadth in dif- 

 ferent parts of the system, and the number of chains which compose 

 it is by no means uniform throughout. 



Passing the eye over a map of the system, we at once distinguish 

 in its longitudinal extent two principal curvatures : the one at the 

 north, from Gaspe to New York, the concavity of which is turned 

 towards the south-east ; the other at the centre, from the Hudson to 

 New River in Virginia, with its concavity also towards the south-east; 

 the third from New River to the south-west extremity of the system, 

 the direction of which is nearly straight, or forming a gentle curve 

 concave to the north-west. These three divisions, diminishing in 

 extent from north to south, are well marked at the north by the Mo- 

 hawk and Hudson Rivers, which break through the Appalachian sys- 

 tem to its basu an 1 across its entire breadth ; and at the south by the 

 deep valley of the New River. 



The northern die Is ion is much the most isolated; it is also geologi- 

 cally the most ancient, since its upheavals appear coeval with the 

 Silurian and Devonian epochs, and are thus much anterior to the 

 rest of the system, which only emerged after the deposit of the car- 

 boniferous rocks which it has elevated. Four hundred feet more of 

 water would separate all the vast territory of the northern division 

 from the American continent. One hundred and forty feet would 

 convert into an island all New England and the British Possessions 

 as far as Gaspe ; for the bottom of the valley occupied by Lake Cham- 

 plain and the Hudson does not in any part exceed this level. 



The 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 one 

 hundred to three hundred feet in Pennsylvania, and five hundred in 

 Virginia near James River, it is one thousand to one thousand two 

 hundred feet in the region of the sources of the Catawba. In the 

 interior of the mountain region the deepest valleys retain an altitude 

 of two thousand to two thousand seven hundred feet. 



In the region of country comprised between the Blue Ridge and 

 the great chain of the Iron, Smoky, and Unaka Mountains which 

 separate North Carolina from Tennessee, through an extent of more 

 than one hundred and fifty miles, the mean height of the valley from 

 which the mountains rise is more than two thousand feet ; the moun- 

 tains which reach six thousand feet are counted by scores, and the 

 loftiest^ peaks rise to six thousand seven hundred feet ; while at the 

 north, in the group of the White Mountains, the base is scarcely one 

 thousand feet, the gaps two thousand feet, and Mount Washington, 

 the only one which rises above six thousand feet, is still four hundred 

 feet below the height of the Black Dome of the Black Mountains of 



