38 OEIGIN OF SOILS 



when calcareous deposits such as tufa, travertine, sinter, result ; or by 

 evaporation, by which gypsum, rock-salt and the Stassfurt deposits 

 were probably formed. Then, too, by the agency of animals, phosphatic 

 deposits, e.g., guano coprolites and bone-earth, have been produced, 

 while the remains of plants have given rise to the important rocks 

 coal, lignite and peat. 



Metamorphic rocks partake of the nature of both igneous and sedi- 

 mentary rocks, many having been formed from the latter by chemical 

 and physical changes produced by great pressure or high temperature. 

 As a rule they tend to show a crystalline character and are often hard 

 and very similar to true igneous rocks. Sandstones become changed 

 by metamorphism into quartzites. Shales become slates or even 

 gneiss. Limestones are converted into marble. 



FORMATION OF SOILS. 



The inorganic portion of a soil is really the insoluble portion of 

 the debris resulting from the weathering of the rock on which it rests. 

 It is, in fact, the disintegrated rock which has not yet been carried 

 away to the final resting-place of all products of denudation the ocean. 



By the decay of igneous rocks there result, as has already been de- 

 scribed, the materials which, when separated according to the order in 

 which they settle out from suspension and solution in water, would 

 form grits, sandstones, shales and limestones. Soil formed by the de- 

 cay of such a rock might be expected to have the composition of such 

 a mixture, and to a great extent this is actually the case, except that 

 the soluble products of denudation, viz., the carbonates of potash, soda,, 

 lime and magnesia, have been to a great extent carried off in the drain- 

 age water. 



By the decay of sandstones there results a soil composed very 

 largely of grains of silica, but generally containing, in addition, 

 whatever fragments of other minerals there might have been in the 

 rock, most commonly particles of felspar, mica, oxide of iron and clay. 

 Such soils are usually light and friable and poor in the main inorganic 

 constituents of plant food, with the exception of potash, which is 

 sometimes sufficiently abundant because of the felspar or other 

 potash-containing minerals present. 



Shales, consisting essentially of the very plastic hydrated silicate of 

 alumina, when disintegrated, tend to yield heavy clay soils, in most 

 cases sufficiently well provided with potash, but often deficient ini 

 phosphates and lime. 



Calcareous rocks, including chalk, limestone and marble, are^ 

 rapidly eroded by the combined action of water and carbon dioxide, 

 their calcium carbonate being removed in solution, and the foreign 

 bodies, e.g., flint, sand, clay and oxide of iron, left behind on the surface- 

 It thus often happens that the surface soil on limestone is almost free 

 from calcium carbonate and would be benefited by the application of 

 lime. In the case of many limestone soils, the actual inorganic matter 

 in the soil probably does not exceed 1 per cent of the amount of lime- 

 stone which must have been denuded in order to leave it. 



