DAVIS: THE GRAND CANYON OF THE COLORADO. 173 
tural plain, from which more than six thousand feet of strata have been 
removed. There have, then, been two periods of essentially completed 
erosion, marked by the floor of the Grand canyon series and by the floor 
of the palzozoic series, and one period of far advanced erosion, marked 
by the skyline of the Kaibab ; and three alternate periods of enormous 
erosion elsewhere to supply the strata locally deposited in the Grand 
canyon series, the palzozoic series, and the mesozoic series; and the 
work done in any one of these six periods far outranks that thus far 
accomplished in the erosion of the canyon. Excepting the periods of 
mesozoic deposition and erosion, all this history is recorded in the canyon 
with the clearness that one ordinarily sees only in’colored diagrams on a 
blackboard, and with infinitely greater detail and impressiveness. 
The Two Unconformities. — The double unconformity associated with 
the ‘‘ wedge” of the Grand canyon series (Figure 14) deserves special 
attention. The floor on which these strata rest is of remarkable even- 
ness, in spite of the great deformation of the fundamental schists. It is 
exposed in the section on the northern wall of the canyon for the greater 
part of a mile, dipping under the river at the lower (eastern) end and 
terminated at its upper (western) end by the surface of the second un- 
conformity. The line here seen may be taken as a fair sample of a large 
area of the floor on which the Grand canyon series rests, because it is 
exposed by the chance section made by the river, whose course was 
originally selected with no regard whatever for the then deeply buried 
crystalline foundation of the region. The whole surface must have been 
much larger than the part seen in the canyon ; for if the crystallines 
were evenly truncated here, they must have been similarly worn down 
over a large extent of adjoining territory ; and, moreover, a formation 
that measures ten thousand feet in thickness, like the Grand canyon 
series, cannot be of merely local development. The conclusion seems 
compulsory that before the deposition of the Unkar strata (the lower 
members of the Grand canyon series, Walcott, e, p. 506) the crystalline 
rocks were reduced to a plain of admirable evenness, either by marine or 
by subaerial forces ; and however many cycles or partial cycles of erosion 
were devoted to this ancient task, the last cycle must have been undis- 
turbed until it was very far advanced. 
The floor on which the palzozoic strata lie was formed by extensive 
erosion after the tilting of the crystalline schists with their heavy cover 
of the Grand canyon series, the compound mass being planed down to 
an almost even surface. The Kaibab section of this floor is over fifty 
miles in length along one side of the river, or about forty miles in a 
