It is so enormously heavy that the sides of the cone cannot bear its weight, and give way low down.

And then, through ashes and cinders, the melted lava burrows out, twisting and twirling like an enormous fiery earth-worm, till it gets to the air outside, and runs off down the mountain in a stream of fire.

And so you may see (as are to be seen on Vesuvius now) two eruptions at once--one of burning stones above, and one of melted lava below.
And what is lava?
That, I think, I must tell you another time.

For when I speak of it I shall have to tell you more about Madam How, and her ways of making the ground on which you stand, than I can say just now.

But if you want to know (as I dare say you do) what the eruption of a volcano is like, you may read what follows.