Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/408

Rh 378 C 11 A L 8 but reefs are also found more or less largely developed in the Indian Ocean, the Persian Gulf, the Red Sea, the coasts of Zanzibar, Madagascar, and Mauritius, the Gulf of Panama, the coast of Brazil, around the West Indian Islands and the shores of Florida, and around the Bermudas. According to the classification of Darwin, which is essen tially the same as that adopted by other authorities, coral- reefs may be separated into three principal groups, viz. : Fringing-reefs, Barrier-reefs, and Atolls. Fringing-reefs are shallow- water reefs, found in the immediate neighbour hood of land, either surrounding islands or skirting the shores of continents. These shore-reefs have no channel of any great depth of water intervening between them and the land, and the soundings on their sea-ward margin indicate that they repose upon a gently-sloping surface. Barrier-reefs, like the preceding, may either encircle islands or skirt continents. They are distinguished from Fringing-reefs by the fact that they are placed at a much greater distance from the land, that there intervenes a channel of comparatively deep water between them and the shore, and that soundings taken close to their seaward mar gin indicate profound depths of water outside them. The Barrier-reefs which surround islands are termed &quot; encircl ing Barrier- reefs,&quot; and they occasionally form a complete ring, though more usually discontinuous and broken at intervals. The Barrier-reefs which skirt continents attain a greater size. As an example of these may be taken the succession of reefs which form the great &quot; barrier &quot; on the north-east coast of Australia. These run, with occasional breaches in their continuity, for a distance of over 1000 miles, their average distance from the shore being between 20 and 30 miles, the depth of the inner channel being from 10 to 60 fathoms, and the sea outside being sometimes over 2000 feet in depth. Atolls are ring-shaped reefs usually oval or circular in form, which enclose a central expanse of water or lagoon, without any land. Occasionally (as in &quot;Whitsunday Island) the entire circle of the Atoll may have been raised above the water. More commonly the ring is not complete, but is breached by one or more openings, which are always placed on the leeward side of the Atoll, or on the side most com pletely sheltered from the prevailing winds. In their struc ture Atolls are identical with &quot; encircling Barrier-reefs,&quot; from which they differ only in the fact that the lagoon which they enclose does not contain an island in its centre. Many coral-reefs are constantly submerged below the sea, and are not laid bare even at extreme low water ; others are exposed to view by the recession of the tide, and are covered at high water ; others, again, are partially raised above the level of the highest tides, and thus constitute dry land. If we examine a reef of the last class say a portion of an Atoll or an encircling Barrier-reef the fol lowing are the general phenomena which may be noticed. The general form of the reef is approximately triangular, as seen, in section, with a steep and abrupt seaward face, and a long and gentle slope towards the inner lagoon or channel. The extreme outer margin of the reef is the only portion of the whole which is composed of actually living coral, and this part is not exposed to view even at extreme low water. Soundings outside this line always indicate a more or less considerable depth of water, and the outer margin of the reef is usually exposed to the beating of a tremendous surf, in which the coral-polypes find their most congenial home. Immediately inside the line of breakers is a broader or narrower platform of dead coral and coral- rock, which is only laid bare at low water, and which may be bounded internally by a ledge of brecciated coral-rock only reached by the waves at high water. Finally, the inner portion of the reef rises to the height of a few feet above the level of high water, and constitutes dry land. It is composed of blocks of coral more or less completely cemented together by the percolation of water holding car bonate of lime in solution, along with blown sand derived from the disintegration of the coral. The land generally Flo. 9. Section of Keeling Atoll. (After Darwin.) a, Level of the sea at low water; 6, outer edge of that flat part of the reef which dries at low water; c, flat of coral-rock, covered at high water; d, low pro jecting ledge of brecciated coral-rock, washed by the waves at high water; e, slope of loose fragments reached by the sea only during gales (the upper part, which is from C to 12 feet high, is clothed with vegetation ; the surface of tha islet slopes gently to the lagoon) ; f, level of the lagoon at low water. bears a luxuriant vegetation, and slopes with a prolonged and gentle inclination to the inner lagoon. The beach of the Atoll is covered with coral-sand, and with fragments of coral, which are often cemented together by the percolation of water so as to form compact oolitic or brecciated lime stones. The bottom of the inner lagoon usually supports many living corals, along with accumulations of fine chalky mud, apparently largely formed from the excreta of animals, which, like the Scari and Holothurians, feed upon the living corals. Outside the reef, at depths greater than 25 fathoms, the bottom seems to be covered with coral-sand and dead coral. The general method of formation of a coral-reef becomes readily intelligible on a consideration of the conditions which are requisite for the existence and welfare of the coral-polypes. The reef-building corals, in the first place, flourish most vigorously in depths up to 10 fathoms, and appear to be incapable of existing at all at depths exceeding 25, or at the utmost 30 fathoms. It follows from this that no coral-reef can begin to be formed on a sea-bottom covered by more than 30 fathoms of water. In the case, however, of Atolls and Barrier-reefs, we have reefs rising out of profound depths, soundings on their seaward margin indicating depths of from 100 up to more than 1000 fathoms, at points not far removed from the actual edge of the reef. Originally it was believed that the reef had been raised from these great depths to the surface by the exertions of the polypes themselves ; but the extremely limited bathymetrical range of these animals renders this view wholly untenable. The true explanation of this problem was first afforded by the masterly researches of Mr Darwin, who showed that the production of Barrier-reefs and Atolls is really to be ascribed to the subsidence by slow degrees of the foundations on which they rest. Thus a Fringing-reef surrounding an island may be formed in depths of from 10 to 15 fathoms, and may grow till it reaches the level of low water. If, now, such a reef be supposed to sink gradually beneath the sea by a sufficiently slow subsidence, the upward growth of the corals will neutralize the downward movement of the land, so that the reef will appear to be stationary, whilst it is really growing upwards. Whilst the reef will remain to all appearance unaffected in its form, position, and size, the island which it surrounds will gradually diminish in size as the subsidence goes on, and a wide and deep channel will be formed between it and the reef. If the depression should be continued still further, the island will be reduced to a mere peak in the centre of a lagoon, and the reef, from a &quot; Fringing-reef, will have become converted into an &quot; encircling Barrier-reef.&quot; Simultaneously, we should find that there is now deep water all round the reef, on its