148 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
(Dutton, c, p. 200, Atlas, sheet I[.). The Trias must have once ex- 
tended eastward beyond the line on which the fault was broken ; and 
the 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 MonociinaL Fiexures. — 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 gradnal 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.), nearly 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- 
Prat 
