BASELEVEL, GRADE AND PENEPLAIN 107 
but on later pages he uses the phrase ‘cycles of gradation” or 
simply ‘‘cycles”’ instead of ‘‘gradation periods.’’ It matters 
little which of these terms is used; but it would certainly be an 
advantage that only one should be retained to express the single 
idea here considered. 
Denudation and degradation. \t seems worth while to call 
attention in this connection to the desirability of a more careful 
discrimination than is customary between the terms, denudation 
and degradation. Denudation might be advisedly used as the 
name of those active processes, chiefly operative in the youth 
and maturity of a cycle, by which rock structures are laid bare, 
literally denuded, because their waste is removed as fast as it is 
formed. Degradation on the other hand is more appropriately 
associated with those liesurely processes, characteristic of the 
later stages of the cycle, in which a graded slope is reduced to 
fainter and fainter declivity, although maintaining its graded 
condition all the while. Aggradation is naturally the opposite of 
degradation, and implies the deposition of rock waste by trans- 
porting agencies, the built-up surface being always kept essen- 
tially at grade. Thus defined, denudation would accompany the 
early work of downward corrasion by streams, and the longer- 
lasting work of valley widening by weathering and washing. It 
would be systematically transmuted into degradation as the pro- 
cesses that operate on the various lines of down-slope streaming 
attained the graded condition; the large rivers first, the smaller 
branch streams later; the headwater streamlets and the hillside 
waste streams later still. Retreating cliffs and summit ledges, 
the last strongholds of denudation, would pass into the phase 
of degradation when they are reduced under the graded waste 
cover in the stage of subdued relief, characteristic of late 
maturity and early old age; and thenceforward all further ero- 
sion would be by degradation alone. 
Conclusion.—It may seem at first reading that this essay is 
concerned with words rather than with facts; but such is not my 
intention. My object has been primarily to secure a just and 
accurate recognition of facts, and only secondarily to attach 
