246 
THE GRAND CANON DISTRICT. 
the edges of each stratigraphic series, would not be possible otherwise. To 
appreciate this it is only necessary to glance at the efFects which have been 
produced by the monoclines which at wide intervals interrupt the continuity 
and constancy of the cliffs. No wilder scene can be imagined than the 
pinnacles, towers, and domes which bristle upon the flank of a great flex- 
ure, like the Echo Cliff or Water Pocket folds. 
III. The condition of horizontal homogeneity with vertical heteroge- 
neit}^ presents considerations some of which are as obvious as the foregoing, 
while others are extremely complex. It is clear that if the strata now form- 
ing the escarpment of the Vermilion Cliffs had varied rapidly from point to 
point in respect to the thickness of individual members, or in respect to 
their lithological characters, the action of weathering would have varied 
accordingly. The profiles would undergo rapid changes of form along the 
front. In truth, there is a certain amount of variability of just this charac- 
ter in that escarpment. The Vermilion Cliffs in the valley of the Virgen 
differ notably in detail from what they are at Kanab and near the Paria. 
The enormous sandstone member dwindles in thickness from west to east, 
and other members less conspicuous gradually lose identity, and the resem- 
blance of the two widely separated portions of the wall is only general. 
But the change along the front is very slow and is nowhere abrupt. 
The vertical heterogeneity is the character which gives complexity to 
the profile. Where the beds are numerous and where they differ among 
themselves as to durability, the profile becomes very complicated, like a 
very elaborate series of horizontal moldings. The extreme of simplicity 
is found in the Jurassic sandstone, where the cliff consists of a single massive 
stratum 800 to 1,000 feet thick and homogeneous from top to bottom. 
Those more complex results which follow the action of weathering upon 
vertically heterogeneous strata will be discussed in the next chapter. 
IV. The effects of an arid climate are by no means simple nor intelli- 
gible at a glance. They appear only upon analysis, and the analysis must 
take cognizance of a wide range of facts. I cannot do better here than to 
have recourse to the excellent analysis of Grilbert : 
“All the processes of erosion are affected directly by the rainfall and 
by its distribution through the year. All are accelerated by its increase 
