472 



CORAL 



forms may be noted the blue coral (Heliopora 

 caerulea] of many Pacific coral-reefs, the horny and 

 beautifully coloured sea-fan (Gorgonia flabellum), 

 and the familiar organ-pipe coral ( Tubipora musica ). 



More important, however, in practical interest is 

 the red coral (Corallium rubrum) of commerce. In 

 this coral the limy spicules are fused to form the 

 familiar solid axis. The most important banks 

 are those of Algiers and Tunis, and off the Sar- 

 dinian, Sicilian, and Balearic islands. Great 

 quantities are fished near Naples, and at Sciacca 

 in Sicily. The coral is roughly dredged, freed 

 from its rind, assorted according to colour, and 

 manufactured in Naples, Leghorn, Genoa, &c. 

 Pale rose-coloured coral, not spoilt by the boring of 

 worms and other intruders, has fetched 12, 10s. 

 per ounce. The industry is a very important one, 

 though in recent years it has seriously declined. 

 The most important markets are in Asia, Africa, 

 parts of Russia, Japan, and South America. 



CORAL ISLANDS is the name given to certain 

 low islets which are composed for the most part of 

 the calcareous skeletons of various kinds of corals. 

 An atoll or typical coral island consists of a some- 

 what ring-shaped reef inclosing a lagoon. Such reefs 

 vary in size from less than a mile up to 90 miles 

 long and may be 10 miles wide the breadth of the 

 annular reef being on an average about a quarter 

 of a mile. Barrier-reefs, which have a similar 

 breadth, are found surrounding islands or bordering 

 mainlands, from which they are separated by a 

 navigable channel. Fringing-reefs, on the other 

 liana, extend outwards from the shore, with no 

 separating lagoon-channel. In the case of atolls 

 and barrier-reefs there is generally a navigable 

 passage through the reef into the lagoon or lagoon- 

 channel, which is kept open by the scour of the tide. 

 It is upon atolls or barrier-reefs that islands occur. 

 Only very rarely, however, are these islands co- 

 extensive with the reef generally they appear as 

 longer or shorter belts, forming a series of islets 

 straggling along its surface. As reef- building corals 

 do not flourish in water having a lower temperature 

 than 68 F., they are necessarily restricted to tropi- 

 cal and subtropical seas from many regions of 

 which, however, they are excluded by the presence 

 of cold currents coming from extra-tropical lati- 

 tudes, by the muddy character of the water oppo- 

 site the mouths of large rivers, and by other causes. 



The rock of a coral-reef is a white limestone com- 

 posed chiefly of masses of corals, coral debris, and 

 sand, together with the hard parts of molluscs, 

 echinoderms, bryozoans, and calcareous alga?. Not 

 infrequently this rock has been changed by the 

 chemical action of percolating water into a compact 

 or crystalline mass, which has lost all, or nearly 

 all, trace of its organic structure. Chamisso (q.v., 

 known also as a poet), the naturalist who accom- 

 panied Kotzebue the Russian navigator in his 

 voyage of discovery into the South Sea in 1815-18, 

 appears to have been the first to show how such 

 reefs are converted into dry land. He describes 

 how the polyps cease to grow when they reach 

 the surface of the sea, and how the reef, exposed 

 at low-tide, by and by becomes disintegrated, 

 while broken coral debris is heaped up by the 

 action of the breakers so as eventually to form 

 a high bank which can only be covered during 

 spring-tides. This bank of coral blocks and debris 

 solidifies in time, and remains exposed to the fierce 

 heat of the sun, so that the mass shrinks and 

 cracks, and flakes are detached from it, and raised 

 one upon another by each returning tide. ' The 

 always active surf throws blocks of coral (frequently 

 of a fathom in length and 3 or 4 feet thick) and 

 shells of marine animals between and upon the 

 foundation-stones ; after this the calcareous sand 

 lies undisturbed, and offers to the seeds of trees 



and plants cast upon it by the waves a soil upon 

 which they rapidly grow to overshadow its dazzling 

 white surface. Entire trunks of trees, which are 

 carried by the rivers from other countries and 

 islands, find here at length a resting-place after 

 their long wanderings : with these come small 

 animals, such as lizards and insects, as the first 

 inhabitants. Even before the trees form a wood, 

 the real sea-birds nestle here ; strayed land-birds 

 take refuge in the bushes ; and at a much later 

 period, when the work has been long since com- 

 pleted, man also appears, builds his hut on the 

 fruitful soil formed by the corruption of the leaves 

 of the trees, and calls himself lord and proprietor 

 of this new creation.' According to Professor L.^ 

 Agassiz, the large blocks torn off by the breakers 

 are loosened not by shrinkage under the sun's heat, 

 as Chamisso supposed, but by the innumerable 

 perforations of various boring molluscs. 



The origin of coral-reefs is a question which has- 

 at various times given rise to discussion. Accord- 

 ing to Chamisso, the corals commenced to grow on 

 shoals in the sea on the tops of submarine moun- 

 tains in short. The circular form of the atoll and 

 its basin-shaped lagoon he believed to be due to the 

 natural growth of the coral. The most vigorous 

 growth took place upon the outward edges of the 

 reef, where the largest and most massive corals 

 flourished. In the interior the coral growth was 

 hindered by the accumulation of shells, coral 

 debris, and sand swept forward by the waves. The 

 reef, therefore, as it approached the surface would 

 gradually assume a basin-shape the outer edges, 

 or peripheral and actively growing portion, forming 

 the reef the interior part, where growth was 

 retarded or prevented by sediment, &c., forming 

 the lagoon. Other writers attempted to account 

 for the annular form of atolls by supposing that the 

 reefs had grown upwards from the crests of sub- 

 marine volcanic craters. The not infrequent occur- 

 rence of volcanoes and volcanic islands in the coral 

 regions of the Pacific Ocean was thought to favour 

 this supposition, but the enormous diameter of 

 many atolls seemed to preclude the possibility that 

 these larger atolls, at all events, could be founded on 

 the lips of submarine craters. And when at length 

 it was discovered that reef-building corals flourish 

 only in comparatively shallow water, this hypo- 

 thesis was rejected, since it was extremely unlikely 

 that so many submarine volcanic mountains should 

 rear their summits just to the limits within which 

 the corals could begin their superstructures. 



Darwin, during the famous voyage of the 

 Beagle, was naturally fascinated by the problem of 

 the origin of these remarkable reefs, and while he 

 admitted that certain atolls might very likely have 

 been formed in the manner described by Chamiseo,, 

 yet he could not believe it possible that a broad 

 mountain -summit lay buried at the depth of a few 

 fathoms beneath every atoll, and nevertheless that 

 throughout the immense areas occupied by many 

 of those reefs not one point of rock should project 

 above the surface. The theory of subsidence which 

 he subsequently advanced seemed to account for 

 all the phenomena, and until recently it com- 

 manded nearly universal assent. According to 

 Darwin, each atoll has passed through the succes- 

 sive stages of fringing-reef and barrier-reef. Since 

 reef-building corals do not thrive at greater depths 

 than 100 feet or thereabout, it is evident that the 

 foundations of a coral reef could not have been laid 

 in deeper water. And as such moderate depths 

 occur only round islands and off the shores of con- 

 tinents, the reef-builders would begin their work 

 by forming at first a fringing-reef. Slow subsid- 

 ence of the sea-bottom is supposed to have super- 

 vened ; but while the foundations were being carried 

 down, the corals continued to grow upwards, the 



