375 
[Davis and Wood. 
ago, the latter explanation would probably have been accepted with- 
out a question. At present, many students would almost as un- 
hesitatingly accept the former. But it must be admitted that, as 
far as our inquiry has progressed, both are adequate, and some bet- 
ter method of discriminating between them than the fashion of the 
time must be devised. We find no established guides in this in- 
quiiy, but the following considerations may be suggested. 
A subaerial baselevel plain is gradually completed by the action 
of ordinary forces on all parts of its surface. Reduction to base- 
level is slowest along the divides on the harder rocks, and quickest 
along the streams on the softer rocks. The valley bottoms there- 
fore approach and practically reach baselevel long before the in- 
terstream areas are reduced so low. 
A submarine platform is essentially completed strip by strip, 
once for all, as far as it goes ; its advance is rapid at first, very slow 
at last. Its landward margin is surmounted by a sinuous cliff or 
slope with a level base, facing the sea and separating an interior 
of greater or less relief from a smooth sea-bottom, uncon formably 
veneered over with the deposits from the land. If the transgres- 
sion of the sea over the land be aided by a depression of the land, 
many inequalities of the surface might be preserved beneath the un- 
conformable cover of marine deposits. Such a surface, when again 
lifted and somewhat denuded, might be indistinguishable from one 
that had not been submerged. The occurrence of unconformable 
deposits on an even foundation cannot alone be taken as evidence 
that the foundation is a surface of marine denudation ; it may be 
a subaerial baselevel plain depressed below sea-level and covered 
with sediments from an adjacent portion of the same that was not 
submerged. 
The ultimate forms of the two kinds of plains are probably much 
alike, and it may be hopeless to seek to distinguish them after they 
have been elevated and roughened by subsequent erosion. But the 
penultimate forms of the two might be separated ; one would be 
gently rolling, its residual inequalities being of the hill and valley 
type ; the other would be smoother, and might be very smooth if its 
veneer be regarded as its surface, but it would have a definite mar- 
gin, beyond which the penultimate subaerial plain would be found. 
The two forms are of course often associated. 
9. Value of contoured maps in geographic study. We have 
tried to detect on the restored surface of the old Highland pene- 
