592 C. R. KEYES RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



from the center of a basin any sporadic waters that might come down 

 from the inner side of the mountain rim and carry them into a lower and 

 outside plain. This is well shown by the Tijeras Canyon, which bisects 

 the Sandia range east of Albuquerque and which, while it would be 

 expected to drain eventually the great Estancia Plains into the Eio 

 Grande Valley, will probably do nothing of the kind. It merely carries 

 off the surface storm waters from the eastern slope of the range. The 

 Palomas Canyon, in the Sierra de los Caballos, is a similar illustration. 

 The Southern Pacific Eailway, in traversing the deserts of southern 

 New Mexico, Arizona, and California, gives opportunity to observe dozens 

 of like cases. The Santa Fe Eailway, in Arizona and California, does 

 likewise. Through means of deflation the soft central parts of the in- 

 termont plains are more than keeping pace with the lowering of the rims 

 of hard mountain rock. 



According to recent observations in the deserts of Nevada, California, 

 Arizona, and Mexico, the actual levels above mean tide of contiguous 

 intermont plains is perfectly independent of general drainage and largely 

 also of recent deformation. This is particularly well shown in the Death 

 Valley region.^^ It may be that the position of Death Valley itself, now 

 500 feet below sealevel, is as much the result of deflation as it is of tec- 

 tonics; and the same may be suggested concerning the Imperial Valley, 

 the bottom of which is below sealevel and is partly occupied by the Salton 

 Sea. 



ARID CYCLE INITIATED IN A PLAINS REGION 



The descriptions of Passarge and of Bornhardt of the South African 

 Inselberglandschaft appear to be based on the general desert-leveling 

 effects of a surface that originally was a typical plains plateau with small 

 contrasts of relief. These authors do not recognize a distinctly staged 

 arid cycle comparable to the divided cycle of the moist region. It is not 

 likely that under the conditions existing in that country there ever could 

 be worked out a genetic scheme in the same sense as it is understood in a 

 wet climate. In the case of the Mexican tableland and of the adjoining 

 country of southwestern United States, the possibility of the existence of 

 a vast plains surface, a peneplain perhaps, at the beginning of the arid 

 period is not to be overlooked. Indeed it is probable, as already noted, 

 that this factor must be one of the main conditions to be reckoned with. 



In the initiation of an arid period on an upraised peneplain, the degra- 

 dational processes, of whatever nature they may be, would be espected to 

 start reducing the country toward ultimate baselevel just the same as 



»> Keyes : Bulletin of the Geological Society of America, vol. 19, 1908, p. 



