178 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
appearing falls and cliffs will remain in evidence for a much longer time 
if they are up-stream from the master ledge, for there they must recede 
until they are concealed beneath the backward extension of the gently 
sloping graded reach or platform below them. Thus a master ledge 
promotes the development of a high fall or long talus-slope beneath it, 
and of a long reach or broad platform above it ; and when this relation 
is established, the number of separate elements of form in valley 
bottom or on valley side is much less than it was in earlier youth. 
The great escarpments of the High plateaus exhibit the concentration 
of all the large and small cliffs of youth in a relatively small number 
of great cliffs in late maturity ; and thus still again confirm Dutton’s 
opinion that the erosion of the plateaus and the erosion of the canyon 
took place in different cycles, separated by a strong uplift of the region. 
The general principles here reviewed are of wide application, but in 
regions of moderate relief and moist climate, the forms of valley sides 
are seldom analyzed: certainly they are frequently overlooked in 
geographical descriptions. In the Grand canyon district, where the 
relief is on a huge scale, and where the arid climate lays bare every 
topographical detail, the elements of form assumed under moving 
streams of waste on valley sides are conspicuous ; they are glorified by 
mere magnitude so that one is tempted to treat them as a new class of 
topographic forms, until it is recognized that they are only new varia- 
tions on an old class, examples of which are to be found in all regions 
of horizontal structure. The canyon walls in the Kaibab and the great 
mesozoic ‘‘ terraces” that overlook the plateau from the north exhibit 
the earlier and later stages of all these forms with great clearness. 
Cirques, Cusps, and Niches. — The many variations in the horizontal 
pattern of the cliffs in the canyon walls have been briefly described by 
Dutton (e, pp. 258, 259). The cliff outline, as seen in plan, has two 
expressions (Figure 16). In some cases, the re-entrants of a cliff are 
sweeping concave curves, but little notched, while the intervening 
salients are sharply attenuated cusps. In other cases, the re-entrants 
are narrow and acute notches, while the salients are broad and rounded 
spurs. The difference between these two cases seemed to depend partly 
on the drainage area of the uplands, whose waters are shed over the 
cliffs from higher levels, and partly on the amount of erosion that the 
cliffs have suffered ; these two factors being indirectly connected. 
Where an upland sheds a stream over a cliff, the cliff will be cut back 
much faster by the stream than it will be weathered back on the inter- 
stream front; here the re-entrant must be an acute notch; while the 
