24 THE ORIGIN OF SOILS [chap. 



results of solution, and represent the lines along which 

 the drainage of the rain water has been more active, 

 owing to a joint or fissure in the rock below. 



Other minerals which do not constitute any large 

 proportion of the earth's crust, but still play some 

 part in the soil, are apatite, glauconite, selenite, limon- 

 ite, and iron pyrites. 



Apatite, or crystallised phosphate of lime, — 

 Ca 5 (P0 4 ) 3 F, — is present in small quantities in many 

 of the fundamental rocks, and is probably the ultimate 

 source of the phosphoric acid of soils. Apatite also 

 occurs massive in some of the older strata, and has been 

 worked as a raw material, for the manufacture of phos- 

 phatic manures, in Norway and Canada. 



Selenite, hydrated sulphate of lime, CaS0 4 , 2H 2 0, 

 termed gypsum when massive, is not a fundamental 

 mineral, but occurs in most clay rocks in well de- 

 veloped crystals. Diffused through the soil and dis- 

 solved in soil water, selenite doubtless provides most of 

 the sulphur required by plants. 



Limonite ) hydrated oxide of iron, occurs in lumps 

 and bands in many of the sedimentary rocks; in a 

 diffused state it is the main colouring matter of soils ; 

 in heavy, undrained soils it often forms a layer or " pan " 

 some inches below the surface. It is deposited from 

 water containing bicarbonate of iron on exposure to the 

 air ; the rusty deposits and stains from chalybeate springs 

 and wells consist of limonite. The action appears to be 

 as follows — the hydrated peroxides of iron in the soil 

 when in contact with humus (decayed vegetable matter) 

 and water charged with carbonic acid become first 

 reduced to the ferrous state by the organic matter, and 

 then dissolved as bicarbonate. On exposure to the 

 air, the excess of carbonic 1 *' acid escapes by diffusion, 

 the ferrous carbonate, as it is precipitated, is also oxi- 



