EROSION AND THE SUMMIT LEVEL OF THE ALPS 9 



has been stronger and greater, and where the mountain crests are 

 sharpest; but the stage of the first cycle to which the preglacial 

 central Alps correspond is not specified. In any event, the initial 

 conditions of the first cycle must be modified before they can repre- 

 sent the case of the Alps by assuming, not an antecedent lowland, 

 but a region of subdued mountain forms, which Penck finds reason 

 to believe had been developed there in an early cycle of erosion 

 before the uplift by which the cycle of preglacial carving was intro- 

 duced; this early cycle of erosion — or a still earlier one — having 

 destroyed the great inequalities of altitude due to the huge Alpine 

 overthrusts. 



It must, however, be a difficult matter to determine whether 

 the conditions of uplift and erosion postulated in Pencks' first ideal 

 cycle really represent those under which the Alps gained their 

 preglacial forms; for apart from the highly specialized relation of 

 unrelated factors which, as pointed out above, is necessary for the 

 persistence of graded streams and of graded ridge slopes during 

 progressive upheaval, there remains the large uncertainty as to 

 whether those parts of the high Alps in which no trace of earlier- 

 cycle forms now survive may not have gained a considerably 

 greater altitude than now at an early stage of the preglacial cycle 

 in virtue of a rapid upheaval — such an upheaval, for example, as 

 that which, acting on a much larger scale, gave the Himalayas their 

 towering heights of today; and also an uncertainty as to whether 

 the rough equality of present Alpine altitudes may not be com- 

 petently explained chiefly by erosion after upheaval had ceased, 

 as Penck formerly supposed, rather than by a delicate balance of 

 erosion and upheaval as he now suggests. The idea of a constancy 

 of height being imposed on mountain crests by a balance between 

 upheaval and degradation is a beautiful one, and the ingenuity of 

 the analysis by which the idea is carried out is not to be questioned, 

 but its appHcation to Alpine summits does not seem fully assured. 

 As already noted, the region of the Alps was not a lowland at the 

 time preceding the upheaval by which the preglacial cycle was 

 introduced, but as Penck himself shows, a region of subdued 

 mountains. The upheaval was not broadly uniform, as the scheme 

 of the first ideal cycle tacitly assumes, but, as Penck himself again 



