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:—
| Kimeridge Clay |
Corallian Rock |
“Coralline Oolite” |
Upper Calcareous Grit Coral Rag and Upper Limestone Middle Calcareous Grit |
A. plicatilis. | |||
| Oxford Clay |
Lower Limestone Passage Beds Lower Calcareous Grit |
A. perarmatus. |
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.)
CORAL-REEFS. Many species of coral (q.v.) are widely
distributed, and are found at all depths both in warmer and colder
seas. Lophohelia prolifera and Dendrophyllia ramea form dense
beds at a depth of from 100 to 200 fathoms off the coasts of
Norway, Scotland and Portugal, and the “Challenger” and other
deep-sea dredging expeditions have brought up corals from great
depths in the Pacific and Atlantic oceans. But the larger
number of species, particularly the more massive kinds, occur
only in tropical seas in shallow waters, whose mean temperature
does not fall below 68° Fahr., and they do not flourish unless
the temperature is considerably higher. These conditions of
temperature are found in a belt of ocean which may roughly be
indicated as lying between the 28th N. and S. parallels. Within
these limits there are numerous reefs and islands formed of coral
intermixed with the calcareous skeletons of other animals, and their formation has long been a matter of dispute among naturalists and geologists.
Coral formations may be classed as fringing or shore reefs, barrier reefs and atolls. Fringing reefs are platforms of coral rock extending no great distance from the shores of a continent or island. The seaward edge of the platform is usually somewhat higher than the inner part, and is often awash at low water. It is intersected by numerous creeks and channels, especially opposite those places where streams of fresh water flow down from the land, and there is usually a channel deep enough to be navigable by small boats between the edge of the reef and the land. The outer wall of the reef is rather steep, but descends into a comparatively shallow sea. Since corals are killed by fresh water or by deposition of mud or sand, it is obvious that the outer edge of the reef is the region of most active coral growth, and the boat channel and the passages leading into it from the open sea have been formed by the suppression of coral growth by one of the above-mentioned causes, assisted by the scour of the tides and the solvent action of sea-water. Barrier reefs may be regarded as fringing reefs on a large scale. The great Australian barrier reef extends for no less a distance than 1250 m. from Torres Strait in 9.5° S. lat. to Lady Elliot island in 24° S. lat. The outer edge of a barrier reef is much farther from the shore than that of a fringing reef, and the channel between it and the land is much deeper. Opposite Cape York the seaward edge of the great Australian barrier reef is nearly 90 m. distant from the coast, and the maximum depth of the channel at this point is nearly 20 fathoms. As is the case in a fringing reef, the outer edge of a barrier reef is in many places awash at low tides, and masses of dead coral and sand may be piled up on it by the action of the waves, so that islets are formed which in time are covered with vegetation. These islets may coalesce and form a strip of dry land lying some hundred yards or less from the extreme outer edge of the reef, and separated by a wide channel from the mainland. Where the barrier reef is not far from the land there are always gaps in it opposite the mouths of rivers or considerable streams. The outer wall of a barrier reef is steep, and frequently, though not always, descends abruptly into great depths. In many cases in the Pacific Ocean a barrier reef surrounds one or more island peaks, and the strips of land on the edge of the reef may encircle the peaks with a nearly complete ring. An atoll is a ring-shaped reef, either awash at low tide or surmounted by several islets, or more rarely by a complete strip of dry land surrounding a central lagoon. The outer wall of an atoll generally descends with a very steep but irregular slope to a depth of 500 fathoms or more, but the lagoon is seldom more than 20 fathoms deep, and may be much less. Frequently, especially to the
