kept him from a retort which might have brought on a quarrel.

On reaching the bottom, Clem and Anton set out along the man-way together. It was a walk of nearly a mile underground from the main shaft of the mine to the distant "room" or square hole in the seam, where Clem was to dig away the coal face, and which was one of the rooms from which Anton was loading coal.

This Ohio colliery was being worked on what is known as the pillar-and-room method. This consists in dividing the seam of coal into squares like a chessboard, taking out the coal from each alternate square, leaving the intervening squares of coal intact to act as pillars in holding up the roof. They do not look like pillars to a careless observer, often being blocks of coal thirty yards square.

"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.

"Some mines show much bigger changes. In the famous coal basin of Mons, in Belgium, there are 157 layers of coal, of which 120 are thick enough to be workable. The Saar basin, on the left bank of the Rhine, which has played so important a part in the international troubles following the end of the World War, has 164 seams, with 77 of them workable, giving a thickness of 240 feet of coal. However, as the lowest layers are nearly four miles deep, they will probably never be worked."

"Why not?"

"To start with, the cost of haulage to the top would be enormous. But, aside from that, a good many mining engineers figure that the temperature at that depth would be above boiling point. You know, in general, the farther you go down in a mine, the hotter it gets."

"What do you mean by a seam being 'workable'?" the boy queried. "Can't all coal be dug out?"

"Not by a long shot. At least not so as to be worked at a profit. Suppose a seam of coal is only a few inches thick, how is a miner going to dig it out? He couldn't crawl in such a seam, let alone using his tools there."

"He could cut out enough rock at the top and bottom to give him a chance to get in."

"A miner is paid for digging coal, not digging rock," was the answer. "What's more, according to your scheme, so much shale or sandstone would be mixed with the coal that it would be useless for burning.

"Even seams two feet thick are so hard to work that most of them are left alone, and a seam three feet thick means extra expense in getting out the coal because of the difficulty of labor in hewing and transporting the coal from the face to the shaft. The ideal thickness is between six and eight feet, where a man can stand upright and can reach to the roof with a slate bar. That height, too, makes timbering easy.

"Very thick seams have their own difficulties. The worst of these is the supporting of the roof. Take a seam 30 or 40 feet thick, for example. Look at the size of the hole that is left when the coal is dug away! Timbering becomes a real problem, there, for the longer a prop is, Anton, the weaker it is. Coal managers in mines like those have to do some careful figuring, or the cost of the timber they put into the mine would be more than the value of the coal they take out."

"How do they handle it then?"

"As if it were a quarry, rather than a mine. The seam is worked on successive levels, but, even then, it is impossible to prevent constant accidents from the fall of coal or the sudden collapse of a roof. Take it the world over, and ten miners are killed every day in collieries alone. I told you coal mining was dangerous."

"But are there any of those thick seams in the United States?"

"None of the really thick ones. There's a 40-foot anthracite seam in Pennsylvania. But in France, near the famous Creusot works, there's a bed of coal which is 130 feet thick. It's a basin, though, rather than a seam.

"So you see, Anton, every coal mine is different, with its layers or seams of coal of different thicknesses and at varying distances apart. Some pits are near the surface, some are very deep; some coal is full of gas, other has very little; some coal is so hard that every bit of it has to be blasted, in other mines the coal is so soft that the hewer spends half his time spragging the face so that the coal doesn't fall on him when he's undercutting or holing. Don't you make the mistake of thinking that all a miner has to do is to use his pick! He's got to know his business thoroughly or he's useless to the mine boss and a danger to all his fellow-workmen.

"And that isn't all, Anton, not by a good deal!

"Coal mining might be bad enough, even if the coal seams always ran level. But it's very seldom that they do. They run up-hill and down-hill in all sorts of fashions and play hide-and-go-seek in a way that's fairly bewildering.

"Nearly all coal seams are broken up by faults. The coal suddenly seems to stop, and, when you go to hewing it the pick suddenly hits against a rock wall, right on the level of the seam. In the North Gallery of this very mine, there's a fault like that. You know where the 'snagger' is?"

"Sure," agreed Anton, "you mean where the cars have to be hitched on to a