36 ELEMENTARY AGRICULTURAL CHEMISTRY 



been formed. It therefore supplies potash to plants. Its 

 physical properties are very important, and greatly influence 

 soils in which it is abundant, 



ROCKS ' 

 The igneous rocks are the oldest, and it was from the debris 

 of igneous rocks that sandstones, shales and (indirectly) 

 limestones were formed. 



Sandstones, Grits and Conglomerates consist of the larger 

 fragments of the waste resulting from the denudation of 

 igneous rocks — e.g., granite — which, in consequence of their 

 size and weight, have been deposited at or near the mouths 

 of rivers. Their main ingredient is silica, the grains of sand 

 consisting largely of quartz crystals, but in many cases frag- 

 ments of felspar, mica and other minerals are present. The 

 grains are cemented together either by calcium carbonate (in 

 " calcareous sandstone"), clay ("argillaceous sandstone"), ferric 

 oxide (*' ferruginous sandstone "), or colloidal silica (" siliceous 

 sandstone"). Soils produced by the decay of sandstones are 

 (ight and friable and poor in plant food, unless there be 

 present potash-containing minerals, e.g., felspar and mica. 



Shales consist essentially of the plastic, hydrated aluminium 

 silicate, kaolin, Al2O3.2SiO3.2H2O, but may contain any other 

 extremely finely divided matter obtained by the erosion of 

 the original rock. Particles of undecomposed or partially 

 decomposed felspar are often present, and these are important 

 because of the potash they contain. Soils formed from shale 

 are "heavy" and clayey, generally sufficiently rich in potash, 

 but poor in phosphates and in calcium carbonate. 



Limestones, in which term chalk and magnesian limestone 

 may also be included, have been formed largely by the abstrac- 

 tion from solution by living organisms — e.g., coral polyps and 

 shellfish — of calcium and magnesium carbonates. They often 

 contain small quantities of clay, ferric oxide, silica and nearly 

 always calcium phosphate in comparatively large quantities 



