: FIG. 6. Origin and development of coal. After
Gilbert.]

This exhibit shows the successive chemical stages in the
evolution of coal. The striking qualities of the original are
lost in the reproduction through the use of designs in the
place of realistic coloring, but the effect is retained
sufficiently to indicate the nature of the sequence and the
directness with which it leads back to an origin in vegetal
accumulations. The evolutionary process is seen to take the
form of increasing density through the progressive expulsion
of volatilizable matters in the course of geologic time. This
inference is substantiated beyond reasonable question by the
actual presence of organic remains in coal beds.

Grasses, trees, and other plants growing in swamps and bogs decay and
form a vegetable mold in the nature of _peat_. A peat bog from the top
downward consists of (1) living plants, (2) dead plants, and (3) a dense
brownish-black mass, of decayed and condensed vegetable material, in
which the vegetable structure is more or less indistinct. Peat consists
chiefly of fixed carbon and volatile matter, also of sulphur, moisture,
and ash. The volatile matter consists mainly of various combinations of
hydrogen and carbon, called hydrocarbons; it goes off in gas or smoke
when the peat is heated to a red heat. The fixed carbon is the carbon
left after the volatile matter has been driven off. The ash represents
the more incombustible mineral matter, usually of the nature of clay or
slate. The moisture in peat may be as high as 90 per cent.

The essential condition for thick accumulation of peat seems to be
abundance of moisture, which favors luxuriant growth and protects the
plant remains from complete oxidation or decay. Without moisture the
vegetable material would completely oxidize, leaving practically no
residue, as it does in dry climates. For the formation of thick peat
beds, there seems to be implied some sort of a balance between the slow
building up of organic accumulations and the settling of the area to
keep it near the elevation of the water table. Present day bog deposits
are known in some cases to have a thickness of forty feet. This
thickness is not enough to account for some of the great coal seams
within the earth; but there seems to be no escape from the conclusion
that the same sort of deposits, formed on a larger scale in the past,
were the first step in the f