84 SLOPE SPO kK. SRO ENTS, 
dently desirable to associate ‘‘baselevel’’ with at most only a 
few of these meanings, preferably with only one; and to leave 
the others unnamed orto associate them with other terms. It 
seems to me advisable to limit ‘‘baselevel”’ to the first meaning, an 
imaginary level surface, and to define it simply as the level base 
with respect to which normal subaerial erosion proceeds; to 
employ the term “grade” for the balanced condition of a mature 
or old river; and to name the geographical surface that is devel- 
oped near or very near to the close of a cycle, a “‘peneplain,”’ or 
‘plain of gradation.” The following paragraphs may make the 
need of this discrimination clearer. 
A full understanding of the development of land forms can be 
gained only by tracing the progressive changes of a generalized 
example from the initial stage through the various sequential 
stages to the ultimate stage of an ideal geographical cycle. This 
problem is encountered in an elementary form at the beginning 
of the study of land sculpture in physiography or geology, and 
at the very outset it is necessary to make definite and simple 
statement regarding the limit with respect to which the proc- 
esses of normal subaerial erosion (weather and streams, without 
significant aid from wind or ice) may act. If the limit is defined 
in terms of the slopes that the streams of the region will have 
gained when they have reached a maturely balanced condition, 
the definition will be of no service to beginners; indeed, a limit 
thus defined is elusive and difficult of conception even by 
experts. The limiting surface is certainly of so great impor- 
tance to the beginner that it must be briefly defined for him in 
terms of known factors, and the definition thus framed should 
remain serviceable through all later study. These conditions are 
satisfied when the beginner is told that the limit of subaerial 
erosion is the ‘‘level base” or ‘‘baselevel,”’ drawn through a land 
mass in prolongation of the normal sea level surface. The fact 
that rivers erode their channels near their mouths below sea level 
and that special processes of erosion (winds and glaciers) may 
work below sea level, does not invalidate the general statement 
at the opening of the whole discussion; but these special condi- 
