132 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



moval of waste cm 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 (6, 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 dilferent 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 Sheepti'ough to Workman 

 spring (see Atlas of Grand Canon 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 

 Duttou'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 

 l)i 'tween these two examples re-enforces the suggestion that the bare 

 scarps may have been much better clothed with waste during some 



