"But," we might say, "aren't they satisfied? We are. It's all plain enough to us now that one kind of rock does change into another. Then why do these geologist people go on getting more evidence when they've already got enough? It's like a boy learning two lessons when he only has to recite in one; and whoever _heard_ of such a thing!" THESE BOYS JUST LOVE TO STUDY The answer is that this "going on" is one of the many delights of study, particularly in Nature's books, when once you get the habit. [Illustration: _From a photograph by Frith & Co., Ltd., Reigate_ THE MARBLE ROCKS AT JABALPUR The gorge of the "Marble Rocks," near Jabalpur, India, is a mile long and of an unearthly beauty of which even this little picture will give you some idea. The walls gleam white and golden in the sun. They are not really marble but limestone, which, as you will learn in this chapter, is the stone that becomes marble in "the fairyland of change." It looks as if nature had begun the making of marble columns in those cliffs, doesn't it? This is because the cliff is cut up by joints. You can also make out in one of the "pillars" the strata, or horizontal divisions of the rock, as it was laid down in the sea. ] Among other things, the scientists search the pockets of the rocks, so to speak, for further evidence as to whether one kind changes into another. Chemistry is a great help in doing this, and, of course, the microscope. They find in this way that rocks that are full of crystals, such as granite and marble, and that look so different from the rocks that are not crystallized--such as limestone and sandstone--have in them the very same substances--silica, lime, potash, iron, and so on. And again they put the oysters on the witness stand. (You remember how, long ago, oysters helped tell that mountains were once a part of the sea bottom.) They put a piece of limestone in a certain acid, and it bubbles and gives off a certain kind of gas. Then they do the same thing to an oyster-shell, and it gives out the same kind of gas. Then they try it on a piece of marble and out comes that very gas again! So all three--the limestone, the oyster-shell, and the marble--must be pretty close relations. Marble is just oyster and other shells warmed up and then allowed to cool. But they don't stop here--these students of the rocks. It isn't enough that all these facts point to one conclusion. They want to actually _try it out_. So what do they do but change chalk--which is a kind of very soft limestone--into marble in the laboratory? This they do by heating the chalk and then cooling it under immense pressure. III. The Fairies of the Fairyland of Change