From what I little that I have found, this is an obsolete, little-used term for an observed discontinuity in the velocity of seismic waves within the mantle around a depth of 400 kilometers. Daly (1940) interpreted it as “the top of the mesosphere at a depth of 200-480 kilometres.” This term was apparently coined by Daly (1940). The Jeffrey Discontinuity is more commonly known as the “20 degree discontinuity” within seismology. Siesmologists and other geologists currently argue that the Jeffrey Discontinuity (20 degree discontinuity) and other observed seismic velocity discontinuities, which occur at about 410- and 660-kilometers in depth within Earth’s upper mantle, are caused by phase changes in olivine and other minerals created by the increasing pressure with increasing depth as discussed by Shearer and Flanagan (1999). These phase changes include the phase change from olivine pyroxene to spinel garnet at about 410 kilometers and spinel garnet to Mg-perovskite and Mg-wustite at 660