382 W. M. DAVIS 



part in an intentionally and avowedly deductive manner, but they 

 are checked by facts from stage to stage. My especial indebtedness 

 to Passarge is stated below. 



The arid climate. — The essential features of the arid climate, as 

 it is here considered, are: so small a rainfall that plant growth is 

 scanty, that no basins of initial deformation are filled to overflowing, 

 that no large trunk rivers are formed, and hence that the drainage 

 does not reach the sea. 



The agencies of sculpture and their opportunities for work in arid 

 regions are peculiar in several respects. The small rainfall and the 

 dry air reduce the ground water to a minimum. In its absence, 

 weathering is almost limited to the surface, and is more largely physi- 

 cal than chemical. The streams are usually shorter than the slopes, 

 and act as discontinuously at their lower as at their upper ends. The 

 scarcity of plant growth leaves the surface relatively free to the attack 

 of the winds and of the intermittent waters. Hence, in the production 

 of fine waste, the splitting, flaking, and splintering of local weather- 

 ing are supplemented rather by the rasping and trituration that go 

 with transportation than by the chemical disintegration that charac- 

 terizes a plant-bound soil. 



No special conditions need be postulated as to the initiation of 

 the arid cycle. The passive earth's crust may be (relatively) uplifted 

 and offered to the sculpturing agencies with any structure, any form, 

 and any altitude, in dry as well as in moist regions. 



Initial stage. — Let consideration be given to an uplifted region of 

 large extent over which an arid climate prevails. Antecedent rivers, 

 persisting from a previous cycle against the deformations by which 

 the new cycle is introduced, must be rare, because such rivers should 

 be large, and large rivers are unusual in an arid region. Consequent 

 drainage must prevail. The initial slopes in each basin will lead the 

 wash of local rains toward the central depression, whose lowest point 

 serves as the local baselevel for the district. There will be as many 

 independent centripetal systems as there are basins of initial deforma- 

 tion ; for no basin can contain an overflowing lake, whose outlet would 

 connect two centripetal systems: the centripetal streams will not 

 always follow the whole length of the centripetal slopes ; most of the 

 streams of each basin system will wither away after descending from 



