phacolith

1. (geology) A lens-shaped mass that occurs in an anticlinal crest or synclinal trough.Examples: "This body is a phacolith intruded along the gneiss-schist boundary in the trough of a syncline that plunges 16° southeast."; "This bedding forms a distinctive stratigraphic sequence that may be traced from phacolith to phacolith throughout an area of 1,200 square miles."; "Corndon Hill, which lies east of Montgomery in Powys, Wales, is a circular phacolith made of Ordovician dolerite (Figure 5.3b)."