EXCAVATION OP THE CHASM. 
245 
WEATHERING. 
We have seen that the work of corrasion, pure and simple, is limited 
to the cutting of deep and narrow gashes in the strata and to the grinding up 
of the larger fragments brought into the channels. The widening of these 
cuts into the present configuration of the canon and the sculpture of the 
walls are the work of the process which is termed weathering. By far the 
greater part of the material removed in the total process of the excavation 
has been broken up and comminuted by the action of atmospheric reagents, 
and their mode of operation is worthy of careful study. In the remaining 
part of this chapter I shall treat briefly of the more general conditions and 
features of this process, and devote the final chapter to the consideration of 
some of its striking details. 
The peculiar cliff forms of the Grand Canon, and indeed of the prov- 
ince at large, would hardly be possible in any other country, for no other 
country presents all of the conditions which are necessary for them. These 
conditions may be summarized as follows: (1) The great elevation of the 
region. (2) The horizontality of the strata. (3) A series of strata con- 
taining very massive beds, which differ greatly among themselves in respect 
to durability, but each member or subordinate group being quite homo- 
geneous in all its horizontal extent; in a word, heterogeneity in vertical 
range and homogeneity in horizontal range. (4) An arid climate. 
I. It is at once apparent that great elevation is essential to the produc- 
tion of high reliefs in the topography by the agency of erosion. Only in 
a high country can the streams corrade deeply, and it is by corrasion that 
the features of this region have been originated and blocked out. The 
elevation, however, is a condition whose immediate consequences are asso- 
ciated with corrasion, while it affects weathering only secondarily or 
remotely. The principal effect is the determination of the heights of the 
cliffs and the magnitudes of the topographical reliefs in general. All this 
seems so obvious that discussion is superfluous. 
II. No less obvious is the effect of the horizontality of the strata. The 
long flat crestlines, the constant profiles maintained for scores of miles along 
