OEIGIN OF BUTTES, CUSPS, AND GABLES. 
259 
resent the locus of the edge of any stratum high in the series at successive 
epochs of the development by parallel equidistant lines, we shall have a 
series of curves running up the stream nearly parallel to its course, then cir- 
cling around its head and retiu’ning to its mouth. As these curves recede 
from the stream the more nearly do they approach to arcs of circles. The 
same recession is going on in the walls of the main amphitheater, into which 
the minor one opens. Where two minor or major amphitheaters are situated 
near each other the recession of their walls at length obliterates a portion 
of the summit stratum which divided them, and at later stages successively 
obliterates lower and lower members. The great cloister-buttes are formed 
by the recession of the walls of any two parallel major amphitheaters, and 
the wings of these buttes by the recession of the walls of the minor amphi- 
theaters. The minor recesses exhibit the rounded contours at their upper 
ends. Everywhere is disclosed an approximate uniformity in the rate of 
recession. Where the expanding curves of recession from any two adjoin- 
ing recesses meet or intersect, the included mass between them is carved 
sometimes into a cusp or sharp “spur,” sometimes into a stately gable, ac- 
cording to the relative positions of the axes of the curves. The spurs are 
frequently very narrow and sharp, and in an advanced state of erosion such 
a spur in the Red Wall band breaks into a row of needles or pinnacles. 
Many of the gables are of most noble form and of wonderful symmetry. 
Human conception cannot surpass their beauty even if it can rival it. 
The cusp contour is also repeated on a minor scale in the wall faces, 
whei’e it appears as a minor decoration or fretting of the edges of the strata. 
It is especially conspicuous in the Kaibab. It appears to have its origin in 
minor corrasion by storm sluices, which at first scour out at frequent and 
regular intervals deep notches, which widen out by weathering, and once 
started the recesses or alcoves thus opened perpetuate themselves. The con- 
tour line along the face of the cliff thus curves inward and the intersection 
of two curves at their ends forms the cusp. 
In the faces of the great limestone member of the Red Wall may be 
seen large niches or panels of veiy regular form spanned by circular arches 
above. These panels must be more than 600 feet high and 500 to 600 feet 
wide. Sometimes they are very deeply recessed in the fa 9 ade, sometimes 
