DAVIS: THE GRAND CANYON OF THE COLORADO. 151 
district (Dutton, a, Atlas, sheet 6, sections 1, 2, 3) show the Sevier 
plateau to consist of a heavy body of early Tertiary sedimentary strata, 
covered by a heavy volcanic series. This pair of rock masses ascends 
by an eastward rising monocline to the Fish-lake plateau, where the 
volcanic series has been largely removed, and even the underlying Ter- 
tiaries are reduced to much less than their normal thickness. So un- 
equal a measure of erosion in two adjoining plateaus of similar structure 
suggests that the rocks of the higher one were greatly consumed during 
a former lower stand of the land, when the destructive processes halted 
at a horizon that is now found by drawing a line over the plateau tops ; 
and that the existing valleys are chiefly the work’of a revival of erosion 
after a broad uplift had introduced the present cycle ; this broad uplift 
being perhaps associated with that by which the canyon cycle was intro- 
duced in the Grand canyon district. True, the displacement between 
the two plateaus here referred to is a flexure, not a fault; but if a former 
cycle of erosion is indicated by the unequal erosion on the flexed masses, 
it is possible that the faults which elsewhere dislocate the High plateau 
blocks may, as well as the flexures, have been produced in that cycle, 
and not in the present one. In this case, it is manifestly impossible to. 
date the faults by the erosion that has occurred in the present cycle; 
they may therefore be as old in the High plateaus as in the Grand 
canyon district. 
The relation of flexures and fractures appears to be the same in the 
two districts. Speaking of the High plateaus, Dutton instances three 
faults that are younger than the flexures which they traverse. He 
adds: “It is a most curious circumstance that where we find this two- 
period displacement the motion of the fault’ is often reversed, — the 
lift of the first period is the throw of the second. It is not always so, 
but I believe it to be true in a majority of cases where the double move- 
ment has been detected” (a, p. 43). But it is of course not intended 
by this to imply that the High plateau flexures, which affect the Eocene 
beds, are as old as the Waterpocket flexure, which was eroded and uncon- 
formably buried by the Eocene (a, pp. 43, 44). 
Origin of the Drainage System. 
GENERAL EXPLANATION BY ANTECEDENCE. — Powell classified all streams 
as consequent, antecedent, or superimposed (now generally called super- 
1 The word “fault” is here, as in some other passages, used to include both 
flexure and fracture. See, for example, Dutton’s account of the ‘east Kaibab 
fault” (a, p. 52). 
