was made by the sand and mud which
were brought in by the water. These shaly rocks split easily into
sheets and show beautiful fossil impressions of ferns. There are also
impressions of the bark and fruit of trees, together with shells,
crinoids, corals, remains of fishes and flying lizards, and some few
trilobites,--crablike animals with a shell somewhat like the back of
a lobster, but marked into three divisions or lobes, from which its
name comes.

Since the crust of the earth was so thin and yielding, it wrinkled up
as the earth cooled, much as the skin of an apple wrinkles when the
apple dries. This brought some of the strata of coal to the surface,
and after a while people discovered that it would burn. If a vein of
coal cropped out on a man's farm, he broke some of it up with his
pickaxe, shoveled it into his wheelbarrow, and wheeled it home. After
a while hundreds of thousands of people wanted coal; and now it had to
be mined. In some places the coal stratum was horizontal and cropped
out on the side of a hill, so that a level road could be dug straight
into it. In other places the coal was so near the surface that it
could be quarried under the open sky, just as granite is quarried.
Generally, however, if you wish to visit a coal mine, you go to a
shaft, a square, black well sometimes deeper than the height of three
or four ordinary church steeples. You get into the "cage," a great
steel box, and are lowered down, down, down. At last the cage stops
and you are at the bottom of the mine. The miners' faces, hands,
overalls, are all black with coal dust. They wear tiny lamps on their
caps, and as they come near the walls of coal, it sparkles as it
catches the light. Here and there hangs an electric lamp. It is doing
its best to give out light, but its glass is thick with coal dust. The
low roof is held up by stout wooden timbers and pillars of coal. A
long passageway stretches off into a blacker darkness than you ever
dreamed of. Suddenly there is a blaze of red light far down the
passage, a roar, a medley of all sorts of noises,--the rattling of
chains, the clattering of couplings, the shouts of men, the crash of
coal falling into the bins. It is a locomotive dragging its line of
cars loaded with coal. In a few minutes it rushes back with empty cars
to have them refilled.

All along this passageway are "rooms," that is, chambers which have
been made by digging out the coal. Above them is a vast amount of
eart