REPORT OF THE CHIEF ASTRONOMER 641 



SESSIONAL PAPER No. 25a 



down stream, slowly attain a lower and lower level. Local notches or cols may 

 be gnawed in the ridge, but all the summits must be roughly accordant, though, 

 of course, not uniform, in altitude. Other things being equal, the more mature 

 the dissection, the more perfect the summit-level accordance; but the principle 

 may be applied to alpine ranges. In those ranges the actual imperfect degree of 

 accordance may often match the imperfectly matured state of dissection. 



SUMMARY. 



The form of the preceding discussion has been analytical, but its main 

 point has been to emphasize the synthetic nature of the process of mountain 

 sculpture. Seven different conditions of erosion ivorh together to produce accord- 

 ance of summit levels in an ideal alpine range undergoing its first cycle of 

 physiographic development. Isostatic adjustment and simultaneous, differential 

 degradation of rising blocks tend to bring about rough accordance of summit 

 levels in the range as ' originally ' formed. Later differential erosion and conse- 

 quent further isostatic adjustment, the influence of metamorphism and intrusion, 

 the sculpture due to high-level glaciation, the normal existence of a high-level 

 tree-line, and, finally, the compound process of river spacing and slope gradation" 

 — all these may combine their effects and render more perfect. the accordance of 

 levels inherited from the early, growing period of the range. 



This composite explanation must, therefore, be considered very carefully 

 in discussing the origin of the present relief in an alpine range where there are 

 no remnant plateaus directly referable to a common, uplifted and dissected pene- 

 plain. Such accordance may give a comparatively even sky-line in views from 

 any dominating point, but the full force of the composite explanation is directed 

 against the reference of that even sky-line to the direct or inherited profile of a 

 peneplained surface. 



General Conclusions on the Physiographic History of the Cordillera at the 



Forty-ninth Parallel. 



Mild deformation of Eocene, Oligocene, and Miocene formations in contrast 

 vvith strong deformation of Upper Cretaceous and older formations at many 

 points in the middle and northern parts of the Cordillera represents a combina- 

 tion of facts which clearly date the last general orogenic revolution at the close 

 of the Cretaceous period. The findings at the Forty-ninth Parallel in this matter 

 corroborate the conclusion of Dawson, McConnell, Dana, and others who have 

 devoted special attention to these mountains. 



The topography resulting from the Laramide revolution was one of great 

 relief and of most intricate design. 



The dominance of quartzites, hard schists, massive limestones and dolomites, 

 and granites in the Forty-ninth Parallel section explain the obvious fact that 

 the Cordillera is here a gigantic highland unit which is specially stubborn 

 against attack by erosive agencies. 



