The occurrence of what is probably clear secondary zircon on detrital grains of this mineral seems first to have been noticed by Butterfield (1936), in heavy residues from the Millstone Grit of Yorkshire. He showed that the outgrowths were too sharp and delicate to have withstood transport, and were, therefore, formed after the deposition of the rock. He also gave some account of their frequency and mode of occurrence in relation to the crystals to which they were attached. Smithson (1937) found similar outgrowths on zircons from the Middle Jurassic of Yorkshire, remarking that until we know of other occurrences it may be unwise to generalize, but that Butterfield's Millstone Grit zircons and his own Jurassic examples occurred in deposits which were either deltaic in origin, or closely associated with deltaic conditions.