BORD cs a? VESTAL—FOOTHILLS VEGETATION 355 
temperatures and probably greater insolation, gives a drier aspect 
to the southern foothills; pinyons are abundant, instead of rock 
pine, as in the northern part of the state. Mean annual tempera- 
tures in the foothills vary from 40 to 50° F.; mean summer tem- 
peratures 60 to 70° (ROBBINS 17). Evaporating power of the air 
is great, though much less than on the plains. Local variations 
in atmospheric conditions, due to surface configuration, are con- 
siderable. Insolation is much greater on summits and south-facing 
slopes than on north-facing slopes or in ravines. The drying winds 
are usually from the west, and their effects are greatest on summits 
and the flatter uplands. 
Local and topographic factors are extremely and very locally 
variable. Position with reference to surroundings, and slope, its 
amount and direction, may perhaps be called the two master 
factors of the topographic complex. Slope and position are most 
important in determining conditions of soil deposition or removal 
(consequently depth and texture of the soil); conditions of absorp- 
tion and run-off, drainage, or seepage from above, or possible 
seepage from snowdrifts; presence or absence of plant remains, 
which may contribute humus, or locally form a mulch (pine needles) ; 
and conditions of exposure to sun and wind with its attendant 
effects. The granitic hills in general (fig. 1) are in an early-mature 
Stage of topographic development (for a mountain region of 
resistant rocks); the side-slope profiles are nearly straight, with 
comparatively little detritus covering the bottom of slopes; the 
tops of the hills are usually rounded, and hardly more subject to 
erosion than the sides. In general, the foothills are so well drained 
that bogs, marshy flats, and moist-soil meadow areas are infrequent, 
even in canyon-bottoms. Hilltops and side-slopes are covered, 
usually thinly, with rock-débris or with decomposed granite soils, 
varying in texture from coarse gravelly material of angular frag- 
ments’ to fine black loamy humus. Wide dikes and occasional 
outcrops of more resistant rock have only a little soil, in joints and 
crevices, hardly any elsewhere. 
* The writer has not been able to find a distinctive name for this coarse angular 
débris resulting from decomposition of granite; it is not re & Os tone Ce word 
usually implies; in this article it will be called “ te-gravel.” 
