DENUDING AGENTS 303 



maturity, and old age. Oscillations of level introduce new condi- 

 tions and cause the work of denudation to start afresh with re- 

 newed energy, or, if the movement be one of depression, it will 

 check the work already in progress. The cycles of development 

 are thus partial rather than complete, and a given region may dis- 

 play topographical forms dating from very different and widely 

 separated cycles. The more resistant rocks retain the features 

 acquired in an earlier cycle, while the weaker and more destructible 

 rocks have already taken on the forms due to a later cycle. A 

 landscape thus often includes features of different geological dates, 

 and it is in the identification of these that the value of the physio- 

 graphical method to historical geology consists. 



In the production of new topographical forms, old ones are 

 more or less completely destroyed, and thus, the farther back in 

 time we go, the fewer subdivisions are recognizable, and only the 

 outlines of the great cycles can be followed. Very ancient features 

 would be quite obliterated in the successive cycles of develop- 

 ment, were they not sometimes buried under the sediments of an 

 encroaching sea. A subsequent reelevation of the area into land, 

 and a stripping away of the covering of newer sediments by the 

 agencies of denudation, will again bring to light the ancient land 

 surface which had been buried for ages. 



In Part I we have already studied the agencies of denudation, 

 but there we concerned ourselves principally with the modes of 

 operation of those agencies, and their efficiency in destroying old 

 rocks and in furnishing material for the construction of new. 

 We have now to consider these agencies from a somewhat differ- 

 ent point of view ; to determine their relative shares in the work 

 of cutting the land down to base-level, and the characteristic 

 forms of land sculpture which they produce at the various stages 

 of their work. There are some differences of opinion among 

 geologists regarding the relative efficiency of the various denuding 

 agents. English authorities, for example, attribute more impor- 

 tance to the work of the sea than do the French or American, who 

 regard the sea as an agency altogether subordinate to those of the 

 atmosphere and running waters. Thus, de Lapparent calculates 



