188 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
escarpments”’ (a, p. 204). It may be urged against this view that 
escarpments of a strength appropriate to their capping layers occur in 
better-watered regions, as in the Catskill mountain front of eastern 
New York, and in the Swabian Alp of southern Germany; and that 
the cause of the great cliffs of the Grand canyon district is to be found 
chiefly in the massive thickness of the resistant strata that guide them, 
and in the weakness of the underlying strata that sap them. Truly, 
the sharpness of the cliffs is highly suggestive of aridity, but a relatively 
short arid period would suffice to sharpen the cliff profiles, even if they 
had been somewhat dulled by a previous humid period. 
The narrowness of the canyons, so generally explained by the aridity 
of the plateaus through which the vigorous Colorado flows, certainly 
finds a large part of its explanation also in the recency of the uplift by 
which the canyon cutting was initiated, and in the massiveness of the 
resistant layers by which the canyon walls are defended. The side 
canyons by which the plateaus are dissected are not insufficient in num- 
ber even for a moist climate; they are much more numerous than the 
perennial side streams. The latter are notoriously rare: the former are 
present in good number, and in wet weather they are actively washed 
by their temporary floods. It is true that the side canyons are of so 
steep a descent along their floors that they lose much of their depth at 
a moderate distance back from the main river; but this may be a conse- 
quence of recent uplift as well as of slow corrasion. The side canyons 
branch frequently enough to satisfy an active drainage system, and cer- 
tainly there is no deficiency of ramifying valleys on the higher uplands 
where the surface is so thoroughly and maturely dissected. It is the 
steep grade of the waste on the floors of these valleys that suggests a 
development under an arid climate, and such a grade would be soon 
acquired under a dry climate even if the valleys had once been cut 
somewhat deeper under a moist climate. Hence, even in the more 
recent past of the canyon cycle, a humid climate seems no more impos- 
sible than unnecessary, and in the more distant past of the plateau 
cycle, climates of any and all kinds might have prevailed, as far as the 
present topography of the Grand canyon district is concerned. 
Moist Miocene and Arid Pliocene Climates. — In contrast to Powell, 
Dutton concluded that the Miocene (plateau cycle) was humid. This 
opinion seems to have been based partly on the occurrence further north 
of extensive fresh-water deposits of Miocene age, usually interpreted as 
having been laid down in large lakes, whose existence pointed to a good 
supply of rainfall (c, p. 223), and also on the apparent desiccation near 
inner SO 
