172 W. N. THAVER 
Columbia River across the Cascade Mountains, and that of the Skagit 
River across the Skagit Mountains were outlined upon the sur- 
face of a Tertiary peneplain and have been maintained in spite 
of uplift to the present time. Most of the smaller streams are sub- 
sequent and have had their courses determined by structure or the 
relative hardness of rocks. Many of these latter have also had their 
courses altered by lava flows. 
The Coast Range of British Columbia has a structure analogous 
to that of the Sierra Nevada and Cascade mountains,’ and its 
topography is strikingly similar. These mountains are also the 
remnants of an uplifted and dissected (Tertiary) peneplain, the 
relief of which has been increased by later “warping, flexure, or 
displacement.”? They are from 60 to 1oo miles in width and 
of fairly uniform height. Many residual peaks rise along the 
crest line to a considerable elevation above the general level. — 
Where erosion has removed the overlying sedimentary rocks a 
number of great batholiths are exposed, which may be regarded 
as connecting the Coast Range structurally with the southern 
members of the system. The eastern slope of the mountains is 
very gentle, as in the Northern Cascades, and in many places 
it merges insensibly with the Interior Plateaus.* 
The principal rivers which flow across the Coast Range from the 
interior are antecedent to the uplift, and have maintained to the 
present time courses which they originally established on pene- 
planed surface that sloped westward to the ocean. Chief among 
these are the Fraser, Stikine, and Taku. 
It is difficult to give a clear summary statement of the topog- 
raphy of the Alaskan section on account of the small amount of 
geological work and mapping that has been done. Suffice it to 
say, however, that the Chigmit and Alaska ranges are bold, moun- 
tainous features, and the available evidence indicates that they have 
been carved from a Tertiary peneplain after differential uplift. 
1A. C. Spencer, U.S. Geol. Survey, Bull. 287, pp. 10, 11; also G. O. Smith, U.S. 
Geol. Survey, Folio 86. 
2 A. C. Spencer, Bull. Geol. Soc. Am., XIV, 117-32. 
3R. G. McConnell, Geol. Survey Canada, Guide Book No. 10, pp. 7-11. 
4A. H. Brooks, op. cit., p. 271. 
