126 A. C. SPENCER — PACIFIC MOUNTAIN SYSTEM 



into the Pacific ocean. The most southern of these streams is the 

 Columbia river, which Russell * has described as antecedent to the uplift 

 of the Cascade range. The Fraser river also, according to the observa- 

 tions of Dawson.t maj^ be considered as having maintained its early 

 course regardless of the upraising of the Coast range across its path. 



The origin of the Skeena, Naas, Stikine, and Taku rivers of British 

 Columbia and southeasternmost Alaska has not been discussed hitherto. 

 They are, however, believed by the writer to be surely antecedent 

 streams, for the reason that the peneplain of the Coast range may be 

 identified as continued from the Fraser river northwestward, across each 

 valley in turn, to the vicinit}^ of Lynn canal. Moreover, the directly 

 observable equivalency of the summit and inland plateaus in the Taku 

 region has already been indicated, from which it follows as a strong 

 probability that the Taku river developed on a continuous peneplain 

 sloping toward the sea, and that the Coast range was uplifted across its 

 previously established course. The other rivers named are by analogy 

 regarded as of similar origin. 



Passing to a consideration of the Alsek river, it has been described b}' 

 Brooks I as antecedent to the uplift of the southeastern wing of the Saint 

 Elias range. This river has lost much of the drainage which at one time 

 belonged to it, through northwestward tilting in the region of the upper 

 White river, and probably also because of ice dams which once existed 

 across its course. A wide valley-like depression was traced from the 

 vicinity of Dezadeash lake approximately north of the mouth of Alsek 

 river northeastward to the head of the Tanana river, and evidence tends 

 to show that this valley was once occupied by a great river which 

 drained into the present Alsek. At present the drainage is not conform- 

 able with this valley, showing that fundamental readjustments have 

 taken place, but the lower river course is still maintained as a w^aterway 

 from the interior to the coast. 



The headwaters of the Copper river do not penetrate the Yukon 

 plateau as do the other rivers which have been mentioned, but the river 

 drains an extended basin, separated from the interior plateau region by 

 the northwestward continuation of the Saint Elias range and lying back 

 of the Chugach mountains, which form the western coastwise prolonga 

 tion of the Saint Elias range. 



A sketch map of the Copper river and its tributaries has been prepared 



*" A reoonnaissance in central Washington." Bull. U. S. Geo). Survey, no. lOS, 1893, p. 97. 



t G. M. Dawson : The later physiographical geology of the Rocky Mountain region in Canada, etc. 

 Trans. Roy. Soc. Canada, vol, iii, 1890, p. 17. 



^Alfred H. Brooks : A reconnaissance from Pyramid Harbor to Eagle City, Alaska, etc. Twenty- 

 first Ann. Rept. U. S. Geol. Survey, pt. 2, 1900, p. 354. 



