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CORALLIAN (Fr. Corallun)
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See also:CORALLIAN (Fr. Corallun) , in See also:geology, the name of one of the divisions of the See also:Jurassic rocks . The rocks forming this See also:division are mainly calcareous grits with oolites, and rubbly See also:coral See also:rock—often called "Coral Rag"; ferruginous beds are fairly See also:common, and occasionally there are beds of See also:clay . In See also:England the See also:Corallian strata are usually divided into an upper See also:series, characterized by the ammonite Perisphinctes plicatilis, and a See also:lower series with Aspidoceras perarmatus as the zonal fossil . When well See also:developed these beds are seen to See also:lie above the See also:Oxford Clay and below the Kimeridge Clay; but it will See also:save a See also:good See also:deal of confusion if it is recognized that the Corallian rocks of England are nothing more than a variable, See also:local lithological phase of the two See also:clays which come respectively above and below them . This caution is particularly necessary when any See also:attempt is being made to co-See also:ordinate the See also:English with the See also:continental Corallian . The Corallian rocks are nowhere better displayed than in the cliffs at See also:Weymouth . Here Messrs See also:Blake and Huddleston recognized the following beds: Upper Coral Rag and Abbotsbury See also:Iron Ore . Sandsfoot Grits . Sandsfoot Clay . Trigonia Beds . Osmington Oolite (quarried at Marnhull and Todbere) . Bencliff Grits .
Lower Corallian j Not he Clay
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Nothe Grit
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In See also:Dorsetshire the Corallian rocks are 200 ft. thick, in See also:Wiltshire See also:ioo ft., but N.E. of Oxford they are represented mainly by clays, and the series is much thinner
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(At Upware, the " Upware See also:limestone " is the only known occurrence of beds that correspond in See also:character with the Coralline oolite between Wiltshire and See also:Yorkshire)
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In Yorkshire, however, the hard rocky beds come on again in full force
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They appear once more at Brora in See also:Sutherlandshire
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Corallian strata have been proved by See also:boring in See also:Sussex (241 ft.)
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In See also:Huntingdon, See also:Bedfordshire, parts of See also:Buckinghamshire, See also:Cambridgeshire and See also:Lincolnshire the Corallian series is represented by the "Ampthili Clay," which has also been called " Bluntesham " or " Tetworth" Clay
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Here and there in this See also:district hard calcareous inconstant beds appear, such as the Elsworth rock, St Ives rock and Boxworth rock
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In Yorkshire the Corallian rocks differ in many respects from their See also:southern equivalents
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They are subdivided as follows:
c Upper Calcareous Grit
Kimeridge 8 Coral Rag and Upper See also:Lime-
Clay a - I See also: Among the fossils in the English Corallian rocks See also:corals play an important part, frequently forming Iarge calcareous masses or " daggers "; Thamnastrea, Thecosmilia and Isastrea are prominent genera . See also:Ammonites and belemnites are abundant and gasteropods are very common (Nerinea,Chemnitzia, Bourgetia, &c.) . Trigonias are very nufaserous in certain beds (T. perlata and T. mariani) . See also:Astarte ovata, See also:Lucina aliena and other pelecypods are also abundant . The echinoderms Echinobrissus scutatus and Cidaris florigemma are characteristic of these beds . Rocks of the same See also:age as the English Corallian are widely spread over See also:Europe, but owing to the See also: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 See also:term Corallian altogether . According to A. de See also:Lapparent's See also:classification the English Corallian rocks are represented by the Sequanien See also: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 See also:French author in his Oxfordien stage . For the table showing the relative position of these stages see the See also:article JuxnsslC . See also " The Jurassic Rocks of See also:Great See also:Britain," vol. i . (1892) and vol. v . (1895) (See also:Memoirs of the See also:Geological Survey) ; Blake and Huddleston, " On the Corallian Rocks of England,' Q.J.G.S. vol. xxxiii . (1877) . (J . A . H.) CORAL-REEFS . Many See also:species of coral (q.v.) are widely distributed, and are found at all depths both in warmer and colder seas . Lophohelia prolif era and Dendrophyllia ramea See also:form dense beds at a See also:depth of from See also:loo to zoo fathoms off the coasts of See also:Norway, See also:Scotland and See also:Portugal, and the " Challenger," and other deep-See also:sea dredging expeditions have brought up corals from great depths in the Pacific and See also:Atlantic oceans . But the larger number of