H2 REGINALD A. DALY 



iently naming and emphasizing a principal epoch in the early history 

 of the range. 



At first sight one may be surprised to find this accordance of sum- 

 mit levels among high mountains of complex structure. Surprise 

 should be tempered, however, by the consideration that the original 

 relief was not even approximately determined by constructional 

 profiles deducible from existing structures. 



It is, for example, highly improbable that the "reconstruction" of a 

 great alpine anticline through a study of its denuded roots can represent 

 the original height of its crest above sea-level. Nor is it legitimate 

 to conclude from the great shortening of the transverse axis of the 

 range by the enormous tangential pressures that orogenic blocks of 

 indefinite height could have been produced. Overthrusting, upthrust- 

 ing, folding, mashing, and igneous intrusion have often occurred on 

 such a scale, that were it not for other and inhibiting causes, differen- 

 tial elevations perhaps forty or fifty thousand or more feet in relative 

 height might have resulted. No geologist believes that local blocks 

 of such height have entered into the construction of any terrestrial 

 range. Erosion during the absolutely slow, though relatively rapid, 

 growth of the range has often been appealed to as sufficient to explain 

 the lack of such heights in even the youngest alps of the world. But 

 not sufficient emphasis has been placed on the quite different control 

 of isostatic adjustment accompanying and following the paroxysmal 

 uplift of orogenic blocks. Single steep slopes of possibly thirty 

 thousand feet might, indeed, then exist if they were underlain by the 

 strongest granite, which likewise formed the underpinning of the 

 whole adjoining district, that granite being throughout at the tem- 

 peratures of ordinary rock-crushing experiments. But such towering 

 masses are highly improbable for weaker rocks which would crush 

 down under the supposed conditions, and wholly impossible for 

 mountain blocks overlying material as plastic as that which composes 

 the original basement of an alpine range. The strength of the main 

 mass of the range is diminished by the inevitable rise of subsurface 

 temperatures with crumpling and mashing. It is the rule with alpine 

 ranges that intrusions of hot magma on a huge scale either accompany 

 or very soon follow the chief paroxysms of folding. In either case, 

 and not only over the areas where denudation has exposed the intru- 



