The sill does not cut across preexisting rocks, unlike dykes. Sills are fed by dykes as they form from a lower magma source. The existing rocks must split to create the planes along which the magma moves in. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing strata. When it cools and crystallises, it is then a sill.
|This article includes text from the public domain 1911 Encyclopaedia Britannica.|