gas and oil. If mountain-making
forces had broken the strata, as in eastern Pennsylvania, the gas and
the oil would have been lost by evaporation.

This is what happened in the anthracite coal-belt.

HOW COAL WAS MADE

The broad, rounded dome of a maple tree shades my windows from the
intense heat of this August day. The air is hot, and every leaf of the
tree's thatched roof is spread to catch the sunlight. The carbon in the
air is breathed in through openings on the under side of each leaf. The
sap in the leaf pulp uses the carbon in making starch. The sun's heat is
absorbed. It is the energy that enables the leaf-green to produce a
wonderful chemical change. Out of soil water, brought up from the roots,
and the carbonic acid gas, taken in from the air, rich, sugary starch is
manufactured in the leaf laboratory.

This plant food returns in a slow current, feeding the growing cells
under the bark, from leaf tip to root tip, throughout the growing tree.
The sap builds solid wood. The maple tree has been built out of muddy
water and carbon gas. It stands a miracle before our eyes. In its tough
wood fibres is locked up all the heat its leaves absorbed from the sun,
since the day the maple seed sprouted and the first pair of leaves
lifted their palms above the ground.

If my maple tree should die, and fall, and lie undisturbed on the
ground, it would slowly decay. The carbon of its solid frame would pass
back into the air, as gas, and the heat would escape so gradually that I
could not notice it at all, unless I thrust my hand into the warm,
crumbling mass.

If my tree should be cut down to-day and chopped into stove wood, it
would keep a fire in my grate for many months.

Burning destroys wood substance a great deal faster than decay in the
open air does, but the processes of rotting and burning are alike in
this: each process releases the carbon, and gives it back to the air. It
gives back also the sun's heat, stored while the tree was growing. There
is left on the ground, and in the ashes on the hearth, only the mineral
substance taken up in the water the roots gathered underground.

If my tree stood in swampy ground and fell over under a high wind, the
water that covered it and saturated its substance would prevent decay.
The carbon would not be allowed to escape as a gas to the air; the woody
substance would become gradually changed into _peat_. In this form it
might remain for thousands of years, and