Clay dikes in the Redstone coal of West Virginia and Pennsylvania are so common that they are believed to result from a definite set of conditions. A similar set of conditions associated with another coal should produce clay dikes. A stratum of plastic clay closely below or above the coal is necessary. A well compacted coal, when stressed tangentially, develops a joint system with one set normal to, and another set paralleling, the direction of stress. This same well compacted coal, when stressed normal to the bedding, acts as a brittle material and forms sets of compressive shear fractures, inclined at some angle to the bedding, averaging 40°–50°. Fractures passing through the coal penetrate the under clay or an overlying clay to form a line of weakness for clay to flow through, equalizing differential pressures, thus forming clay dikes.

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