PLIOCENE AGE OF THE CHASM. 
99 
deuce that this was really the case. The Toroweap Valley is the modified 
channel of an ancient river. On the west side of the Uinkaret is another. 
A third is seen upon the south side of the Colorado, directly opposite the 
Toroweap, and a few others may be easily designated. It appears that all 
these rivers dried up before the inner gorge was excavated. For if they 
had continued to carry water we may be sure that they would have cut 
their chasms as 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 Colo- 
rado and of its forks, the Grand and Green, to be 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 during 
the prevalence of an arid climate. The platform of the country adjoining 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 cafion 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 tr’Mh, 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 enor- 
mous erosion which took place here before the excavation of the chasm was 
