1G6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Basin," and ends on reaching the strong flexure by which the strata are 

 turned up into the Coconino ; but it is probable that in so far as the 

 terms Kaibab and Coconino have a geographical application, their areas 

 will be separated by the canyon rather than by the flexure that 

 separates their uplifts. 



The further northwestward course of the river may have been made 

 inviting if a relative depression or failure of uplift occurred in the 

 eastern Kanab plateau when the west Kaibab faults were formed, and 

 Dutton gives some evidence that this was the case when he describes 

 the northward slope of the Kanab along the southwest border of the 

 Kaibab (c, p. 184) ; but I am at loss to find any conclusive proof of 

 such a condition, apart from the behavior of the river itself. The 

 south beud on the Shivwits may plausibly be explained as a displace- 

 ment from a once more direct course by reason of the volcanic out- 

 pourings which now culminate in Mt. Dellenbaugh. 



Speculative Character of the Preceding Sections. — All this is avowedly 

 speculative ; and it is presented rather as a combination of various 

 possibilities than as embodying the only permissible explanation for the 

 course of the Colorado. Other stages of the great denudation, now 

 unsuspected, may yet be discovered in the development of the Grand 

 canyon district ; new processes of river development, as imperfectly 

 known to-day as was the growth of subsequent streams at the time of 

 the earlier exploration of the canyon, may be added to the growing 

 resources of physiographic study ; and all of these elaborate stages and 

 processes will deserve as careful consideration as has been given to the 

 simpler conception of the antecedent river, marked out by the deeper 

 lines of the Eocene lake floor, and remaining unaltered, save for deep- 

 ening, ever since the ancient lake became dry land. 



Nothing less than extended studies over a large area of the Cordil- 

 leran region will suffice to determine what value should be given to the 

 various possibilities thus suggested. It is not my intention to discount 

 such studies by attempting to announce their result at once ; but only 

 to emphasize the opinion that the facts now on record, combined with 

 such knowledge of the region as our party was able to gather last sum- 

 mer, warrant the consideration of at least one hypothesis alternative to 

 the 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, but, on the other hand, 

 such an origin does not seem compulsory. The chief objection to the 

 theory of antecedence is not that rivers cannot saw their way through 





