112 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
when it is remembered that his route led him across the southern 
plateaus where the great displacements weaken and disappear as they 
come down from the north. He did not demand two periods of erosion 
for the sculpture of the plateaus and the narrow canyon ; difference of 
resistance in the upper and lower strata seemed to him to account for 
these contrasts in the amount of destructive work (p. 62), but he inferred 
a more active erosion in former times than at present ; “ everything 
indicates that the table-lands were formerly much better watered than 
they now are” (p. 47, also pp. 62, 76). 
Powell in his adventurous expedition down the canyon (1869) and in 
his journey over the northern plateaus (1870), discovered the double 
unconformity in the Kaibab section of the Grand canyon (a, pp. 212, 
213), gave many new details concerning the rock series, and em- 
phasized the production of the canyons by erosion in his announce- 
ment of the “antecedent” origin of certain rivers (p. 163). He 
presented a clear account of the great displacements by faults and 
fiexures which divide the Grand canyon district into huge “ blocks,” 
trending north and south (a, pp. 185-190, Figure 73), as well as of the 
great cliffs of erosion or retreating escarpments, north of the canyon, 
facing south and trending irregularly east and west (a, pp. 190, 191, 
Figure 74) ; “the cliffs of erosion are very irregular in direction, but 
somewhat constant in vertical outline ; and the cliffs of displacement 
are somewhat regular in direction, but very inconstant in vertical 
outline” (a, p. 191). Powell does not seem to have felt the necessity 
of supposing an uplift of the region between the great denudation of the 
uplands and the incision of the narrow canyons (pp. 206, 213), but he 
states that “the carving of the cafons .. . is insignificant when com- 
pared with the denudation of the whole area, as evidenced in the cliffs 
of erosion” (a, p. 208). The date of the displacements is not very 
sharply defined; when the great denudation began “there were no 
faults and no benches” (a, p. 200). The first displacements occurred 
after the erosion of valleys had been begun, the displacements were 
long continued, and must have been slower than the erosion of valleys 
by the principal streams, for the displacements did not modify the 
stream courses (a, p. 201). ‘‘ Though the entire region has been folded 
and faulted on a grand scale, these displacements have never determined 
the course of the streams. ... All the facts concerning the relation 
of the water-ways of this region to the mountains, hills, cafions, and 
cliffs lead to the inevitable conclusion that the system of drainage was 
determined antecedent to the faulting and folding” (a, p. 198). The 
