158 bulletin: museum of comparative zoology. 



crown : thus the evidence from drainage confirms that derived from 

 structure and erosion. 



The Origin of the Colorado in the Grand Canyon District. — In 

 view of the various origins other than antecedent that may be ascribed 

 to the branches of the Colorado in the Grand canyon district, it does 

 not seem legitimate to adduce these streams in support of the antecedent 

 origin of the trunk river. That question must be settled by itself, and 

 it is by no means free from difficulty. The antecedent origin of Green 

 river in its passage through the Uinta mountains has been seriously im- 

 pugned by Emmons (1877), who maintains that it is a superposed river 

 (a, pp. 194, 205; b). The antecedent origin of certain parts of the 

 Colorado in the Grand canyon district has later been questioned by 

 Jefferson (1897), who points out the southward bends of the river 

 around the Kaibab and Shivwits plateaus, and suggests that these 

 deflections may be consequent on local uplifts instead of regardless of 

 them. To these doubts must be added a whole series of considerations 

 which had no place in the discussions of Powell and Dutton regarding 

 the spontaneous rearrangement of watercourses during the dissection 

 of a region which has suffered repeated movements and heavy denuda- 

 tion. It is true that, as the Colorado runs for the most part on nearly 

 horizontal strata and in a general way transverse to the displacements 

 that its basin has suffered, it cannot be classed with subsequent rivers, 

 for they always follow the strike of a weak stratum in a series of tilted 

 rocks. But the studies of Hayes and Campbell on the migration of 

 certain divides in a region of uearly horizontal strata that has repeatedly 

 suffered slight tilting during the development of its rivers deserve se- 

 rious consideration in the plateau province, where they have as yet found 

 no exponent. Until this new aspect of the problem shall have been dis- 

 cussed by some one who has a wide acquaintance with the region, it 

 does not seem safe to regard even the trenchant Colorado in its course 

 through the Grand canyon as a purely antecedent river. 



In the mean time I cannot resist the temptation of speculating some- 

 what freely as to the possible development of the great river across the 

 Grand canyon district, especially in view of what has been said in 

 previous sections as to the successive movements that the region has 

 suffered, and in view of the many ways in which drainage lines may be 

 modified during and after such movements. Certain considerations that 

 I wish to bring forward in this connection concern not only the Grand 

 canyon district, but also the adjacent district of the High plateaus and 

 the province of the Basin ranges (or the Great basin) which have not 



