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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 eflfect 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 eflFect, 

 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 



