CONTRASTED CHARACTERISTICS OF ARID AND HUMID STAGES 545 



the moist climate, should in a dry climate have little direct influence if 

 an eolic hypothesis be followed. This inference all observations seem 

 to support. 



The only evidences of antecedent drainage persisting against regional 

 deformation and aridity are presented by the few very largest rivers 

 which have their headwaters beyond arid limits and merely cross the 

 desert on their way to the sea. They receive little or no augmentation 

 to their waters within the area of the dry region. Entirely apart from 

 the desert should these through-flowing streams be considered. The Eio 

 Colorado, the Eio Grande, and the Rio Pecos in the arid country of 

 southwestern United States, the Xile in northeastern Africa, and simi- 

 lar rivers really exert small influence in the general lowering of the 

 lands through which they pass. In the cases of all other streams which 

 in a humid climate would be classed as antecedent rivers all vestiges 

 would be soon lost with the initiation of the arid cycle. Their disap- 

 pearance would be not only because they had merely dried up, but for 

 the reason that their entire valleys had blown away. 



CONSEQUENT DRAINAGE FEATURES 



Consequent drainage, which according to the humid-climate idea must 

 prevail, is in several respects peculiar. It is doubtful whether it should 

 be called consequent drainage at all. It is certainly not consequent 

 drainao^e as it is understood in a moist-climate reofion. In a hiofh-lvins: 

 mountainous desert, such as is displayed in the province already noticed 

 and in the Mexican tableland, whatever drainage there may be is mainly 

 of the sheetflood order.^^ Even the streams coming down from the moun- 

 tains tend to assume this character as soon as they reach the plains of 

 the piedmont, as has been so graphically described by McGee.^^ 



Certain peculiarities presented by these streams of the mountains are 

 more fully discussed further on. 



CENTRIPETAL DRAINAGE 



The development of the present so-called consequent drainage of arid 

 regions may not be necessarily, as has been urged, through the withering 

 away of the lower reaches of streams belonging to a previous moist cycle. 

 Xeither may the independent centripetal systems belong to as many 

 basins of initial deformation ; that they should so belong is a necessar}- 

 deduction of the moist-climate h}'pothesis. On the basis of an arid cli- 

 mate and a development of intermont basins through deflation instead 



3^ Bull. Geol. Soc. America, vol. 19, 1908, p. 78. 

 so Bull. Geol. Soc. America, vol. 8, 1897, p. 87. 



