Lay and Helmberger and their co‐workers have recently proposed models of Earth structure in which the S‐velocity increases discontinuously with depth by about 3% at the bottom of a postulated region of abnormally low (near zero) gradient between depths of about 2300 and 2600 km in the lower mantle. Such models involve drastic departures from previously proposed models of Earth structure in this region. By use of Kirchhoff wave theory it is shown that the pertinent data may be interpreted alternatively as an effect of wave propagation in an inhomogeneous lower mantle. On this alternative interpretation the S‐velocity need nowhere sharply increase with increasing depth at any level in the lower m