4-26 



NATURE 



[Feb. 28, 1889 



there is little, if any, doubt in my mind that the food supply is 

 a most important factor in relation to the growth of corals in the 

 different portions of a reef. Actual observations were made on 

 the feeding of corals at a good many places, as well as numerous 

 observations on the stomach contents. These observations have 

 been confirmed by Alexander Agassiz. 



It is as yet impossible to state in what form the lime, which is 

 secreted as carbonate in such enormous quantities by marine 

 organisms exists in the ocean. 



Dana, in " Coral and Coral Islands," considers it "unnecessary 

 to inquire whether the lime in sea-water exists as carbonate or 

 sulphate, or whether chloride of calcium takes the place of 

 these. The powers of life may take from the element present 

 whatever results the function of the animal requires." 



In connection with this question an interesting series of ex- 

 periments are being conducted at the Scottish Marine Station, 

 Granton, which go far to prove that 'the above hypothesis is 

 correct. 



The following table shows the average composition of sea- 

 water salts, the acids and bases being combined in the way 

 usually adopted by chemists — 



Average Composition of Sea-Sali. 



Chloride of sodium 

 Chloride of magnesium 

 Sulphate of magnesium 

 Sulphate of lime 

 Sulphate of potash 

 Bromide of magnesium 

 Carbonate of lime 



777S8 

 10-878 

 4737 

 3 600 

 2-465 

 0-217 

 0-345 



In the actual ocean water there are probably traces of every 

 icnown element, and it is impossible to say what is the precise 

 amount of the respective chlorides, sulphates, and carbonates 

 present. Theoretically, every base may be combined with every 

 acid, and the whole solution must be in a continual state of flux 

 as to its internal composition. While the quantity of sea-salts 

 in a given volume of water varies with position, yet it has been 

 shown by hundreds of analyses that the actual ratio of acids 

 and bases — that is, the ratio of the constituents of sea-salts — is 

 constant in waters from all regions and depths, with one very 

 significant exception— that of lime — which is present in slightly 

 greater proportion in deep water. 



The total amount of calcitini in a cubic mile of sea-water is 

 estimated at nearly 2,000,000 tons. The amount of the same 

 element present in a cubic mile of river-water is nearly 150,000 

 tons. At the rate at which rivers carry down water from the 

 land it is estimated that it would take 680,000 years to pour 

 into the ocean an amount of calcium equal to that now held by 

 the ocean in solution. 



The amount of calcium existing in the 40,000,000 square 

 miles of the typical calcareous deposits of the ocean exceeds, 

 however, that at present held in solution if we merely take them 

 to have an average thickness of 30 feet, and from 'this calcula- 

 tion we might say that, if the secretion and solution of lime in 

 the other regions of the ocean be exactly balanced, and the cal- 

 cium in the ocean remain always constant, those calcareous 

 deposits of the thickness indicated would require between 

 600,000 and 700,000 years to accumulate. There is good 

 evidence, however, that the rate of accumulation is much more 

 rapid in some positions. 



The lime thus carried down to the sea is originally derived 

 from the decomposition of anhydrous minerals, and comes from 

 the land in the form of carbonate, phosphate, and sulphate of 

 lime — the carbonate being in the greatest abundance in river- 

 water. On the other hand, the sulphate of lime very greatly 

 predominates in sea-water, the carbonates being present in small 

 quantity. We are not in a position to say whether or not the 

 coral Polyps take the whole of the material for their skeletons 

 from the carbonates, as is generally believed, or indeed to say 

 what changes take place during the progress of secretion by 

 organisms. 



