The water melts more lime than it can carry, and drops some of it again, making fresh limestone grain by grain as it drips from the roof above; and fresh limestone again where it splashes on the floor below: till if it dripped long enough, the stalactite hanging from above would meet the stalagmite rising from below, and join in one straight round white graceful shaft, which would seem (but only seem) to support the roof of the cave.

And out of that cave--though not always out of the mouth of it--will run a stream of water, which seems to you clear as crystal, though it is actually, like the Itchen at Winchester, full of lime; so full of lime, that it makes beds of fresh limestone, which are called travertine--which you may see in Italy, and Greece, and Asia Minor: or perhaps it petrifies, as you call it, the weeds in its bed, like that dropping-well at Knaresborough, of which you have often seen a picture.

And the cause is this: the water is so full of lime, that it is forced to throw away some of it upon everything it touches, and so incrusts with stone--though it does not turn to stone--almost anything you put in it.

You have seen, or ought to have seen, petrified moss and birds' nests and such things from Knaresborough Well: and now you know a little, though only a very little, of how the pretty toys are made.
Now if you can imagine for yourself (though I suppose a little boy cannot) the amount of lime which one of these subterranean rivers would carry away, gnawing underground centuries after centuries, day and night, summer and winter, then you will not be surprised at the enormous size of caverns which may be seen in different parts of the world--but always, I believe, in limestone rock.

You would not be surprised (though you would admire them) at the caverns of Adelsberg, in Carniola (in the south of Austria, near the top of the Adriatic), which runs, I believe, for miles in length; and in the lakes of which, in darkness from its birth until its death, lives that strange beast, the Proteus a sort of long newt which never comes to perfection--I suppose for want of the genial sunlight which makes all things grow.