214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



the nature and causes of these different results have not always been 

 easy to understand or to explain. 



THE IMPORTANT ALKALI SALTS. 



The so-called alkali salts include sodium chloride, or common salt, 

 sodium sulphate, sodium carbonate, and sodium bicarbonate, to- 

 gether with some calcium and magnesium and even some potassium, 

 combined usually as the carbonate or sulphate. There is wide varia- 

 tion in the proportions in which these salts occur, but they are usu- 

 ally all present. When sodium carbonate is abundant, it often causes 

 a dark colored deposit on the soil surface, in consequence of which 

 the term " black alkali " is used in reference to this salt. Sodium 

 sulphate, on the other hand, often produces a white efflorescence on the 

 surface of the soil and is known as " white alkali." 



THE ORIGIN OF ALKALI SALTS. 



These salts are derived from the disintegration and weathering 

 of rocks in the process of soil formation. They are formed not 

 only in arid regions but in humid regions as well. In humid re- 

 gions, however, they are leached from the soil by the rain as soon as 

 they become soluble and are carried away by the rivers to the sea In 

 arid regions where the rainfall is insufficient to leach the soil com- 

 pletely the salts remain and accumulate. 



In the course of time these soluble salts are moved about by such 

 rain as falls. As the rains gather into small streams that form tem- 

 porary pools or lakes, the salts are carried into these and left behind 

 when the water evaporates. In some places it is possible to trace 

 very clearly the course of events in the formation of the soil in the 

 dry country. A cross section of the soil will show alternate layers 

 of salt and earthy material. It is clear that this material was laid 

 down in a basin that was subject to flooding after torrential rains. 

 These floods of muddy water spread out over the floor of the basin 

 and the water was soon evaporated, leaving the mud and salt behind. 

 As the last of the water evaporated from the surface of the mud layer 

 it left a crust of salt on this surface. The next flood brought down 

 more mud, which covered the previous layer, and some of the less 

 soluble salts remained between the successive layers of mud, while the 

 more soluble salts were redissolved, only to be redeposited again at 

 the new surface. 



With the changing conditions of flood channels and drainage these 

 surface deposits of salt have been reworked many times even in our 

 most recent geological periods. Thus we find in arid regions that 

 the soluble salts which we call alkali are abundant in some spots 

 while virtually absent in large areas of the better-drained land. 



