Aiigiist 12, 1880] 



NA TURE 





Thomson suggested, carbonic acid. Analysis shows that carbonic 

 acid is most" abundant in sea-water, and especially so in deep 

 water. Pteropod and Heteropod shells are very much larger 

 than the Foraminifera, yet are very much thinner ; and hence, 

 for the quantity of lime contained in them, they present a much 

 greater surface to the action of the sea-water. This seems to 

 be the reason why all large and thin shells are first removed 

 from the deposits with increasing depth, and not the fact that 

 some shells are composed of arragonite and some of calcite, as 

 has been suggested. 



There is a continual struggle in the ocean with respect to the 

 carbonate of lime. Life is continually secreting it and_ moulding 

 it into many varied and beautiful forms. The carbonic acid of 

 ocean-waters attacks these when life has lost its hold, reduces 

 the lime to the form of a bicarbonate, and carries it away in 

 solution. In all the greater depths of the ocean these surface 

 shells are reduced to a bicarbonate either during their fall 

 through the water or shortly after reaching the bottom. 



In the shallower depths— on the tops of submarine elevations 

 or volcanoes— the accumulation of the dead siliceous and cal- 

 careous shells is too rapid for the action of the sea-water to have 

 much effect. Long before such a deposit reaches sufficiently 

 near the sm-face to serve as a foundation for reef-forming corals, 

 it is a bank on which flourish numerous species of Foraminifera, 

 Sponge3,"iHydroids, deep-sea Corals, Annelids, Alcyonarians, 

 Molluscs, Polyzoa, Echinoderms, &c. All these tend to fix and 

 consolidate such a bank, and add their shells, spicules, and skele- 

 tons to the relatively rapid accumulating deposits. Eventually 

 coral-forming species attach themselves to such banks, and then 

 commences the formation of 



Cora! Atolls. — Mr. Darwin has pointed out that " reefs not to 

 be distinguished from an atoll might be formed " ' on submerged 

 banks such as those here described. However, the improbabihty 

 of so many submerged banks existing in the open ocean caused 

 liim to reject this mode of formation for atolls. As here stated, 

 recent deep-sea investigations have shown that submerged _ banks 

 are continually in process of formation in the tropical regions of 

 the ocean, and it is in a high degree probable that the majority 

 of atolls are seated on banks formed in this manner. 



Mr. Darwin has also pointed out tliat the corals on the outer 

 margin of a submerged bank would grow vigorously, whUst the 

 growth of those on the central expanse would be checked by 

 the sediment formed there, and by the small amount of food 

 brought to them. = Very 'early in the history of such an atoll, 

 and whUe yet several fathoms submerged, the corals situated on 

 the central parts would be placed at a disadvantage, and this would 

 become greater and greater as the coral plantations approached thS 

 surface. When the coral plantation was small there was a rela- 

 tively large periphery for the supply of food to the inner parts, 

 and also for the supply of sediment ; and hence in small atolls 

 the lagoon was very shallow, and was soon filled up. For the 

 same reasons coral islands situated on long and narrow banks 

 have no lagoons. An atoll one mile square has a periphery of 

 four mUes. In an atoll four miles square — the periphery in- 

 creasing in arithmetical progression and the area as the square^ 

 we have for each square mile only a periphei-y of one mile over 

 which food may pass to the interior, and from which sediment is 

 supplied for filling up the lagoon. 



With increasing size, then, the conditions become more and 

 more favourable to the formation of lagoons, and as a conse- 

 quence we have no large or moderate sized coral islands without 

 lagoons. Tow-net experiments always showed very much less 

 Pelagic life (food) in the lagoon waters than on the outer edge 

 of the reef. The lagoon becomes less favourable for the growth 

 of all the more massive kmds of coral as the outer edge of the 

 reef reaches the surface, and cuts off the free supply of ocean- 

 waters. Many species of corals die.^ Much dead coral, coral 

 rock, and sediment is exposed to the solvent action of the sea- 

 water. Larger quantities of lime are carried away in solution 

 as a bicarbonate from the lagoon than are secreted by the ani- 

 mals which can still hve in it ; the lagoon thus becomes widened 

 and deepened.'' 



On the other hand a vigorous growth and secretion of lime 

 takes place on the outer margins of the reef ; and when the 

 water outside becomes too deep for reef-forming corals to live, 



' "Coral Reefs." p. ii3. = "Coral Reefs," p. 134. 



3 There are no living corals or shells in some small lagoons, the waters of 

 which become highly heated, and in some cases extremely saline. 



< Complete little Serpula-atoUs, with lagoons from 3 to 50 feet in diame- 

 ter, and formed in this way without subsidence, were numerous along the 

 shores of Bermuda. 



these still build seawards on a talus of their own dibris : — the 

 whole atoll expands somewhat after the manner of . a fairy 

 ring. 



