414 



NATURE 



{March i, 1888 



and right, though it may take millions of years to prove that 

 right is right. I have the same faith in Nature ; and, taking my 

 stand on this scientific faith, I believe that natural selection 

 must in the end prove rational selection, and that what has 

 vaguely been called the survival of the fittest will have to be 

 interpreted in the end as the triumph of reason, not as the mere 

 play of chance. F. Max Muller. 



Oxford, February 21. 



"Coral Formations." 



Captain Wharton's paper on coral formations in last 

 week's Nature (p. 393) will have been read with great interest 

 by all who have examined and studied coral reefs. It is unlikely 

 that any objections will be raised to the illustrations he has 

 brought forward of how the coral plantations may be built up 

 from deeply submerged banks, and eventually formed into com- 

 plete atolls and barrier reefs at a great distance from continental 

 and other shores. The mode of formation has been dwelt upon 

 by Le Conte and Guppy in the case of barrier reefs, and I have 

 pointed out the same thing in my remarks about the Maldlve and 

 similar atoll groups. The instances cited by Captain Wharton 

 are of great value, especially as he has been able to consult large 

 manuscript plans. 



Captain Wharton apparently considers that the solution of 

 carbonate of lime by sea-water plays no important part in deepen- 

 ing, widening, and modifying the form of such atolls and barrier 

 reefs ; in this I cannot agree with him. 



By reference to what is now taking place in Nature, as well as 

 to experiments conducted in the laboratory, it has been shown 

 that the solution of the carbonate of lime of dead shells and 

 skeletons by the sea is as constant and universal as its secretion 

 by the living organisms. From some considerations which I 

 recently laid before the Royal Society of Edinburgh, it is prob- 

 able that there is moi-e secretion and deposition of carbonate of 

 lime in the ocean, as a whole, than removal by solution, and it 

 is almost certain that at the present time there is a vast accumu- 

 lation of carbonate of lime going on within the coral-reef regions 

 of the ocean. The amount of secretion becomes less with in- 

 creasing depth beyond one hundred fathoms, and laboratory 

 expei-iments under great pressures have shown that the rate of 

 solution becomes greater with increasing depth ; but both 

 processes are always in action wherever there are life and growth, 

 death and decay. In some regions secretion is in excess, and 

 there is a formation of calcarous deposits ; in others solution is 

 equal to secretion, as over the red clay areas of the ocean. 

 Again, solution may be in excess of secretion, as in the larger 

 and more perfect coral lagoons. The role of carbonate of lime 

 in the ocean may not inaptly be compared to that of aqueous 

 vapour in the atmosphere over land surfaces. Where precipita- 

 tion is in excess of evaporation, fresh-water lakes are formed, and 

 rivers carry the surplus water down to the ocean ; where evapora- 

 tion is in excess, there is a formation of inland drainage areas, 

 deserts, and salt lakes. 



In small coral atolls the periphery is large relatively to the 

 size of the lagoon, and the secretion of lime and formation of 

 coral sand are greatly in excess of the solution that takes place, 

 hence the lagoon becomes filled up ; in it are frequently found 

 deposits of sulphate of lime, guano, magnesian and phosphatic 

 rocks. On the other hand, when a coaiparatively large atoll 

 reaches the surface, the periphery being small relatively to the 

 size of tlie lagoon, there is less secretion and formation of coral 

 sand by the living outer surface than is removed in solution from 

 the lagoon ; it is in consequence widened, deepened, and re- 

 duced to a more or less uniform appearance, while the islands 

 on such reefs never, so far as I know, contain deposits of sulphate 

 of lime, guano, magnesian or phosphatic rocks. On open 

 banks, such as the Macclesfield and Tizard Banks, the coral 

 sand is generally largely made up of bottom-living Foraminifera, 

 Polyzoa, Serpulae, and Calcareous Algse, and the bank may be 

 rising from the secretions of these organisms ; but when the peri- 

 pheral reefs reach the surface the conditions become more or less 

 inimical to vigorous growth, and in a perfect atoll the fine 

 calcareous mud is removed at a relatively rapid rate. 



