1911 Encyclopædia Britannica/Corallian

From Wikisource
Jump to: navigation, search

CORALLIAN (Fr. Corallien), in geology, the name of one of the divisions of the Jurassic rocks. The rocks forming this division are mainly calcareous grits with oolites, and rubbly coral rock — often called “Coral Rag”; ferruginous beds are fairly common, and occasionally there are beds of clay. In England the Corallian strata are usually divided into an upper series, characterized by the ammonite Perisphinctes plicatilis, and a lower series with Aspidoceras perarmatus as the zonal fossil. When well developed these beds are seen to lie above the Oxford Clay and below the Kimeridge Clay; but it will save a good deal of confusion if it is recognized that the Corallian rocks of England are nothing more than a variable, local lithological phase of the two clays which come respectively above and below them. This caution is particularly necessary when any attempt is being made to co-ordinate the English with the continental Corallian.

The Corallian rocks are nowhere better displayed than in the cliffs at Weymouth. Here Messrs Blake and Huddleston recognized the following beds: —



Upper Corallian
{
Upper Coral Rag and Abbotsbury Iron Ore.
Sandsfoot Grits.
Sandsfoot Clay.
Trigonia Beds.
Osmington Oolite (quarried at Marnhull and Todbere).
Lower Corallian { Bencliff Grits.
Nothe Clay.
Nothe Grit.


In Dorsetshire the Corallian rocks are 200 ft. thick, in Wiltshire 100 ft., but N.E. of Oxford they are represented mainly by clays, and the series is much thinner. (At Upware, the “Upware limestone” is the only known occurrence of beds that correspond in character with the Coralline oolite between Wiltshire and Yorkshire). In Yorkshire, however, the hard rocky beds come on again in full force. They appear once more at Brora in Sutherlandshire. Corallian strata have been proved by boring in Sussex (241 ft.). In Huntingdon, Bedfordshire, parts of Buckinghamshire, Cambridgeshire and Lincolnshire the Corallian series is represented by the “Ampthill Clay,” which has also been called “Bluntesham ” or “Tetworth” Clay. Here and there in this district hard calcareous inconstant beds appear, such as the Elsworth rock, St Ives rock and Boxworth rock.

In Yorkshire the Corallian rocks differ in many respects from their southern equivalents. They are subdivided as follows: —


Britannica Corallian Yorkshire Subdivisions.png


These rocks play an important part in the formation of the Vale of Pickering, and the Hambleton and Howardian Hills; they are well exposed in Gristhorpe Bay.

The passage beds, highly siliceous, flaggy limestones, are known locally as “Greystone” or “Wall stones”; some portions of these beds have resisted the weathering agencies and stand up prominently on the moors — such are the “Bridestones.” Cement stone beds occur in the upper calcareous grit at North Grimstone; and in the middle and lower calcareous grits good building stones are found.

Among the fossils in the English Corallian rocks corals play an important part, frequently forming large calcareous masses or “doggers”; Thamnastrea, Thecosmilia and Isastrea are prominent genera. Ammonites and belemnites are abundant and gasteropods are very common (Nerinea, Chemnitzia, Bourgetia, &c.). Trigonias are very numerous in certain beds (T. perlata and T. mariani). Astarte ovata, Lucina aliena and other pelecypods are also abundant. The echinoderms Echinobrissus scutatus and Cidaris florigemma are characteristic of these beds.

Rocks of the same age as the English Corallian are widely spread over Europe, but owing to the absence of clearly-marked stratigraphical and palaeontological boundaries, the nomenclature has become greatly involved, and there is now a tendency amongst continental geologists to omit the term Corallian altogether. According to A. de Lapparent's classification the English Corallian rocks are represented by the Séquanien stage, with two substages, an upper Astartien and lower Rauracien; but this does not include the whole Corallian stage as defined above, the lower part being placed by the French author in his Oxfordien stage. For the table showing the relative position of these stages see the article Jurassic.

See also “The Jurassic Rocks of Great Britain,” vol. i. (1892) and vol. v. (1895) (Memoirs of the Geological Survey); Blake and Huddleston, “On the Corallian Rocks of England,” Q.J.G.S. vol. xxxiii. (1877). (J. A. H.)