CONTINENTAL ZONES 159 



had upon humidity, and upon temperature symmetry. The 

 two are intimately interwoven. The lowering of tempera- 

 ture 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 grass- 

 land 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 Eocky 

 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 systems 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, forest, which would prevail were it not 

 for these barriers. This actual series of major zones under- 

 goes 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 Appala- 

 chians. Its result is to carry the polar deserts of the north 

 far southward along the crests of the mountains, and to ex- 

 tend 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 occassional appearance of an iso- 

 lated peak. In the western ranges, it produces two sym- 

 metrical series of minor mountain zones, forest, alpine 

 grassland or desert, and forest, to say nothing of the foot- 

 hill and timber line zones of thicket. 



There seems to be no good reason for distinguishing the 

 zones of mountains as regions. The term itself is inappli- 



