Robinson — Tertiary Peneplain of the Plateau District. 129 



theory of antecedence, as an explanation for the origin of the 

 drainage lines in the Grand Canyon District. I do not, on the 

 one hand, consider the antecedent origin of the Colorado 

 disproved, hut, on the other hand, such an origin does not seem 

 compulsory." 



It would seem, however, that certain facts, such as the 

 thorough peneplation of the Grand Canyon District, with the 

 uplifting and warping of the peneplain by later faulting, the 

 dependence of the course of the Colorado upon structure, as 

 described by Gilbert, and the consequent origin, so far as 

 known, of all branch streams, furnished sufficient evidence for 

 definitely abandoning the idea of the antecedent origin of the 

 Colorado River and for substituting a consequent origin in 

 which superposed and subsequent conditions are both present. 



While it is believed that the general question of the origin 

 of the drainage system of the Grand Canyon District rests on 

 a rather definite basis, it is realized that the number of facts is 

 at present much too small to allow exact reasons being given 

 for the location of considerable portions of the courses of the 

 Colorado and Little Colorado, though not of the smaller 

 streams. The problem involves the study of a large area, and 

 it is hoped that whoever may have occasion to visit the region 

 will succeed in gathering information that will assist in its 

 correct solution. 



Yale University, New Haven, Conn., June, 1907. 



