BASELEVEL, GRADE AND PENEPLAIN 103 
brought down absolutely to the level of the sea, but only nearly to that 
level (1895, 34, 35). 
Salisbury states that ‘‘no definite degree of slope can be 
fixed upon as marking a baselevel.’’ Geikie defines baselevel 
as ‘‘a level not much above that of the sea.” Brigham writes: 
The great river first cuts its bed close to the sea level, and we say 
that a portion of the valley is reduced to baselevel. It lacks a little of 
it, but the difference is so small that we neglect it. 
These qualified statements are apparently the result of 
attempting to define a surface in terms of its variable fea- 
ture, slope, instead of in terms of its constant feature, bal- 
ance of activities. Whatever slope is agreed upon must change 
to a fainter slope if more time is allowed; but the balance, 
once struck, is always maintained as long as the cycle endures. 
Another cause of difficulty in definition seems to have arisen 
from giving the same name to a variable and to its lmit. 
Both the imaginary warped surface and the actual peneplain 
are essentially variables; their variations are similar and sys- 
tematic; they both approach, but never reach the limiting 
base of subaerial erosion. The latter is essentially a con- 
stant, accurately definable from the beginning, and remain- 
ing unchanged while the variable surfaces approach it. It 
may be defined as the limit of either of these variables in a 
strictly mathematical fashion. The baselevel is the level: base 
toward which the land surface constantly approaches in accord- 
ance with the laws of degradation, but which it can never reach. 
Plains and peneplains— The names for surfaces of ultimate 
and penultimate subaerial erosion deserve brief consideration. 
My own preference, prejudiced, perhaps, by a share that I 
have had in making up names, would be to avoid ‘“base- 
level’? as a technical name for any geographical form, to 
use “plain” sparingly for surfaces of erosion, because of 
the rare occurrence of complete or ultimate planation; and 
be) 
usually to employ ‘‘ peneplain’”’ as the name for the penultimate 
form developed in a cycle of erosion. It was in order to 
avoid the implication of complete erosion, and the objections 
