The Petrology of Underclays

Underclays are non-laminated clay beds which often occur below the coal beds of the Pennsylvanian System. They are found almost everywhere that Pennsylvanian coals occur, not only in this country but also in eastern Canada and in western Europe. Although one may occur without the other, the association of coals and underclays is much too common to be fortuitous. If the unaccompanied clay or coal is traced laterally, the missing clay or coal will often be seen in its proper place ... The common association of coal and underclays has been a source of speculation for the past century ... Yet, except for the studies of the clays and shales of Illinois ... no general study of the regional and stratigraphic variations of underclays has been made. This paper is intended to fill this need ... Ten underclay horizons and the zone below the Mississippian-Pennsylvanian contact were studied ... Samples were taken from nearly all parts of the Appalachian and Mid-Continent basins and from those parts of the Eastern Interior Basin in Indiana and western Kentucky.