BASELEVEL, GRADE AND PENEPLAIN Q7 
of that very slow and long-continued decrease of river slope 
which continues through late maturity and oldage. Furthermore, 
if at any time in the cycle a change of climate should occur, new 
slopes would have to be developed by the streams in order to 
bring about a new balance between erosion and transportation 
under the new relation of load and volume. If, for example, the 
changes were from humid to arid conditions, all the valley floors 
would have to be steepened by aggradation. If from arid to 
humid, the graded valley floors would be sharply trenched, and 
in time reduced to lower slopes. There can be little doubt that 
under an increased rainfall the ‘‘baselevel’”’ described in south- 
eastern California by Fairbanks would be sharply dissected, 
quite independent of any elevation of the region. I have 
already discussed certain aspects of this problem (1896, 377). 
The clearest account that I have found of the normal varia- 
tion of the graded slope is inthe paper by Johnson already cited. 
He says that the graded slope ‘continually alters its inclination. 
There is a slow departure from equilibrium, and there is closely 
following readjustment toward recovery of it.” The graded 
slope passes through its ‘‘transformations with a slowness com- 
parable to that of mountain wear. .... It keeps pace with the 
slow growth of the débris mass following upon mountain lower- 
ing.” But, if a stream be deprived of the greater part of its load 
by some abnormal changes, it ‘‘ would at once attack its former 
slope of equilibrium, and rapidly, though at a progressively slow- 
ing rate, lower it. Onthe other hand, its load largely increased, 
the stream would rapidly build up the slope. In either case, 
however, it would come toa stand at a new grade of equilib- 
Givin (Ooi, .O2i): 
The same author attaches much importance to the effect of 
climatic changes on graded river slopes. When describing the 
dissection of the High plains, he says: 
It is not necessary, in order to account for change in behavior of the tra- 
versing streams, to appeal to deformation. A sufficient cause may be looked 
for in change of climate. There is record of erosion, with reversal to deposi- 
tion and rebuilding, and reversal again finally to erosion, and there is reason 
