The gradual wasting of plant-tissue in the formation of peat, lignite,
coal, etc., may be estimated as averaging for peat, 20 to 30 per cent.;
lignite, 30 to 50 per cent.; coal, 50 to 70 per cent.; anthracite, 70
to 80; and graphite, 90 per cent. of the original mass. The evolved
products ultimately represent the entire organic portion of the
wood--the mineral matter, or ash, being the only residuum. These evolved
products include both liquids and gases, and by subsequent changes,
solids are produced from some of them. Carbonic acid, carbonic oxide,
nitrogenous and hydrocarbon gases, water, and petroleum, are mentioned
above as the substances which escape from wood-tissue during its
decomposition. That all these are eliminated in the decay of vegetable
and animal structures is now generally conceded by chemists and
geologists, although there is a wide difference of opinion as to the
nature of the process.

It has been claimed that the evolved products enumerated above are the
results of the primary decomposition of organic matter, and never of
further changes in the residual products; i.e., that in the breaking-up
of organic tissue, variable quantities of coal, anthracite, petroleum,
marsh gas, etc., are formed, but that these are never derived, the one
from the other. This opinion is, however, certainly erroneous, and the
formation of any or all the evolved products may take place throughout
the entire progress of the decomposition. Marsh gas and carbonic acid
are seen escaping from the surface of pools where recent vegetable
matter is submerged, and they are also eliminated in the further
decomposition of peat, lignite, coal, and carbonaceous shale. Fire damp
and choke-damp, common names for the gases mentioned above, are produced
in large quantities in the mines where Tertiary or Cretaceous lignites,
or Carboniferous coals or anthracites are mined. It has been said that
these gases are simply locked up in the interstices of the carbonaceous