DAVIS: THE GRAND CANYON OF THE COLORADO. 



135 



tmctly towards the base of the scarp ; several upper branches may unite 

 in a single lower trunk, and thus the lower ravine receives the waste 

 from a considerable length of cliff face ; at the same time, many spurs 

 fail to reach the base of the scarp, but terminate acutely between the 

 forking ravines, as in Figure 9. On the other hand, where a projecting 

 scarp or promontory stands forth, the ravines diverge, and a single spur 

 at the top of the scarp may broaden and repeatedly divide in its descent, 

 assuming a sprawling paw-like form, and including many short ravines 

 that begin within it. Variations of this kind are repeated systematically 

 over and over again in the Permian scarps. When the entire height of 

 the spurs is only some four hundred feet, it may be hardly worth while 

 to develop any special terminology for their different forms ; but when 



•wi-x 



Figure 9. 



Diagram of Permian spurs under Shinarump cliff. Tapering spurs on the left, split spurs 

 on the riffht. Constructed from rouirh sketches. 



one finds that very similar spurs with heights to be measured in thousands 

 of feet occur on the dissected slopes of Mt. San Francisco, the value of 

 a simple terminology becomes more apparent. 



There seems to be a curious alternation in the generation of spurs and 

 ravines where a scanty or insufficient supply of resistant talus fragments 

 from the cliff-maker is associated with an active erosion of the strata in 

 the under-slope. As the coarse waste weeps and washes down from the 

 cliff, it tends to avoid the spurs and to accumulate in the ravines. The 

 bare spurs are then eroded more rapidly than the protected ravines, and 

 thus a new ravine may come to be worn down the axis of what was a 

 spur, while a spur stands forth where the ravine was before. A new 

 supply of waste then gathers in the new ravine, while the older waste is 



