"Yes, fossils. They are the remains of plants and animals that lived a very long time ago. Many rocks are almost entirely made of fossils. Fish and shells also have been covered with soft clay and left their imprints. Great beasts have walked in the mud, and we now find their footprints in the hard stone. Coral—you have seen coral?—is often found in limestone. It is made of the shells of little animals, called the polyp, which live in the sea."
[Illustration: Coral]
"So you see that the firm ground under foot is made of rock, some of which has been ground up over and over again. But there is something else besides rock that makes the world"
"Water," said Donald promptly as he looked down upon the river.
"Yes, the water is just as much a part of our world as the solid rock and the soil. There is water in the soil and in the solid rock, too. It comes out to us in——"
"Springs," said Donald.
"Water fills hollows in the earth——"
"Ponds and lakes," said Frank.
"Water runs down the slopes—"
"Streams," said Frank.