38 W. M. DAVIS 



the visible forms associated with eventual, imminent, recent, and 

 long-past river captures, as in various drawings of Exercise VII, 

 and especially in Plate 26, Figures 11 and 12. If these figures are 

 compared the figures illustrating corresponding problems in 

 advanced textbooks — -for instance, the three block diagrams 

 illustrating river capture, in Figure 130, Volume III, of Passarge's 

 " Landschaf tskunde " — the value of elaborated graphic aids in the 

 teaching of land forms will be apparent. But whatever opinion is 

 held on this aspect of the erosion-cycle problem, there cannot be^ 

 any question that the exercises above cited embody precisely the 

 scheme that Penck recommends, namely, that cycles of erosion 

 are opened by the uplift of a pre-existent surface, that they are 

 continued for a longer or shorter time by the joint operation of 

 upheaval and erosion, and that they are completed after upheaval 

 ceases by the operation of erosion alone, which eventually produces 

 a lowland of degradation. Moreover, the recommended scheme 

 is presented with much greater detail in these exercises of 1908 

 than it is in the pages on the Erosionzyklus in Penck's chapter on 

 "Die Eroberflache" in Scobel's Handbuch of the same date. The 

 " Gipfelflur" essay might, therefore, be more appropriately regarded 

 as an extension of the author's own brief treatment of the interaction 

 of upheaval and erosion in that chapter than as a correction of my 

 fuller treatment of the problem. 



The explanatory description of land forms. — ^A similar treat- 

 ment of the interaction of upheaval and erosion is also presented 

 in my Erkldrende Beschreibung der Landformen (Leipzig, 191 2), 

 which embodies the lectures that I gave in the winter semester of 

 1908-9 at the University of Berlin from Penck's chair, while he 

 was absent as visiting professor at Columbia University. The first 

 explanation of the scheme in this book is limited to the case of normal 

 erosion; it is presented in ideal form and is based on the elementary 

 assumption of rapid uplift; the idea of slow upHft with accompany- 

 ing erosion is presented more fully later, although a brief state- 

 ment at the outset says that upheaval may be imagined to be 

 either slow or rapid (p. 30). The chapter in which this ideal 

 scheme is presented is followed by one in which its deduced conse- 

 quences are confronted with a variety of facts, as a test of its 



