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Phacolith
Lens-shaped igneous intrusion / From Wikipedia, the free encyclopedia
A phacolith is a pluton of igneous rock parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline. In rare cases the body may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that in cross section it has an S shape. In intensely folded terrain the hinge of folds would be areas of reduced pressure and thus potential sites for magma migration and emplacement.
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The term was coined and initially defined by Alfred Harker in his The Natural History of Igneous Rocks in 1909.