i 5 6 



University of California. 



[Vol. 2. 



term beveling. A later article by Prof. W. M. Davis,* in reply to 

 Professor Tarr, fails (partly through misinterpretation) to meet fully 

 his objections to the established theory, or to overthrow his argu- 

 ments for a new one, and the outcome of the discussion leaves it 

 clear that we should not continue to hold to the peneplain idea 

 without a further sifting of the evidence both for and against it. 



It remains, therefore, to be determined how far the accepted 

 theory of peneplains may still be applied, and how far the phenom- 

 ena usually classed under that head may or should be accounted 

 for by laws of erosion working under ordinary conditions. To offer 

 some suggestions on these two points is the object of the present 

 paper. 



DEFINITION OF THE TERM PENEPLAIN. 



It is well, first of all, to review the definition of this term, since 

 it has been used loosely by some writers. Davis f has defined it 

 as "a nearly featureless plain" produced by subaerial denudation. 

 The use of the term is thus strictly limited to those topographic 

 forms, whether of local or greater extent, which are produced by 

 subaerial erosion alone, excluding all other forms of erosion, and all 

 topographic forms due to deposition alone. It is apparent, from 

 further discussion of the term, that the essential point of the defini- 

 tion is the wearing down of the land to an attitude approaching 

 base-level — and therefore no plains, however formed, which do not 

 fulfil this condition, can be called peneplains, even though other- 

 wise similar in character. Finally, although the term does not 

 include plains of submarine erosion or deposition, peneplains 

 formed near the border of a continent would, of necessity, be asso- 

 ciated with these other features to a greater or less extent. 



The conception of peneplanation on which the following argu- 

 ments are based may be stated thus: the reduction of a limited 

 portion of land, by subaerial processes alone, to a condition of very 

 moderate relief, near ultimate base-level. By " a limited portion of 

 land " is meant here such an area as would form a natural geo- 

 graphical unit. This will usually be a mountain range, or other 



*The Peneplain, Am. Geol., Vol. XXIII, April, 1899, pp. 207-239. 

 t Am. Journ. Sci., 1889, Ser. Ill, Vol. XXXVII^p. 430. 



