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. aqua, water), 

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

 upon the beds of rivers. The chief kinds of aqueous or 

 sedimentary rocks are clay, sand, and chalk. 



1. Clay, when pure, consists of very small particles of 

 alumina (kaolin), a mineral which is derived from the 

 weathering of granite rocks. When wet, the particles of 

 which the clay consists 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. 



2. Sand, when pure, consists almost entirely of loose 

 particles of quartz (silica). The particles vary in size 

 from large pebbles to the finest grains. A soil consisting 

 of quartz-sand is quite sterile unless it contains humus 

 or is mixed with other soils ; but, as a rule, fragments of 

 other minerals which may form a source of plant-food are 

 present. 



3. Chalk is an amorphous form of carbonate of lime ; 

 when crystalline, it is limestone. Soils derived from chalk 

 or limestone rocks are called calcareous, and they are 

 naturally alkaline and rich in lime. Carbonate of lime 

 forms rock-masses of great thickness in the earth's crust. 

 They all have an aqueous origin, being formed under 

 water in shallow seas. Some have been chemically pre- 

 cipitated from water rich in lime, but most calcareous 

 rocks have been derived from the waste of coral-reefs or 

 from the comminuted shells of marine organisms. In 

 rocks which owe their origin to the latter source, a small 

 amount of sand is present, derived from organisms with 

 siliceous skeletons. 



Loams are mixtures of sand and clay ; when sand pre- 

 dominates, the soil is a sandy loam ; when clay, 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. 



