120 GttAND CANON DISTRICT. 



deep as the Grand Canon itself— just as the Little Colorado, Kanab 

 Creek, and Cataract Creek have done. For we have only to look at the 

 great multitude of lateral chasms of the upper courses of the Colorado 

 and of its forks, the Grand and Green, to he deeply impressed with the 

 fact that so long as a tributary river carries, we will not say a living 

 stream, but even occasional floods, its channel will be scoured down to 

 the same level as the trunk river itself. It is apparent, then, that the 

 Toroweap dried up before the cutting of the inner gorge of the Grand 

 Canon began, and hence we infer that the arid climate which caused it 

 to dry up existed before the beginning of the inner gorge. 



By the application of other homologous facts, and by the same method 

 of reasoning, we infer that the outer chasm has also been excavated dur- 

 ing the prevalence of an arid climate. The platform of country adjoin- 

 ing the canon is at present devoid of lateral chasms, yet traces are 

 often found of ancient channels which became dry at about the time the 

 excavation of the outer canon began, or very soon thereafter. They are 

 cut to comparatively slight depths— from one hundred to three or four 

 hundred feet. That they are not of recent origin is proved by the fact 

 that they often have slopes away from the river, though it is clear that 

 they formerly sloped towards it. In truth, the entire chasm betrays 

 everywhere the continued action of an arid climate through the entire 

 period of its formation. This arid period is limited, approximately, to 

 Pliocene and Quaternary time. The general tenor of the facts is to the 

 effect that the Miocene was a humid period and the Pliocene a dry one 

 throughout the greater part of the West. This is one of the reasons 

 which lead us to the very probable conclusion that the age of the Grand 

 Canon is not older than the beginning of Pliocene time. We might also 

 draw a similar inference from a consideration of the enormous erosion 

 which took place here before the excavation of the chasm was begun. 

 The denudation of the Mesozoic system was an incomparably greater 

 work, and yet that denudation could not have begun until the last 

 strata (the Lower Eocene) were deposited. If these inferences are welj 

 founded, we may assign the greater part of Eocene and the whole of 

 Miocene time for the principal denudation of the Mesozoic, and the Plio- 

 cene and Quaternary for the excavation of the entire canon. The pro- 

 portion thus suggested between the portions of the work done and the 

 divisions of time required to accomplish them seems very fair and rea- 

 sonable. But the strongest evidence of all it would be almost impos- 

 sible to recite here in detail. In general terms, it may be characterized 

 as that internal evidence which appears when a vast array of facts, at 

 first disjointed and without obvious relation, are subsequently grouped 

 aright into a coherent system. Each constituent fact is then seen to 

 admit of one intelligible interpretation and no other; and each sub- 

 sidiary proposition ha« an overwhelming justification and an evidence 

 of verity far stronger than any which could be summoned if we en- 

 deavored to prove it independently. 



