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PURBECKIAN

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Originally appearing in Volume V22, Page 658 of the 1911 Encyclopedia Britannica.
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PURBECKIAN , in See also:

geology, the highest and youngest member of the See also:Jurassic See also:system of rocks. The name is derived from the See also:district known as the Isle of Purbeck in See also:Dorsetshire where the strata are splendidly exposed in the cliffs See also:west of See also:Swanage. The rocks include See also:clays, shales and marls with marly, tufaceous and shelly limestones and occasional oolitic and sandy strata. Nodules of chert are See also:present in some of the limestones. The Purbeck beds follow the See also:line of the Jurassic outcrop from Dorsetshire, through the Vale of Wardour, See also:Swindon, Garsington, See also:Brill and See also:Aylesbury; they have been proved by borings to See also:lie beneath younger rocks in See also:Sussex; in See also:Lincolnshire they are represented in See also:part by the Spilsby Sands, and in See also:Yorkshire by portions of the Speeton See also:Clay. The thickness of the See also:series in See also:Wiltshire is 8o to 90 ft., but in Dorsetshire it reaches nearly 400 ft. In most places the Purbeckian rests conformably upon the See also:Portland beds and it is conformably overlaid by the See also:Wealden formations; but there are in some districts distinct indications that the Portland rocks were uplifted and worn to some extent See also:prior to the deposition of the Purbeck beds. The Purbeckian in See also:England is divisible into three subdivisions, viz. Upper, See also:Middle and See also:Lower. The Upper Purbeck comprises 50-6o ft. of fresh-See also:water clays and shales with limestones, the " Purbeck See also:marble " and Unio-See also:bed, in the lower part. The Middle See also:division (50-15o ft.), mainly thin limestones with shaly partings, contains the See also:principal See also:building stones of the Swanage district; near the See also:base of this subdivision there is a 5-in. bed from which an interesting See also:suite of mammalian remains has been obtained; in this portion of the Purbeck series there are some marine bands. The Lower Purbeck (95-160 ft.) consists of fresh-water and terrestrial deposits, marls, and limestones with several fossil soils known as " dirt beds." This division is very extensively exposed on the Isle of Portland, where many of the individual beds are known by distinctive names.

The See also:

chief building stones of Upway belong to this part of the Purbeckian. No zonal fossil has been recognized for the See also:British Purbeckian strata, but the See also:horizon is approximately See also:equivalent to that of Perisphinctes transitorius of the See also:European See also:continent. The Purbeckian equivalents of Spilsby and Speeton are in the See also:zone of Belemnites lateralis. Other marine fossils are Hemicidaris purbeckensis and Ostrea distorta, the latter being abundant in the ' Cinder bed " of the Middle Purbeck. The fresh-water See also:mollusca include Viviparus (Paludina), Planorbis, Melanopsis, Unio, Cyrena. A large number of See also:insect genera has been found in the Middle and Lower Purbeck beds. Dinosaurs (See also:Iguanodon, Echinodon), crocodiles (Goniopholis, Petrosuchus), Cimoliosaurus, the plesiosaurs and the chelonians (Chelone, Pleurosternum) , are representative See also:reptiles. The mammals, mostly determined from lower jaws, found in the beds mentioned above include Plagiaulax, Amblotherium, Stylodon, Triconodon, Spalacotherium and several others. The isopod crustacean Archeoniscus Brodei is very See also:common in the Purbeck of the Vale of Wardour. The silicified stumps and trunks of cycads and coniferous trees, often surrounded by See also:great masses of calcareous concretions (Burrs), are very noticeable in the dirt beds of Portland and near Lulworth. Chara is found in the fresh-water cherts of the Middle Purbeck. Many geologists have ranged the Purbeck beds with the overlying Wealden formation on See also:account of the similarity of their fresh-water faunas; but the marine fossils, including the fishes, ally the Purbeck more closely with the Upper Jurassic rocks of other parts, and it may be regarded as the equivalent of the Upper Volgian of See also:Russia.

The Purbeckian is present in the neighbourhood of See also:

Boulogne; in See also:Charente it is represented by thin limestones with Cyrena and by gypsiferous marls; in See also:north-west See also:Germany three subdivisions are recognized, in descending See also:order Purbeck See also:Kalk, Serpulit and Mitnder Mergel. The building stones of the Purbeck beds have already been mentioned ; the Purbeck or Paludina marble, a See also:grey or greenish See also:limestone full of shells, was formerly extensively employed in cathedrals and churches. See also:Stone tiles or " slatts " were once used locally for roofing from the Lower Purbeck of Portland, Swanage and Swindon. See also:Gypsum was formerly worked from the Lower Purbeck at Swanage. See JURASSIC; also The Jurassic Rocks of Great See also:Britain (1895), vol. v. and " The Geology of the Isle of Purbeck and See also:Weymouth," See also:Memoirs of the Geol. Survey (1898).

End of Article: PURBECKIAN

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