EVOLUTION OF THE KAIBAB PLATEAU. 
187 
conforms very closely to the upper surface of the Aubrey limestones. The 
principal inequalities have their origin in the unequal amounts of uplifting 
which have prevailed in a platform of beds that once were horizontal. In 
other w'ords, they are almost wholly structural, and not imposed upon the 
mass by unequal ei’osion. 
The principal points in the history of the evolution of the Kaibab may 
be discerned by a comparative study of the drainage and displacements. 
The argument here adopted is not a new one, but it is to be canied to an 
unprecedented extent. Its validity can only be judged by the result. In 
other countries — in regions where the geological data are more obscure — its 
application would be very doubtful and perhaps impossible; but here the 
data are exceptionally plain and definite, and enable us to co-ordinate the 
facts and draw conclusions with a high degree of confidence. 
It has repeatedly been remarked that the principal structural features 
of the Grand Canon district are comparatively recent in their origin, prob- 
ably going back no further than the early Pliocene, while the principal 
drainage channels are as old as the epoch at which the lacustrine period 
terminated. We infer that the establishment of the drainage is older than 
the structural features, because if we suppose otherwise, then the drainage 
would have been very differently distributed from that which now exists. 
For example, let us suppose that the Grand Canon were filled up evenly 
with the summit of the cliffs which overlook it. The Colorado River beinsr 
o 
thus dammed, a large lake would be formed. When the inflowing waters 
had filled this lake the outlet would be at the northern extremity of the 
Kaibab near Paria. Here is the lowest barrier which would keep the water 
from the ocean. Supposing Kanab Canon to be also filled up, the course 
of the waters would be to the westward, and they would finally pour over 
the Hurricane Ledge at a point near that where the Virgen crosses it. This 
is the course which the Colorado would take if the present structural con- 
figuration were to determine it anew. The inevitable inference is that the 
course of the river could not have been determined by the existing structure, 
and must have been fixed by a configuration older than the present one. 
The mind therefore recurs to the earliest period, far back in the Eocene, 
when the waters of the lake receded and left a river running in the deepest 
