CHANGE-PRODUCING AGENCIES 



655 



on the delta it remains 

 caught in the vegetation 

 and it, too, enters as a 

 loess constituent into the 

 delta structure. 



Of the two essential fac- 

 tors — mountain-rising and 

 precipitation — we may, I 

 think, take the rising of 

 the mountains to average a 

 constancy adequate to the 

 maintenance of a relatively 

 constant grade. On the 

 other hand, we will prob- 

 ably be right in dealing 

 with considerable periods 

 of time, if we assume that 

 precipitation is a factor of 

 more varying intensity. It 

 is evident that, other 

 things remaining equal, 

 the amount of detritus 

 brought from the moun- 

 tains will be proportionate 

 to the amount of precipi- 

 tation to supply the vol- 

 ume of water needed to 

 move it. 



After this detritus 

 emerges from the moun- 

 tains, the manner in which 

 it builds up the delta de- 

 pends largely on the rela- 

 tion between the secularly 

 maintained volume of 

 water and the established 

 grade. 



The tilting of the edge 

 of the plain favors ero- 

 sive action and deepen- 

 ing of the channels of 



