98 STUDIES FOR STUDENTS 
for believing that this series of interruptions of the gradation cycle was an 
effect of climatic oscillation rather than of earth movement (1go1I, 628). 
This is by far the most striking actual example of varying 
graded slopes on record: an example that is easily defined in 
terms of changing grade, but not in terms of changing baselevel. 
It is much more satisfactory to describe the High plains in terms 
of stronger or fainter graded slopes than to consider them ‘‘ near 
baselevel,” as has been done in spite of their standing at alti- 
tudes of 4,000 feet and more. 
The conception of grade must therefore include the concep- 
tion of different and changing slopes in large and small streams, 
in mature and old streams, in streams dissecting weak and strong 
rocks, in streams of arid and humid regions. The conception is 
of the greatest value as a supplement to the simpler idea of base- 
level; but it is so intricate that it cannot be fully apprehended 
until the whole course of the cycle is patiently worked through. 
Yet a still further extension of the conception remains to be con- 
sidered. 
Baselevels are of only two kinds, general ( permanent) and local 
(temporary) ; Grade includes not only the balanced condition of large 
and small, mature and old water-streams, but that of all kinds of 
waste streams as well_—aA final reason for giving different names 
to the limiting base of subaerial erosion and to the balanced 
condition of the mature and old streams that are working with 
respect to the limiting base is found in the essential identity of 
conditions in graded water streams and in graded waste streams, 
and in the strong unlikeness between the attitude of a baselevel 
surface as defined in any of the above citations and the slopes 
often assumed by graded waste streams. These are points to 
which the geologists and geographers of the older schools gave 
little or no attention: indeed it is only about fifty years since 
some of the leaders of our science taught that rounded hills 
could not be formed by subaerial erosion and that they must 
be the work of the sea. It is now well understood, however, 
that slopes covered with soil of local or up-hill derivation are 
really “drained” by an association of many graded waste 
