This term refers to the composition of the upper layer of the Earth’s crust, which consists mainly of sedimentary rocks, granite and metamorphic rocks. The Conrad discontinuity divides the crust into the upper crust and lower crust. The upper crust is a silica-aluminium-rich layer, while the lower crust is mainly a silica-magnesium-rich (sima) layer. The silica-aluminium-rich layer is the lower crustal lithospheric layer and is composed of basic rocks and metamorphic rocks. Because the Conrad discontinuity has an intermittent pattern globally, the upper and lower crusts are not always discretely divided. Silica-magnesium-rich layers are widespread globally and exist in both the continents and oceans, while silica-aluminium-rich layers mainly exist in the continental crust but tend to be very thin, or even missing, in the oceanic crusts. The continental crust is an average of 35 km thick and can reach 60–70 km thick, but the oceanic crust is only 5–15 km thick.