Anyhow here I am, a little freckled granite pebble talking myself red in the face because I've got so much to say, such wonderful things to tell, and only a few hundred pages to tell it in!
II. How Do They Know?
But, after all, how do they know that one rock changes into another? No one ever caught a rock doing this, did they?
Not quite, but almost. To explain, I must first tell you about the fossils that are found in stone. Haven't you often noticed in marble curious figures that reminded you of sea-shells? They were sea-shells but have been turned to stone, and things similarly changed while still keeping their original form are called "fossils."
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.
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?