660 CHARLES RK. KEYES 
regarded as pre-eminently plains-forming. On this account, mani- 
festly, it is that in the arid regions of our western country there is the 
vast general plains-surface, a veritable and illimitable sea of earth 
out of which, isle-like, rise the myriads of lofty desert ranges whose 
foundations are the harder rock-masses. 
In the absence of distinctive stream-action in the desert regions 
there has appeared no delimiting factor controlling the general lower- 
ing of an arid area, comparable to that establishing a base-level of 
erosion postulated for every humidland. The difficulties arising from 
this want of a general planation-surface to which all relief features 
of the desert could be referred are especially noted by Passarge’ in 
his masterly treatment of the South African “Inselberglandschaft.”’ 
Penck? also appreciates the same difficulty when he suggests 
that so long as the ocean be held back from a desert eolian erosion 
might go on indefinitely below sea-level. In his discussion of the 
geographic cycle in an arid climate Davis apparently recognizes the 
force of this desideratum, but refers this most conspicuous plains- 
characteristic to the results of perfected drainage and general lowering, 
much as they are accomplished under conditions of humid climate, 
though at an infinitely slower rate. ‘That there is really a level below 
which eolian erosion in arid regions cannot go and which for all 
practical purposes corresponds to the base-level of stream-corrasion 
in a moist country, seems amply demonstrated by recent observations 
made in Death Valley and in the Imperial Valley, in California, areas 
lying below sea-level. The testimony seems fully corroborated in 
other desert valleys of the region. 
Until lately the origin of the larger and most characteristic relief- 
features of the great desert regions of the globe has remained without 
adequate or satisfactory explanation. Direct genesis upon the basis 
of ordinary tectonics, or of normal erosion during former wet-climate 
periods, or of water-action under present conditions, has always met 
with unsurmountable obstacles. When recently4 the physiognomy 
of our western country was briefly considered from the viewpoint of 
t Zeitsch. d. deut. geol. Gesellsch., LVI. Bd. (1904), Protokol, p. rot. 
2 Am. Jour. Sci. (4), XIX (1905), 165. 
3 Journal of Geology, XIII (1905), 382. 
4 Ibid., XVI (1908), 434. 
