But how is the swallow-hole sure to end in a cave?

Because it cannot help making a cave for itself if it has time.

Think:  and you will see that it must be so.  For that water must run somewhere; and so it eats its way out between the beds of the rock, making underground galleries, and at last caves and lofty halls.  For it always eats, remember, at the bottom of its channel, leaving the roof alone.  So it eats, and eats, more in some places and less in others, according as the stone is harder or softer, and according to the different direction of the rock-beds (what we call their dip and strike); till at last it makes one of those wonderful caverns about which you are so fond of reading—­such a cave as there actually is in the rocks of the mountain of Whernside, fed by the swallow-holes around the mountain-top; a cave hundreds of yards long, with halls, and lakes, and waterfalls, and curtains and festoons of stalactite which have dripped from the roof, and pillars of stalagmite which have been built up on the floor below.  These stalactites (those tell me who have seen them) are among the most beautiful of all Madam How’s work; sometimes like branches of roses or of grapes; sometimes like statues; sometimes like delicate curtains, and I know not what other beautiful shapes.  I have never seen them, I am sorry to say, and therefore I cannot describe them.  But they are all made in the same way; just in the same way as those little straight stalactites which you may have seen hanging, like icicles, in vaulted cellars, or under the arches of a bridge.  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.