140 
UTAH PENEPLAINS 
soon will be when the last lingering vestiges of their summital 
plains are obliterated by the wasting away of their lava caps. 
Their existence then becomes only a memory, and rests wholly on 
hypothesis. The Duttonian conception is faulty for reason of the 
fact that only a very circumscribed area was considered, and no 
attempt is made to view the problem in its larger aspects. This 
might have easily been done since as a part of the same regional 
depletion abundant evidences are retained in the highlands lying 
to the eastward. There in the great denudation interval the 
ancestral Cordilleras thrice reared their heads as majestically above 
the Great Plains as do the Rockies of today, and as often were 
completely razed to the level of the sea. But in Dutton's day the 
conception of a geographic cycle finds not faintest suspicion, and a 
distinct desert geology is yet to be formulated. 
We have to gather the missing chapters of the denudation 
history of the Grand Canyon region from the preserved records 
of neighboring districts. There is every reason to believe, there¬ 
fore, that over the Navajo Dome all four of the principal pene¬ 
plains once extended in full force as they ,did over the southern 
Rockies and over the High Plateaux region. 
Whether any of the four great planations are really reflected 
in the relief of the Grand Canyon itself is difficult to say at this 
time. It may be that the esplanade widening may be eventually 
correlated with the Maya peneplain and the terrepleins of the 
High Plateaux. This being the case it leaves the long history of 
the Grand Canyon unrecorded there, concerning which nothing has 
yet been even indicated. Or, the Colorado River may be one of 
those desert streams which arise from the introduction of pluvial 
climate on the contiguous highlands because of their rapid eleva¬ 
tion above the intermontane spaces. In this case its history has 
to be entirely rewritten. 
I 
