Kimeridge Clay, named from the village of Kimeridge in the Isle of Purbeck (Dorsetshire), is an important division of the Upper Jurassic (q.v.), giving the name "Kimeridgian" to the series. It is a bluish-grey or sometimes yellow clay, generally shaly, containing beds of bituminous shale, lignite, crystals of selenite, septarian nodules, and, near its base, sandy beds with clay-ironstone. It is of marine origin, and has a maximum thickness apparently of 660 feet, passing conformably downwards into the Corallian, or, as in Bedfordshire, in the Fens, and in the Sub-Wealden boring near Battle, into clays continuous with the Oxford Clay. To these passage-beds the name Ampthill Clay has been given. The Kimeridge Clay is overlaid conformably by the sands and limestones of the Portlandian. The outcrop of the clay extends from Dorset and Somerset, through North Wilts, Oxfordshire, and the Fens, into Norfolk, Lincolnshire and the Vale of Pickering, forming broad, flat areas of cold, stiff pasture-land. As the oak flourishes on this soil it was named "Oaktree Clay" by William
Smith, as by confusion were other clays. The pyrites in the bituminous beds has sometimes by its decomposition produced spontaneous combustion, as in the. so-called "Volcano" in Ringstead Bay, which burnt'for some years. The bituminous shales have been distilled for paraffin, and in prehistoric times a lignite, almost as lustrous as jet, was turned into ornaments near Kimeridge, where the wastrel of the lathe is still found and is known as "Kimeridge coal-money," being supposed to have been used as coin. The Kimeridge Clay has been subdivided into an upper and a lower division. The upper or Virgulian group of foreign authors, characterised by the small oyster, Exogyra viryula, consists of paper-shales, bituminous shales, cement-stones, and clays, best developed in the south of England. The lower, or Astartian, with Astarte minima and Ostrea deltoidea, better developed in the north, is often sandy with ferruginous "doggers." Other characteristic fossils are Ammonites biplex and numerous saurian" remains. On the Continent a middle division, or Pterocerian, with Pteroceras oceani, is distinguished. The fine-grained limestone, long quarried for lithographic purposes at Solenhofen, near Munich, in which the oldest-known fossil-bird, Archceopteryx (q.v.), was found, together with numerous well-preserved saurian, insect, and other remains, belongs to the Kimeridgian.