IOWA ACADEMY OP SCIENCE 
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the immediate valleys of the rivers only need be regarded as filled up. As 
defiation progresses the belts of hard and soft rocks would be, perhaps, brought 
into somewhat sharper contrast than they commonly are at corresponding 
stages in a wet climate. The geologic structures would also be more accentuated. 
The rock-fioor would be cleaner swept. The areas of weak rocks would be re- 
moved somewhat faster. At all times the plains aspect would be more strik- 
ingly dominant. 
The country upon which an arid climate is imposed might be in the beginning 
either of plain or mountainous relief. Whether the one, or the other, there 
would be a tendency for the different surfaces to become soon very much alike. 
Crustal uprising in an arid climate does not initiate a new and distinct cycle of 
down-cutting; it merely allows greater or longer erosion. In this respect the 
soil is different from that of the erosion cycle in a moist climate. 
In general, four fundamental factors have to be considered: (1) Whether 
in the introduction of an arid climate the general relief was mountainous or of 
the plains type; (2) whether the epeirogenic movement was slight or great; 
(3) whether before the epeirogenic movement there was much or little folding 
and faulting; and (4) whether subsequent to the epeirogenic movement there 
was extensive deformation. 
Applying these tests to a specific region, as arid United States, for example, 
I recently suggested* that in the high-lying plain of the Mesa de Maya there 
is strong reason for believing that it is the last remnantal representative of a 
great pepeplain upraised in Mid or Late Tertiary time. It has been long 
universally conceded that the epeirogenic movement was great. 
In the arid country it seems. that all four mutations or combinations above 
mentioned are well represented. The Great Basin appears to be an area which 
both prior and subsequent to the last epeirogenic movement suffered severe 
folding and faulting. There was much early, but little late deformation in 
the northern part of the Mexican tableland. The Colorado plateau appears to 
have been little effected by either ancient or modern faulting and folding; which 
is also true of the South African region. In southern California there is little 
to indicate very extensive pre-epeirogenic deformation but much evidence of 
great and wide-spread post-epeirogenic orogeny. 
In the existence of plateau-plains at many different levels in the broad belts 
of the less resistant rocks there thus appears to be furnished one of the 
strongest proofs of the eolic character of the general erosive activities; since in 
situations of this kind not only are rain-fall and water-action very deficient and 
wholly inadequate to produce the relief effects presented, without the time-ele- 
ment be vastly and unreasonably prolonged, but conditions are such in many 
oases as absolutely to preclude the intervention of stream-work. 
The summit plains of the continent in New Mexico is a region of continual, 
high winds and constant sand-storm. Nowhere else in the desert of the South- 
w'est is windscour in active operation so advantageously viewed. Nowhere else 
in this country are deflative effects and desert-leveling so well displayed. No- 
where else in all the world is general lowering of a country by the wind so 
strikingly presented. Pew places there are on this continent where stream- 
action as a general erosional power is so manifestly utterly impotent. 
*Bull. Geol. Soc. America, Vol. XIX, p. 76, 1908; also, Proc. Iowa Acad. Sci., Vol. 
XIII, p. 221, 1908. 
