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PORTLANDIAN

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Originally appearing in Volume V22, Page 122 of the 1911 Encyclopedia Britannica.
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PORTLANDIAN , in See also:

geology, a subdivision of the Upper See also:Jurassic See also:system that includes the strata lying between the Kimeridge See also:Clay and the Purbeck beds. These rocks are well exposed on the isle of See also:Portland, See also:Dorsetshire, where they have been quarried for more than Doc years. J. See also:Mitchell appears to have been the first to use the See also:term " Portland See also:lime " in See also:geological literature (1788); T. See also:Webster spoke of the " Portland Oolite " in 1812. In See also:England the strata are very variable; the upper See also:part consists principally of limestones, shelly, oolitic or compact, or in places very closely resembling See also:chalk (Upway, Portisham, See also:Brill, Chilmark). Nodules and layers of chert are well See also:developed in some of the limestones of Dorsetshire and elsewhere; and a silicified oolite occurs near St See also:Alban's See also:Head. About See also:Swindon, beds of See also:sand are See also:common in the Upper Portland beds with layers of calcareous See also:sandstone (Swindon See also:stone). Marly and sandy beds occur also at Shotover See also:Hill. The See also:lower portion is usually sandy and shows a See also:gradual passage into the underlying Kimeridge Clay. W. H.

See also:

Fitton in 1827 gave the name " Portland Sand" to this See also:division. The Upper Portlandian in Dorsetshire is 130–170 ft. thick; the Lower Portlandian in the same See also:district is 100–120 ft. These rocks See also:crop out from See also:South Dorsetshire into See also:Wiltshire, See also:Oxfordshire and See also:Buckingham-See also:shire, and possibly extend beneath younger rocks into See also:Bedford-shire and See also:Cambridgeshire. They have been proved by borings in See also:Sussex and See also:Kent, and in See also:Yorkshire they are represented by part of the Speeton See also:Clays, and in See also:Lincolnshire by part of the Spilsby Sand. At Swindon and See also:Aylesbury a conglomeratic layer with small pebbles of lydite and phosphatized fossils lies at the See also:base of the Portland Stone. The Upper Portlandian of England is characterized by the ammonite Perisphinctes giganteus, along with Cytheria (Cyrena) rugosa, Trigonia gibbosa, Perisphinctes boloniensis and Trigonia incurva as subzonal forms. Olcostephanus gigas is the zonal ammonite in the Lower Portlandian, associated with Trigonia Pellati, Cyprina Brongniarti, Exogyra brantrutana and See also:Astarte Saemanni as subzonal indices. Other characteristic fossils are Cerithiunt portlandicum, the casts of which See also:form the See also:familiar " Portland See also:screw,"Isastraea,oblonga, the Chelonian Stegochelys; the remains of saurians Pliosaurus and Cimoliosaurus and others are found; Mesodon, Ischyodus and other fishes occur in this formation. The Portland limestones have been much in demand for See also:building purposes; at Portland the " See also:Top See also:Roach," the " Whit See also:Bed" or top freestone, and the " Best Bed" (or Base Bed) are the best known. In the Vale of Wardour the lower Portlandian has been largely quarried; the stone from this neighbourhood is often described as Wardour, Tisbury or Chilmark stone. Swindon stone is a calcareous sandstone that occurs in the sands of the Upper Portland beds near Swindon. Rocks of Portlandian See also:age are well developed on the See also:continent of See also:Europe, but the grouping of the strata is different in some respects from that adopted by See also:English geologists.

In See also:

France the " Portlandian " is usually taken to include the See also:Purbeckian as well as the equivalents of the English Portland beds, and some authors, e.g. E. See also:Renevier, have included more or less of the See also:Kimeridgian in this division. The Portlandian of See also:north-See also:west See also:Germany includes the Eimheckhauser Plattenkalk and the Lower Portland See also:Kalk. See also:Oppel's " Tithonian " (tithonic) division, embracing Upper Kimeridge beds, Portlandian and Purbeckian beds in the Alpine district, is now recognized as a deeper See also:water See also:deposit of this See also:time with many points of resemblance to the See also:Russian development to which the name " Volgian " has been applied by S. See also:Nikitin. The Portlandian beds of Yorkshire are more nearly related to the Volgian phase than to the beds of the same age in the south of England. The term Bononian (= Bolonian) was suggested by J. F. See also:Blake in 188i for a part of the Portlandian See also:series, from their occurrence at See also:Boulogne (See also:Bononia) where they are similar to the beds of See also:Dorset. He limited the name Portlandian to the Purbeckian and Upper Portlandian (Portland stone), while he placed the Portland Sands and upper part of the Kimeridge Clay in his Bolonian division: this See also:scheme has not been accepted in England. See JURASSIC.

End of Article: PORTLANDIAN

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