The Conrad discontinuity (named after the seismologist Victor Conrad) is considered to be the border between the upper continental crust and the lower one. It is not as pronounced as the Mohorovičić discontinuity, and absent in some continental regions. Up to the middle 20th Century the upper crust in continental regions was seen to consist of felsic rocks such as granite (sial, for silica-aluminium), and the lower one to consist of more magnesium-rich mafic rocks like basalt (sima, for silica-magnesium). 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.