150 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The faults have already been referred to the plateau cycle; the flex- 
ures must therefore belong still earlier in that cycle. Hence the period 
of the great denudation, thus far undifferentiated, should now be divided 
into a pre-flexure cycle, an inter-flexure-and-fault cycle, and a post-fault 
cycle. It is believed that a complicated scheme of this kind is much 
nearer the truth than the simple scheme of time division thus far postu- 
lated ; but it still remains true that the post-faulting quiescent period 
must have been long enough for a strong excess of cliff-recession to occur 
in the heaved blocks, before the relatively modern erosion of the canyon 
was excited by a broad uplift of the region. 
THE DIspLACEMENTS OF THE Higa PuLateaus. — Although our excur- 
sion did not lead us into the district of the High plateaus, it seems 
necessary to examine what has been written about them in order to see 
how far what has now been inferred as to the date of the faults and 
flexures in the Grand canyon district may find application in the adjoin- 
ing district on the north. Our source of information is again chiefly in 
one of Dutton’s reports, from which it appears in the first place that the 
faults of the two districts are to be considered as a single group of dis- 
placements ; and, in the second place, that the uplift by which the canyon 
cycle was introduced probably affected the district of the High plateaus 
also (a, pp. 27, 28, 45). The faults of the plateaus are dated as younger 
than the youngest formations that they dislocate ; namely, younger than 
the middle Eocene sediments and heavy lava sheets and conglomerates 
of a somewhat later epoch; and as old enough to have allowed a certain 
moderate amount of erosion since their production. The erosion on the 
borders of the faulted High plateau blocks seems small compared to that 
by which the recession of the bordering cliffs on the south has been 
accomplished, and Dutton therefore decides to ‘“ place the age of the 
principal displacements in a period which had its commencement in the 
latter part of Pliocene time, and extended down to an epoch which, even 
in a historical sense, may not be extremely ancient, and which certainly 
falls on this side of the glacial period ” (a, p. 35). It seems, however, 
that in reaching this conclusion, no explicit consideration was given to 
the possibility that the faults might have occurred during a former cycle 
of erosion, when the district stood much lower than now; that the forms 
then initiated by faulting may have been much reduced or even nearly 
obliterated by the erosion of this earlier cycle ; and that the erosion on 
the borders of the blocks, by which the faults have been dated, has 
taken place only since a general uplift has revived the erosive processes. 
There is some evidence that such is the case. Certain sections of the 
