EROSION AND THE SUMMIT LEVEL OF THE ALPS 37 



the ideal cycle. These omissions may be justified on two grounds : 

 first, the essence of the problem under discussion does not demand 

 their inclusion; and second, the presentation of a discussion before 

 a learned Academy, in which all sciences are represented, does 

 demand, if the speaker wishes to be generally understood, that he 

 should simpHfy his problem to the utmost; for deeply learned as 

 academicians are in their own subjects they must not be expected 

 to know much about the subjects of their colleagues. 

 > The deficiency regarding belts of strong and weak rocks in my 

 exercise on mountains is, however, made up in large measure by 

 later exercises on rivers and valleys, based on a series of eight draw- 

 ings, the first of which shows a larger and a smaller river crossing 

 a lowland district of inclined hard and soft strata, the course of the 

 rivers being at right angles to the strike of the strata; the second and 

 third show the district progressively upwarped with accompanying 

 erosion; and the others show successive stages of still-stand degrada- 

 tion with an appropriate growth of subsequent streams and capture 

 of different segments of the smaller river by branches of the larger 

 river; the last drawing represents the greater part of the district 

 again reduced to a lowland. 



Various opinions may perhaps be held as to the correctness and 

 intelligibility of these drawings and as to the efficiency or teaching- 

 value of the accompanying questions; but the exercises have 

 nevertheless been found worthy of translation into German.^ 

 From my own perhaps prejudiced point of view, I am inclined to 

 believe that the three elementary exercises above cited, as well as 

 several others that are not here cited, treat a variety of problems 

 more clearly and intelHgibly than they are treated in most advanced 

 textbooks. For example, the development of platforms in the 

 walls of canyons eroded in plateaus, where a high-level thin cHff- 

 making stratum retreats more rapidly than a stronger, underlying 

 cliff-maker, as in Plate 9, Figure 3, and Plate 10, Figure 4; the 

 pattern of cliff-rimmed plateau margins as seen in plan, Plate 7, 

 Figures 5, 8, 9; the relation of an antecedent river to the consequent 

 streams of an up-arched mountain belt, as in Plate 12, Figure 2; 



' W. M. Davis and K. Oestreich, Praktische Uebungen in Physicher Geographie, 

 Leipzig, 1918. 



