JET, a mineral substance belonging to the carbonaceous group, and generally regarded as a compact variety of lignite, or wood-coal, impregnated with bitumen. The word jet (German Gagat) is corrupted from gagates, the name applied to it, or to a similar substance, by Greek and Roman writers, and derived, according to Pliny (H. A r ., xxxvi. 34∖ from the river Gagas in Lycia, where the mineral was originally found. Its occurrence in Britain is mentioned by Solinus; but it was certainly used there in pre-Roman times. Barrows of the Bronze Age have yielded beads, buttons, rings, armlets, and other personal ornaments of jet. The early supply was probably obtained from the Yorkshire coast, near Whitby—a locality which still yields the finest varieties. The Whitby jet occurs in isolated masses, of irregular shape, but frequently more or less lenticular, imbedded in bituminous shales near the base of the Upper Lias. The particular horizon of the jet-rock is known to geologists as the zone of Ammonites serpentinus. Opinion is divided as to the exact nature and origin of the jet: some regard it as a variety of lignite, others as a kind of cannel coal, and others again as a hardened form of bitumen. There is little doubt that the jet has in all cases resulted from the decay of organic matter. Microscopic sections of jet frequently reveal a ligneous structure, in most cases of coniferous type. It has been suggested that masses of wood brought down by a river have drifted out to sea, where, becoming water-logged, they have sunk and have gradually been covered with a deposit of fine black mud, beneath which the decay has slowly proceeded! Possibly bituminous matter may have been distilled from this decaying vegetation, and deposited between the layers of shale in its ...