"It seems silly," said Anton, after they had walked on a minute or two, "to leave all this coal near the shaft and to go digging a mile away. Why not take all the coal that is handy first?"
"And have the roof come down and block up all the coal that is beyond? That would be just throwing away the wealth of the mine."
"Timber the roof, then!"
"It would cost too much, for one thing," Clem explained, "and, for another, all the timber in the world won't hold up a roof if the excavation is made too big. There's millions of tons of rock pressing down on a mine roof. Judging by the way you talk, Anton, I don't believe you understand what a coal formation is, yet."
"Isn't it like Otto said, then?"
"Only in a way. Otto's description of the coal forests was near enough—in spite of his ideas about goblins and sprites—and he was correct in saying that the forests decayed under water and turned into coal after they were pressed down by rock. But it wasn't the Flood that did that, at least not the Flood that Otto was speaking of. The coal forests existed millions of years before Noah.
"What's more, it wasn't only just once that the forests were covered by a deluge. That happened several times, a hundred or more, in some places.
"For centuries at a time, these gloomy and steaming forests grew in boggy land, only a few inches above the level of the sea. Gradually the land sank, the sea came in, the trees fell and decayed under the water, and a layer of mud or sand was deposited over them. Then gradually the land rose again just above the level of the sea, and a new forest grew. Once more the land sank below the water, the second forest fell into decay and upon that layer a new deposit of mud or sand was laid. That gave two layers or seams of coal-forest-bog, to be turned later into coal by pressure; and two layers or strata of mud or sand, to be turned into shale and slate or into sandstone, also by pressure.
"When a long time elapsed between the swampings, several centuries of coal forests had made a deep bed of bog, which, ages after, became a thick seam of coal. When the swampings happened close together, the layer of bog was shallow, producing a thin seam of coal. In the same way, the layers of shale or sandstone are thick or thin according to the length of time that the land was under the water.
"Because of that, Anton, in nearly every colliery there is not just one layer or seam of coal, but a number of them. There are sixteen different seams in this mine, showing that the land rose and fell sixteen times, probably in the course of a million years.