CORAL ISLANDS.

Otago Witness, Issue 1917, 17 August 1888, Page 35

CORAL ISLANDS.

By F. G. S. Of all the enchanting scenes wliich the pen of the tropical traveller has described, ' there is none more alluring than The Circular Coral Kccf, or Atoll, outside whose walls the waves? dash and roar in their fury, and inside whose protecting rim the pellucid water lies calm and tranquil, a very haven of rest for the ship-wrecked mariner, and one frequently made use of by the historian of adventures "on tho deep. Everyone is familiar with the fact that these coral reefs are built by the ceaseless labour of. many myriads of busy little insects ; but fe^Y possibly- have paused to think under wha,t conditions these minute organisms are capable of existing, and whence they obtain the materials for their structure. Nor must we imagine that the delicate branching coral so frequently exhibited as a drawing room ornament is a fair sample of the islands on 1 which grow palms and tropical plants of considerable magnitude. The mass of a coral island is a hard, cornpacfc, sometimes crystalline rock, suitable for permanent buildings, a,nd in many cases indistinguishable from, an ancient limestone. B,efore.,pfoceed i ingjo lv discuss the formation of coral, it is requisite to pause and inquire of what it is composed. Fortunately its composition ia so simple that we have in this inquiry to encounter no difficulty of any magnitude, ordinary limestone of our quarries, the. ma^e^ial which so substantially increases the £eit\lifcy of the soil, is simply a , chemical combination of carbonic acid and a j metal called calcium ; chemically expressed, calcic carbonate, or more usually carbonate of lime, which occurs very plentifully in nature, and forms the > substance of which coral is composed, Now calcic carbonate is slightly soluble in water, and, curiously enough, much more so in cold than in boiling water ; for while of the former only 730 parts are required, of the latter 1300 are requisite to take up one- part of the solid. This solubility is of great importance in the economy of nature, for every stream which traverses the surface of the ground takes up a certain amount of carbonate, of lime is\

solution, and bears it to the ocean. The solvent properties of rain water are much increased ty the carbonic acid which ifc obtains in its passage through the air, and thus vast quantities of carbonate of lime are borne down to the sea ; the Rhone alone has been estimated by competent authorities to carry every year sufficient to make three hundred and thirty- two thousand million oysters. Many organisms, particularly those which "inhabit the sea, have the power of secreting lime in the hard parts of their bodies, and of this action the coral insect affords a very good c? am pie, while its body is formed of soft material, its skeleton, which is outsideand forms a protecting shield, is composed of pure carbonate of lime. And now let us consider under what conditions these lowly organisms live. The old and poetic legends of ocean depths, where brilliant fishes darted iv and out among waving, bright-hued seaweeds, and the coral groves were alive with the. many -coloured inhabitants of the sea, have not survived the searching light of science. It is now a well-known fact that the coral insects cannot exist at a greater depth than 90ft to 120 ft, and that they are Killejl by Exposure to the Sun and Air. Their sphere of action is thus limited to about 100 ft vertically, and we are at once met with the query : How is ifc that if these organisms can ply their trade only in so limited a zone, we have islands, apparently of solid coral, rising from the great ocean abysses? A natural solution, prompted no doubt by the circular form of many coral islands, and one still to a great extent accepted, was that the craters of extinct volcanoes had formed the foundation of the organic superstructure ; but then another difficulty arose. How could these insects, in their limited sphere of action, build reefs many fathoms in thickness ? The late Mr Darwin, who devoted a great deal of attention to thi^ subject, and whose wonderful powers of observation caused his conclusions to be accepted with great readiness by scientific men, propounded a theory which from its apparent concurrence with all' the known ! facts, and its wonderful ingenuity, for many years satisfied at once the requirements of geologists and biologists, and received a world -wide credence. This theory, briefly stated, assumed that as the coral polyp grew and built, and died, and was succeeded by fresh geneiations who gradually raised the reef to the limit of the habitable zone, so the surface on which they had founded their colony sank gradually further, and further below the ocean, arid by this means great thicknesses of coral rock were formed. This theory was the more interesting as affording evidence of long-continued- depression extending over a great area in the Pacific Ocean j but, like many time-honoured* and ingenious hypotheses, it has at length been... to a great extent abandoned. Dr Murray, of the Challenger expedition, has given an explanation which d.oes away with the necessity for a gradually sinking sea bottom, and accounts for the great thickness of coral rock on other and more simple "rounds. The -reef, ho says, when the insects . have built to their extreme vertical limit, is subject to - • ■ Enoi'mous Action of the Waves, and the fragments of coral which are broken off are partly carried up by the water to form dry land, and are pajitly allowed to sink below the surface, there to be cemented by coral sand and calcareous mud into a foundation on which the reef is laterally extended. By this means a great thickness of rock is built up, the lower portion of which is formed of coral fragments, cemented together in a calcareous matrix, while the upper portion is in actual process of growth ? and we are not compelled to take refuge in a theory of universal subsidence, which, though : possibly correct in some instances, is nob I always admissible. 1 In order that the coral insect may thrive j and multiply ib must be supplied with plenty 1 of food, and as this is brought by the ocean currents which impinge on the outside of the reef, this portion grows and 1 extends itself on the fragments broken off by the waves, and the interior margin not only fails to increase, but is actually dissolved by the . water which washes it. We have now a circular ring of calcareous material, formed by the joint action of the waves and the polypes,' but as yet bearing no vegetation or terrestrial aniraal life, and .unfit for the habitation of m?.u. By degrees however, a change takes place. Drifted seaweeds and plants are stranded on the beach ; thrown up by some wave, they decay and form soil, on which seeds borne by the winds, or perhaps by some wandering bird, maytake root ; the fertility of the soil is ensured by the presence of lime, and as generationafrer generation of plants grow and die and rot, it becomes more plentiful and rich. Gradually the reef, so recently barren and desolate and awash afc high tide, becomesthe abode of beautiful tropical plants, and the resting place of birds and possibly of beasts. Ship -wrecked mariners may find there a refuge from the inhospitable waste of waters, and sufficient sustenance to maintain life. It is not only in these later days that tho coral insect plies his busy trade. Long, long ago, in the Palaeozoic periods of geology, wefind it written in language that cannot bemisread that the warm ocean currents carried food to millions upon millions of these-little-creatures, wliich worked and lived and died as they do to-day, and that changes in the/ conditions of the seas which they inhabited | caused them to be covered with mud or sand, and thus entombed their remains in a grave from which we may .exhume them, wonderfully preserved in all the, beautiful detail of their microscopic structure.