For days, and weeks, and months this goes on; even it may be for hundreds of years: till a great cone is formed round the steam vent, hundreds or thousands of feet in height, of dust and stones, and of cinders likewise.

For recollect, that when the steam has blown away the cold earth and rock near the surface of the ground, it begins blowing out the hot rocks down below, red-hot, white-hot, and at last actually melted.

But these, as they are hurled into the cool air above, become ashes, cinders, and blocks of stone again, making the hill on which they fall bigger and bigger continually.

And thus does wise Madam How stand in no need of bricklayers, but makes her chimneys build themselves.
And why is the mouth of the chimney called a crater?
Crater, as you know, is Greek for a cup.

And the mouth of these chimneys, when they have become choked and stopped working, are often just the shape of a cup, or (as the Germans call them) kessels, which means kettles, or caldrons.