BASE-LEVEL OF EOLIAN EROSION 661 
purely eolative origin, the present lofty mountains and the immeasur- 
able intermont plains were regarded simple, differential effects of 
wind-scour. Extended observations seem conclusively to show that 
there once existed at a level of about 5,000 feet above the present 
general plains-surface an old uplifted peneplain. Out of this were 
sculptured existing highland and lowland, the belts of more resistant 
rock-masses forming the mountain ranges and the belts of weak rocks 
the intermont plains. Compared with the usual operations of the 
geologic processes under normal moist conditions the most noteworthy 
effects of those under conditions of an arid climate are the prevalency, 
constancy, and high efficiency of wind-scour action, deflation, or 
eolation, the very subordinate, local, and sporadic character of water- 
action, and the remarkable plains-forming tendency which eolian 
erosion imparts to the landscape." 
In the South African deserts Passarge has devoted special atten- 
tion to the work of the winds. The importance of his great general- 
ization lies in the suggestion that it is possible under conditions of an 
arid climate for the general planation of vast areas to go on without 
regard to sea-level. Fora long time this author? found great difficulty 
in accounting for the great plains-surfaces by wind action alone, for 
the reason, as he explains, that the wind has no base-level of erosion 
and must continue the work of excavation and removal wherever the 
rock-floor is not resistant. There appears to be, however, a limit 
even to this desert-leveling and eolian excavation. The ground- 
water level in a structurally inclosed basin must finally put a stop to 
wind-action by keeping the surface moist, either giving rise to salinas 
or forming a basin into which sporadic storm-waters find a long 
resting-place.s This would seem to be the case of the Death and 
Imperial valleys of California, the basin of Lake Eyre in Australia, 
the Aral and other basins of western Asia, and many of the great 
depressions of the Sahara. 
The absolute independence of one another of neighboring inter- 
mont plains, such as everywhere occur in the American arid regions, is 
t Pop. Sci. Mon., LXXIV (1909), 23. 
2 Zeitsch. d. deut. geol. Gesellsch., LVI. Bd. (1905), Protokol, p. 108. 
3 Am. Jour. Sci. (4), XVI (1903), 377- 
4 Bull. Geol. Soc. America, XIX (1908), 91. 
