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 palaeozoic 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 palaeozoic 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 palaeozoic 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 



