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-flex ure-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 High Plateaus. — Although our excur- 

 sion did uot 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 (n, 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 



