What is "cone sheet"

A cone sheet is a type of high-level igneous intrusion found in partly eroded central volcanic complexes. Cone sheets are relatively thin inclined sheets, generally just a few metres thick, with the geometry of a downward-pointing cone. Viewed from above, their outcrop is typically circular to elliptical. They were originally described from the Ardnamurchan, Mull and other central complexes of the British Tertiary Volcanic Province (now recognised as part of the North Atlantic Igneous Province).