108 GROVE KARL GILBERT— DAVIS [M ™ olR \voZ l xxi, 



But the possibility of such reduction is at once denied: The law of uniform slope "is never 

 free to work out its full results; for it demands a uniformity of conditions which nowhere exists" 

 (115). 



It is chiefly in connection with the balanced action of maturely graded rivers, as they 

 would to-day be described, that the evolution of drainage systems and of the associated land 

 forms is most directly intimated. One there finds statements like the following : 



As the soft rocks are worn away the hard are left prominent. The differentiation continues until an equi- 

 librium is reached through the law of declivities. When the ratio of erosive action as dependent on declivities 

 becomes equal to the ratio of resistances as dependent on rock character, there is equality of action (115, 116). 



Yet the time element is not specifically treated here. Even the beautiful generalization 

 concerning the interdependence of slopes is introduced abruptly, although it is a condition 

 which is only reached after an immense amount of preparatory work has been accomplished; 

 for it involves not only the degradation of every cascade in all the streams, but also the oblitera- 

 tion of every outcropping ledge on all the interstream slopes. The introduction is : 



The tendency to equality of action, or to the establishment of a dynamic equilibrium, has already been 

 pointed out in the discussion of the principles of erosion and of sculpture, but one of its most important results 

 has not been noticed (123). 



Interdependence of slopes being thus presented only as the result of a tendency, the reader 

 gains little understanding of the many changes through which the tendency has been realized. 



The failure to give fuller consideration to the time element and to the changes of action and 

 of form that go with it occasionally leads to minor errors. Thus it is announced under the law 

 of structure: 



Erosion is most rapid where the resistance is least, and hence as the soft rocks are worn away the hard 

 are left prominent (115, 116). 



Yet this statement evidently applies only in the early stages of erosional degradation, 

 while the weaker structures are j 7 et to be worn down. It is reversed in the later stages, after 

 the weaker structures have been worn down; for then, during the remainder of an uninterrupted 

 cycle of erosion, the prominences composed of the harder strata will be worn down faster than 

 the surface of the depressions already degraded on the weak strata; hence in these later stages 

 erosion is not most rapid where rock resistance is least. True, the eventual degradation of 

 hard-rock prominences will be much slower in the later stages of a cycle than the preliminary 

 degradation of the belts of weak strata was in the early stages, but it will be faster than the 

 contemporary degradation of the weak strata. Again, under "planation" it is stated that 

 the "downward wear [of streams] ceases when the load equals the capacity for transportation" 

 (126); and this overlooks the long continuation of a very slow "downward wear" through the 

 late maturity and old age of a drainage system, as the result of an equally long continued and 

 correspondingly slow decrease of "load" from its maximum value at the time of full maturity. 

 Revival of erosion by renewal of uplift is also omitted from this discussion as it was from the 

 Wheeler reports 



PROGRESS IN PHYSIOGRAPHY 



But the reader may ask, if not exclaim : Why point out these shortcomings in a study that 

 had so many excellencies? The answer is: Partly to reinforce the lesson of the preceding 

 section that progress is not made all at once; but even more to spur on those discouraged 

 physiographers of to-day who seem to fear that their science is now completely developed, and 

 that no new progress is to be expected. The same fear might have been expressed with as 

 much justice when the Henry Mountains report came out, for Gilbert surely carried the formu- 

 lation of physiographic laws as far as he could in the final chapter of that memorable volume; 

 and his readers of that time might have regarded his contributions as a final, beyond which no 

 further advance was to be expected. Yet as a matter of fact even his lesser followers have 

 gradually built higher on the firm foundations that he then laid; and if their superstructure 

 seems to any of them a new finality, that is only because no new Gilbert has yet arisen to show 

 them where to build next. 



