SOILS FKOM VAKIOUS ROCKS. 43 



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 

 qiiartzites. Shales become slate or even gneiss. Limestones 

 are converted into marble. 



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 described, the materials which, when separated accord- 

 ing 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 decay 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 drainage 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 be 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 excep- 

 tion of potash, which is sometimes sufficiently abundant be- 

 cause 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, generally sufficiently well provided with potash, but often 

 deficient in 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, c.y., flint, sand, clay, oxide of iron, &c., left 

 behind on the surface. It thus often happens that the surface 

 soil on limestone is almost free from calcium carbonate and 