It is not necessary to call in disseverment of large atolls in 

 order to explain the appearances presented in the Great Maldiva 

 group of atolls.^ The coral fields rising from very many parts 

 of these extensive submarine banks form atolls. The marginal 

 atolls have from the first the advantage of a better supply of 

 food. They elongate in the directpon of the margin of the bank 

 where the water is shallower than to seaward. Many of these 

 marginal atolls have coalesced, and as this growth and coales- 

 cence have continued, a large part of the food-fupply has been 

 cut off from the small atolls situated towards the interior of the 

 bank. Ultimately a large atoll-like Suadiva atoll would be 

 formed. The atolls in the interior would be perhaps wholly 

 removed in solution, and the atoll-like character of small mar- 

 ginal but now coalesced atolls \\ ould be wholly or partially lost 

 by the destruction of their inner sides." A study of the charts 

 shows all the stages in this mode of development. 



In the case of the Lakadivh, Caroline, and Chagos archipela- 

 gos we have submarine banks at various stages of growth towards 

 the surface, some too deep for reef-forming species of coral, 

 others with coral plairtations, but all submerged several fathoms, 

 and scattered amongst these some of the oldest and most com- 

 pletely-formed atolls and coral islands. It is most difficult to 

 conceive how these submerged banks could have been produced 

 by subsidence, situated as they are in relation to each other and 

 with respect to the perfectly-formed atolls, barrier or fringing 

 reefs of the groups. 



It is a much more natural view to regard these atolls and sub- 

 merged banks as originally volcanoes reaching to various heights 

 beneath the sea, and which have subsequently been built up to 

 and towards the surface by accumulations of organic sediment 

 and the growth of coral on their summits. It is a remarkable 

 fact that in all coral atoUs which have been raised several 

 hundred feet above the sea, the base is generally described as 

 composed of solid limestone, or "of various kinds of coral 

 evidently deposited after life had become extinct." ^ This base 

 is probably often made up of such a rock as that brought by the 

 missionaries from New Ireland, and described by Prof. Liver- 

 sidge,'' as composed chiefly of Pelagic foraminifera, the same as 

 those taken by the Challenger in the surface waters of the 

 Pacific. 



Microscopic sections of a rock taken from 50 feet below sea- 

 level at Bermuda show that a deposition of carbonate of lime is 

 going on. The small shells are filled with, and the broken pieces 

 of shflls and corals are cemented by, calcite. The wells in coral 

 islands rise and fall with the tide, so that the whole atoll is filled 

 like a sponge with sea-water. This water is very slowly inter- 

 changed, and by the solution of the smaller and thinner particles 

 becomes saturated, and a deposition of lime follows. In this 

 way we may explain the absence of many of the more delicate 

 shells from some limestones.'' 



Barrier Reefs. — During the visit of the Challenger to Tahiti a 

 careful examination was made of the reefs by dredging, sounding, 

 &c. , in a steam pinnace, both inside and outside the reefs. 

 Lieutenant Swire, of the Challenger, made a careful trigono- 

 metrical survey of the profile of the outer reefs on six different 

 lines ; and while associated with him in this work the author was 

 indebted to that officer for many valuable suggestions. 



A ledge ran out from the edge of the reef to about 250 yards, 

 where we got a depth of from 30 to 40 fathoms. It was covered 

 with a most luxuriant growth of coral bosses and knobs. 



Between 250 and 350 yards from the edge of the reef "there 

 was generally a very steep and irregular slope ; about 100 

 fathoms was got at the btter distance, and the angles between 

 these last-mentioned distances often exceeded 45 degrees. The 

 talus here appeared to be composed of huge masses and heads of 

 coral, wliicli had been torn by the waves from the upper ledge 

 and piled up on each other. They were now covered with 



1 Mr. Darwin's application of his theory to this group— where the dis 

 severment of large atolls is called in, and a destructive power attributed to 

 oceanic currents, which it is very unlikely they can ever possess— has often 

 been considered unsatisfactory. 



2 " In speaking of Bow Island, Belcher mentions the fact that several of 

 its points had undergone material change, or were no longer the same when 

 visited after the lapse of fourteen years. These remarks refer particularly 

 to islets situated within the lagoon, I could myself quote many instances of 

 the same description." — " Wilkes' Exploring Expedition," vol iv. p. 27r. 



3 "U.S. Ex. Exp ," vol iv. p. 269. ■• Geol Mag., December, 1877. 



5 Fuchs, "Ueber die;EntstehungderAptychenkalke," J'fV-*. derk.Akad 

 dtr WUscTisch.t 1877. 



