168 A, Guyot on the Appalachian Mountain System. 
River to the southwest extremity of the system, the direction of 
which is nearly straight or forming a gentle curve concave to 
wards the northwest. These three divisions, diminishing in ex- 
tent, from the north to the south, are well marked, at the north, 
by the deep valleys of the Mohawk and the Hudson, which 
break through the Appalachian system to its base and across its 
entire breadth; at the south, by the New River whose deep val- 
ley with vertical walls also separates regions whose orographi¢ 
ebaracters present remarkable differences. 
The northern division is much the most isolated; it is geolog~ 
ically 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 carboniferous 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 hundrec 
and forty feet would conyert into an island all New England 
and the British possessions as far as Gaspé; for the bottom of 
the valley occupied by Lake Champlain and the Hudson does 
not in any part exceed this level. 
I distinguish in this northern portion three physical regions; 
Ist, the triangular plateau of the Adirondack, with its mountain 
chains more or less parallel, between Lake Champlain and the 
St. Lawrence, Lake Ontario and the Mohawk: 2nd, New Eng: 
narrow about New York, presents towards the centre, 12 
very 
Pennsylvania, its greatest breadth which again diminishes to 
wards the south. It is composed of a considerable number of — 
tion, 
to 25 
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