ANCIP:NT BASE-LEVELS OF EROSION. 
119 
more complex. The comparative smoothness of the Glrand Canon platform, 
therefore, is a problem which seems to demand special explanation. Such 
an explanation may, I think, be given, and in the sequel it will be offered. 
Just here it may be sufficient to indicate it only in the briefest manner. 
We may suppose that this entire region, at the epoch at which the great 
denudation of the Mesozoic system approached completion, occupied a level 
not much above the sea. Under such circumstances it wmuld have been at 
what Powell terms base-level of erosion. The rivers and tributaries would 
no longer corrade their channels. The inequalities which are due to land 
sculpture and the general process of erosion would then no longer increase, 
and the total energy of erosion would be occupied in reducing such ine- 
qualities as had been previously generated. During periods of upheaval, and 
for a considerable time thereafter, the streams are cutting down their chan- 
nels, and weathering widens them into broad valleys with ridges between. 
The diversification so produced reaches a maximum when the streams have 
nearly reached their base-levels. But when the streams can no longer cor- 
rade, and if the uplifting ceases, these diversifications are reduced and 
finally obliterated. Such, I conceive, was the case here. Somewhere about 
the close of the Miocene the principal denudation had been nearly com- 
pleted. The Grand Caiion platform then may have lain near sea-level, and 
the remnants of Mesozoic beds which we may imagine to have been scattered 
over it were gradually obliterated, and the entire region was planed down 
to a comparatively smooth surface. Subsequently a new epoch of upheaval 
set in, and the Grand Canon was begun. Undei’ an ordinary climate this 
new upheaval would have set at work an intricate plexus of streams, carving- 
out anew the uplifted platform into deep valleys and lofty ridges and mesas. 
But a new condition intervened. The climate had now become an arid one. 
There were but few streams to corrade, and as the aridity increased these 
few became only three or four along the entire drainage system of the Grand 
Gailon platform. No new inequalities were generated other than those we 
now see, because there were no rivers to carve them out. 
This explanation ma}^ seem at first to be a gratuitous assumption for 
the explanation of a single fact. But as the discussion proceeds we shall 
find ourselves brought frequently to the inference of an epoch when the 
