REPORT OF THE CHIEF ASTRONOMER 25 



SESSIONAL PAPER No. 25a 



geological history. It is possible, if not indeed probable, that the ranges imme- 

 diately bounding the belt of Interior Plateaus have had a common history with 

 it; they certainly include the same rock formations as occur in the interior 

 plateaus. If the peneplain theory be finally accepted for the latter, it may 

 ultimately prove best to treat the Coast range and other ranges in terms of the 

 same theory. The only feasible scheme of subdivision at the present day must 

 be based on topography only. 



Mere hypsometry will not serve alone; the ranges of summit altitudes is too 

 slight, their variation too unsystematic, for that. Dawson found that continuity 

 of crest-lines and the position of the greater erosion valleys formed the most 

 available basis of classification. As field work progresses in British Columbia, 

 it becomes more and more certain that this double principle is the best that 

 could be devised for present use. Many of the larger valleys are undoubtedly 

 located on structural breaks, but it is evident that the strength of most of the 

 valleys is the more direct result of fluviatile and glacial erosion. Owing to the 

 peculiarly complicated rearrangements in the drainage of the Cordillera, 

 \vhether due to glacial, volcanic, or crustal disturbances, or to spontaneous river 

 adjustments, the valleys of British Columbia are in size very often quite out of 

 relation to their respective streams. For example, the longest depression in the 

 whole Cordillera is occupied by relatively small streams, the headwaters of the 

 Kootenay, Columbia, Eraser, etc. Each of the rivers named, in its powerful 

 lower course, flows through narrow canyons. Erosion-troughs rather than rivers 

 have, therefore, been selected by Dawson and other explorers as the natural 

 lines of demarcation between most of the constituent ranges of the Cordillera 

 in these latitudes. The procedure is not new, but it is noteworthy as the most 

 wholesale application of the principle on record. It stands in contrast to the 

 more structural treatment, not only possible, but enforced by the orographic 

 conditions in the United States. 



In the course of his own work, the writer has become convinced of the 

 permanent value of Dawson's early and consistently held general view of the 

 British Columbia mountains. But there has arisen the necessity of extending 

 it to cover the Boundary mountains which, for the most part unvisited, were 

 left unnamed by Dawson. The task of systematizing them is simple only in 

 the stretch from the Great Plains to the Kootenay river at Tobacco Plains, 

 a width of about seventy-five miles. The remainder, or five-sixth?, of the Cor- 

 dillera on the international line is not generally grouped into organic units at 

 all; or, where so grouped, the names of the groups are not universally accepted. 

 In attempting to supply this lack of system, the writer's aim has been to develop 

 a system of grouping and nomenclature largely founded on names and concepts 

 alreadv in use, but not generally applied to the mountains so far south as the 

 Boundary. 



TRENCHES AND GREATER VALLEYS. 



A point of departure is readily found. Within the Cordillera on the 

 Eorty-ninth Parallel, there are four principal longitudinal valleys which 

 serve as convenient lateral boundaries for leading members of the system. 



