i6o 



The National Geographic Magazine 



layers of the substratum, being usually 

 dry in arid regions, are readily shifted 

 and worn by winds. 



THE CAUSES OF A DESERT 



The term desert may be applied to 

 areas of the earth's surface which sup- 

 port a sparse vegetation of a more or 

 less specialized character, owing to in- 

 adequate rainfall or to the unsuitable 

 composition or lack of soil. Of these 

 conditions, scanty water supply may be 

 regarded as of the greatest importance, 

 and it is to this factor that most deserts 

 owe their existence. Desert conditions 

 arise in any region in which the rain- 

 fall is markedly less than the amount of 

 water that evaporates from the surface 

 of this liquid in the open air. As the 

 amount of evaporation naturally in- 

 creases from the polar regions toward 

 the tropics and is affected by winds and 

 elevation, it follows that no arbitrary 

 amount of rainfall may be designated 

 as an invariable cause or accompani- 

 ment of arid or desert conditions. Thus 

 in certain portions of the tropics a rain- 

 fall of less than 70 inches results in 

 aridity, while some of the most fertile 

 agricultural districts in the north and 

 south temperate zones receive scarcely 

 one-third this amount. 



Regions in which precipitation is less 

 than evaporation are characterized by a 

 lack of running streams or of a perma- 

 nent run-off, although in some instances 

 these districts may be traversed by large 

 rivers which have their sources in dis- 

 tant mountain ranges, as in the case of 

 the Nile in Africa and of the Colorado 

 River in America. The rainfall in a 

 desert may be so heavy at certain sea- 

 sons as to produce torrents of great vol- 

 ume, which, rushing downward over 

 the slopes and mountain sides, wear 

 distinct streamways extending out into 

 the plains below, in some instances for 

 miles ; but the flow soon ceases after 

 the rains have passed, and the stream 

 beds become dusty channels until the 



next rainy season. Striking examples 

 of such streamways are to be seen in 

 the great Sonoran desert in northwest- 

 ern Mexico. It is evident that districts 

 in which the average rainfall is not 

 much greater than the evaporation are 

 in a very critical condition, since in 

 seasons of minimum precipitation the 

 amount of water received may be less 

 than that lost, and drouth may result, 

 often with direful effects on agricultural 

 operations and economic conditions in 

 general. 



The seasonal distribution of the rain- 

 fall is a matter of importance in regions 

 where evaporation is nearly as great as 

 precipitation. If the rainfall occurs 

 within a brief period, the remainder of 

 the year must be extremely dry, and 

 the region will show distinct desert con- 

 ditions, with a tendency on the part of 

 the native plants to develop marked 

 storage capacity for water. The dis- 

 tribution of the scanty rainfall through- 

 out the year in any region will favor 

 the development of slowly growing 

 xerophytic forms. 



Arid deserts occur in all of the great 

 land divisions and reach an enormous 

 extent in Africa, Asia, and Australia. 

 The most pronounced desert conditions 

 of South America are found on the west- 

 ern slopes and benches of the Andes. 

 One locality, that of Copiapo, has an 

 average precipitation of 0.4 inch per 

 year, and, so far as known, is the driest 

 spot on the earth's surface. The des- 

 erts of North America are confined to 

 the Cordilleran region and occupy pla- 

 teaus and plains east and west of the 

 main ranges to an extent of more than 

 a million square miles. 



THE AMERICAN DESERTS 



A study of the physiographic, floristic, 

 and meteorological features of western 

 North America has resulted in delimit- 

 ing two great desert areas by the geog- 

 rapher, botanist, and meteorologist. 

 The outlines of these might be roughly 



