When the plants and the shell creatures of the sea die they fall to the bottom, and mud and sand settles over them and closes them in, much as you shut leaves and flowers between the pages of a book. But while the book presses the leaves of flowers out of shape these bodies of the water-plants and shell creatures are slowly enclosed in a soft mass of mud that doesn't change their shapes at all. Then the particles that go to make up the soft bodies of these buried things are slowly dissolved away, and the minerals in the water and mud above them soak in and take their places. It's like passenger after passenger in a car getting up and other passengers taking the vacant places. Finally this mass of limey shells becomes buried deep under the sea, is turned to limestone, and when in course of time this part of the seashore rises--as we know shores have a way of doing--or is wrinkled up into a mountain, this limestone becomes a part of the face of the land. [Illustration: _From a photograph by the American Museum of Natural History_ STORY OF THE LITTLE JEWEL-BOX A kind of jewel-box? Yes, the kind geologists call a "geode." It began as a piece of limestone in which the underground waters had dissolved a cavity. But these waters had already, in solution, quartz which they had dissolved from quartz rock, and this quartz, deposited little by little in the cavity, formed into crystals. The quartz also made the surrounding walls more solid, so that when the mass of limestone containing this pocket was cut away by erosion this jewel-box remained, and, being rolled about in streams or by the lap and plunge of waves, it was rounded. ] WOULDN'T WE SAY THE SAME THING? Now suppose where some great granite rock stood up through layers of other kinds of rock--looking as if it had pushed itself through like the great granite boss on which Edinburgh Castle stands--you found that wherever this intruder touched the other rock that rock was crystallized. If we had just found all this out for ourselves, as the geology people found it, we would say, just as they said: [Illustration: FATHER, GRANDFATHER, AND THE CHILDREN IN THE PORPHYRY FAMILY In this piece of porphyry you see three generations, all living under one roof, as it were. Notice that six-sided crystal near the centre? Compare it with other good-sized crystals that haven't any distinctive shape. The reason for the difference is that the shapeless ones have had some of their substance taken away to form the smaller crystals. The dark mass is lava. In it the big crystals formed. Then, from most of the big crystals the lava reabsorbed material, and this material later turned into little crystals--the "grandchildren" of the three generations. ]