lo W. M. DAVIS 



shows, was significantly greater in certain areas than in others. 

 He indeed suggests that certain longitudinal valleys of today 

 represent belts of smaller and slower upheaval than the areas of 

 lofty and sharp peaks; hence the generaHzed surface of upheaval, 

 instead of being a single very broad arch from south to north, 

 appears to have been an undulating arch; and, if that were the case, 

 a moderate increase in one of the convexities, as the result of a 

 locally strengthened and accelerated but relatively brief upheaval, 

 might produce the great initial altitudes above suggested. It may 

 be noted in this connection that Solch, professor of geography at 

 Innsbruck, has recently pointed out certain difficulties that stand 

 in the way of accepting Penck's views.^ 



Let it be added that it is physiographically immaterial to which 

 one of the three sharp-crested stages in Penck's first ideal cycle the 

 preglacial Alps correspond; all three stages would look alike. 

 Similarly, it is physiographically immaterial whether young moun- 

 tains with flat interstream uplands and subdued mountains with 

 rounded crests are to be explained by his first or his second ideal 

 cycle, which in the early and late stages develop similar forms. 

 Again, it is physiographically immaterial whether old mountains — • 

 that is, masses of deformed structure, once lofty but now reduced 

 to undulating lowlands — are to be explained by his first, second, or 

 third ideal cycle, or by a cycle introduced by a very rapid upHft, 

 except in so far as a prevailing adjustment of drainage to weak 

 structures would suggest the operation of a rapid uplift, or of 

 the first instead of the third of Penck's cycles, as noted above. 



PART II. THE SCHEME OF THE EROSION CYCLE 



Physiographic principles of the erosion cycle.— In order more 

 fully to appreciate the novelty of Penck's discussion, a brief review 

 may be made of the leading physiographic principles that are 

 associated with the scheme of the erosion cycle. One of these 

 principles is that highlands and mountains in existence today are 

 not necessarily the unconsumed residuals of a single primitive 

 upheaval, but that they may be and in many cases are the residuals 



I "Grundfragen der Landformung in den nordostlichen Alpen," Geogr. Annalen, 

 Vol. IV (Stockholm, 1922), pp. 147-93; see p. 187. 



