All eyes were gazing at the moon as it seemed to rise above the trees. The great face of the man in the moon became distinct as he looked down upon the rolling earth. "A beautiful and wonderful world," continued Uncle Robert, "but probably not a bit more wonderful than the countless worlds we see up there. "Just think! we are on a great round ball, and it is moving on its axis from west to east toward the moon. The moon, you know, does not really move over our heads as it seems to do. The round earth rolls upon its axis, and that makes the moon seem to rise higher and higher, and then sink away below the western horizon." "To-morrow night it will come up in the east a little later," said Frank. "Round and round we go upon our ball of earth. The sun seems to rise and set just as the moon does, but it is the world itself that makes the sun and moon seem to rise and set," said Uncle Robert. "What is our earth made of?" asked Donald. "Just what you see before you," answered Uncle Robert. "Under our feet we have the ground, the soil, gravel, sand, and loam, which is made of--" "Ground-up rock," said Frank. "And underneath the soil there is--" "The solid rock," said Frank. "And underneath that?" asked Mr. Leonard. "We do not know, but it is quite certain the solid earth is made of ground-up rock and rock that may be ground. The mills are all at work, grinding all the time." "The mills!" said Susie. "Where are the mills?" "I know one," said Donald. "The river is a great mill. Don't you remember about the pebbles?" "And the glaciers are mills, too," said Frank. [Illustration: Glaciers on the Coast of Norway] "Yes, the rivers, the ice rivers or glaciers, the wind, the frost, heat and cold, all grind masses of rock into bowlders, pebbles, and sand." "The rock has been ground so long I should think there would be nothing left but soil," said Frank. "You saw the limestone down by the spring?" asked Uncle Robert. "Yes," the children said together. "That limestone was once soft mud spread out upon the bottom of the ocean in shallow water." "How do you know that is so, uncle?" questioned Frank. [Illustration: Fossil fish.] "There are many proofs, but the best proof is that in the limestone are found shells of animals that live in the sea," said Uncle Robert. "Fossils," said Mrs. Leonard. "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"