GEOLOGY. 281 



the eastern part which faces the ocean, and of an extended plateau 

 which prevails towards the west and north-west and descends grad- 

 ually towards the inland valleys of the St. Lawrence, the Lakes Erie 

 and Ontario, and the Ohio River. 



The base on which this large belt of mountains rests, and which 

 may be considered as bounded by the Atlantic Ocean on one side and 

 by the Ohio and St. Lawrence Rivers on the other, is formed in the 

 east by a plain slightly inclined towards the Atlantic. The width 

 of that plain in New England does not vary much from fifty miles. 

 Near the mouth of the Hudson, however, in New Jersey, it nearly 

 disappears, but gradually increases towards the south to a width of 

 over two hundred miles. Its elevation above the sea, at the foot 

 of the mountains, is in New England from three hundred to five 

 hundred feet. From the neighborhood of the Bay of New York, 

 where it is nearly on a level with the ocean, it rises gradually tow- 

 ards the south to an altitude of over one thousand feet. On the 

 west the table-lands which border upon the Ohio River, and which 

 may be considered as the general base of the system, preserve a 

 mass elevation of a thousand feet or more, in the thickness of which 

 the river-bed is scooped out to the depth of from four hundred to six 

 hundred feet, thus reducing the altitude of the Ohio River full one- 

 half from that of the surrounding lands. 



The vast belt of the Appalachian highlands forms the marginal 

 barrier of the American continent on the Atlantic side, and deter- 

 mines the general direction of the coast line, which in general runs 

 parallel to the inflections of its chains with remarkable regularity. 



When from any point we traverse the Appalachian system from the 

 Atlantic, we encounter first a zone which shows that they are but the 

 last undulations due to the action of the same forces which have up- 

 heaved and folded that region, and which have raised at the same 

 time the mass of these more uniform plateaus. Thus, when from any 

 point we traverse the Appalachian system from the Atlantic, we en- 

 counter first a plain more and more undulated and gradually ascend- 

 ing to the foot of the mountains ; then a mountainous zone with its 

 ranges parallel and its valleys longitudinal ; at length a third zone of 

 uniform plateaus slightly inclined towards the north-west, and cut 

 with deep transverse valleys. 



Another conspicuous feature of the Appalachian system is a large 

 central valley which passes through the entire system from north to 

 south, forming, as it were, a negative axis through its entire length. 

 This is what Mr. Rogers calls the Great Appalachian Valley. At 

 the north it is occupied by Lake Charnplain and the Hudson River ; 

 in Pennsylvania it bears the name of Kittatinny, or Cumberland 

 Valley. In Virginia it is the Great Valley ; more to the south it is 

 called the Valley of East Tennessee. At the north-east and at the 

 centre its average breadth is fifteen miles ; it contracts in breadth 

 towards the south, in Virginia, but reaches its greatest dimensions in 

 Tennessee, where it measures from fifty to sixty miles in breadth. 

 The chain, more or less compound, which borders this great valley 

 towards the south-east is the more continuous, and extends without any 

 great interruption from Vermont to Alabama. In Vermont it bears 

 the name of Green Mountains, which it retains to the borders of New 

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