finally be changed into coal.

Whether it was burned while yet in the condition of peat, or millions of
years later, when it was transformed into coal, the heat stored in its
substance was liberated by the burning. The carbon and the heat went
back to the air.

Every green plant we see spreads its leaves to the sun. Every stick of
wood we burn, and every lump of coal, is giving back, in the form of
light and heat, the energy that came from sunshine and was captured by
the green leaves. How long the wood has held this store of heat we may
easily compute, for we can read the age of a tree. But the age of coal
we cannot accurately state. The years probably should be counted by
millions, instead of thousands.

The great inland sea that covered the middle portion of the continent
during the Silurian and the Devonian periods, became shallow by the
deposit of vast quantities of sediment. Along the lines of the deposits
of greatest thickness, a crumpling of the earth's crust lifted the first
fold of the Alleghany Mountains as a great sea wall on the east, and on
the western shore another formed the beginning of the Ozark Mountain
system in Missouri. An island was lifted out of the sea, forming the
elevated ground on which the city of Cincinnati now stands. Various
other ridges and islands divided the ancient sea into much smaller
bodies of water. Hemmed in by land these shallow sea-basins gradually
changed into fresh-water lakes, for they no longer had connection with
the ocean, and all the water they received came from rain. After
centuries of freshets, and of filling in with the rock debris brought by
the streams, they became great marshes, in which grew water-loving
plants. Generation after generation of these plants died, and their
remains, submerged by the water, were converted into peat. In the
course of ages this peat became coal. This is the history of the coal
measures.

There is no guesswork here. The stems of plants do not lose their
microscopic structure in all the ages it has taken to transform them to
coal. A thin section of coal shows under a magnifier the structure of
the stems of the coal-forming plants. Moreover, the veins of coal
preserve above or below them, in shales that were once deposits of mud,
the branching trunks of trees, perfectly fossilized. There are no better
proofs of the vegetable origin of coal than the lumps themselves. But
they are plain to the naked eye, while the coal tells its story