1911 Encyclopædia Britannica/Purbeckian

From Wikisource
Jump to navigation Jump to search

PURBECKIAN, in geology, the highest and youngest member of the Jurassic system of rocks. The name is derived from the district known as the Isle of Purbeck in Dorsetshire where the strata are splendidly exposed in the cliffs west of Swanage. The rocks include clays, shales and marls with marly, tufaceous and shelly limestones and occasional oolitic and sandy strata. Nodules of chert are present in some of the limestones. The Purbeck beds follow the line of the Jurassic outcrop from Dorsetshire, through the Vale of Wardour, Swindon, Garsington, Brill and Aylesbury; they have been proved by borings to lie beneath younger rocks in Sussex; in Lincolnshire they are represented in part by the Spilsby Sands, and in Yorkshire by portions of the Speeton Clay. The thickness of the series in Wiltshire is 80 to 90 ft., but in Dorsetshire it reaches nearly 400 ft. In most places the Purbeckian rests conformably upon the Portland beds and it is conformably overlaid by the Wealden formations; but there are in some districts distinct indications that the Portland rocks were uplifted and worn to some extent prior to the deposition of the Purbeck beds. The Purbeckian in England is divisible into three subdivisions, viz. Upper, Middle and Lower. The Upper Purbeck comprises 50–60 ft. of fresh-water clays and shales with limestones, the “ Purbeck marble” and Unio-bed, in the lower part. The Middle division (50–150ft.), mainly thin limestones with shaly partings, contains the principal building stones of the Swanage district; near the base of this subdivision there is a 5-in. bed from which an interesting 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 chief building stones of Upway belong to this part of the Purbeckian.

No zonal fossil has been recognized for the British Purbeckian strata, but the horizon is approximately equivalent to that of Perisphinctes transitorius of the European continent. The Purbeckian equivalents of Spilsby and Speeton are in the 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 mollusca include Viviparus (Paludina), Planorbis, Melanopsis, Unio, Cyrena. A large number of insect genera has been found in the Middle and Lower Purbeck beds. Dinosaurs (Iguanodon, Echinodon), crocodiles (Goniopholis, Petrosurhus), Cirnoliosaurus, the plesiosaurs and the chelonians (Chelone, Plcurosternum), are representative 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 common in the Purbeck of the Vale of Wardour. The silicified Stumps and trunks of cycads and coniferous trees, often surrounded by 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 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 Russia. The Purbeckian is present in the neighbourhood of Boulogne; in Charente it is represented by thin limestones with Cyrena and by gypsiferous marls; in north-west Germany three subdivisions are recognized, in descending order Purbeck Kalk, Serpulit and Münder Mergel.

The building stones of the Purbeck beds have already been mentioned; the Purbeck or Paludina marble, a grey or greenish limestone full of shells, was formerly extensively employed in cathedrals and churches. Stone tiles or “ slatts ” were once used locally for roofing from the Lower Purbeck of Portland, Swanage and Swindon. Gypsum was formerly worked from the Lower Purbeck at Swanage.

See Jurassic; also The Jurassic Rocks of Great Britain (1895), vol. v. and “The Geology of the Isle of Purbeck and Weymouth,” Memoirs of the Geol. Survey (1898).