species, particularly the more massive kinds, occur only in tropical seas in shallow See also: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 See also:belt of ocean which may roughly be indicated as lying between the 28th N. and S. See also: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 See also:long been a See also:matter of dispute among naturalists and geologists . Coral formations may be classed as fringing or See also:shore reefs, barrier reefs and atolls . Fringing reefs are platforms of coral rock extending no great distance from the shores of a See also:continent or See also:island . The seaward edge of the See also:platform is usually somewhat higher than the inner part, and is often awash at See also:low See also:water . It is intersected by numerous creeks and channels, especially opposite those places where streams of fresh water flow down from the See also:land, and there is usually a channel deep enough to be navigable by small boats between the edge of the See also:reef and the land . The See also: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 See also:sand, it is obvious that the outer edge of the reef is the region of most active coral growth, and the See also: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 See also:action of sea-water . Barrier reefs may be regarded as fringing reefs on a large See also:scale . The great Australian barrier reef extends for no less a distance than 1250 M. from Torres Strait in 9.5° S. See also:lat. to See also: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 See also:York the seaward edge of the great Australian barrier reef is nearly 90 M. distant from the See also:coast, and the maximum depth of the channel at this point is nearly 20 fathoms . As is the See also: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 See also:time are covered with vegetation . These islets may coalesce and form a See also:strip of dry land lying some See also: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 See also: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 See also:complete See also:ring . ' An See also:atoll is a ring-shaped reef, either awash at low See also:tide or surmounted by several islets, or more rarely by a complete strip of dry land surrounding a central See also: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 Upper Corallian Clay A. plicatilis . A. perarmatus . leeward See also:side of an atoll, there may be one or more navigable passages leading from the lagoon to the open sea . Though corals flourish everywhere under suitable conditions in tropical seas, coral reefs and atolls are by no means universal in the torrid See also:zone . The Atlantic Ocean is remarkably See also:free from coral formations, though there are numerous reefs in the See also:West See also:Indian islands, off the See also:south coast of See also:Florida, and on the coast of See also:Brazil . The See also:Bermudas also are coral formations, their high land being formed by sand accumulated by the See also:wind and cemented into rock, and are remarkable for being the farthest removed from the See also:equator of any See also:recent reefs, being situated in 32° N. lat . In the Pacific Ocean there is a vast See also:area thickly dotted with coral formations, extending from 5° N. lat. to 25° S. lat., and from 1300 E. long. to 145° W. long . There are also extensive reefs in the westernmost islands of the Hawaiian See also:group in about 25° N. lat . In the Indian Ocean, the Laccadive and Maldive islands are large See also:groups of atolls off the west and south-west of See also:India . Still farther south is the See also:Chagos group of atolls, and there are numerous reefs off the north coast of See also:Madagascar, at See also:Mauritius, See also:Bourbon and the See also:Seychelles . The Cocos-Keeling Islands, in 12° S. lat. and 96°E. long., are typical atolls in the eastern part of the Indian Ocean . The remarkable charactersof barrier reefs and atolls, their isolated position in the midst of the great oceans the seemingly unfathomable depths from which they rise their peaceful and shallow lagoons and inner channels, their narrow strips of land covered with coco-See also:nut palms and other vegetation, and rising but a few feet above the level of the ocean, naturally attracted the See also:attention of the earlier navigators, who formed sundry speculations as to their origin . The poet-naturalist, A. von See also:Chamisso, was the first to propound a definite theory of the origin of atolls and encircling reefs, attributing their See also:peculiar features to the natural growth of corals and the action of the waves . He pointed out that the larger and more massive species of corals flourish best on the outer sides of a reef, whilst the more interior corals are killed or stunted in growth by the See also:accumulation of coral and other debris . Thus the outer edge of a submerged reef is the first to reach the See also:surface, and a ring of land being formed by materials piled up by the waves, an atoll with a central lagoon is produced . Chamisso's theory necessarily assumed the existence of a great number of submerged See also:banks reaching nearly, but not quite, to the surface of the sea in the Pacific and Indian oceans, and the difficulty of accounting for the