SUMMIT LEVELS AMONG ALPINE MOUNTAINS in 



the lines of the peneplain theory is at present not so necessary as a 

 critical inquiry into alternative hypotheses. For the same reason, 

 a concrete criticism of the views on which have been based the attempts 

 to establish the peneplain theory for special alpine ranges will here 

 be left in abeyance. 



The other possible explanations of accordance, including those 

 already published as well as others which have occurred to the writer 

 in the course of field-work, have one important feature in common — 

 a feature which places all of them in opposition to the peneplain 

 hypothesis. That hypothesis demands at least two cycles of erosion 

 in the history of the mountain range; one cycle essentially completed 

 at the time of penultimate extinction of relief, with a second cycle, 

 the present one, advanced to the mature stage of dissection. Involved 

 with this premise of multiple cycles is the conception that the present 

 cycle has been initiated by a quite different kind of mountain-building 

 from that which first gave the range its great altitude. Broad, rela- 

 tively gentle warps, producing on the average an arch elongated in 

 the axis of the existing range, form the kind of movement demanded 

 in the uplift of the peneplained area, while intense plication, thrusting, 

 and blocking gave the range its internal structures and its original 

 relief. In short, the peneplain hypothesis stands in contrast with 

 all the other hypotheses in placing peneplanation and subsequent 

 warping among the necessary stages in the development of the existing 

 mountains. Unequal in strength as these alternative hypotheses 

 may be, they have the common characteristic of excluding a great 

 denudation and a specialized kind of crustal movement from the list 

 of complications in the history of the range. It is most important 

 to observe that this common characteristic, coupled with the fact that 

 the alternative explanations are not mutually exclusive, gives them 

 cumulative force against the peneplain hypothesis, when applied to 

 truly alpine mountains. 



2. Hypothesis of original rough accordance of summit levels, due 

 to isostatic adjustment. — Basal to all of the alternative hypotheses 

 is the inquiry as to the original form of the range at the geological 

 moment when paroxysmal folding of its rocks was practically com- 

 pleted. It is self-evident that the term "original" is here used arbi- 

 trarily, but the strain on language may be permitted in thus conven- 



