subsurface geology and coal resources of the pennsylvanian system ...

SUBSURFACE GEOLOGY AND COAL RESOURCES
OF THE PENNSYLVANIAN SYSTEM IN CERTAIN
COUNTIES OF THE ILLINOIS BASIN
HE PRESENT VOLUME contains the
T second of a series of reports that deal
with the subsurface geology and coal resources
of the Pennsylvanian strata in the
various counties (fig. 1) in the Illinois
basin. The first report was issued in 1944.'
The general history and purposes of the
investigation were set forth by M. M.
Leighton in the introduction to the first
series of papers. The methods used in the
studies were also described in a separate
paper in that report.
INTRODUCTION
BY
GILBERT H. CADY
It seems desirable to present here certain
general explanations in regard to items that
otherwise would have to be explained in each
paper. These have to do particularly with
the matter of key beds, the selection and use
of beds as a basis for structure maps, the
methods of construction of such maps for
this series of reports, and acknowledgments
to those who assisted in the work of the
project.
KEY BEDS
In these investigations a key bed signifies
one whose identity is reasonably definite
over large areas in the Illinois basin. It is
a bed by reference to which the position of
other less widespread beds can be conveniently
defined.
The stratum which satisfies this definition
of a key bed most satisfactorily in the
Illinois coal field as a whole, as well as in
the Illinois basin, is the Herrin (No. 6)
coal bed, the top of which is used in this
report as the boundary between the Mc-
Leansboro and Carbondale groups, the coal
bed being in the Carbondale group.2 This
coal bed is not universally present in the
Illinois coal field but it is present in a large
portion of the field, particularly in the
southern part.
Where No. 6 coal bed is not present there
is generally no other bed of comparable usefulness
for reference, and its absence makes
FIG. 1.-Index
map of areas studied in this
report.
l Progress reports on subsurface studies of the Pennsylvanian
system in the Illinois basin: Illinois Geol. Survey
Rept. Inv. 93, 87 pp., 1941.
Because the cyclothem division as used by the Illi~loi~
State Geological Su~\ey has not been considered appropriate
to the plan of description in thia leport, the top of the
M~Leansboro group has been placed at the top of the
Herrin (No. 6) coal in the present publication, rather than
it the top of the cyclothem contain~ng the Herrin (No. 6)
coal, as acccpted by the Survey.