132 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
moval of waste on the graded platform beneath ; and the relation must 
evidently be one of equality. The slopes have been described as 
“bare,” and so they are in a first general view, but in reality they are 
cloaked with somewhat loosened clay or shale, a little displaced from its 
normal position, still revealing the general attitude of the strata, yet 
masking the finer details of bedding. It must be supposed that the 
creep and wash of loosened waste from the slopes just suffices to keep 
the wet-weather streams busy when they are running in the channels on 
the graded floor; and hence that we here have one of the many exam- 
ples of a natural equilibrium between capacity for work and work to be 
done. Similar examples of equilibrium probably occur between the 
descent of waste on certain dissected mountain slopes and the removal 
of the waste across the graded rock-floor platforms that stretch forward 
from the mountain base, as described by Penck for the subarid moun- 
tains north of Madrid in central Spain (p. 132) and by McGee for the 
arid mountains of the Sonoran district of Arizona and Mexico (4, p. 91, 
Plate 12). The basal angle between the slope and the platform seem 
to be more sharply defined in arid than in humid climates, probably 
because of the different values of the various agencies for soil produc- 
tion and removal in the two cases. 
THe PERMIAN SCARPS UNDER THE SHINARUMP CLIFFS. — The compara- 
tively small quantity of waste on the Permian scarps beneath the 
Shinarump cliffs of certain fine mesas in the northern part of the 
Uinkaret plateau suggests a recent revival in the process of sapping ; 
for it is difficult to understand how cliffs that have receded as far as 
these — forty miles from the canyon — should not by this time have 
their under-slopes well sheeted over with waste, if the retreat had been 
in a single cycle. The Permian scarps are not always bare: for exam- 
ple, those enclosing a valley opened in the Shinarump mesa ten miles 
southeast of Toquerville, and extending from Sheeptrough to Workman 
spring (see Atlas of Grand Cajion district, sheet XX.), are cloaked with 
a considerable covering of waste, which here and there bears a mask 
of vegetation ; but in this case the processes of erosion have been much 
delayed by a recent flow of lava on the valley floor (not shown on 
Dutton’s map). The southern scarps of the same group of mesas, only 
about ten miles further south and without lava at their base, are very 
bare, exhibiting many delicately colored strata that maintain a horizon- 
tal course across beautifully varied spurs and ravines. The contrast 
between these two examples re-enforces the suggestion that the bare 
scarps may have been much better clothed with waste during some 
