EROSION AND THE SUMMIT LEVEL OF THE ALPS 25 



are like strengthened or reset rungs or newly added upper rungs 

 in a physiographic ladder, on the lower rungs of which a good 

 measure of ascent has already been made above the empirical level 

 of the science fifty years ago. It is not to be questioned that 

 various special cycles have yet to be worked out in order to develop 

 form-sequences appropriate to pecuHar structures and processes; 

 and it is greatly to be desired that systematic studies of this kind 

 should be combined with the observational studies of trained 

 geographers in regions of unlike climates. And now after this long 

 detour away from the Alps, return may be made there in order to 

 show that Penck's explanation of the similar summit altitudes 

 involves the elaboration of precisely such a special sequence of 

 forms as contributes to the fuller development of the cycle scheme; 

 but his study unfortunately includes an element of destructive 

 criticism to which attention must also be called. 



PART III. penck's criticisms OF EARLIER STUDIES 



Progress in Alpine physiography. — It is profitable to read in 

 connection with Penck's Gipfelflur der Alpen Heim's chapter on 

 the denudation of mountains which, already referred to as published 

 in his Mechanismus der Gehirgshildung half a century ago, marked 

 the farthest advance reached at that time in the most difl&cult 

 problem of land physiography. It was then tacitly assumed that 

 the present cycle of Alpine erosion had been introduced by the great 

 crushing which produced the greatly deformed Alpine structures; the 

 possibiKty of successive more or less complete cycles of erosion, 

 introduced by simple upHft and following after an advanced stage 

 of the earher cycle introduced by the crushing, was not thought of. 

 Nor was the scheme of one-cycle erosion then carried to its legitimate 

 end of peneplanation; if such a possibiHty was imagined it was 

 probably dismissed as an extravagance. Furthermore, the impor- 

 tant share that glacial erosion has taken in the sculpture of the Alps 

 had not then been learned. 



Many are the modifications of those early views that have 

 since been accepted. It is now recognized that the Alps are no 

 longer in their first cycle of erosion, but that the present cycle was 

 preceded by another — ^whether that one was the first need not be 



