REPORT OF THE CHIEF ASTRONOMER 635 



SESSIONAL PAPER No. 25a 



discussed as alternative with, and as against, the peneplain hypothesis for truly 

 alpine ranges. By the peneplain hypothesis, the accordance of summit-levels was 

 most perfect in the initial stage of the physiographic cycle begun by the upwarp- 

 ing of the peneplain; by that hypothesis mature dissection of the range tends 

 to destroy sometbing of the initial accordance. The alternative, composite 

 explanation, already in part outlined, involves the conclusion that the accordance 

 tends to become more and more perfect as the stage of mature dissection of the 

 lewly folded range is reached. The question remains whether the accordance 

 inherited from the forms original from the epoch of plication may be so much 

 further developed by subsequent erosion in the physiographic cycle initiated by 

 that plication, as to give the amount of accordance actually observed in the 

 existing range. 



II. SPONTANEOUS DEVELOPMENT OP SUMMIT-LEVEL ACCORDANCE. 



1. Spontaneous development by isostatic adjustment. — The last paroxysm of 

 crumple and upthrust in the young alpine range has occurred. Henceforth' its 

 forms are to be determined chiefly by erosive processes — yet not altogether so. 

 Several authors have suggested that the levelling influence of gravity is not only 

 manifest in the piecemeal carriage of rock fragments out to the piedmonts, or 

 finally to the sea; but that also the very accordance of summit levels is in large 

 part related to gravitative adjustment on a large scale. Where, for any cause or 

 causes, denudation significantly lowers a localized area of the range faster than 

 neighbouring areas of the same altitude, the former area will tend to rise, the 

 surrounding region to sink, so as to reproduce conditions of equilibrium in the 

 range. This view entails belief once again in the principle of isostasy. The 

 appeal to the principle in the present case is all the more worthy because of the 

 long continuance of the special plasticity belonging to the very slowly cooling 

 basement of a recently folded alpine range.. 



2. Metamorpliism and igneous intrusion in relation to the degradation of 

 mountains. — It is a truism that the rocks of any alpine range vary enormously 

 in composition and structure. It is quite as true that their resistance to weather- 

 ing and wasting is far less variable. 



Secondly, the original upper surface of the zone of intense metamorphism 

 is almost certainly much less uneven than the outer surface of the original 

 range. 



Thirdly, many of the great intrusive bodies of alpine ranges had originally 

 themselves a demonstrably dome-like form with broad, flatfish tops. If the writer 

 is correct in holding that the patches of Anarchist schists and other of the older 

 rocks occurring within the area of the Similkameen batholith are true roof- 

 pendants, it seems probable that the present erosion surface on this large mass 

 is not far from the position of the original roof of the batholith. The reader will 

 recall similar cases described or mapped in the Sierra Nevada and elsewhere. 



The foregoing statement of a difficult theme is brief, but it suffices to suggest 

 the bearing of metamorphism and intrusion on the question of accordance. In 

 what has been defined as its original state, an alpine range was composed of a 



