PHYSIOGRAPHIC EXTENSION OF UNITED STATES 175 
produced according to the peneplain theory of Davis. Daly' 
opposes this idea, however, in so far as it applies to the region under 
discussion, and explains the accordance of summit levels on a 
different principle. Without attempting to pass upon the relative 
merits of the theories, it may be remarked that the general topo- 
graphic features are at least consistent with the peneplain 
hypothesis. 
The sequence of physiographic events in the Alaskan section 
is difficult to determine. A probable summary is as follows: first, 
crustal disturbance in the late Mesozoic era and the opening of 
numerous volcanic vents at about the same time; second, pene- 
planation toward the close of the Tertiary period; and third, later 
differential uplift and erosion that in a large measure contributed 
to the present topography.” Though this statement should be 
accepted as subject to revision as our knowledge of Alaskan geog- 
raphy and geology increases, it appears probable that the major 
events of physiographic history in Alaska have followed the schedule 
described for the remainder of the province. 
THE INTERMONTANE PLATEAUS 
East of the mountains of the Pacific System lies a broad belt 
of country which, though characterized in many places by moun- 
tains and valleys or basins, presents on the whole a plateau surface, 
in part degraded, in part constructional. This belt of plateaus 
extends from Mexico northward to the Bering Sea and includes the 
Great Basin, Colorado and Columbia plateaus of the United States, 
the Interior Plateaus of British Columbia, and the Yukon Plateau 
of Alaska. It is bounded throughout its entire extent on the east 
by the Rocky Mountains. 
Although there is some diversity of surface features among the 
several units of this intermontane belt, particularly between widely 
separated units, there is sufficient similarity among them in the 
relation of each to the adjoining provinces on the east and west, 
in their principal structural features, and in their records of 
t™R. A. Daly, Geol. Survey Canada, Mem. 38, pp. 631-41. 
2 A. H. Brooks, op. cit., pp. 290-95. 
