574 ^^^^ American Naturalist. [July, 
one and the others. But the aid from geology is conclusive; 
for West Virginia and a large area around it is made up of hori- 
zontal layers of bedded rocks that once were at the bottom of the 
sea, and that still retain the essentially horizontal attitude in 
which they were laid down : the whole mass of horizontal layers 
has simply been raised with respect to the surface of its parent 
ocean/ This elevation is so long ago that the immaturity such 
as still characterizes the Red River plains is here long past ; the 
adolescence seen in the Carolina plains is also long ago lived 
through. In West Virginia we have maturity ; there can be no 
greater variety of form than is here presented. The relief of the 
surface is at its highest value, for while the inter-stream hills have 
not lost much of their original height, the valleys have been sunk 
about as low as they can be, and hence there is the greatest pos- 
sible difference of altitude between hill-top and valley-bottom. 
The streams have become very numerous, and can hardly be 
more so ; every part of the surface is intersected by them. There 
is no room for more. 
From this time on the form of the surface becomes less pro- 
nounced. As the destructive changes progress further, the 
valleys can deepen but little, although the hill-tops must be 
reduced, and the valley-slopes must widen out, and all the topo- 
graphic expression must weaken as old age is approached. This 
is the character of central Kentucky, and appears in the third 
model of the set. Excepting where the valleys are enclosed in 
especially hard rocks, they are wide open, and the variable height 
of the intervening hills makes it clear that they retain no longer 
all of the height that they once possessed. They are weakening, 
passing into forms of less and less emphasis, losing variety, 
becoming old and feeble. 
In the next stage, we may expect to find the valleys so far 
widened that they should form broad plains, smoothly rolling, 
essentially a low-land of faint relief, but occasionally diversified 
1 In speaking here of relative changes of level between land and sea, I do not wish 
to raise the question as to how the level was changed ; except to say that the teachings 