existence of so many of these led C . See also:Darwin to reject his views and bring forward an explanation which may be called the theory of subsidence . Starting from the well-known premise that reef-building species of corals do not flourish in a greater depth of water than 20 fathoms, Darwin argued that all A: f l"A See also:Diagram showing the formation of an atoll during subsidence . (After C . Darwin.) The lower part of the figure represents a barrier reef surrcunding a central See also:peak . A,A, outer edges of the barrier reef at the sea-level ; the coconut trees indicate dry land formed on the edges of the reef . L L, lagoon channel . A',A', outer edges of the atoll formed by upgrowth, of the coral during the subsidence of the peak . L', lagoon of the atoll . The See also:vertical scale is considerably exaggerated as compared with the See also:horizontal scale . coral islands must have a rocky See also:base, and that it was inconceivable that, in such large tracts of sea as occur in the Pacific and Indian oceans, there should be a vast number of submarine peaks or banks all rising to within 20 or 30 fathoms of the surface and none emerging above it . But on the supposition that the atolls and encircling reefs were formed See also:round land which was' undergoing a slow See also:movement of subsidence, their structure could easily be explained . Take the case of an island consisting of a single high peak . At first the coral growth would form a fringing reef clinging to its shores . As the island slowly subsided into the ocean the upward growth of coral would keep the outer rim of the reef level with or within a few fathoms of the surface, so that, ' as subsidence proceeded, the distance between the outer rim of the reef and the sinking land would continually increase, with the result that a barrier-reef would be formed separated by a wide channel from the central peak . As corals and other organisms with calcareous skeletons live in the channel, their remains, as well as the accumulation of coral and other debris thrown over the outer edge of the reef, would maintain the channel at a shallower depth than that of the ocean outside . Finally, if the subsidence continued, the central peak would disappear beneath the surface, and an atoll would be See also:left consisting of a raised margin of reef surrounding a central lagoon, and any pause during the movement of subsidence would result in the formation of raised islets or a strip of dry land along the margin of the reef . Darwin's theory was published in 1842, and found almost universal See also:acceptance, both because of its simplicity and its applicability to every known type of coral-reef formation, including such difficult cases as the Great Chagos See also:Bank, a huge submerged atoll in the Indian Ocean . Darwin's theory was adopted and strengthened by J . D . 
See also:Dana, who had made extensive observations among the Pacific coral reefs between 1838 and 1842, but it was not long before it was attacked by other observers
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In 1851 See also: Thus the reef will rise to the surface as an atoll, and the nearer it approaches the surface the more will the corals on the exterior faces be favoured, and the more will those in the centre of the reef decrease, for experiment has shown that the minute pelagic organisms on which corals feed are far less abundant in a lagoon than in the sea outside . Eventually, as the margin of the reef rises to the surface and material is accumulated upon it to form islets or continuous land, the coral growth in the lagoon will be feeble, and the solvent action of sea-water and the scour of the tide will tend to deepen the lagoon . Thus the considerable depth of some lagoons, amounting to 40 or 5o fathoms, may be accounted for . The observations of Guppy in the See also:Solomon islands have gone far to confirm Murray's conclusions, since he found in the islands of Ugi, See also:Santa See also:Anna and See also:Treasury and See also:Stirling islands unmistakable evidences of a See also:nucleus of volcanic rock, covered with soft earthy bedded deposits several hundred feet thick . These deposits are highly fessiliferous in parts, and contain the remains of pteropods, lamellibranchs and echinoderms, embedded in a foraminiferous See also:deposit mixed with volcanic debris, like the deep-sea muds brought up by the "Challenger." The flanks of these elevated beds are covered with coralline limestone rocks varying from too to 16 ft. in thickness . One of the islands, Santa Anna, has the form of an upraised atoll, with a mass of coral limestone 8o ft. in vertical thickness, resting on a friable and sparingly argillaceous rock resembling a deep-sea deposit . A . Agassiz, in a number of important researches on the Florida reefs, the See also:Bahamas, the Bermudas, the See also:Fiji islands and the Great Barrier Reef of See also:Australia, has further shown that many of the peculiar features of these coral formations cannot be explained on the theory of subsidence, but are rather attributable to the natural growth of corals on banks formed by prevailing currents, or on extensive shore platforms or submarine flats formed by the erosion of pre-existing land surfaces . In See also:face of this accumulated