IOWA ACADEMY OF SCIENCE 
KiO 
our own country Spurr*, Crossf, and Blackwelder^: have also called attention 
to certain phases of wind-work in general erosion. 
In the western arid country recent observations seem to point strongly to the 
fact that the wind is to be regarded as the chief erosive agent§; w’ater-action 
playing distinctly only a very secondary role. According to the conclusions 
thus reached, the wind must he considered in a dry climate fully as effective 
in general erosion and leveling as is water in a normal wet region. In the 
arid country wind appears not only the most potent of the gradational agencies, 
but its efficiency as an erosive force is probably greater than that of all other 
erosive processes combined. Within the confines of the desert itself its main 
activities are severely degradational in character; its constructive effects are 
local and relatively inconsequential. 
Under favorable conditions wind-iaction is, in its broader effects of reducing 
a country to a lower plains-level, not so very unlike water-action. The areas 
occupied by the less resistant rock-belts are removed faster than the areas of 
more indurated rock-masses, dividing the region into belts of lowland and belts 
of highland. The former unite in the general plains-surface; the latter con- 
stitute the mountain ranges which, owing to ancient deformation, now have the 
aspect of tilted or upthrusted blocks. 
Physiographically the ultimate goal of general erosion is the plain. Whether 
accomplished by marine denudation, stream-corrasion, or deflation, a general 
plains-surface is the final product of a completed geographic cycle. Between 
general leveling in moist climates and in dry ones the chief difference is that 
in the one case the greater part of each geographic cycle is spent in attaining 
the general plains-surface, or peneplain; while, from the very beginning, in 
the other the plains-surface is the dominant relief-form. In the case of the 
latter, as Passarge|| has pointed out in the instance of the South African 
desert lands, and as has been urged for the arid regions of western America**, 
there is, without base-leveling, leveling more complete than that of peneplana- 
tion. The interment plains and the general plains-surface of the desert are 
smoother than any known pepeplain. 
It may be here observed that eolian activities under the favorable conditions 
of an arid climate are pre-eminently plains-forming. In a region of alternating 
belts of non-resistant and hard rocks the materials of the first are much more 
rapidly removed than in the case of the second. As general lowering of the 
country goes on the plains character is constantly extended. This seems to be 
largely accomplished by the air-currents heavily charged at the bottom with 
sands which cut most vigorously at the line where plain and mountain rneetft^ 
much in the same way as the sea works in, carving out of its shore a sub- 
marine shelf. In comparing the effects of a large stream in time of flood, and 
a gale of wind, it was recently estimated that in the cases of the river, and of 
the lower 20 feet of a desert air-stream, there are equal amounts of fine rock- 
waste moving in like areas of cross-section; but the air-stream moves 40 miles 
*U, S. Geol. Surv., Prof. Pap. No. 42, p. 110, 1905. 
tBull. Geol. Soc. America, Vol. XIX, p. 53, 1908. 
Uournal of Geology, Vol. XVII, p. 443, 1909. 
SJournal of Geology, Vol. XVII, p. 31, 1909. 
ilZeitsch. d. deut. geol. Gesellschaft, LVI Bd., Protokol, pp. 193-209, 1904. 
’^*Bull, Geol. Soc. America, Vol. XIX, p. 86, 1908. 
ftScience, N. S., Vol. XXIX, p. 7-53, 1909. 