My answer to Captain Wharton's question is that in all normal 

 conditions the extent of surface in the shell, coral, or fragment 

 of coral sand exposed to the action of sea- water compared with 

 the mass determines the rate at which these organisms will dis- 

 appear in solution. It is improbable that this action is extremely 

 slow at the bottom of the deep lagoons. Independently of the 



mixing by convection currents, even a very slight wind over the 

 surface of the lagoon will set the whole water in motion. This 

 is clearly shown by my observations in the western lochs of 

 Scotland, which are much deeper than any lagoon ; a moderate 

 breeze produces motion at a depth of sixty fathoms in a very 

 short space of time. The water mixed up with the mud at the 

 bottom is thus changed long before the point of saturation is 

 reached. 



I have never seen any wide extent of fringing reef but what was 

 very deeply cut upwith channels,and from Captain Wharton's own 

 description this appears to be the case at Rodriguez. That a ship 

 channel has not there been formed is probably due to the shallow 

 water surrounding the island and the probably rapid growth out- 

 ward of the reef ; the average depth outside the reef is usually less 

 than ten fathoms, and at a distance of two miles seaward it is 

 only from twenty to thirty fathoms. In some instances the large 

 proportion of Calcareous Algse on the reefs appears to compensate 

 for the removal in solution, and thus to retard the formation of 

 ship channels. 



I doubt if any recent writer has attempted to give an *' ex- 

 planation which will fully account for the almost infinite variety 

 of coral formations." It is unnecessary to state that each reef 

 must have peculiarities depending on the nature and form of i's 

 foundation, and the meteorological and other conditions of the 

 seas in which the reef is situated ; it is only by a careful and 

 detailed study of all these conditions that the peculiarities of any 

 individual reef can be fully explained. At the same time it 

 appears to me beyond doubt that the general and well-known 

 characteristic features and form of coral reefs can be accounted 

 for by reference to certain general considerations, chief among 

 these being the vigorous growth of reef-forming species in posi- 

 tions and at depths where the supply of pelagic oceanic organisms, 

 which form their food, are most abundant, and the removal of 

 dead coral and coral debris wherever this is exposed to the action 

 of sea-water. 



Captain Wharton calls attention to our imperfect knowledge 

 of the coral groups of the Pacific, but he understates the case 

 in saying "that the waters of the Fiji and the Society Islands 

 are the only ones which can be said to be in any sense surveyed." 

 Cook, Kotzebue, Duperrey, Beechy, and Wilkes have given 

 running surveys of many of the Paumotus, and we know some- 

 thing about the depths inside and outside of a good many of 

 them. We know much about the islands containing guano. The 

 French have made some excellent charts of the New Caledonia 

 reefs, and the Americans have done the same for some of the 

 Hawaiian Islands. Captain Wharton will acknowledge that we 

 have a splendid survey of the Maldives, the most extensive 

 group of atolls in the world ; the islands marked with names in 

 this British Survey number 602. Other groups in the Indian 

 Ocean are well surveyed, and nearly all the Atlantic reefs have 

 been correctly laid down on charts. 



I feel sure that all who take an interest in this subject will 

 hope for many more contributions from Captain Wharton's pen 

 on coral formations. John Murray. 



I have read with great interest the article on ciral formations 

 in your last number (p. 393), by Capt. W^harton. It is not 

 because I wish to claim to have anticipitated the views which he 

 gives as to the formation of atoll lagoons and barrier reef 

 lagoons that I am writing to state that at the very date of the 

 publication of Capt. Wharton's article I was engaged in writing 

 a paper on coral formations, based upon a study of living corals 

 at Uiego Garcia, and on a consideration of the great submerged 

 atolls known as the Great Chagos Bank and the Pitt and 

 Centurion Banks, situated north and west of that island, in 

 which I arrive at conclusions nearly identical with his. It has 

 seemed to me, as it has to him, that the solution of dead coral 

 rock in the interior of a reef does not sufficiently account for 

 the formation of lagoons, and that the true cause of the atoll 

 and barrier lagoons surrounded either by a reef which is awash, 

 or by a strip of low land, lies in the peculiarly favourable con- 

 ditions for coral growth present on the steep external slopes of 

 the reef. In Diego Garcia I observed that although the shore 

 reefs are for the most part covered with I or 2 feet of water, even 

 at the lowest spring tides, yet their flat surfaces are nearly in- 

 variably barren of growing coral. Just at their edges, however, 

 and on the steep external slopes beyond th^ edges, reef-building 

 corals grow luxuriantly. According to Capt. Moresby, quoted 

 by Mr. Darwin in his book on "Coral Reefs," the flat surface 

 of the rim of the Great Chaijos Bink is barren of living corals, 



