KIMERIDGIAN

kimeridgian, in geology, the basal division of the upper oolites in the jurassic system. the name is derived from the hamlet of kimeridge or kimmeridge near the coast of dorsetshire, england. it appears to have been first suggested by t. webster in 1812; in 1818, in the form kimeridge clay, it was used by buckland. from the dorsetshire coast, where it is splendidly exposed in the fine cliffs from st alban's head to gad cliff, it follows the line of jurassic outcrop through wiltshire, where there is a broad expanse between westbury and devizes, as far as yorkshire, there it appears in the vale of pickering and on the coast in filey bay. it generally occupied broad valleys, of which the vale of aylesbury may be taken as typical. good exposures occur at seend, calne, swindon, wootton bassett, faringdon, abingdon, culham, shotover hill, brill, ely and market rasen. traces of the formation are found as far north as the east coast of cromarty and sutherland at eathie and helmsdale. in england the kimeridgian is usually divisible into an upper series, 600-650 ft. in the south, dark bituminous shales, paper shales and clays with layers and nodules of cement-stones and septaria. these beds merge gradually into the overlying portlandian formation. the lower series, with a maximum thickness of 400 ft., consists of clays and dark shales with septaria, cement-stones and calcareous "doggers." these lithological characters are very persistent. the upper kimeridgian is distinguished as the zone of _perisphincles biplex_, with the sub-zone of _discina latissima_ in the higher portions. _cardioceras alternans_ is the zonal ammonite characteristic of the lower division, with the sub-zone of _ostrea deltoidea_ in the lower portion. _exogyra virgula_ is common in the upper part of the lower division, and the lower part of the upper kimeridgian. a large number of ammonites are peculiar to this formation, including _reineckia eudoxus_, _r. thurmanni_, _aspidoceras longispinus_, &c. large dinosaurian reptiles are abundant, _cetiosaurus_, _gigantosaurus_, _megalosaurus_, also plesiosaurs and ichthyosaurs; crocodilian and chelonian remains are also found. _protocardia striatula_, _thracia depressa_, _belemnites abreviatus_, _b. blainvillei_, _lingula ovalis_, _rhynchonella inconstans_ and _exogyra nana_ are characteristic fossils. alum has been obtained from the kimeridge clay, and the cement-stones have been employed in purbeck; coprolites are found in small quantities. bricks, tiles, flower-pots, &c., are made from the clay at swindon, gillingham, brill, ely, horncastle, and other places. the so-called "kimeridge coal" is a highly bituminous shale capable of being used as fuel, which has been worked on the cliff at little kimeridge. the "kimeridgien" of continental geologists is usually made to contain the three sub-divisions of a. oppel and w. waagen, viz.:-- / upper (virgulian) with _exogyra virgula_ kimeridgien < middle (pteroceran) with _pteroceras oceani_ \ lower (astartian) with _astarte supracorallina_; but the upper portion of this continental kimeridgian is equivalent to some of the british portlandian; while most of the astartian corresponds to the corallian. a. de lapparent now recognizes only the virgulian and pteroceran in the kimeridgien. clays and marls with occasional limestones and sandstones represent the kimeridgien of most of northern europe, including russia. in swabia and some other parts of germany the curious ruiniform marble _felsenkalk_ occurs on this horizon, and most of the kimeridgien of southern europe, including the alps, is calcareous. representatives of the formation occur in caucasia, algeria, abyssinia, madagascar; in south america with volcanic rocks, and possibly in california (maripan beds), alaska and king charles's land. see "jurassic rocks of britain," vols. v. and i., _memoirs of the geological survey_ (vol. v. contains references to literature up to 1895). (j. a. h.)