24 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



component ranges by broad intermontane plains of mountain waste, or of lava 

 filling vast structural troughs or basins. Nothing quite comparable is to be 

 seen anywhere in the Canadian portion of the Cordillera. Near the latitude 

 of Spokane, the whole mighty group of ranges is marshalled into a solid phalanx 

 of closely set mountains which sweep on in substantial unity north-westward 

 through Yukon territory into Alaska. The area of British Columbia alone 

 would enclose twenty-four Switzerlands. For purposes of exposition this moun- 

 tain sea must be divided and subdivided. How shall it be done? 



The remarkable insight and generalizing power of the pioneer in British 

 Columbia geology, G. M. Dawson, early supplied what seem to be the only 

 fruitful principles. His classification applies chiefly to southern British Col- 

 umbia, but it is probable that its principles must be extended throughout the 

 Canadian Cordillera. In 1879 Dawson announced the possibility of a natural 

 division of the mountains between the Forty-ninth and Fifty-fifth parallels into- 

 three broad belts paralleling the coast. 



The middle belt, the ' Interior Plateau,' afterwards described in some detail, 

 has the special style of topography characteristic of closely folded mountains 

 once reduced by denudation to mere rolling hills or an imperfect plain, since- 

 uplifted and cut to pieces by streams. In other words, the Interior Plateau is, 

 by Dawson's definition, an uplifted, dissected peneplain, a region of plateaus 

 and hills remnant from the old surface of denudation. Yet Dawson himself 

 concluded that, while many of these tabular reliefs may be correlated into the 

 ancient facet of denudation, other similar reliefs in the belt are structural, 

 and due, namely, to the erosion of wide, flat-lying lava flows that flooded the 

 country after the peneplanation. Another and simpler explanation of the topo- 

 graphy makes the lava flooding anterior to peneplanation. Still, a third history 

 may, on further investigation, turn out to be the true one. At the present time 

 it is impossible to decide between the rival views. 



A safer definition of the region is purely topographic; it may thus be 

 called the Belt of Interior Plateaus, or, briefly, the Interior Plateaus. (Plate 3,) 

 This slight change in Dawson's name lays stress upon the individual tabular 

 reliefs so characteristic of the region. These reliefs are facts ; the peneplain and 

 the involved assumption that the myriad individual reliefs belong to a physio- 

 graphic unit, a single uplifted peneplain, are matters of theory. The pluralizing 

 of the word ' plateau ' in the title not only changes the emphasis, but, in so 

 doing, restores the term to its more advisable definition of a tabular relief 

 bounded by strong downward slopes. The Interior Plateau as defined by Daw- 

 son is bounded on all sides by the strong upward slopes of the enveloping 

 mountain ranges. 



The belt of Interior Plateaus having thus been differentiated on special 

 grounds, we may pass to the subdivision of the remaining two parts of the 

 British Columbia complex. Those two belts separated by the plateau belt are 

 rugged, often alpine, and, as a rule, do not show tabular reliefs. Present 

 knowledge of the vast field cannot provide a rational treatment of these moun- 

 tains rigidly on the basis of either rock composition or structural axes or 



