188 bulletin: museum of comparative zoology. 



escarpments " (a, p. 204). It may be urged against this view that 

 escarpments of a strengtli 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 

 chieflv 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 pex-iod. 



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 

 hiving 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 



