82 BRITISH PLANTS 



Rocks and Soils. 



The rocks from which soil is derived are either igneous 

 or aqueous in origin. Igneous rocks are those which have 

 been erupted from volcanoes (lavas), or which have cooled 

 down within the earth from a molten condition (granites). 

 Aqueous rocks, as the name implies (Lat. aqtm, water), 

 have been deposited under water, in seas and lakes, or 

 upon the beds of rivers. Most soils are derived from rocks 

 of aqueous or sedimentary origin, the chief of which are 

 clays, shales, and slates, which give rise to clay soils; 

 sands and sandstones, from which sandy soils are derived ; 

 and chalk and limestone, which form calcareous soils. 



The physical characters of a soil depend mainly on the 

 size of its particles and to some extent on the presence 

 or absence of humus (see p. 83). In describing the soil- 

 particles it is usual to fix a more or less arbitrary limit to 

 the size. Those which range in diameter from 1 mm. to 

 0-05 mm. are designated " sand," those from 0'05 mm. 

 to 0-005 mm. " silt," and those below 0-005 mm. " clay." 

 In a heavy clay soil, the small particles greatly predomi- 

 nate, two-fifths, perhaps, being clay, one-third silt, and 

 less than one-tenth sand. A coarse sandy soil, on the 

 other hand, may contain as much as four-fifths of sand, 

 only about 5 per cent, of silt, and under 2 per cent, of 

 clay. The rest of the soil will consist mainly of humus 

 with a certain amount of chalk and moisture. Soils 

 intermediate in character between the heavy clay and 

 coarse sand are termed loams; when sand predominates 

 the soil is a sandy loam; when clay and silt, a clayey 

 loam. A clayey loam containing chalk is called marl; 

 if the amount of chalk exceeds 20 per cent, it is a cal- 

 careous marl. 



Clay consists mainly of kaolin (hydrated siHcate of 

 alumina), which is derived from the felspar of igneous 

 rocks by the action of weathering. China clay is practi- 

 cally pure kaolin; but in most clays small quantities of 

 quartz, mica, and other minerals are also present. When 

 wetj the particles of clay stick together, forming a wet 

 mass, greasy to the touch, very difficult to dry, and almost 

 impermeable to water. In drying, clay cracks into hard 

 consolidated masses, which the roots of plants cannot 

 penetrate. 



