ACADEMY OF SCIENCES] WHEELER SURVEY 49 



force and speed, and ceased forever"; for although he was then one of the first to recognize the 

 important but negelected phenomenon of soil creep, it was not until 10 years later that he 

 became a uniformitarian so far as to explain the excavation of valleys by slow erosion. On the 

 other hand, Powell, earlier by a few years in the western field than Gilbert, but of the same date 

 with him in the delayed publication of his first important report, was satisfactorily explicit 

 regarding the possibilities of subaerial processes; his report on "Exploration of the Colorado 

 River of the West" (1875) explained that "the first work of rain and rivers is to cut channels 

 and divide the country into hills, and perhaps mountains, by many meandering grooves or 

 watercourses, and when these have reached their local base levels, under the existing conditions, 

 the lulls are washed down, but not entirely carried away" (204). The clear recognition here 

 given to the natural sequence of valley erosion and hill degradation, as well as to the relation 

 of both to "base level," marks the real opening of the modern epoch of rational physiography. 



TWO LAWS OF EROSION 



Gilbert's early contributions to the problem of subaerial planation are chiefly as follows; 

 An important approach to the first generalization noted at the beginning of the preceding 

 section was made in his second report by formulating, in simple but sharply pointed phrases, 

 two "laws of erosion" or "general principles everywhere manifested. The first is that soft 

 material is worn more rapidly than hard and the second that high points are worn more rapidly 

 than low — or, more strictly, that steep acclivities suffer more than gentle. The tendency of 

 the first principle is to variety of surface, of the second, to uniformity; and the two are com- 

 plementary" (554). The value of these two principles, which Gilbert himself characterized 

 as "familiar" rather than as novel, was to him largely physiographic rather than geologic, for 

 they were illustrated much more by examples of eroded forms than of erosional processes. It 

 is especially significant in the present connection that explicit mention was made of the tendency 

 of the second principle to bring about a uniformity of surface; that is, to produce plains of 

 degradation. Moreover, the final clause regarding the complementary relation of the two 

 principles should be interpreted to mean that the ridges or highlands of resistant rock which 

 survive for a time between valleys or lowlands excavated on belts of weak rock in accordance 

 with the first principle will later be themselves worn down to lowlands in accordance with the 

 second principle. But unfortunately no examples of this kind were brought forward, and it 

 may be questioned whether the readers of 50 years ago gathered, without the illumination that 

 explicit examples would have given, the full meaning that is latent in the overterse final clause. 

 If these principles are to-day regarded as elementary, that only shows how much progress has 

 been made in the half century since they were formulated. Yet helpful as their early presenta- 

 tion and illustration was, their value was lessened by the omission of direct statement as to 

 the attitude in which a plain of degradation should stand with respect to the imaginary surface 

 of control which physiographers, following Powell's lead, later came to know as the "baselevel 

 of erosion." 



The tendency of erosional processes eventually to produce uniformity of surface had already 

 been shown in Gilbert's first report to be realized over the vast extent of surface occupied by 

 the lowest one of the great "terraces" or "benches" of the plateau province, across which the 

 Colorado Canyon is trenched. This "terrace" measures about 130 by 300 miles, and its pro- 

 duction by the removal of overlying strata, thousands of feet in thickness, involved the "immen- 

 sity of denudation" (81), to which reference has already been made, although the principles 

 involved in its planation were not then stated. In the second report several additional statements 

 are found concerning realized or nearly realized surfaces of planation. All these surfaces are in 

 areas of relatively weak rocks, yet they seem to show that their observer and describer under- 

 stood that the final result of subaerial erosion acting upon highlands of any structure and of 

 any original form must be a surface of faint relief; not only so, he must also have recognized 

 that this final result was preceded by earlier stages characterized at first by narrow canyons 

 and later by open valleys. Hence he must have been mentally aware of the second generali- 

 zation of the three above stated, although he did not formulate it. Indeed, he had noted in 

 bis first report "the well-recognized fact in the natural history of rivers that their first work 



