Because there are little caps--little patches--of them left on the tops of many hills to the north of London; just remnants which the sea, and the Thames, and the rain have not eaten down. Probably they once stretched right out to sea, sloping slowly under the waves, where the mouth of the Thames is now. You know the sand-cliffs at Bournemouth? Of course. Then those are of the same age as the Bagshot sands, and lie on the London clay, and slope down off the New Forest into the sea, which eats them up, as you know, year by year and day by day. And here were once perhaps cliffs just like them, where London Bridge now stands. * * * * * There, we are rumbling away home at last, over the dear old heather-moors. How far we have travelled--in our fancy at least--since we began to talk about all these things, upon the foggy November day, and first saw Madam How digging at the sand-banks with her water-spade. How many countries we have talked of; and what wonderful questions we have got answered, which all grew out of the first question, How were the heather-moors made? And yet we have not talked about a hundredth part of the things about which these very heather-moors ought to set us thinking. But so it is, child. Those who wish honestly to learn the laws of Madam How, which we call Nature, by looking honestly at what she does, which we call Fact, have only to begin by looking at the very smallest thing, pin's head or pebble, at their feet, and it may lead them--whither, they cannot tell. To answer any one question, you find you must answer another; and to answer that you must answer a third, and then a fourth; and so on for ever and ever. For ever and ever?