III. THE MIDDLE OOLITES.--The central portion of the Jurassic series of Britain is formed by a great argillaceous deposit, capped by calcareous strata, as follows: a, The _Oxford Clay_ (_Terrain Callovien_ and _Terrain Oxfordien_ of D'Orbigny), consisting of dark-coloured laminated clays, sometimes reaching a thickness of 700 feet, and in places having its lower portion developed into a hard calcareous sandstone ("Kelloway Rock"); b, The Coral-Rag (_Terrain Corallien_ of D'Orbigny, "Nerinean Limestone" of the Jura, "Diceras Limestone" of the Alps), consisting, when typically developed, of a central mass of oolitic limestone, underlaid and surmounted by calcareous grits. IV. THE UPPER OOLITES.--a, The base of the Upper Oolites of Britain is constituted by a great thickness (600 feet or more) of laminated, sometimes carbonaceous or bituminous clays, which are known as the _Kimmeridge Clay_ (_Terrain Kimméridgien_ of D'Orbigny); b, The _Portland Beds_ (_Terrain Portlandien_ of D'Orbigny) succeed the Kimmeridge clay, and consist inferiorly of sandy beds surmounted by oolitic limestones ("Portland Stone"), the whole series attaining a thickness of 150 feet or more, and containing marine fossils; c, The _Purbeck_ Beds are apparently peculiar to Great Britain, where they form the summit of the entire Oolitic series, attaining a total thickness of from 150 to 200 feet. The Purbeck beds consist of arenaceous, argillaceous, and calcareous strata, which can be shown by their fossils to consist of a most remarkable alternation of fresh-water, brackish-water, and purely marine sediments, together with old land-surfaces, or vegetable soils, which contain the upright stems of trees, and are locally known as "Dirt-beds." One of the most important of the Jurassic deposits of the continent of Europe, which is believed to be on the horizon of the Coral-rag or of the lower part of the Upper Oolites, is the "_Solenhofen Slate_" of Bavaria, an exceedingly fine-grained limestone, which is largely used in lithography, and is celebrated for the number and beauty of its organic remains, and especially for those of Vertebrate animals. The subjoined sketch-section (fig. 159) exhibits in a diagrammatic form the general succession of the Jurassic rocks of Britain.