"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.