PHYSIOGRAPHIC EXTENSION OF UNITED STATES 179 
in the region adjacent to White and Chilkoot passes, and in the 
vicinity of White and Stikine rivers. In these places the summit 
plateau of the mountains blends with the plateau of the interior." 
The physiographic history of the Intermontane Plateaus is 
coincident with that of the mountain provinces bordering them on 
the west down to Eocene time,” and since the latter has been dis- 
cussed in preceding pages it will not be necessary to repeat it here. 
The Eocene epoch witnessed widespread volcanic action of great 
magnitude in the Columbia Plateau division, and lesser, localized 
action in the northern divisions, accompanied by an uplift of the 
previously subdued surface. An erosion interval followed in the 
Oligocene epoch. Whatever effect it may have had on the Colum- 
bia Plateau is now largely obscured. Drysdale’ records a post- 
Eocene-pre-Miocene erosion interval for the Interior Plateaus and 
Brooks? records a post-Eocene erosion interval for the Yukon 
Plateau. Dawson,’ Spurr,° Spencer,’ and others have correlated the 
peneplains thus produced as belonging to the same period, making 
it appear that between the close of the Eocene epoch and the begin- 
ning of the Miocene epoch there was a period of widespread pene- 
planation. Daly,® however, insists that there was no period of 
general peneplanation, and states that the upland surface was pro- 
duced by several pre-Miocene erosion cycles. ‘The residual moun- 
tains of British Columbia, Yukon Territory and Alaska, and the 
Blue Mountains of Oregon probably represent masses which 
remained unsubdued during this time (one or more cycles as the 
case may be), although they may date back to the early Tertiary 
or late Mesozoic erosion period. 
The first basaltic flows on the Columbia Plateau are assigned 
to the Miocene epoch. Similar flows, more localized, however, 
tA. C. Spencer, op. cit., pp. 125-28. 
2G. O. Smith and F. C. Calkins, U.S. Geol. Survey, Bull. 235, pp. 85-90. 
3 C. W. Drysdale, Geol. Survey Canada, Guide Book No. 8, Part II, pp. 235, 236. 
4A. H. Brooks, op. cit., pp. 278, 279. 
5G. M. Dawson, Trans. Royal Soc. Can., VIII, sec. 4, p. 12. 
6 J. E. Spurr, U.S. Geol. Survey, 18th Ann. Rept., Part III, p. 260. 
7A. C. Spencer, op. cit., p. 128. 
8 R. A. Daly, Geol. Survey Canada, Guide Book No. 8, Part II, p. 164. 
91. C. Russell, op. cit., p. 61. 
