10 GEOGRAPHICAL DISTRIBUTION 



the rainy season the lower parts of these valleys are flooded 

 by the waters of swollen streams; during the dry season 

 this water is practically all evaporated, leaving its soluble 

 material behind. This results in great level bodies of 

 land charged in varying degrees with soluble salts. The 

 composition of these saline deposits depends on the com- 

 position of the country rock through which the streams 

 flow. Very little work up to the present time has been 

 done to reclaim the alkali soils of Mexico. 



South America. No important published material is 

 available on the alkali condition of the soils of South 

 America. It is known, however, that the arid sections of 

 that continent do not differ essentially from those of other 

 arid sections of the world. Practically the entire western 

 part of the continent is arid and throughout this section 

 areas subject to alkali troubles are found. It includes 

 most of the Pacific slope west of the Andes and the greater 

 part of the western plains of Brazil and Argentina east of 

 these mountains. 



The deposits of sodium nitrate in Chile are a conspicuous 

 example of the retention of soluble salts that would be 

 leached out in a humid climate. 



Africa. The distribution of alkali soils in Africa is not 

 the same as in North and South America. It is found 

 over practically the entire northern portion of the con- 

 tinent and also in the southwestern part. The central, 

 and particularly the west-central, portion is practically 

 free. Throughout the Union of South Africa up into 

 Rhodesia alkali soils are found but have not received as 

 much attention as some of the sections of North Africa, 

 particularly in Egypt. The soils of the Sahara as well as 

 many of those of Algeria, Morocco, and Tunis are so 

 contaminated with soluble salts that it was necessary for 



