1911 Encyclopædia Britannica/Portlandian

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PORTLANDIAN, in geology, a subdivision of the Upper Jurassic system that includes the strata lying between the Kimeridge Clay and the Purbeck beds. These rocks are well exposed on the isle of Portland, Dorsetshire, where they have been quarried for more than 200 years. J. Mitchell appears to have been the first to use the term “ Portland lime” in geological literature (1788); T. Webster spoke of the “ Portland Oolite” in 1812. In England the strata are very variable; the upper part consists principally of limestones, shelly, oolitic or compact, or in places very closely resembling chalk (Upway, Portisham, Brill, Chilmark). Nodules and layers of chert are well developed in some of the limestones of Dorsetshire and elsewhere; and a silicified oolite occurs near St Alban's Head. About Swindon, beds of sand are common in the Upper Portland beds with layers of calcareous sandstone (Swindon stone). Marly and sandy beds occur also at Shotover Hill. The lower portion is usually sandy and shows a gradual passage into the underlying Kimeridge Clay. W. H. Fitton in 1827 gave the name “ Portland Sand ” to this division. The Upper Portlandian in Dorsetshire is 130–170 ft. thick; the Lower Portlandian in the same district is 100–120 ft. These rocks crop out from South Dorsetshire into Wiltshire, Oxfordshire and Buckinghamshire, and possibly extend beneath younger rocks into Bedfordshire and Cambridgeshire. They have been proved by borings in Sussex and Kent, and in Yorkshire they are represented by part of the Speeton Clays, and in Lincolnshire by part of the Spilsby Sand. At Swindon and Aylesbury a conglomeratic layer with small pebbles of lydite and phosphatized fossils lies at the 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 gigs is the zonal ammonite in the Lower Portlandian, associated with Trigonia Pellati, Cyprina Brongniarti, Exogyra brantrutana and Astarte Saemanni as subzonal indices. Other characteristic fossils are Cerithium portlandicum, the casts of which form the familiar “ Portland screw," Isastrae 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 building purposes; at Portland the “ Top Roach,” the “ Whit 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 age are well developed on the continent of Europe, but the grouping of the strata is different in some respects from that adopted by English geologists. In France the “ Portlandian ” is usually taken to include the Purbeckian as well as the equivalents of the English Portland beds, and some authors, e.g. E. Renevier, have included more or less of the Kimeridgian in this division. The Portlandian of north-west Germany includes the Eimbeckhäuser Plattenkalk and the Lower Portland Kalk. Oppel's “ Tithonian " (tithonic) division, embracing Upper Kimeridge beds, Portlandian and Purbeckian beds in the Alpine district, is now recognized as a deeper water deposit of this time with many points of resemblance to the Russian development to which the name “ Volgian " has been applied by S. 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. Blake in 1881 for a part of the Portlandian series, from their occurrence at Boulogne (Bononia) where they are similar to the beds of 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 scheme has not been accepted in England. See Jurassic.