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. agua, 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 ihmestone. 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. 
