"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?"