166 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 (¢, 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 bend 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 
