REPORT OF THE CHIEF ASTRONOMER 631 



SESSIONAL PAPER No. 25a 



Cordillera may have been completed during the Cretaceous, when the 

 enormously thick Shasta-Chico beds were accumulated. The writer does, then, 

 favour the peneplain theory as applied for other times and places; but he fears 

 that the hypothesis of late Tertiary or mid-Tertiary peneplanation in the Cas- 

 cades may obscure the essential facts of their post-Laramie geology. 



Before discussing this favoured conception of the Cascades further, it is 

 well to review the correlative explanation of the accordance of summit levels 

 in high mountains. This is an important phase of the argument for one erosion 

 cycle and against two cycles in the Tertiary history of the Cascades, as, indeed, 

 for practically all of the trans-Cordilleran section at the Forty-ninth Parallel. 



Development of Accordance of Summit Levels in Alpine Mountains. 



In 1905 the writer published a paper on this subject with intent to empha- 

 size a ' composite ' explanation of summit-level accordance as the normal pro- 

 duct of the forces which act on a complex mountain range up to the mature 

 stage of its first erosion cycle.* This explanation is opposed to that in terms 

 of two erosion-cycles involving the uplift of a peneplain. In the years which 

 have followed, the writer's additional field and laboratory studies have tended 

 to confirm belief in the ' composite,' one-cycle hypothesis. A digest of the pre- 

 liminary paper will here be given, together with some further illustration of 

 important points, taken from the region covered by the Forty-ninth Parallel 

 survey. 



In the present section the term ' alpine range ' is used to signify a range 

 possessing not only the rugged, peak-and-sierra form of the Swiss Alps, but, as 

 well, the internal structures incidental to intense crumpling, metamorphism, and 

 igneous intrusion as exemplified in the Swiss Alps. 



The word 'accordance' is used advisedly. 'Equality' of heights is not 

 meant by those observers who have given the question the best attention. 

 For limited areas ' subequality ' of the summits is a fact, but over wider 

 stretches, and especially over the whole of a single range, even subequality fails, 

 and the accordance takes the form of sympathy among the peaks whose tops in 

 companies or in battalions rise or fall together in imaginary surfaces often far 

 removed from the spheroidal curve of the earth. In general, the imaginary 

 surface which will include the higher summits of peaks and ridges in an alpine 

 range has the form of a low arch, highest in the interior of the range and 

 elongated in the direction of the main structural axis of the range. Subordinate, 

 but usual and systematic, complications in the form of this imaginary surface 

 are found in transverse crenulations which alternately depress and raise the 

 surface from its average out-sloping position on the margin of the great arch. 

 The axes of these transverse depressions are often suspiciously coincident with 

 existing drainage courses. 



There is, then, at least one orderly element in the ( chaos ' or ' tumbling 

 sea' of mountains visible from a dominating point in any one of a goodly 



*R. A. Daly, Journal of Geology, Vol. 13, 1905, pp. 105-125. 



