EROSION AND THE SUMMIT LEVEL OF THE ALPS 39 



correctness and completeness; and the scheme is thereby found 

 to be in need of various amendments and additions, among which 

 the possible action of various other destructive forces than those of 

 normal erosion and the possible interruption of the ideal cycle by 

 any kind of deformation at any stage in its progress are emphasized. 

 Another chapter is then given to an elaboration of the ideal scheme, 

 and here a special paragraph is given to erosion during upheaval, 

 which may be translated as follows : 



It is important to remember that the erosional processes by no means 

 wait until upheaval has ceased before they begin their attack upon a land 

 surface. A very significant erosional work can take place while upheaval is 

 slowly progressing. An upheaval can, in fact, go on so slowly as to permit a 

 large river to preserve a graded course and gently sloping valley sides, especially 

 if the upheaved mass is of weak structure. Such a river will, therefore, have 

 no youth, but will, Minerva-like, begin its life with maturity. On the other 

 hand many examples can be adduced in which a highland surface preserves 

 its initial form between the stream-cut valleys so little altered in the early 

 stage of a cycle that we are well justified in believing that upheaval in general 

 is accomplished more rapidly than degradation [pp. 146, 147]. 



Various other allusions are made to a slow upheaval and accom- 

 panying erosion ; for example, under coastal plains, it is noted that 

 the rate of their upheaval should be considered, as the amount of 

 dissection during upheaval is thereby determined (p. 207); but 

 this idea is not elaborated. Again in the chapter on the marine 

 cycle, slow changes of level are explicitly but briefly mentioned 

 (pp. 463, 518). The most detailed consideration of the interaction 

 of upheaval and erosion is presented in the chapter on mountains, 

 where several excerpts from the drawings in the above-cited 

 "Practical Exercises" are introduced. Here both a slow and a 

 rapid arching of an initial lowland of generally homogeneous 

 structure is considered; if rapid, the arched surface will be drained 

 chiefly by new consequent streams; if slow, the larger streams may 

 persist in their antecedent courses (pp. 256, 257). It is explained 

 that when the upheaval is only partly accomplished, V-shaped 

 valleys are incised between portions of the upheaved but otherwise 

 little changed initial surface (p. 258), essentially as in the early 

 stage of Penck's first ideal cycle in his "Gipfelflur" essay; when 

 a greater upheaval is accompKshed, the deeper incision of the nearly 



