PAUCITY OF TRIBUTAEIES. 
25 
with the absence of tributaries— this one alone excepted. The cause is 
obviously the arid climate. But there are also no dry canons coming from 
the north, and though dry canons are abundant in the terraces there is a 
conspicuous absence of them in the vicinity of the Grrand Canon. The 
reason for it will appear in the sequel, and it is only remarked here in a 
genei’al wa)^ that the arid climate has prevailed during a very long period, 
and although other tributaries once existed, they have for the most part 
been obliterated or rendered obscure. They ceased to run with the advent 
of this arid climate, which came on when the present Grand Canon had no 
existence, or had just begun to develop. 
