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 famou