<P> Dikes often form as either radial or concentric swarms around plutonic intrusives, volcanic necks or feeder vents in volcanic cones . The latter are known as ring dikes . </P> <P> Dikes can vary in texture and their composition can range from diabase or basaltic to granitic or rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast volumes of mantle - derived magmas through fractured lithosphere throughout Earth history . Pegmatite dikes comprise extremely coarse crystalline granitic rocks - often associated with late - stage granite intrusions or metamorphic segregations . Aplite dikes are fine - grained or sugary - textured intrusives of granitic composition . </P> <P> In contrast to magmatic dikes, a sill is a magmatic sheet intrusion that forms within and parallel to the bedding of layered rock . </P> <P> Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rock layers . They can form in two ways: </P>

Which is older a dike or the rock layers that dike cuts across explain