CHAP, v.] Soil and Situation 95 



Anhydride or Sulphur Trioxide(SO 3 ) and Phosphoric Anhydride 

 or Phosphorus Pentoxide (P., O.) ; whilst the chief Bases 

 are Potash (K 2 O), Soda (Na. 2 O), Lime or Calcium Oxide 

 (CaO), Magnesia or Magnesium Oxide (MgO), Ferrous 

 Oxide (Fe O), Sesquioxide of Iron or Ferric Oxide (Fe 2 O,), 

 Alumina (A1 2 O ,), and Manganese Dioxide (Mn O 2 ). The 

 Silicates comprise quartz, serpentine, talc, felspars, mica, 

 hornblende, augite, and chlorite; the Carbonates include 

 carbonate of lime, chalk, and dolomite ; to the Sulphates 

 belong anhydrite and gypsum or calcium sulphate. When 

 the water of crystallization in many salts composing rocks 

 becomes heated and driven off, the salt crumbles into powder ; 

 and, at the same time, all rocks, being porous, are pervious to 

 water, which not only dissolves certain salts, and holds them in 

 solution ready for absorption by the roots of plants, but also 

 effects transformations in many chemical compounds. Thus 

 water impregnated with carbonic acid dissolves silicates con- 

 taining alkaline earths and ferrous oxide, as also carbonate of 

 lime (Ca CO 3 ) and ferrous carbonate (Fe CO.,) which are easily 

 soluble in it, whilst magnesium carbonate (Mg CO 3 ) is much 

 less so. The products of the decomposition of rocks are mostly 

 alkalies in combination with silica and carbonic acid, together 

 with carbonate of lime, magnesia, and ferrous oxide, whilst 

 soluble sulphate of lime is also found in most soils ; but as 

 these salts act and re-act on each other, many various com- 

 pounds are formed. 



Rocks of complicated structure decompose most rapidly, as, 

 for instance, those rich in felspars, or in compounds of iron, in 

 contrast with silicious slate or quartz-rock ; pure limestone de- 

 composes slowly, but this difficulty diminishes with any increase 

 of alumina and iron. Sandstones and conglomerates decompose 

 in proportion to the cementing matter they contain, and to the 

 degree in which this is affected by moisture. Fine-grained 

 and massive rocks are less apt to fissure and crumble than 

 coarse-grained stones or slaty beds, as they afford fewer points 



