A mine subsidence theory that distinguishes between the manner of fracture of shale and sandstone, holding that the former rock breaks along vertical lines irrespective of the angle of dip, and that the latter has a vertical fracture over a rise face and a fracture at right angles to the bed over a dip face. The theory predicts vertical lines of break in either rock for a level seam, and is, indeed, a compromise between the vertical theory and that of the normal.