ack
lignite," a term that is objectionable because the coal is not
lignitic in the sense of being distinctly woody, and because
the use of the term seems to imply that this coal is little
better than the brown, woody lignite of North Dakota, whereas
many coals of this rank approach in excellence the lowest
grade of bituminous coal. Subbituminous coal is generally
distinguishable from lignite by its black color and its
apparent freedom from distinctly woody texture and structure,
and from bituminous coal by its loss of moisture and the
consequent breaking down of "slacking" that it undergoes when
subjected to alternate wetting and drying.

_Lignite._ The term "lignite," as used by the Geological
Survey, is restricted to those coals which are distinctly
brown and either markedly woody or claylike in their
appearance. They are intermediate in quality and in
development between peat and subbituminous coal.

[Illustration: FIG. 5. Diagrams showing the chemical
composition and heat efficiency of the several ranks of coal. Upper
diagram: Comparative heat value of the samples of coal represented in
the lower diagram, computed on the ash-free basis. Lower diagram:
Variation in the fixed carbon, volatile matter, and moisture of coals of
different ranks, from lignite to anthracite, computed on samples as
received, on the ash-free basis. After Campbell.]

GEOLOGIC FEATURES

Geologic features of coal may be conveniently described in terms of
origin or genesis. Coal has essential features in common with asphalt,
oil, and gas. They are all composed of carbon, hydrogen, and oxygen,
with minor quantities of other materials, combined in various
proportions. They are all "organic" products which owe their origin to
the decay of the tissues of plants and perhaps animals. They have all
been buried with other rocks beneath the surface. The common geologic
processes affecting all rocks have in the main determined the evolution
of these organic products and the forms in which we now find them.
Originating at the surface, they have participated in the constructive
or anamorphic changes of the metamorphic cycle, which occur beneath the
surface, and under these influences have undergone various stages of
condensation, refinement, distillation, and hardening.

All stages in the development of coal have been traced. In brief, the
story is this:

[Illustration