substances are in any way related.

Our earth is very old, and its surface has gone through many
transformations; mountains, plains, and portions of the sea floor
have changed places with one another. Wherever there have been
marshy lowlands, since plants first began to grow luxuriantly upon
the earth, it has been possible for beds of coal to be formed.
We all know how rankly plants grow where there is plenty of heat
and moisture. Many of us have been in swampy forests and have seen
the masses of rotting tree trunks, limbs, and leaves. Now, if we
should form a picture in our minds of such a swamp slowly sinking
until the water of some lake or ocean had flowed over it and killed
the plants, and then washed sand and clay upon the buried forest
until it was covered deeply in the earth, we should understand
how the coal-beds began. Veins of coal that have been opened by
the miners frequently show trunks and stumps of trees, as well
as impressions of leaves and ferns. Underneath the coal there is
usually a bed of clay, while above sand or sandstone is commonly
found.

The oldest coal has been changed the most. It is hard and rather
difficult to ignite, but when once on fire it gives more heat and
burns longer than other coals. This coal, known as anthracite, is
not found extensively in the United States outside of Pennsylvania.
Coal which is younger and has been less changed by the heat and
pressure brought to bear upon it when it was buried deep in the
earth, is known as bituminous. This is the kind of coal which is
found in the Mississippi and Ohio valleys, in the Rocky Mountains,
and upon the Pacific slope. A still younger coal, which is soft
and has a brownish color, is called lignite, and is found mostly
in the South and West.

Still another sort of fuel, known as peat, is found in swamps where
considerable vegetation is now accumulating, or has accumulated
in recent times. Peat is a mass of plant stems, roots, and moss,
partly decayed and pressed together. In countries where wood is
scarce peat is cut out, dried, and used for fuel.

The larger part of the coal in the eastern United States was formed
during the Carboniferous period. That part of our country was then
low and swampy; but the West, which is now an elevated area of
mountains and plateaus, was at that time largely beneath the ocean.

Then, as the surface of the earth continued to change, the ocean
retreated from the Rocky Mountain region, and extensiv