"Used coal," replied a boy from a back seat.
"Yes," said Miss Turner, "if there were any coal, and if the colonists knew where to find it and how to use it. But what is this coal and where does it come from?"
"We owe all our knowledge of the origin of coal to the geologists, who have made a careful study of the surface of the earth," continued Miss Turner. "They tell us that there was a time when human beings did not live on the earth; when not even animals that need to breathe the air could exist. The atmosphere which surrounded the earth in those days was different from the air which we breathe. We need the oxygen that is in our air to sustain life; poor ventilation in our rooms or halls soon renders them uncomfortable and often causes our heads to ache. The reason for this is the presence in the air of too large a quantity of a gas called carbonic acid gas; an extra amount of it makes the air unfit to breathe, but a certain amount is necessary to sustain plant life.
"In the coal-forming or carboniferous age the atmosphere around the earth contained less oxygen than at present and great quantities of carbonic acid gas. For this reason, as I have said, animals did not exist, but plants--large shrubs, great ferns, and huge trees--lived and grew vigorously. If we have ever seen thick woods we need only imagine all the bushes and trees of the forest to be of enormous size in order to have some idea of the vegetable growth of the carboniferous age. The earth was preparing vast quantities of fuel to be ready, thousands of years later, for the millions of men that were to come.
"The growth of the forests was but one step in the preparation of coal. The second step was the submerging of the forests, covering them with water as if at the bottom of the sea. Then the streams brought gravel, sand, and mud into this ocean, and these were hardened into clay and sandstone by the pressure of the water, perhaps aided by the heat of the earth itself. The trees and ferns were bent down and pressed together and driven into the most compact condition possible.
"But again earthquakes came and the water disappeared. The layer of clay and sandstone was covered with soil which became dry enough to produce other forests, growing as rank as the first. These were again overwhelmed and covered first with water, then with rocks and soil, only to be lifted again for another growth. This process was repeated in some cases many times, as we can see with a little study."