108 REGINALD A. DALY 



out, and, again, that the methods of spontaneous development are 

 not yet brought into the proper balance for final discussion or decision 

 on the question. 



I. EXPLANATIONS BY INHERITANCE 



The accordance of summit levels may well suggest the analogy 

 of moderately or maturely dissected plains underlain by rocks of 

 horizontal structure. Often with such plains there is little or no 

 doubt of the original, simple form before erosion had produced the 

 intaglio forms of dissection. The common agreement of altitudes 

 among the hilltops of the sculptured plain is manifestly the effect 

 of inheritance from the early, initial stage of the plain's history. Is 

 there anything comparable in the derivation of existing alpine moun- 

 tain ranges ? Their almost infinite complexity of structure due to 

 folding, blocking, thrusting, igneous intrusion, and metamorphism 

 forbids that the analogy shall be anything more than an analogy; 

 yet the question is raised whether, at some earlier stage in the history 

 of each range, there may not have been produced a more or less 

 perfect accordance of summit levels which would, through ordinary 

 processes of erosion, furnish similar accordance in the later stages 

 of the history, including the present stage. Three answers may be 

 proposed to the question. 



i. The peneplain theory. — The explanation which has, on the 

 whole, won most attention from American students of the problem 

 is that now familiar to physiographers as the peneplain theory. By 

 this view the alpine range is supposed to have passed through the 

 paroxysmal epoch of uplif t by crumble, faulting, and thrusting ; then 

 through a period of denudation so prolonged that the once lofty range 

 was thereby reduced to a gently rolling lowland, the surface of which 

 stood near sea-level or the general base-level of the region. In every 

 full published discussion the author favoring the peneplain theory 

 has regarded it as probable or as certain that subordinate residual 

 hills or mountains, monadnocks, rose above this peneplain. 



A second chief premise necessary to the theory is that of a broad, 

 massive warping of the peneplained surface; the major axis of upwarp 

 being roughly coincident, or parallel, with the present topographic 

 axis of the range. The existing details of relief are then regarded 



