A lovely passage for any geology fans out there. . .
But there are three kinds of earth which, in mixed mass and prevalent quantity, form the world. Those are, in common language, the earths of clay, of lime, and of flint. Many other elements are mingled with these in sparing quantities, but the great frame and substance of the earth is made of these three, so that wherever you stand on solid ground, in any country of the globe, the thing that is mainly under your feet will be either clay, limestone, or some condition of the earth of flint, mingled with both.
Nature seems to have set herself to make these three substances as interesting to us, and as beautiful for us, as she can. The clay, being a soft and changeable substance, she doesn't take much pains about, as we have seen, until it is baked; she brings colour into it only when it receives permanent form. But the limestone and flint she paints, in her own way, in their native state: and her object in painting them seems to be much the same as in her painting of flowers; to draw us, careless and idle human creatures, to watch her a little, and see what she is about -- that being on the whole good for us, -- her children. For Nature is always carrying on very strange work with this limestone and flint of hers: laying down beds of them on the bottom of the sea; building islands out of the sea; filling chinks and veins in mountains with curious treasures; petrifying mosses, and trees, and shells; in fact, carrying on all sorts of business, subterranean or submarine, which it would be highly desirable for us, who profit and live by it, to notice as it goes on. And apparently to lead us to do this, she makes picture-books for us of limestone and flint; and tempts us, like foolish children as we are, to read her books by the pretty colours in them. The pretty colours in her limestone books form those variegated marbles which all mankind have taken delight to polish and build with from the beginning of time; and the pretty colours in her flint-books form those agates, jaspers, cornelians, bloodstones, onyxes, cairngorms, chrysophrases, which men have in like manner taken delight to cut, and polish, and make ornaments of, from the beginning of time; and yet so much babies are they, and so fond of looking at pictures instead of reading the book, that I question whether, after six thousand years of cutting and polishing, there are above two or three people out of any given hundred who know, or care to know, how a bit of agate or a bit of marble, was made, or painted.