THE CYCLE OF EROSION 189 



mature, and old ; and to speak of the early, intermediate, and late stages 

 of a cycle as its youth, maturity, and old age — two of these biological 

 terms having been employed in this connection nearly thirty years ago 

 by Chamberlin and Salisbury in their description of the "Driftless Area 

 of the Upper Mississippi Valley" ('85,229), although what was there 

 described as "old age" is here called "maturity." Evidently a cycle of 

 erosion thus conceived is not a definite measure of time, but simply such 

 a duration as suffices for the accomplishment of the task initiated by 

 uplift; just as the cycle of life for an organic form is not a definite 

 number of days or years, but simply a time sufficient for passing through 

 all the stages of growth in a completed life. Similarly when a certain 

 sequential form is described as young or mature or old, no definite meas- 

 ure of time or number of years is to be understood, but only such a part 

 of the total duration of the cycle concerned as corresponds to its earlier, 

 middle, or later development. A young Sequoia might be older than a 

 mature man, and a mature man much older than an old mushroom, be- 

 cause the terms young, mature, and old, even when indicative of age, are 

 always considered in relation to the normal length of life of the indi- 

 vidual under consideration ; or, as Kant put it over a hundred years ago : 

 "Wenn man wissen will, ob ein Ding alt, ob es sehr alt, oder noch jung 

 zu nennen ist, muss man nicht nach der Anzahl der Jahre schatzen, die 

 es gedauert hat, sondern nach dem Verhaltnis, dass diese zu derjenigen 

 Zeit gehabt haben, die es dauern solte" (Sammtliche AVerke, I, 1867, 

 189). 



Interruptions of the Cycle of Erosion 



The progress of changes through a cycle of erosion may be interrupted 

 at any stage, if the land-mass is displaced so as to take a new position 

 with respect to the controlling baselevel of erosion ; the land-mass may 

 be regarded as thereupon entering a new cycle of erosion, the young 

 forms of the newly introduced cycle for a time combining with and then 

 gradually replacing the previously developed forms of the interrupted 

 cycle. Besides the many complications introduced by crustal movements 

 which, following the terminology here employed, may "interrupt" one 

 cycle and "introduce" another, there are other disturbing complications 

 produced by volcanic eruptions or by climatic changes, which do not 

 necessarily involve any change in the attitude of a land-mass with respect 

 to baselevel. It is my personal habit to speak of these disturbances as 

 "accidents," and to reserve the word "interruptions" for crustal move- 

 ments by which a land-mass is given a new attitude. Willi accidents we 

 here have nothing more to do. With interruptions, which include up- 



