CONTRASTED FEATURES OF TOPQGRiVPHtC JUVENILITY 549 



mate. One of the most serious objections to the development of desert 

 landscapes by water action is the utter discordance between the necessary 

 consequences of the moist-climate hypothesis and the facts actually 

 observed. 



YOUTHFUL ARID STAGE UNDER DEFLATION 



On the basis of wind-scour action little that is ordinarily postulated 

 for the early stage of the geographic cycle really obtains. The potency 

 of water-work is found to be very greatly overestimated. The perma- 

 nency of the deposits transported by the rains from the highland rims 

 to the central lowlands of the intermont plains seems fanciful. The ris- 

 ing of the local baselevels appears to be poorly supported by facts. The 

 development of drainage systems nowhere agrees with direct observation. 



In the early stage of the arid cycle the relief seems to be rapidly in- 

 creased by the hollowing out of broad flat-bottomed troughs or basins, 

 laterally bordered by steep-sided, sharp-ridged mountain ranges. The 

 air-streams accomplishing this work are hundreds of miles wide instead 

 of a few hundreds of feet, as in the case of rivers ; consequently the sur- 

 face worked over is comparable to the channels of broad, shallow streams. 

 Sharply incised topography so characteristic of humid lands is, there- 

 fore, impossible. General lowering is controlled partly by the propor- 

 tions of weak and resistant rocks, partly by the extent and frequency of 

 the faulting and other deformations of all previous time. Local condi- 

 tions are sufficiently variable to enable the general lowering process to 

 go on independently in the different basins, and the general leveling 

 goes on also regardless of the relations of the denuded surface to sealevel. 

 From the very beginning plains-forming is the most characteristic fea- 

 ture of desert-leveling. As the mountains become higher and higher 

 normal water action increases on their sides, imparting to them some- 

 thing of the appearance of stream-graved surfaces. The relatively scant 

 amounts of waste materials brought down and spread out from time to 

 time at their bases are so rapidly removed by the winds that there is at 

 any one time little actual accumulation. Earely do sporadic cloud- 

 bursts carry notable quantities of the finer waste into the centers of the 

 larger basins, there eventually to constitute thick deposits. Moving 

 sand dunes are momentary phenomena. Permanent deposition of the 

 finer waste goes on only beyond the boundaries of the desert in the 

 bordering semi-arid belts or in the adjoining seas. 



In the case of the youthful relief stage in a normally humid land its 

 maturity is commonly regarded as approaching when dissection has gone 

 on until the major drainage divides have lost some of their height and 

 sharpness of outline and all elevations have begun to assume a notably 



