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(Geology)

An intrusive igneous rock formation, formed when magma forces its way between two horizontal rock strata, usually sedimentary rock, forming a layer (1) of more or less even thickness. A sill usually exists in relation to a vertical volcanic conduit(2) that connects it to the magma chamber or batholith (3) beneath it. The conduit usually continues above the sill to the surface; otherwise, magma forced in between the rock layers will have forced the rock layers upwards, forming a laccolith (4) instead of a sill.
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