34 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
similarly, the sharp cusps between the amphitheaters are the later stages 
of the round fronted spurs that once separated the ravines. 
THe PrortLe or SHarp Cusps. — When the red-wall spurs are sharp- 
ened into slender cusps all the overlying Aubrey strata are removed. 
The cusps are then unprotected by waste from above, and they are 
attacked on both sides by the weather. Under these conditions, the 
relatively even edge of the cliff in the amphitheater is exchanged for 
a serrated crest line, a true aréte. Again when two amphitheaters head 
against each other, the isthmus of cliff wall between them is gradually 
narrowed and converted into an aréte that sags in the middle. Arétes 
in Alpine mountains occupy similar positions, but there the amphithea- 
ters are usually cirques of glacial origin, as Richter and others have 
shown. 
The strong color of the red-wall cliffs in the amphitheaters is due to 
staining by wash from the overlying lower Aubrey red beds. Where the 
serrated cusps stand forth, stripped of the Aubrey cover and out of the 
way of descending wash, the cliffs are gray. 
It has been suggested to me by a correspondent, well versed in the 
topography of arid regions, that the amphitheaters of the canyon walls 
are more largely the product of wind action than would be inferred 
from the explanation here given for them. Certainly the wind is 
a powerful agent in regions where vegetation is as scanty as it is on thie 
barren walls of the Colorado canyon, yet I cannot think that wind action 
is largely responsible for the amphitheaters, inasmuch as their slopes 
are always developed in accordance with lines of gravitative action and 
not in sympathy with the flowing lines characteristic of the bottom of an air 
current. The winds must often enough sweep the finer detritus about, 
yet its general disposition is essentially such as creeping and washing 
waste should assume. So with the regularly concave amphitheaters : 
under the control of wind action these forms might occur at various 
altitudes, irrespective of tributary slopes at higher levels. As a matter 
of fact, the concave amphitheaters are developed only in particular sit- 
uations with respect to the wash of waste from higher levels, and there- 
fore, however active the wind may be in shaping them, its activity must 
be subordinate to that of local weathering and of the down-slope move- 
ment of waste under the control of gravity. 
GENERAL RELATIONS OF AMPHITHEATERS AND Cusps. — The correctness 
and generality of this view as to the origin of the amphitheaters and 
cusps are sustained by several lines of evidence. In the first place, the 
earlier forms, from which the cusps and amphitheaters of the red-wall 
