54G W. M. DAVIS — DATES OF TOPOGRAPHIC FORMS. 



constructive features and the elaboration of middle age are both lost ; and 

 if no accident enter to disturb the relation of mass and baselevel, the area 

 will remain indefinitely with insignificant change, maintaining, like the Sibyl, 

 an immortal old age. 



The broad constructional forms of youth, the elaboration of maturity, and 

 the featureless simplicity old age, are all recognizable in many parts of the 

 world. The difficulty of the problem is not in finding warrant for this 

 simple ideal life history, but in the proper consideration of the disturbances 

 that interrupt it. At any time in the cycle of life, an area may be depressed 

 and more or less drowned, or elevated and thereby carried into a new cycle ; 

 the forms acquired before change of level then taking the place of truly con- 

 structional forms as the basis on which further development repeats in many 

 ways the features of youth. The possibility of thus subdividing the history 

 of any district into chapters or partial cycles, each separated by some rela- 

 tively rapid change of elevation or of attitude, would probably not have 

 been questioned when the prevalent teaching of geology gave belief in hurried 

 and spasmodic movements; but with the introduction of the doctrine of uni- 

 formitarianism, and especially with the confirmation of it supposed to be 

 found in the theory of antecedent rivers, doubt would naturally arise as to 

 the possibility of truly dividing the history of any region into long times of 

 quiet and briefer times of relative activity. Yet, so far as I can see, the 

 facts justify exactly such a division. There are several districts whose to- 

 pography has been carefully examined in relation to their geological history, 

 with the result of practically demonstrating that after a long period of rela- 

 tive rest they have passed through a short period of relatively active move- 

 ment; and their history may thus be divided into partial cycles of geographic 

 development, more or less complete, the present form of the surface being 

 the product of the combined actions of destructive and constructive forces in 

 all the cycles. 



An objection to this conclusion that I have sometimes met is based on an 

 exaggerated estimate of the share taken by rivers in denuding the surface 

 of the land. It is overlooked that most of the land is not occupied by streams, 

 and that the wasting of the interstream areas is very slow. Where large 

 rivers are at work they may truly at times almost keep pace with elevation ; 

 but even where this rare relation is found it is characteristic of but a small 

 part of the surface of the land. The interstream surfaces waste with com- 

 parative slowness, and the waste slowly washes and creeps to the streams, 

 to be carried down to the ocean after many halts on the way. It is on these 

 interstream surfaces that the records of one cycle are carried over into an- 

 other. 



Factors controlling Topographic Development. — Variations from the order 

 of development of the ideal example are found in several directions. First 



