If they stayed down below there, they would be of no use; while they will be of use up here in the open air.

For, year by year--by the washing of rain and rivers, and also, I am sorry to say, by the ignorant and foolish waste of mankind--thousands and millions of tons of good stuff are running into the sea every year, which would, if it could be kept on land, make food for men and animals, plants and trees.

So, in order to supply the continual waste of this upper world, Madam How is continually melting up the under world, and pouring it out of the volcanos like manure, to renew the face of the earth.

In these lava rocks and ashes which she sends up there are certain substances, without which men cannot live--without which a stalk of corn or grass cannot grow.

Without potash, without magnesia, both of which are in your veins and mine--without silicates (as they are called), which give flint to the stems of corn and of grass, and so make them stiff and hard, and able to stand upright--and very probably without the carbonic acid gas, which comes out of the volcanos, and is taken up by the leaves of plants, and turned by Madam How's cookery into solid wood--without all these things, and I suspect without a great many more things which come out of volcanos--I do not see how this beautiful green world could get on at all.
Of course, when the lava first cools on the surface of the ground it is hard enough, and therefore barren enough.