EROSION AND THE SUMMIT LEVEL OF THE ALPS t^t, 



criticism that my scheme of the cycle excludes erosion until after 

 upheaval has ceased. There are, however, other and later pubhca- 

 tions yet to be cited, in which the interaction of upheaval and 

 erosion is fully discussed; but with these pubHcations Penck seems 

 to have been unacquainted, for after making the above criticism 

 he goes on to say that an erosion cycle ought to be conceived as 

 including the sequence of forms from an initial lowland to an 

 ultimate plain of degradation (and hence involving the interaction 

 of erosion and upheaval while upheaval continues, and of erosion 

 alone after upheaval ceases), and that such a cycle ought not to 

 begin, like Davis' cycle, with an initial form of completed deforma- 

 tion, but at the moment when deformation first displaces a pre- 

 existent lowland (p. 264). This conception of the cycle is good but 

 it is not new. It has already been realized, as may be seen in various 

 illustrations concerning plateaus, mountains, and valleys in my 

 "Practical Exercises in Physical Geography" (Boston, 1908), and 

 in the chapter on mountains in my Erklarende Beschreibung der 

 Landformen (Leipzig, 191 2). But before entering upon that aspect 

 of the question, a paragraph may be given to Penck's comment, 

 above, on the definition of the initial surface of a cycle of erosion. 

 A number of my diagrams and various passages in my writings 

 may, if taken hterally and alone, have given the impression that 

 the initial surface of an erosion cycle is a surface in its new attitude 

 after upheaval and deformation are completed; this impression may 

 be gained especially from passages where a rapid upheaval is 

 tacitly postulated; in fact in the first account of the cycle in my 

 "Erklarende Beschreibung" the initial surface (Uroberflache) is 

 directly defined as the upheaved surface (p. 30). But inasmuch 

 as other passages and diagrams make it clear that some of the 

 erosional work of an erosion cycle takes place during any upheaval 

 and that much takes place during slow upheavals, a reader who 

 apprehends the spirit of the whole rather than the letter of a part 

 of the cycle problem must soon understand that the true initiation 

 of the cycle is at the beginning of upheaval, and hence that the 

 initial surface should be understood to be, as Penck says, the surface 

 then upheaved. If my writings have given rise to a misunder- 

 standing on this point, it should be noted that a similar misunder- 



