According to Bowen's reaction series, the first crystals to form from a cooling silicate melt are those found in mafic igneous rocks. Mafic rocks are rich in magnesium and iron, and have a relatively low silica content. This means that they have a higher melting point than felsic rocks, which have a higher silica content. As the magma cools, the first minerals to crystallize are those that have the highest melting points, which are the mafic minerals. As the temperature continues to decrease, the minerals that have lower melting points start to crystallize, leading to the formation of felsic rocks.