e are likely to be several
superposed and overlapping seams of coal, separated by sandstones,
shales, or other rocks. In Illinois and Indiana there are nine workable
coal seams, in Pennsylvania in some places about twenty, and in Wales
there are over one hundred, many of which are worked. Some of the seams
are of very limited extent; others are remarkably persistent, one seam
in Pennsylvania having an average thickness of 6 to 10 feet over about
6,000 square miles of its area. Only 2 per cent of the coal-bearing
measures of the eastern United States is actually coal.

Even where not subsequently disturbed by deformation, coal beds are not
free from structural irregularity. They are originally deposited in
variable thicknesses on irregular surfaces. During their consolidation
there is a great reduction of volume, resulting in minor faults and
folds. Subsequent deformation by earth forces may develop further faults
and folds, with the result that the convolutions of a coal bed may be
very complex. The beds of a coal-bearing series are usually of differing
thickness and competency, and as a consequence they do not take the same
forms under folding. Shearing between the beds may result in an
intricate outline for one bed, while the beds above and below may have
much more simple outlines. In short, the following of a coal seam
requires at almost every stage the application of principles of
structural geology. It is obvious, also, that the identification and
location of sedimentary geologic horizons are essential, and hence the
application of principles of stratigraphy.

The folios of the United States Geological Survey on coal-bearing areas
present highly developed methods of mapping and representing the
geologic features of coal beds. On the surface map are indicated the
topography, the geologic horizons, and the lines of outcrop of the coal
seams. In addition, there are indicated the sub-surface contours of one
or more of the coal seams which are selected as datum horizons. The
sub-surface structure, even though complex, can be readily read from one
of these surface maps. With the addition of suitable cross sections and
comparative columnar sections, the story is made complete. In the study
of the occurrence of coal seams, the reader cannot do better than
familiarize himself with one or more of the Geological Survey folios.

The high-grade coals of the eastern and central United States are found
in rocks of Carboniferous