In the greatest depths of the Pacific coral seas there is striking 

 evidence of the solvent power of ocean water. Our dredges 

 bring up from a depth of three or four miles over a hundred ear- 

 bones of whales and remnants of the dense Ziphioid beaks, but 

 all the larger and more areolar bones of these immense animals 



have been almost entirely removed by solution. In a single 

 haul there may also be many hundreds of sharks' teeth, some of 

 them larger than the fossil Carcharoiion teeth, but all that 

 remains of them is the hard dentine. None of the numerous 

 calcareous surface shells reach the bottom, although they are 

 quite as abundant over the red clay areas as over those 

 shallower areas, where they form Globigerina and Pteropod 

 deposits. In consequence of the small amount of detrUal 

 material reaching these abysmal areas distant from con- 

 tinents, cosmic metallic spherules, manganese nodules, highly 

 altered volcanic fragments, and zeolitic minerals, are there 

 found in great numbers. Almost all these things are found 

 occasionally in the other regions of the ocean's bed, but 

 their presence is generally masked by the accumulation of other 

 matters. In some regions Radiolarian and Diatom remains are 

 found in the greatest depths, and they too are subject to the 

 solvent power of sea-water, but to a much less extent than 

 carbonate of lime shells. 



As we ascend to shallower waters, a few fragments of the thicker- 

 shelled specimens are met with at first ; with lesser depths the 

 carbonate of lime shells increase in immber, until in the shallower 

 deposits the remains of Pteropods, Heteropods, and the most deli- 

 cate larval shells are present in the deposit at the bottom. This 

 gradation in the appearance of the shells can be well seen in a 

 series of soundings at different depths around a volcanic cone, 

 such as has been described as forming the base of a coral atoll. 

 There is no known way of accounting for this vertical distribu- 

 tion of these dead shells except by admitting that they have been 

 dissolved away in sinking through the deeper strata of water, or 

 shortly after reaching the bottom ; indeed, an examination of the 

 shells themselves almost shows the process in operation. It is 

 rare to find any trace of fish-bones in deposits other than the 

 otoliths. 



These considerations, as well as numerous experiments in the 

 laboratory, show that everywhere in the ocean dead or amorphous 

 carbonate of lime structures quickly disappear wherever they are 

 exposed to the action of sea-water, and in investigating the evolu- 

 tion of the general features of coral reefs it is as necessary to take 

 cognizance of this fact as of the secretion of carbonate of lime by 

 organisms. At the same time, too much stress cannot be laid 

 upon the fact that carbonate of lime, although markedly soluble 

 in sea-water in the amorphous form in which it exists in con- 

 nection with (organic) life, becomes practically insoluble when 

 after the death of the secreting animal it assumes the crystalline 

 state. 



In a paper read before the Royal Society of Edinburgh, em- 

 bodying some of the results of his investigations on the solubility 

 of carbonate of lime under different forms in sea-water, Mr. 

 Irvine remarks, " It is due to this molecular change that coral 

 deposits, shells, and calcareous plants are able to accumulate in 

 the ocean, ultimately to form beds of limesione rocks," 



The first stage, then, in the history of a coral island is the 

 preparation of a suitable foundation on the submerged volcanic 

 cones, or along the shores of a volcanic island, or the borders of 

 a continent. In the case of the atoll the cone may have been 

 reduced below the level of the sea by the waves and atmospheric 

 influences, or built up to the lower limit of breaker action by 

 the vast accumulation of organisms on its summit. 



A time comes, however, should the peak be situated in a 

 legion where the temperature is sufficiently high, and the surface 

 currents contain a suitable quality of food, that the reef-builders 

 fix themselves on the bank. The massive structure which they 

 secrete from ocean water enables them to build up and maintain 

 their position in the very face of ocean currents, of breakers, of 

 the overwhelming and outrageous sea.^ 



"Coral" with the sailor or marine surveyor is usually any 

 carbonate of lime shell or skeleton or their broken-down parts. 

 "Coral" is used by the naturalist in a much more restricted 

 sense : he limits the term to animals classed as Madrepores, 

 Hydrocorallines, and Alcyonarians. The animals belonging to 

 the first two of these orders comprise those included under the 

 vague term of reef corals. Besides these, however, very many 

 other classes of animals contribute to the building up of coral 



^Dr. Brougham Guppy says, " History can afford us no clue to the first 

 appearance or the age of reefs ; yet in the myths of the Pacific Islanders 

 we find that the savage inhabitants of these regions regard the history of a 

 coral atoll as commencing with -the submerged shoal, which through the 

 agency of God-like heroes is brought up by their fish-hooks to the surface." 

 —Paper, Vict. Inst. 



