The Conrad discontinuity (named after the seismologist Victor Conrad) is considered to be the border between the upper continental (sial, for silica-aluminium) crust and the lower one (sima, for silica-magnesium).
[1] Up to the middle 20th century, the upper crust in continental regions was seen to consist of felsic rocks such as granite (sial), and the lower one to consist of more magnesium-rich mafic rocks like basalt (sima).
Therefore, the seismologists of that time considered that the Conrad discontinuity should correspond to a sharply defined contact between the chemically distinct two layers, sial and sima.
The area of contact during the movement of the Continental plates is on the Conrad discontinuity.
The possibility that it represents the transition from amphibolite facies to granulite facies metamorphism has been given some support from observations of the uplifted central part of the Vredefort impact structure and the surrounding Kaapvaal Craton.