BA SHEE VEL GRADE AND PENEPLAIN 85 
tions must be explicitly considered later, especially those con- 
cerning glacial erosion. It suffices at first to recognize that 
in the ideal undisturbed cycle of normal erosion the base- 
level must be more and more closely approached as time is 
extended. 
This definition of ‘‘baselevel’’ as a level base certainly has 
the advantage of being easily conceived. Once conceived in 
the study of the ideal cycle, it needs no modification so long as 
the relative attitude of land and sea remains fixed. If the land 
rise or fall with respect to the sea, the baselevel takes a new 
position within the land mass, and further progress of erosion 
is then continued with respect to the new limit. 
As the study of the cycle advances it becomes desirable to 
speak of various local or temporary controls of erosion: a rock 
ledge or a lake on a river course, the central basin of a dry 
interior basin either above or below sea level, the surface of 
a lake in such a basin. Nothing can be simpler than to imagine 
a level surface passing through any one of these controls, and 
rising or sinking as the control rises or sinks; and such a surface 
is naturally called a local or temporary baselevel. With the 
enlargement of conceptions that is required when the aggrada- 
tion of depressions is considered at the same time with the 
degradation of elevations, Powell’s more general term, ‘“ grada- 
tion,’’ may replace ‘‘erosion;”’ the merit of this substitution will 
appear more fully in the following pages. 
In view of the importance appropriately allowed to the idea 
of the level base with respect to which the erosion of valleys 
by rivers must proceed, it is curious that earlier writers did not 
give more explicit attention to it; but as far as I have read, 
they were so largely occupied with controverting the various 
older theories of the origin of valleys that it~ did not occur to 
them to give special name to limiting surface of erosion. Their 
understanding of the important principle here involved is to be 
read rather between the lines than in explicit statements. For 
example, Greenwood, in his curious book on Rain and Rivers, 
almost as remarkable for its admixture of jokes and polemics 
