176 <A. Guyot on the Appalachian Mountain System. 
zone of the plateaus. A transverse section from New York to 
Lake Erie shows that the depression of the system does not ex- 
tend to the western zone which preserves in appearance a height 
very nearly uniform, from the plateau of Adirondack in the 
State of New York as far as to the Cumberland mountains in 
Tennessee. There is here no well marked region of subsidence 
as in the eastern zone, but only a tendency to it which is slightly 
manifested upon a line between the maximum of eastern depres- 
sion and Pittsburg. It is towards that central line of depression 
that the Allechany and Monongahela rivers flow from opposite 
1600 feet. Towards the south also the plateaus rise to the sources 
of the Monongahela. In Virginia and Tennessee they appear’ 
to reach 2000 or 2500 feet, at least near the mountains, but the 
measurements which I possess are too few in number and too 
uncertain to allow me to speak with certainty on this subject. 
This remarkable depression of the Appalachian system in 
t 
e : 
ter, causes a great part of the continental plains, which form 
ei natural base of the mountain folds, to disappear under 
bathe the very base of the mountains, and the region of plains 
fades away on the frontiers of New Jersey and New York, while 
towards the south the emerged portion enlarges gradually #8 
it rises according to the law of gradual increase indicated above, 
so that it reaches a breadth of more than 200 miles in the Caro- : 
linas. This depression seems to be due to a local subsidence of 
: 
New York through the Narrows and adyanees far out under the 
waters of the ocean. It is not possible to suppose that such 4 
channel which is constantly liable to be oblite 
bani i 
