I.] ROCK-FORMING MINERALS 27 



bacteria. The presence of an iron pan or of rusty 

 stains and deposits from the drainage water or in the 

 ditches is a sure sign of a sour, unhealthy soil, which 

 requires lime or chalk before it can be made fertile. 



Glaucom'te is a hydrated silicate of iron, alumina, 

 and potash with a little lime and magnesia, which 

 occurs as dark green grains in many sedimentary rocks, 

 especially of the Cretaceous age. It is to the presence of 

 this material that the Greensand formations owe their 

 name ; it is sometimes also to be seen in chalk and 

 in the tertiary sandstones. It readily weathers to 

 brown oxides of iron. 



Zeolites. Akin to glauconite are certain hydrated 

 double silicates of aluminium and the alkalis or alkaline 

 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 O f 

 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. 



