84 
SECOND BOOK. 
by being shifted from place to place by running 
water. But this work is not enough to make a 
fertile soil. There is yet another way in which the 
soil is greatly changed and added to; that is, by 
the growth and decay of plants and trees. In their 
growth these rob the soil of some substances, and 
in their decay they give back more than they took 
away. 
2 . But they do even more than this: their roots 
enter the cracks and crevices of rocks, and, as they 
grow larger, they act like a wedge in causing the 
rock to split and break up. Again, when roots 
decay, little passages or tunnels are left where they 
grew, and these make it easy for water to pass 
through the soil. 
3. There are some soils that have been almost 
wholly made by the decay of plants. Imagine a 
very wet place, where mosses and water-plants 
flourish well. These grow and die, crop after crop, 
until there is a good store of ‘ leaf-mould ’ formed 
by their decay. Then larger and stronger plants 
appear, and these add rapidly to the new soil, caus- 
ing it to become deep and plentiful. This kind of 
soil is known as Peat, and is sometimes spoken of 
as ‘ vegetable soil or as humus. Some of it is 
present in all fertile soils. 
4. But what are the soils that are formed by the 
wearing down of the rocks. From this cause we 
get clay, sand, and lime or chalk. These mixed 
