soil: its origin and nature. 17 



ment are termed schists. Gneiss and mica-schist are common 

 examples. Yet another great class of rocks are known as 

 derivative rocks. These are so called because they have been 

 derived from the breaking-up of pre-existing rocks, and the 

 deposition of the materials in layers and beds. Characteristic 

 rocks of this group are ordinary sandstones and argillaceous 

 shales. To the same group belong the great marine limestones, 

 which consist chiefly of the debris of corals, sea-lilies, shells, 

 and other organisms. 



I. Igneous Rocks. — The soils resulting from the disintegration 

 and decomposition of igneous rocks show considerable variety. 

 As typical examples we may cite granite and basalt. 



Granite, as we have seen, is essentially an aggregate of 

 three minerals — quartz, felspar, and mica or some other ferro- 

 magnesian species. While quartz is insoluble, the other 

 constituents are in time decomposed by rain, and resolved into 

 clay and certain alkalies and alkaline earths, which tend to be 

 removed in solution as bicarbonates. When thoroughly de- 

 composed, therefore, granite yields a clay-soil, containing a 

 variable admixture of grit and sand (quartz). From this clay 

 all the soluble ingredients — potash, magnesia, and lime — may 

 be completely leached out. In general, however, larger and 

 smaller fragments of the decomposing rock are disseminated 

 more or less plentifully through the soil and subsoil, and from 

 these fresh supplies of the alkalies and alkaline earths are 

 continually set free.^ Other minerals besides those mentioned 

 may be present in granite — the most notable being apatite, from 

 the decomposition of which phosphoric acid is added to the 

 soil. Under favourable conditions, therefore, granite yields a 

 fertile clay-soil. In this country, however, the rock usually 

 occurs at considerable elevations, where the climatic conditions 

 and the form of the ground are against the retention in place of 

 its decomposing debris. The disintegrated materials, unprotected 

 by vegetation, fall a prey to wind, rain, and melting snow, and 

 are carried more or less rapidly away. Steep declivities thus 

 either remain bare or are curtained with sheets of loose boulders, 

 blocks, and fragments of all shapes and sizes, while more gently 



^ The common felspar of typical granite contains potash. The soil over 

 this rock, therefore, may be expected to contain a good supply of that sub- 

 stance. Not infrequently the mineral hornblende is more or less plentifully 

 present, from the decomposition of which lime is derived. 



VOL. XXI. PART I. B 



