40 THE SOIL. III. 



characteristics. Thus in felspar, K 2 O.Al. 2 3 .6Si0 2 , as it is 

 usually represented, the potash is almost always replaced to 

 some extent by soda, and the mineral can be traced through 

 all intermediate stages to albite, with the ideal composition 

 Na 2 O.Al. 2 8 .6Si0. 2 . So, too, in calcite, which is theoretically 

 CaC0 8 , traces of magnesium are invariably present, and the 

 replacement of Ca by Mg may go on until the composition of 

 the substance would be more correctly represented by MgCO H . 



Minerals important in agriculture because of their abun- 

 dance are : 



Quartz. This is, practically, pure siljca, Si0. 2 , and is 

 extremely abundant. It occurs in granite and many other 

 igneous rocks. It is 'practically insoluble in water and so is 

 little affected by \veather. Usually, however, the other con- 

 stituents of the rocks in which it occurs (e.g., the felspar in 

 granite, the cementing material calcium carbonate, clay, or 

 ferric oxide in sandstone, &c.) disintegrate by weathering, and 

 the quartz fragments become detached and are thus removed. 

 Quartz, though the most abundant constituent in most soils, 

 is of practically no value as a plant food. 



Felspar may be orthoclase, K 2 O.AL 2 O 8 .6Si0. 2 

 albite, Na,O.Al.A.6Si0. 2 

 oligodasc, 2(Na, 2 :Ca)O.2Al. 2 0,-6Si0. 2 

 or labradorite, (Na 2 :Ca)O.AL 2 O 8 -3SiO. 2 . 



Orthoclase, the most important of these, is very abundant, 

 forming an essential ingredient in granite, gneiss, syenite, and 

 many other rocks. Though a hard substance, felspar is very 

 easily decomposed by the influence of the weather. Water 

 containing carbonic acid attacks it readily, removing the larger 

 portion of the potash and a portion of the silica in a soluble 

 form and leaving eventually a residue containing pure clay or 

 kaolin, Al 2 H .2H./).2Si0. 2 . Clay, however, is usually contami- 

 nated with partially decomposed felspar containing still a 

 portion of its potash. Felspar furnishes a considerable pro- 

 portion of the potash of a soil. 



Mica, 3Al. 2 8 .K 2 0.4SiO. 2 , always contains considerable quan- 

 tities of ferric oxide, which partially replaces the alumina ; 

 magnesia, soda, and lime, also, are usually present. This 

 mineral occurs in many rocks, notably in granite and gneiss. 



