170 
EOLIAN GEOLOGY 
None of the supporters of the dissected fault-block idea of form¬ 
ation of the desert ranges ever explain the steep, straight scarp 
on the down-slope side of the tilted mass. It seems never to have 
been observed; yet it is one of the constantly recurring phenomena 
among arid mountains. Gilbert, clambering over many a range 
displaying the feature in magnificent proportions, never notes 
it. Davis, visiting numerous mountains of this type gives no rea¬ 
son why there should be profound fault on the one side and none 
on the other. In truth the presence of fault-scarp seems to be one 
of the yet undetermined premises of desert range structure. 
The formation of the encircling shelf of the desert range appears 
to be a direct result of natural sand-blast action, or the cutting 
process of strong, sand-armed winds, the chief abrading zone of 
which is always immediately above the surface of the ground. 
This is the reason why in the desert plain meets mountain as sharp¬ 
ly as the strand line of the ocean. 
^ Keyes. 
Multiplanary Relief of American Deserts. Presence of multi¬ 
tudinous plains-levels, so striking a feature of western arid coun¬ 
try, is highly confusing because of attempted transfer of moist 
climate notions to tracts where water is a negligible quantity. The 
peneplain being the ultimate physiographic goal towards which all 
pluvial depletion tends, intitiation of a new erosion cycle, or re¬ 
juvenescence of an old cycle can only take place where there is 
regional uplifting of the earth’s crust. Unlike under conditions 
of pluvial climate, in arid regions planation early assumes large 
importance in the general lowering and depletion of the land sur¬ 
face, and the plain rather than the hill is the dominant feature of 
landscape throughout the erosion cycle. 
In moist lands general denudation procedes through the down¬ 
cutting of the streams. In countries of little rain planation takes 
place horizontally; stream-work is conspicuous because of its ab¬ 
sence. On the one hand the hills are mainly attacked by the 
erosional agencies and are lowered most rapidly. On the other 
hand, the valleys are the objects of chief attack and form the belts 
of fastest lowering, while the hills about increase their stature. 
These are the contraposed products of erosion by rain and by wind. 
Variance of level of the myriad remnantal plains of mesa-land 
is really an accidental consequence of regional depletion inde- 
