282 THE FORMATION 
furnishes striking proof of the truth of this. Conforming to the gradual 
decrease of temperature and water-content northward, three primary belts 
of vegetation stretch across the continent from east to west. These are 
forest, grassland, and polar desert. The first is further divided into the 
secondary zones of broad-leaved evergreen, deciduous, and needle-leaved 
forests. At right angles to this temperature-water symmetry lies a symmetry 
due to water alone, in accordance with which forest belts touch the oceans, 
but give way in the interior to grasslands, and these to deserts. It is at 
once evident that the mutual interruption of these two series of zones has 
produced the primary features of North America vegetation, i. e., tropical 
forests where heat and water are excessive, deserts where either is unusually 
deficient, grassland when one is low, the other moderate, and deciduous and 
coniferous forests, where the water-content is as least moderate and the 
temperature not too low. Such a simple yet fundamental division has been 
modified, however, by the disturbing effect which three continental moun- 
tain systems have had upon humidity and upon temperature symmetry. The 
two are intimately interwoven. The lowering of temperature due to altitude 
produces the precipitation of the wind-borne moisture upon those slopes 
which look toward the quarter from which the prevailing winds blow. A 
mountain range thus makes an abrupt change in the symmetry, and renders 
impossible the gradual change from forest to grassland and desert. The 
Appalachian system is not sufficiently high to produce a pronounced effect, 
and forests extend far beyond it into the interior before passing into prairies 
and plains. On the other hand, the influence of the Rocky mountains and the 
Sierra Nevada is very marked. The latter rise to a great height relatively 
near the coast, and condense upon their western slopes nearly all of the. 
moisture brought from the Pacific. The Rocky mountains have the same 
effect upon the much drier winds that blow from the east, and the two sys- 
tems in consequence enclose a parched desert. This series of major zones 
thus becomes, starting at the east, forest, grassland, desert, and forest, in- 
stead of the more symmetrical series, forest, grassland, desert, grassland, for- 
est, which would prevail were it not for these barriers. This actual series of 
major zones undergoes further interruption by the action of these mountain 
systems in deflecting northern isotherms far to the south. This action is 
greatest in the high ranges, the Rocky mountains and the Sierras, and least 
in the lower Appalachians. Its result is to carry the polar deserts of the 
north far southward along the crests of the mountains, and to extend the 
boreal coniferous forests much further south along their slopes. In the 
Appalachians, this means no more than the extension of a long tongue of 
conifers into the mass of deciduous forests, and the occasional appearance 
of an isolated peak. In the western ranges, it produces two symmetrical 
