I.] ROCK-FORMING MINERALS 25 



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dised by the oxygen of the air, and deposited as Hmonite. 

 It will be noticed that stones taken from peaty land 

 are always bleached white, through the removal of iron, 

 and the surface sand of heathy land is always simi- 

 larly bleached. On examining a section of any purely 

 sandy formation, the surface soil will be found to be 

 bleached below the layer of vegetable matter to the 

 depth of a foot or more. Then comes a layer an inch or 

 two thick nearly black in colour, where the sand is more 

 or less cemented together by Hmonite, and below this 

 the normal brown or yellow sand begins. The black 

 band is formed at the depth to which the air usually 

 penetrates the soil ; it consists of Hmonite deposited 

 at the evaporating surface of the soil water, which 

 contains the iron dissolved from the bleached surface 

 sand. In a similar manner arises the hard layer of 

 Hmonite, the " iron pan " or " moor-band pan," found just 

 below the cultivated soil on many undrained lands, and 

 again the deposit of " bog iron ore " which is generally 

 to be seen beneath the black peaty accumulation in any 

 swampy place. The solution of iron as bicarbonate, 

 and its precipitation as Hmonite, do not occur in soils 

 containing any calcium carbonate, being essentially a 

 sign of an acid condition of the soil and its need for 

 lime or chalk. 



Glauconite 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 



