BASELEVEL, GRADE AND PENEPLAIN 10] 
Until the imaginary surface is thus realized, it is hardly worth 
while to attempt to conceive it; for as far as the control of ero- 
sive processes is concerned, that is better exercised (as has been 
stated already) by the visible skeleton of the surface that is seen 
in graded streams than by the surface itself. It is always with 
reference to the graded course of the main river that the side 
streams are graded; it is with reference to all the graded streams 
that the slopes of the interfluves are graded. These relations 
of branch to trunk and of side slopes to streams are of the very 
greatest importance and must be considered with the utmost 
care; but the imaginary surface passing from river to river 
under the hills of the interfluves has relatively little importance 
as a control of the processes of erosion. With every extension 
headwards along the graded channels of branching streams, the 
surface becomes more warped and wrinkled, more difficult to 
conceive, more likely to differ as conceived by different minds. 
It must be not only irregularly warped when first defined, but it 
must vary slowly in form and slope. Just as no limit can be set 
to the headward part of the graded main stream or to the number 
of graded branch streams to be taken as guides for the imaginary 
warped ‘“‘baselevel,’’ so no limit can be set between the graded 
stream courses and the graded waste slopes of their head or 
along their sides. The imaginary surface should, if conceived 
at all, follow the lead of al the graded lines and surfaces as 
fast as they are developed; it should be extended as they are 
extended, modified as they are modified. But if this be agreed 
to, part of the imaginary surface becomes a real surface, and 
the rest may be neglected until it also is realized. 
Just as every reach in a stream is graded with respect to the 
next down-stream barrier or local baselevel, so every waste slope 
is graded with respect to the ledge or cliff at its lower margin; 
the lowest reach, ending at the river mouth, is graded with refer- 
ence to the general baselevel or the ocean; the lowest graded 
slope of a valley side is graded with reference to the stream (or 
flood plain) in the valley bottom. Again, every bare ledge on 
a mountain side will, in time, be graded (or consumed ) by the 
