L. That sounds to me more like three questions than one, Mary. If it is only one, it is a wide one.
Mary. I did not say anything about the width of it.
L. Well, I must keep it within the best compass I can. When rocks either dry from a moist state, or cool from a heated state, they necessarily alter in bulk; and cracks, or open spaces, form in them in all directions. These cracks must be filled up with solid matter, or the rock would eventually become a ruinous heap. So, sometimes by water, sometimes by vapour, sometimes nobody knows how, crystallisable matter is brought from somewhere, and fastens itself in these open spaces, so as to bind the rock together again, with crystal cement. A vast quantity of hollows are formed in lavas by bubbles of gas, just as the holes are left in bread well baked. In process of time these cavities are generally filled with various crystals.
Mary. But where does the crystallising substance come from?
L. Sometimes out of the rock itself; sometimes from below or above, through the veins. The entire substance of the contracting rock may be filled with liquid, pressed into it so as to fill every pore;—or with mineral vapour;—or it may be so charged at one place, and empty at another. There's no end to the 'may be's.' But all that you need fancy, for our present purpose, is that hollows in the rocks, like the caves in Derbyshire, are traversed by liquids or vapour containing certain elements in a more or less free or separate state, which crystallise on the cave walls.
Sibyl. There now;—Mary has had all her questions answered: it's my turn to have mine.
L. Ah, there's a conspiracy among you, I see. I might have guessed as much.
Dora. I'm sure you ask us questions enough! How can you have the heart, when you dislike so to be asked them yourself?
L. My dear child, if people do not answer questions, it does not matter how many they are asked, because they've no trouble with them. Now, when I ask you questions, I never expect to be answered; but when you ask me, you always do; and it's not fair.
Dora. Very well, we shall understand, next time.