26 THE ORIGIN OF SOILS [chap. 



earths, called generically zeolites, which play a very 

 important part in the soil, though they may not be 

 present in large amounts. These bodies, which result 

 from the weathering of the felspars, contain a consider- 

 able proportion of water, loosely combined and readily 

 displaced, but their distinguishing feature is the ease 

 with which the secondary bases they contain, the calcium, 

 magnesium, sodium or potassium, are replaced by other 

 metals, whenever their salts are brought into contact 

 with the zeolites. Little is known of the actual nature of 

 the zeolitic bodies in the soil, but certain zeolites occur 

 from time to time in a pure state. The best known of 

 them is natrolite, which crystallises in fine needles 

 possessing the composition — Na 2 0, A1 2 3 , 3Si0 2 , 2H 2 0, 

 a little calcium being generally present also. 



Iron Pyrites, FeS 2 , occurs in small brass yellow 

 cubic crystals in many of the older rocks, especially 

 those of a clay character; another form, in fibrous 

 masses of a lighter colour, is called marcasite, and is 

 common in the more modern clays, especially the 

 London clay, and again in round balls in the chalk. 

 Marcasite readily oxidises in moist air to ferrous sul- 

 phate and sulphuric acid : and many clay soils contain 

 basic sulphates, soluble in dilute acids but not in water, 

 that have arisen in this way. Selenite and the soluble 

 sulphates present in well waters, especially in clay soils, 

 are probably secondary products arising from the oxida- 

 tion of marcasite. In a finely divided condition iron 

 pyrites forms the colouring matter of many dark green 

 or olive rocks and clays. 



Soil and Subsoil, 



Although the transition from soil to subsoil is 

 gradual, the distinction between the two is, as a rule, 

 easy to be made; the change begins an inch or so 



