PHYSIOGRAPHIC EXTENSION OF UNITED STATES 181 
of the forty-ninth parallel. Daly limits the use of the term to 
what is really the Canadian extension of the Front Ranges of the 
United States and fails to recognize that the term ‘‘ Rocky Moun- 
tains”’ is used in a very much broader sense by both popular and 
scientific writers in the United States. Ransome? applies the term 
“to the whole of that part of the Laramide System which extends 
from the Bering Sea to the southern ends of the San Juan and 
Sangre de Cristo ranges in Colorado.’”’ Fenneman? makes his 
Rocky Mountain division within the United States include prac- 
tically the same area as Ransome’s, though the latter wrote of 
general geology rather than physiography. 
There is something to be said, of course, in favor of each of these 
definitions, as well as of the earlier nomenclature proposals of 
Dawson and Dana, more or less perhaps as one lives north or south 
of the forty-ninth parallel. However, in spite of all that may be 
written, and in the face of frequent inconsistencies, popular usage 
will always be the final arbiter in questions of geographic names, 
and as Ransome’s and Fenneman’s definition conforms more nearly 
to popular usage than the others it will be used in this paper. 
The Canadian divisions of the Rocky Mountain System are 
easily correlated with the divisions lying within the United States; 
in-fact, the so-called ‘‘ Northern Rockies”’ of the United States and 
the ranges of British Columbia belong to one and the same 
province. This province may be further subdivided into four oro- 
graphic units, separated among themselves and from adjacent prov- 
inces by five roughly parallel, structural, north-south lines, namely, 
(1) the western edge of the Great Plains; (2) the Rocky Mountain 
trench; (3) the Purcell trench; (4) the Selkirk valley; and (5) the 
eastern edge of the Intermontane Plateaus.‘ 
The Rocky Mountain trench is a long, narrow, intermontane, 
structural depression or trough that extends from Flathead Lake 
in Montana northward almost to the boundary between British 
Columbia and Yukon Territory, a distance of about 990 miles. 
™R. A. Daly, Geol. Survey Canada, Mem. 38, p. 27. 
2 F. L. Ransome, Problems of American Geology, p. 201. 
3. N. M. Fenneman, op. cit., pp. 119-24 and pl. 2. 
4R. A. Daly, op. cit., p. 26. 
