14 W. M. DAVIS 



solution (in limestone areas), or weather and waves; and some of 

 these various processes may interact in the same region. This third 

 principle is closely associated with a fourth, namely, that at any 

 stage in the systematic advance of a cycle of erosion, the various 

 elements of form then developed are reasonably associated with 

 one another in a manner appropriate to the structures and processes 

 concerned; and herein Hes the chief value of the scheme of the 

 cycle of erosion in the explanatory description of land forms. 

 It advances land physiography from an empirical study of unrelated 

 facts to a rational, evolutionary, genetic study of closely correlated 

 facts. The contributions to the establishment of these two princi- 

 ples have been so numerous that it is impossible to list them here; 

 but one of the earliest and most helpful is to be found in a chapter 

 on the erosion of Alpine valleys in Heim's famous Untersuchungen 

 iiber den Mechanismus der Gebirgsbildung,^ a work which marked at 

 its time a great advance in the study of the erosional development of 

 valleys during the progress of mountain carving, and which may be 

 appropriately cited here in order, as will be more fully shown below, 

 to measure the great additional advance that has been made in 

 that most difficult and most important of all physiographic problems 

 during the following half century. It may be further noted that 

 my own share in developing the scheme of the erosion cycle has been 

 chiefly in the way of systematizing the ideas brought forward by 

 others. 



The terminology of the erosion cycle. — A section may be given to 

 some items of terminology in the scheme of the erosion cycle. 

 The use of the word, cycle, in this connection is my own suggestion. 

 A complete cycle denotes the lapse of time between the initiation 

 of the uplift of a land mass by any kind of deformation and its 

 ultimate degradation. Two ordinary words — interrupt, meaning 

 the closing of a cycle at any stage of its progress by a movement 

 of the land mass, and introduce, meaning the opening of a new 

 cycle by such movement — are not technical terms, but it is con- 

 venient to employ them systematically in the sense just indicated. 

 They have been so used above. The disturbance of the progress of a 



'Vol. I, pp. 293-301, Basel, 1871. 



