112 bulletin: museum of compakative 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 

 flexures 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 canons ... 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, canons, and 

 cliffs lead to the inevitable conclusion that the system of drainage was 

 determined antecedent to the faulting and folding " (a, p. 198). The 



