50 GROVE KARL GILBERT— DAVIS [UEilolR \v£™xi, 



of erosion, where they have rapid fall, is upon their beds, and that it is only when they have 

 so far reduced their grades as greatly to reduce their transporting and cutting power, that they 

 begin wearing their banks and widening their channels, so as to render flood-plains possible" 

 (67); therefore it seems fair to interpret a later statement, that certain worn-down areas "have 

 been denuded evenly, instead of being deeply scored along the chief lines of drainage" (554), 

 as meaning that the early stage of deep scoring had been past before the late stage of even 

 degradation was reached. Nevertheless Gilbert's statement of this aspect of the problem is 

 not so clear as Powell's, quoted above. 



EXAMPLES OF SUBAERIAL DEGRADATION 



Besides the Carboniferous "terrace" or plateau just cited, the following examples of worn- 

 down areas may be instanced, chiefly from the southern part of the plateau province, where a 

 large part of the surface is occupied by strata of moderate or small resistance. Certain Cre- 

 taceous areas in western New Mexico are described, above which stand the lava-capped tables 

 of the Mount Taylor and Acoma Plateaus, "and from the entire surface of which it is demon- 

 strated a thousand feet of rock have been razed" in the production of their present surf ace of 

 small relief; and to this it is added that, when "standing upon the edge of one of these tables 

 and viewing a broad stretch of country . . . one can appreciate the fact that erosion is the 

 great agent in the production of all details of surface, and that the disposition and hardness of 

 rocks are only modifying conditions" (554, 555). An account of the Zuni Mountains, which 

 have already been cited as occupying an elongated dome of upheaval, and in which it is noted 

 that "the antagonism and the concurrent result of the two laws of erosion are illustrated," is 

 still more significant, although the manifest meaning of its lines constantly arouses the wish 

 that the deeper meaning between them had been more fully written out. "On every side the 

 strata dip away from the axis, and the soft [Mesozoic] formations that have been eroded from 

 the dome now outcrop in a series of concentric elhptical belts" (563), the weakest members of 

 which are shown in accompanying sections to have been worn down to faint relief. "If only 

 the law of altitude were obeyed, (which would have been the case if the Paleozoic and Archaean 

 rocks were no harder than the Mesozoic), there could be no mountain at all, and the uplift 

 would be marked only by concentric annular outcrops of the several strata" (555). It is 

 difficult to imagine that the writer of these lines, in which an explanation of the present form of 

 the mountains was the first consideration, did not perceive that, given more time, the Paleozoic 

 and Archaean rocks, resistant as they may be, would also be worn down low; and yet in the 

 absence of a completed statement to that effect one must remain in some doubt as to how 

 fully Gilbert had then solved the problem of subaerial planation. 



A passage in the chapter of the second report on the basin ranges tends to resolve this doubt 

 in Gilbert's favor. The passage concerns the "sterile and remote" Pyramid Range in south- 

 western New Mexico, composed of eruptive rocks traversed by quartz veins. "The whole 

 range has an appearance of great antiquity, being reduced nearly to the level of the surrounding 

 plain by an erosion, the present progress of which is of exceeding slowness. . . . The purest 

 quartz veins, resisting the destructive agents by which the country rock is degraded, project 

 above the ground surface in long, ragged walls"; but as the planation here accomplished is 

 explained by "the easy disintegration of the ancient lava" (514), it hardly reaches the case of 

 resistant rocks. The planation of such rocks is best exemplified, not in any surface forms of 

 to-day, but in the ancient land surface of small relief on which, in both the plateau and the 

 basin range provinces, the Paleozoic formations rest in strong unconformity. An example of 

 such a surface in the southeastern part of the basin range province shows that, where the Archean 

 and Paleozoic rocks are in contact, the pre-Paleozoic "degradation of the Archaean mountain 

 was carried so far" as to produce "a plane and originally level [but now tilted] surface"; yet 

 here an additional clause concerning the exposure of the ancient land "to the waves of the 

 Paleozoic shore" harks back to the idea of marine abrasion (510). The account of the same 

 great unconformity revealed deep in the Colorado Canyon has already been cited; it may be 

 more confidently interpreted as recognizing the possibility of subaerial planation of resistant 

 rocks, although no explicit statement to that effect was made. 



