PHYSIOGRAPHIC EXTENSION OF UNITED STATES 171 
probability the system also includes the volcanic Aleutian Islands. 
In the absence of adequate data on the mountains of Alaska it should 
be remembered that any classification is merely tentative and is 
subject to revision as knowledge of the country becomes more 
thorough. However, there is considerable evidence to support the 
present classification. This will be given in the discussion of the 
topography and physiographic history of the region. 
In a general way the topography of this province, particularly 
Sierra Nevada and Cascade mountains, is that of an uplifted 
(Tertiary) peneplain, which has been deformed by folding and 
faulting, deeply dissected by erosion, and covered deeply in places 
with volcanic products." 
The Sierra Nevada is a bold, continuous, and deeply dissected 
range, about 75 miles wide, with a crest line of well-defined 
residual peaks. The mountains are delimited on the east for hun- 
dreds of miles by a high and steep fault-scarp, but descend gradually 
to the Great Valley on the west across a gently sloping plateau. 
The Cascade Mountains have also, in most places, a broad, 
maturely dissected summit of about the same width as that of the 
Sierras, above which rises a straight north-south line of several 
scores of volcanic peaks. Long, broad, flat-topped spurs, analogous 
in some respects to the crest of the High Sierras, diverge from these 
peaks. The eastern slope of the Southern Cascades is bold and the 
western slope gentle. In these respects also they resemble the 
Sierras. In the Northern Cascades, however, the eastern slope 
loses its abruptness and the peneplain of the mountains descends 
gradually to the plateau of the Columbia River. 
Three types of volcanic products have contributed to the mak- 
ing of the present topography of the Sierra Nevada and Cascade 
mountains: (1) batholithic granite and diorite intrusives, (2) flows 
of basaltic lava, and (3) andesitic cones which rise above the general 
level and dominate the view from many points. 
The drainage of these sections of the province is characteristic 
of its type of topography. The courses of the forks of Feather 
River across the crest of the Sierra Nevada, the course of the 
tJ. S. Diller, U.S. Geol. Survey, Bull. 353, p. 9; also I. C. Russell, U.S. Geol. 
Survey, 20th Ann. Rept., Part II, p. 140. 
