Din-row.] DETAILS OF SCULPTURE. 165 



a free discharge to the fragments into the depth below. There is no 

 cheek to the descent of the talus; the amount of protection given by it 

 to all the beds of the Lower Aubrey is very nearly uniform, and the 

 slope becomes straight. But whenever, as sometimes happens, the top 

 of the Red Wall precipice stands at an unusual distance from the Lower 

 Aubrey, the curvature of the profile of the latter appears, and its em- 

 phasis is proportional to the distance which separates the vertical 

 planes of the Red Wall and of the cross-bedded sandstone. 



Many details of repetitive or systematic sculpture are presented in 

 the great chasm, and they may be explained as readily as the profiles. 

 Only one other feature can be alluded to here, and the allusion will be 

 brief. It concerns the plau or horizontal projections of the component 

 features of the Kaibab division, the blocking out of the cloister buttes 

 and the temples, and their reduction to their present forms. In a gen- 

 eral way it is apparent that these have been originated by the profound 

 corrasion of short lateral tributaries of the Colorado and the subsequent 

 widening of the cuts into the present amphitheaters and alcoves ; the 

 buttes and temples being the residual masses between them. But the 

 contours of the latter are striking and peculiar in the extreme. They 

 are explained by observing that wherever recession of the cliffs takes 

 place it proceeds with great uniformity along the entire front. It starts 

 along the line of a stream which is tortuous, but as it proceeds it car- 

 ries back the cliff in a succession of curves, and in process of time minor 

 inequalities are obliterated. Eaeh larger bend of the stream gives rise 

 to its own curve in the trend of the wall, and where successive curves 

 intersect they form very sharp cusps. Everything here depends upon 

 uniformity in the rate of the recession of all parts of the cliff. Where 

 the outward spreading circles of erosion from two distinct alcoves or 

 amphitheaters meet by recession in opposite directions, a butte is cut off 

 and a saddle or "col" is formed. The cusps between two intersecting 

 circles are exceedingly sharp and well formed, and three circles gener- 

 ally give rise to a fine gable. 



The peculiar cliff-forms of the Plateau country would hardly be pos- 

 sible in any other, for no other presents those conditions which are nec- 

 essary for them. These conditions may be summarized as follows: 

 ( 1.) The great elevation of the region. ( 2.) The horizontally of the 

 strata. ( 3.) A series of strata containing very massive beds which dif- 

 fer greatly among themselves in respect to hardness, but each member 

 being very homogeneous in all its horizontal extent; in a word, heter- 

 ogeneity in vertical range and homogeneity in horizontal range. (4.) 

 An arid climate. The great elevation is essential to high reliefs in the 

 topography. Only in a high country can the streams corrade deeply, 

 and it is by corrasion of streams that the features are originated and 

 blocked out. The effect of horizontality of the strata is self-evident. 

 With regard to vertical heterogeneity, it is apparent that it is essential 

 to give diversity to profiles. If the rocks were homogeneous in vertical 



