122 
UTAH PENEPLAINS 
of continuous and notable upraising. The hypothesis of isostasy 
which was the chief outcome of Dutton’s meditations was just 
beginning to take on tangible form. 
Be that as it may, it is not the theory of isostatic compensation 
that is now under rigid scrutiny. The main consideration con¬ 
cerning Utah’s High Plateaux is the character of the testimony 
which they offer on the course of the physiographic development 
of the larger tract around the southern end of the Rocky Cordil¬ 
lera. Bearing directly upon this point they seem to supply the 
very links missing from other parts of the continent. They carry 
us back immediately one cycle farther than any with which we 
were previously cognizant. They bring into orderly relationship 
a host of scattered observations which heretofore had no satis¬ 
factory place of abode, and little meaning. They seem to enable 
us to picture forth the former existence and nature of a great 
peneplain the expanse of which was the greater portion of the 
southwestern United States. As last lingering traces of a former 
cycle of geographic sculpturing the High Plateaux of Utah call 
forth exceptional consideration. 
With the high powers to resistance to differential erosion the old 
lava sheets comprising the substructure of the Plateau terreplains 
serve to retard in the general degradation of the country early 
effacement of the spots which they cover. The surfaces on which 
the lavas rest may not, of course, be exactly the original plains- 
level on which the geographic cycle was initiated; but if not they 
are certainly very close to it. Like the tops of the Basin ranges 
they may actually represent a sub-summital level. Elsewhere in 
the arid region the essential phenomena of the High Plateaux recur 
again and again at different and lower levels. Far around the 
Rocky Mountain uplift lofty lava-capped plateaux continue to 
represent the same prevailing conditions. To this type belongs 
the famed Mesa de Maya far to the eastward near the Texas line. 
When, a decade ago, the development of the geographic cycle 
in an arid climate was under discussion, I argued for an initial 
plains-surface, or peneplain, instead of a mountainous configura¬ 
tion, as had been commonly assumed. At that time the sharp 
desert sierras of New Mexico, Arizona and Old Mexico were 
alone in mind. The Utah High Plateaux now seem strongly to 
support the original plains contention. 
