KINDS OF SOIL. 
83 
Besides, when they dissolve, hollows are left, which 
cause other parts of the rocks to break away. 
10. But what can the air do towards making the 
soil? I will give you an example. When a piece 
of iron is left for some time in the air you know 
that it rusts. That is because the oxygen of the 
air has joined with some of the iron to make a fresh 
substance, which you can rub off in dust. In the 
same way the oxygen or the carbonic acid in the air 
can act on some substances in the rocks, causing 
them to be changed, so that they become either 
crumbled or dissolved, and are then carried away 
by the water. 
11. The great caverns, so common in the lime- 
stone hills of Jamaica, show us what a great work 
the carbonic acid and water can do in this way. 
The caverns have been hollowed out w v here quan- 
tities of the substance of the limestone rock have 
been dissolved and carried off. 
12. The red clay soil seen in some parts of the 
island is another example. What was left after the 
limestone had been washed away has been acted 
upon by the air, causing changes by which the clay 
has become red. 
KINDS OF SOIL. 
l. You have just read how rocks may be powdered 
and crumbled to soil, and how this may be mixed 
