134 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
is true that the Permian scarps of the northern Uinkaret are long, 
being from three to five hundred feet in relief, and that the Shinarump 
cap may be only fifty or a hundred feet thick. Nevertheless, the oc- 
currence of considerable sheets of talus on the scarps in the valley east 
of Workman spring serves to show first that the Shinarump capping 
yields sufficient material to form a cloak; secondly, that the cloak will 
quickly accumulate if erosion is retarded at the base of the slope; 
and thirdly, that aridity of climate cannot account for the absence of 
talus on the bare scarps a few miles further south. Hence it seems 
that the bareness of these Permian scarps must be explained by the 
recent removal of a talus that once covered them. 
Perched Boulders. — There are certain details connected with the 
problem of retreating escarpments that are finely illustrated southwest 
of Lee’s Ferry, where the full height of the Permian scarps with their 
Shinarump capping is revealed. Here, as on the Uinkaret, the scarps 
are bare; the bareness is appropriate to the active recession of the 
scarps already indicated by the landslides of this neighborhood. An- 
other item was noted. We frequently saw large blocks of sandstone, ten 
to thirty feet in diameter, evidently derived from the cliff above, and now 
perched on pedestals of weak shale a little distance forward from the 
base of the scarp, as in Plate 2. The perched blocks correspond to 
rock tables on glaciers. The pedestals are from three to fifteen feet 
in height ; some of the blocks have lately fallen to one side, and the 
pedestals that once supported them are not yet wholly worn away. The 
top of the pedestals represents the height of the graded platform at 
the base of the scarp when the block rolled down from above. Since 
then the platform has been degraded by the height of the pedestals ; 
at the same time, the scarp has been pushed back a decidedly greater 
amount, and at a rapid rate compatible with the absence of a talus cloak 
on its slope. 
Spurs and Ravines. —The minute morphology of the carved scarps 
affords a pleasing study; it is a subject of subordinate importance, 
truly, yet perhaps as deserving of careful attention as the morphology 
of the leaves of plants. Where the mesa scarps have a straight front, 
the ravines descend in relatively direct and parallel courses with few 
branches. Each main ravine receives the waste from but a small length 
of cliff front above. The spurs are correspondingly simple, each one de- 
scending with little variety of form from top to bottom of the scarp, 
although with increasing relief downwards. But where a curved re- 
entrant occurs in a mesa front, the ravines converge more or less dis- 
