COMPOSITION OF SOIL-FORMING MATERIALS 21 



of metallic sulphides; and, partly, undoubtedly, to the 

 formation of the slightly soluble calcium- sulphate. This 

 last, however, is gradually brought toward the surface, 

 and is often found in enormous masses at moderate depths 

 in the soils of arid regions. Undoubtedly the calcium 

 carbonate so generally found in large masses at moderate 

 depths in the soil of arid regions originates in a similar 

 manner. 



" Hydrochloric acid is transported through soils and 

 most absorbing media with comparative ease. Moreover 

 the chlorides of the alkalies and alkaline earths are readily 

 soluble. Chlorides should be expected, therefore, to 

 accumulate in preponderant masses at the surface, which 

 under arid and semi-arid conditions they generally do. 



"The preponderance of sodium chloride above other 

 chlorides is readily explicable. It is well known that when 

 solutions of chlorides are poured through columns of soil 

 or similar substances, offering a large surface of contact 

 to the solution, there is a well-marked selective absorption, 

 the soil tending to withdraw the base from the solution 

 to a decidedly greater extent than the acid, with the result 

 that the leaching generally contains free acid. So far as 

 the experience we have goes, it would seem that, in general, 

 soils absorb potassium most readily, then magnesium, 

 calcium, and sodium in the order named. Supposing the 

 hydrochloric acid when found in the lower layers to be 

 neutralized with a mixture of these bases, as it rises in the 

 capillary movement, there is always a tendency, owing to 

 the selective absorption of the soil, toward a lagging 

 behind of the potassium, a lesser lagging of the magnesium 

 and calcium (these bases probably tending also to form 

 the much less soluble sulphates and carbonates) and a 

 much less lagging of the sodium. In consequence, sodium 



