I.] ROCK-FORMING MINERALS 25 



Sclcnite, 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 iron on exposure to the air ; the rusty 

 deposits and stains from chalybeate springs and wells 

 consist of limonite. 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 similarly bleached. On ex- 

 amining 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 limonite, and below this the 

 normal brown or yellow sand begins. 



On other soils the limonite may form a hard con- 

 tinuous layer, impervious to drainage, the " iron pan," 

 or " moor-band pan," which is found a few inches 

 below the surface, and which must be broken up by 

 deep cultivation before satisfactory crops can be 

 obtained from the land. Analogous are the formations 

 of loose rubbly concretions "crowstone " found below 

 the surface of certain clay soils, and the bright red 

 deposit of "bog iron ore" generally to be seen beneath 

 the black peaty accumulation in swampy places. The 

 factor common to all these deposits is the absence of 



