REPORT OF THE CHIEF ASTROXOMER 633 



SESSIONAL PAPER No. 25a 



the original form of the range at the geological moment when paroxysmal fold- 

 ing of its rocks was practically completed. It is self-evident that the term 

 ' original ' is here used arbitrarily, but the strain on language may be permitted 

 in thus conveniently 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 summit 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, upthrusting, folding, mashing, and igneous intrusion have often 

 occurred on such a scale, that were it not for other and inhibiting causes, 

 differential 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 follow- 

 ing 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 adjoin- 

 ing district, that granite being throughout at the temperatures 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 intrusives, but also over much wider 

 areas about the downwardly expanding bases of the batholiths, the heat of the 

 intrusions still further increases the plasticity of the basement on which the 

 mountains are growing. The weakness of the underpinning is further manifest 

 in the case of such ranges as the Cascades or the Coast range of British 

 Columbia, so largely formed of granitic magma injected in a fluid state during 

 or just after the last great period of plication in those ranges. 



The conclusion seems unavoidable that the tendency of tangential force to 

 erect orogenic blocks projecting much higher into the air than Mount Everest 

 itself is operative only up to a certain critical point. Beyond that point the 



