384 W. M. DAVIS 



above described, and of basins aggraded with the waste from the 

 dissected mountains. Trunk streams are rare. The initial relief has 

 been decreased, although the basin floors are from 3,000 to 5,000 feet 

 above sea-level. Persia and Tibet give further illustrations of the 

 same relation. In the latter region the intermont basins often con- 

 tain saline lakes; but the stage of development there reached is not 

 yet clear, because the origin of the ranges and basins is not, as a rule, 

 considered by Tibetan explorers. It should not, however, be inferred 

 that the separation of the many drainage systems in regions such as 

 those of Persia and Tibet is the result of any special peculiarity in 

 the initial deformation of the surface, essentially unlike the deforma- 

 tion of other regions of normal climate, where large unified drainage 

 systems are the rule. The latter regions may initially have had as 

 many basins of deformation as the former, but the more plentiful 

 rainfall of normal climate has enabled their rivers to cut down the 

 basin rims. This principle has been pointed out by Penck (a, p. 87 ; 

 b, p. 159) and others. The initial relief may be of coarse pattern, 

 as in central Asia, where the vast aggraded plains of eastern and 

 western Turkestan are separated by a broadly uplifted and deeply 

 dissected mountainous area; or of finer pattern, as in the Basin Range 

 province just mentioned, where many small ranges separate nearly 

 as many small basins. The progress of evolution through the cycle, 

 and the arrangement of forms at successive stages, will be much 

 affected by these unlike initial conditions. 



Streams, floods, and lakes are the chief agencies in giving form to 

 the aggraded basin floors, as well as to the dissected basin margins 

 in the early stages of the cycle ; but the winds also are of importance : 

 they do a certain share of erosion by sand-blast action; they do a 

 more important work of transportation by sweeping the granular 

 waste from exposed uplands and depositing it in more sheltered 

 depressions, and by raising the finer dust high in the air and carrying 

 it far and wide before it is allowed to settle. Wind-action is, more- 

 over, peculiar in not being guided by the slopes or restrained by the 

 divides which control streams and stream systems. It is true that 

 the winds, like the streams, tend in a very general way to wear down 

 the highlands and to fill up the basins; but sand may be drifted 

 uphill — dunes may be seen climbing strong slopes and escarpments 



