Bridge Co._

First the steel frame, then the floors, then the stone or brick shell,
then the interior finishing--this is how the building is made.]

THE INDUSTRIAL READERS

BOOK II

DIGGERS IN THE EARTH

I

IN A COAL MINE

Did you ever wonder how beds of coal happened to be in the earth? This
is their story.

Centuries ago, so many thousand centuries that even the most learned
men can only guess at their number, strange things were coming to
pass. The air was so moist and cloudy that the sun's rays had hard
work to get through. It was warm, nevertheless, for the crust of the
earth was not nearly so thick as it is now, and much heat came from
the earth itself. Many plants and trees grow best in warm, moist air;
and such plants flourished in those days. Some of their descendants
are living now, but they are dwarfs, while their ancestors were
giants. There is a little "horse-tail" growing in our meadows, and
there are ferns and club mosses almost everywhere. These are some of
the descendants; but many of their ancestors were forty or fifty feet
high. They grew very fast, especially in swamps; and when they died,
there was no lack of others to take their places. Dead leaves fell and
heaped up around them. Stumps stood and decayed, just as they do in
our forests to-day. Every year the soft, black, decaying mass grew
deeper. As the crust of the earth was so thin, it bent and wrinkled
easily. It often sank in one place and rose in another. When these
low, swampy places sank, water rushed over them, pressing down upon
them with a great weight and sweeping in sand and clay. Now, if you
burn a heap of wood in the open air, the carbon in the wood burns and
only a pile of ashes remains. "Burning" means that the carbon in the
wood unites with the oxygen gas in the air. If you cover the wood
before you light it, so that only a little oxygen reaches it, much of
the carbon is left, in the form of charcoal.

When wood decays, its carbon unites with the oxygen of the air; and so
decay is really a sort of burning. In the forests of to-day the
leaves, and at length the trees themselves, fall and decay in the open
air; but at the time when our coal was forming, the water kept the air
away, and much carbon was left. This is the way coal was made. Some of
the layers, or strata, are fifty or sixty feet thick, and some are
hardly thicker than paper. On top of each one is a stratum of
sandstone or dark-gray shale. This