s 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 chain?"

"Yes, there! The coal seam jumps upwards fifty feet. That's why the
cars, after rolling down nearly a quarter of a mile, by gravity, have
to be pulled up fifty feet by an endless chain, to rejoin the same
seam and then to go rolling on down by themselves."

"Just what are faults?"

"H'm, that's a bit hard to explain to you, Anton, because you don't
know anything about geology, but maybe I can get you to see. Faults
are breaks in the layers of rock, or in the stratification, as it is
called. All coal seams and the rocks above and below them have been
laid down by water. Since water levels everything, these layers of
rock were level, once.

"In ages past, however, the crust of the earth changed a good deal. As
the crust cooled, it contracted, crumpling up these different layers
into all sorts of shapes. Sometimes it bulged them up, sometimes it
hollowed them down so that the edges rose. Quite often a layer of
rock would be cracked right across, and one half would stay level
while the other shot up almost a right ang