"A little over three quarters of a mile," said Mr. Leonard.
"The farther they go down into the crust of the earth, the warmer they find it. I have been down in a mine thirty-two hundred feet, and it was very hot. No one could have lived there if cool air had not been brought down from the surface.
"Some people have thought that inside the crust of the earth the rock is all a molten mass, like melted iron. You have read about volcanoes, and of the lava that is thrown out of them?"
"Does that come out of the inside of the earth?" asked Donald.
[Illustration: Down in a Gold Mine]
"It comes from somewhere in the earth. Some men give their whole lives to the study of these questions, but you know they can not see beneath the crust of the earth. It is thought by some that the weight of the crust would keep the center of the earth a solid mass. So you see there are still many questions unsettled. We know that the crust is moving up and down all the time."
"Oh, I hope the land won't rise here!" said Susie.
"You wouldn't know it, Susie, if it did," said Uncle Robert, laughing.
"Unless there was an earthquake," said Frank.
"Or a volcano," said Donald. "I'd like to see one."