148 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



(Dutton, c, p. 200, Atlas, sheet II.). The Trias must have once ex- 

 tended eastward beyond the line on which the fault was broken ; and 

 tin' uplifted eastern extension of the formation must have been worn 

 away after the faulting. On the other hand, a late movement on the 

 same fault line has been thus far tacitly postulated, for the uplift by 

 which the canyon cycle was introduced in the Grand canyon district 

 does not seem to have extended with equal strength into the Basin 

 range province west of the Grand wash fault line. But as I have not 

 seen this district, it must be passed only with brief mention. 



The Western Monoclinal Flexures. — The above review makes it 

 probable that, however modern certain minor movements on the fault 

 planes may be, the chief movements, by which, as Gilbert said, the 

 adjacent plateau blocks were made subject to " different conditions of 

 denudation," are of considerable antiquity. They must antedate the 

 beginning of the canyon cycle. Yet the first deformation of the Grand 

 canyon district is of still earlier date, for the faults with heave on the 

 east were in a number of cases preceded by monoclinal flexures with 

 heave on the west. The few exceptions to this rule of unlike move- 

 ments do not seriously invalidate it. The east Kaibab monocline is torn 

 into an east-throwing fault near the canyon ; and similar small faults 

 are suspected along the Echo monocline, as noted above. The Escalante 

 flexure dips to the southwest, but its displacement is gradual compared 

 to that of the Waterpocket, Echo (Paria), and east Kaibab flexures, 

 which all dip eastward. A flexure with strong throw on the west is 

 indicated for the western border of the Kaibab in Powell's general sec- 

 tion of the district (a, p. 190, Figure 73) ; the displacement given to it 

 is as great as that of the upper of the two east Kaibab flexures. Two 

 west-dipping flexures on the west side of the Kaibab are shown by Gilbert 

 (a, p. 51) ; but in the more detailed descriptions and sections given in 

 Dutton's report (c, pp. 183-186, Figures 3, 4, and Plate II.), nearty all 

 of the displacement on the west side of the Kaibab is accomplished by 

 two faults, with hardly a tract of flexure ; the gentle westward dip in the 

 western half of the Kaibab highland — more pronounced to the north 

 where faulting is changed for a west-dipping flexure — may suffice to 

 warrant the use of the term " Kaibab arch," but it seems to be even 

 less pronounced than the broad Escalante flexure. Where our party 

 descended westward from the Kaibab by one of the greater ravines, west 

 of Jacob's lake, the horizontality of the strata all the way to the main 

 limiting fault was in strong contrast to the pronounced flexure of the 

 eastern border. It may be, however, that both the western faults be- 