evidence, it must be admitted that the subsidence theory of Darwin is inapplicable to a large number of coral reefs and islands, but it is hardly possible to assert, as Murray does, that no atolls or barrier reefs have ever been developed after the manner indicated by Darwin . The most recent See also:research on the structure of coral reefs has also been the most thorough and most convincing . It is obvious that, if Murray's theory were correct, a See also:bore hole sunk deep into an atoll would pass through some too ft. of coral rock, then through a greater or less thickness of argillaceous rock, and finally would penetrate the volcanic rock on which the other materials were deposited . If Darwin's theory is correct, the boring would pass through a great thickness of coral rock, and finally, if it went deep enough, would pass into the See also:original rock which subsided below the waters . An expedition sent out by the Royal Society of See also:London started in 1896 for the island of Funafuti, a typical atoll of the See also:Ellice group in the Pacific Ocean, with the purpose of making a deep boring to test this question . The first attempt was not successful, for at a depth of 105 ft. the refractory nature of the rock stopped further progress . But a second attempt, under the management of See also:Professor See also:Edgeworth See also:David of See also:Sydney, proved a complete success . With improved apparatus, the boring was carried down to a depth of 697 ft . (116 fathoms), and a third attempt carried it down to 1114 ft . (185 fathoms) . The boring proves the existence of a mass of pure limestone of organic origin to the depth of 1114 ft., and there is no trace of any other rock . The organic remains found in the core brought up by the See also:drill consist of corals, See also:foraminifera, calcareous See also:algae and other organisms . A boring was also made from the See also:deck of a See also:ship into the See also:floor of the lagoon, which shows that under roo ft. of water there exists at the bottom of the lagoon a deposit more than too ft. thick, consisting of the remains of a calcareous alga, Halimeda See also:opuntia, mixed with abundant foraminifera . At greater depths, down to 245 ft., the same materials, Mixed with the re-mains of branching madrepores, were met with, and further progress was stopped by the existence of solid masses of coral, fragments of porites, madrepora and heliopora having been brought up in the core . These are shallow-water corals, and their existence at a depth of nearly 46 fathoms, buried beneath a mass of Halimeda and foraminifera, is clear evidence of recent subsidence . Halimeda grows abundantly over the floor of the lagoon of Funafuti, and has been observed in many other lagoons . The writer collected a quantity of it in the lagoon of Diego See also:Garcia in the Chagos group . The boring demonstrates that the lagoon of Funafuti has been filled up to an extent of at least 245 ft . (nearly 41 fathoms), and this fact accords well with Darwin's theory, but is incompatible with that of Murray . In the See also:present See also:state of our knowledge it seems reasonable to conclude that coral reefs are formed wherever the conditions suitable for growth exist, whether in areas of subsidence, See also:elevation or See also:rest . A considerable number of reefs, at all events, have not been formed in areas of subsidence, and of these the Florida reefs, the Bermudas, the Solomon islands, and possibly the Great Barrier Reef of Australia are examples . Funafuti would appear to have been formed in an area of subsidence, and it is quite probable that the large groups of low-lying islands in the Pacific and Indian oceans have been formed under the same conditions . At the same time, it must be remembered that the atoll or barrierreef shape is not necessarily evidence of formation during subsidence, for the observations of Karl Semper, A . Agassiz, and Guppy are sufficient to prove that these forms of reefs may be produced by the natural growth of coral, modified by the action of waves and currents in regions in which subsidence has certainly not taken See also:place . See A . Agassiz, many publications in the Mem . Amer . Acad . (1883) and See also:Bull . See also:Mus . Comp . Zool . (Harvard, 1889—1899) ; J . D . Dana, Corals and Coral Islands (1853; 2nd ed., 1872; 3rd ed., 189o) ; C . Darwin, The Structure and See also:Distribution of Coral Reefs (3rd ed., 1889) ; H . B . Guppy, " The Recent Calcareous Formations of the Solomon Group," Trans . See also:Roy . See also:Soc . Edinb. xxxii . (1885) ; R . See also:Langenbeck, " See also:Die neueren Forschungen fiber die Korallenriffe," See also:Hettner geogr . Zeitsch. iii . (1897) ; J . Murray, " On the Structure and Origin of Coral Reefs and Islands," Proc . Roy . Soc . Edinb. x . (1879–188o) ; J . Murray and See also:Irvine, " On Coral Reefs and other Carbonate of Lime Formations in See also:Modern Seas," Proc . Roy . Soc . Edinb . (1889) ; W . See also:Savile See also:Kent, The Great Barrier Reef of Australia (London, W . H . See also:Allen & Co., 1893) ; Karl Semper, See also:Animal See also:Life, Internat . Sci . Series," vol. xxxi . (188,); J . S . Gardiner, Nature, lxix . 371 . (G . C . |
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