EROSION AND THE SUMMIT LEVEL OF THE ALPS 13 



must be understood that while the existence of various slightly- 

 dissected plateaus and mountainous highlands proves that in such 

 cases the movement of upheaval was rapid compared to the general 

 processes of subaerial degradation, the occasional occurrence of 

 valleys transecting upheaved masses proves that the antecedent 

 rivers which eroded such valleys were competent to cut down their 

 channels about as fast as the transected mass was upheaved. 

 Hence while an elementary conception of the cycle impHes that 

 erosion was chiefly accomplished after upheaval ceased, a more 

 matured conception recognizes that much erosion may take place 

 while upheaval is in progress. The elementary conception may, how- 

 ever, be nearly realized in regions of resistant rocks, quick upUf t, and 

 weak destructive agencies; the more elaborate conception may be 

 most fully reahzed in regions of weak rocks, slow uplift, and strong 

 destructive agencies. 



It thus came to be more and more generally recognized that 

 every eroding land area exhibits in its actual form the present stage 

 of a succession of changing forms that began when the area was 

 uplifted and exposed to erosion and that will continue, if no dis- 

 turbance takes place, until it shall be worn down to a featureless 

 lowland. With the recognition of this succession for actual land 

 areas came its generalization for imagined land areas. Thus 

 concepts of typical cycles of erosion took their place along with 

 concepts of typical rivers, volcanoes, and so on. Progress since 

 then has been made, on the one hand, by inferring past and future 

 changes of many actual land areas, and on the other, by enlarging 

 the number of imagined cycles and elaborating the discussion of 

 their succession of changes. 



A third principle of the cycle has been thus established, namely, 

 that a systematic sequence of forms is developed during its progress, 

 the sequence varying according to the structure of the mass that 

 is to be eroded and to the nature of the processes by which the 

 erosion is done. The discussion of each structural unit or entity 

 should, therefore, be carried through separately. The structure of 

 such an entity may be uniform or varied, simple or complex, resistant 

 or weak; the processes of erosion may be, as above noted, weather 

 and rivers, weather and glaciers, weather and wind, weather and 



