34 W. M. DAVIS 



standing of the initial surface as the fully upheaved surface would 

 be gained if certain passages and diagrams in Penck's chapter on 

 "Die Erdoberfiache," above cited, were taken without their 

 context. His diagrams of warped, faulted and doomed structures 

 (Sc. Figs. 72, 73, 81) are drawn with non-eroded surfaces. His text 

 includes a statement concerning an initial surface that is very 

 similar to the one above quoted from my "Erklarende Besch- 

 reibung," namely: ''If we review the development of valleys, we 

 recognize clearly that their first course is defined by the presence 

 of an original slope, down which the water flows. This slope is the 

 initial form" (Sc. 143). In other passages one reads that lateral 

 compression produces crustal folds having arches and troughs, 

 like those assumed by a cloth when it is pushed from one side 

 (Sc. 134) ; that a table-like highland is sometimes upheaved between 

 two faults (Sc. 135); that the Black Forest and the Vosges, with 

 the trough of the Rhine between them, are striking examples of 

 the sides of a collapsed arch (Sc. 136); that young fault blocks 

 form plateaus with steep slopes (Sc. 147); that an upheaved 

 peneplain takes the form of a highland (Sc. 176); that a zone of 

 compression runs through its cycle of erosion as soon as the compres- 

 sion ceases (Sc. 176): none of these passages mention erosion as 

 accompanying upheaval. But it would be manifestly unfair to 

 cite such passages without their antidotes ; for example, that arched 

 areas of the earth's surface suffer degradation during their arching 

 (Sc. 136); and that as soon as a surface is uplifted erosion begins 

 to destroy it (Sc. 146); and other similar statements. The fact 

 is that so many elements enter into the problem of the erosion 

 cycle that it is difficult to state them intelhgibly all at once. It 

 might be well to use two terms: the initial surface would then be 

 defined as the surface at the time when a crustal movement began 

 to introduce a new cycle; and the (potential) deformed surface 

 would be defined as the initial surface in its new altitude after 

 movement had ceased, but without taking account of contempora- 

 neous erosion. 



Practical exercises on plateaus and mountains. — The treatment 

 of erosion during upheaval as illustrated in my "Practical Exer- 

 cises" may now be taken up. The exercises are based on an atlas 



