35: 



NATURE 



\_A7lgUSt 12, iSSo 



reef becomes a barrier reef, and the barrier reef an atoll Ijy a 

 continuous process of development. 



Object of the Present Paper. — Prof. Semper,' during his exami- 

 nation of the coral reefs in the Pelew group, experienced great 

 difficulties in applying Darwin's theory. Similar difSculties 

 presented themselves to the author in those coral-reef regions 

 visited during the cruise of the CliaUenger. 



The object of the present paper is to show, first, that, while 

 it must be granted as generally true that reef-forming species of 

 coral do not live at a depth greater than 30 or 40 fathoms, yet 

 that there are other aijencie-; at work in the tropical oceanic 

 regions by which submarine elevations can be built up from 

 very great depths so as to form a foundation for coral reefs ; 

 second, that while it must be granted that the surface of the earth 

 has undergone many oscillations in recent geological times, yet 

 that all the chief features of coral reefs and islands can be 

 accounted for without calling in the aid of great and general 

 subsidences. 



Nature of Oceanic Islands and Submarine Elevations. — It is 

 now known that, with scarcely an exception,^ all oceanic islands 

 other than coral atolls are of volcanic origin. Darwin, Dana, 

 and others have noticed the close restmblance between atolls and 

 ordinary islands in their manner of grouping as well as in their 

 shapes. In a previous paper the author pointed out the wide 

 distribution of volcanic d3ris over the bed of the ocean in 

 tropical regions, and the almost total absence of minerals, such 

 as quartz, which are characteristic of continental land.3 There 

 is every reason for believing that atolls are primarily situated on 

 volcanic mountains, and not on submerged continental land, as is 

 so often supposed. 



The soundings of the Tuscarora and Challenger have made 

 known numerous .submarine elevations : mountains rising from 

 the general level of the ocean's bed, at a depth of 2,500 or 3,000 

 fathoms, up to within a few hundred fathoms of the surface. 

 Although now capped and flanked by deposits of Globigerina 

 and Pteropod ooze, these mountains were most probably originally 

 formed by volcanic eruptions. The deposits in deep water on 

 either side of them were almost wholly made up of volcanic 

 materials. 



Volcanic mountains situated in the ocean basins, which during 

 their formation had risen above the surface of the water, 

 would assume a more or less sharp and pointed outline owing to 

 the denuding action of the atmosphere and of tloe waves, and 

 very extensive banks of the denuded materials would be formed 

 around them. Some, like Graham's Island, might be wholly 

 swept away, and only a bank with a few fathoms of water over 

 it be left on the spot. In this way numerous foundations may 

 have been prepared for barrier reefs and even atolls. 



Those volcanoes which during their formation had not risen 

 above the surface of the sea (and they were probably the most 

 numerous) would a-sume a rounded and dome-like contour,'' 

 owing to the denser medium in which the eruptions had taken 

 place, and the deposits which had been subsequently formed on 

 their summits. 



In order to clearly understand how a submarine mountain, 

 say half a mile beneath the sea, can be built up sufficiently near 

 the snrface to form a foundation on which reef-forming corals 

 might live, it is necessary to consider attentively the 



Pelagic Fauna and Flora of Tropical Regions. — During the 

 cruise of the Challenger much attention was paid to the subject. 

 Every day while at sea tow-nets were dragged through the 

 surface waters ; and while dredging they were sent down to 

 various depths beneath the surface. Everywhere life was most 

 abundant in the surface and sub-surface waters. Almost every 

 haul gave many calcareous, siliceous, and other Alg^ ; great 

 numbers of Foraminifera and Radiolaria, Infusoria, Oceanic 

 Hydrozoa, Medusre, Annelids ; vast numbers of microscopic and 

 other Crustacea, Tunicates, Pelagic Gasteropods, Pteropods, 

 Heteropods, Cephalopods, Fishes, and fish-eggs; larvce of 

 Echinoderms, and of many of the above creatures, &c. 



Most of these organisms live from the surface down to about 

 100 fathoms.^ In calm weather they swarm near the surface, 



' Zeitschr.fiir Wissen. Zootogic, vol. xiii. p. 563. 



- New Zealand, New Caledonia, and the Scyclielles have primitive rocks. 

 It these can be regarded as oceanic islands. Sjme of the islands between 

 JNew Laledonia and Australia may have primitive rocks, and the atolls in 

 these regions may be situated on foundations of this nature. 



iProc. Roy. Soc. Edin., 1876-77, p. 247. 



^ Scrope on " Volcanoes," chap. viii. 

 The Challengeridx, and many of the other members of Hxckel's new 

 Vrfll. """'S- ""^'"'y live deeper, as we never got them in the 



tropics except when the net was sent down to a depth of 200 or 300 fathoms. 



but when it is rotigh they are to be found several fathoms 

 beneath the waves. They are borne along in the great oceanic 

 currents which are created by the winds, and meeting with 

 coral reefs, they supply the corals on the outer edge of the reefs 

 with abundant food. The reason why the windward side of a 

 reef grows more vigorously appears to be this abundant supply 

 of food, and not the more abundant supply of oxygen, as is 

 generally stated. The Challenger researches showed that oxygen 

 was particularly abimdant in all depths mhabited by reef- 

 forming corals. 



When these surface animals die, either by coming in contact 

 with colder water or from other causes, their shells and skeletons 

 fall to the bottom, and carry down with them some organic 

 matter which gives a supply of food to deep-sea animals. The 

 majority of deep-sea animals live by eating the mud at the 

 bottom. 



An attempt was made to estimate the quantity of carbonate of 

 lime in the form of calcareous Algre, Foraminifera, Pteropods, 

 Heteropods, Pelagic Gastropods, in the surface-waters. A tow- 

 net, having a mouth 12.^ inches in diameter, was dragged for as 

 nearly as possible half a mile through the water. The shells 

 collected were boiled in caustic potash, washed, and then 

 weighed. The mean of four experiments gave 2' 545 grammes. 

 If these animals were as abundant in all the depth down to 100 

 fathoms as they were in the track followed by the tow-net, this 

 v.-ould give over 16 tons of carbonate of lime in this form in a 

 mass of the ocean one mile square by 100 fathoms.' 



Bathyntetrieal Distribution of the Calcareous Shells and Skele- 

 tons of Surface Organisms. — Although these lime-secreting 

 organisms areso abundant in tropical surface waters, their cast-off 

 shells and skeletons are either wholly or partially absent from 

 by far the greater part of the floor of the ocean. In depths 

 greater than 3,000 fathoms we usually met with only a few shells 

 of Pelagic Foraminifera of the larger and heavier kinds ; a few 

 hundred fathoms nearer the surface they became more numerous, 

 and we got a few of the smaller kinds and some Coccoliths and 

 Rhabdoliths. At about 1,900 or 1,800 fathoms a few .shells oi 

 Pteropods and Heteropods are met with ; and in all depths less 

 than a mile we have a deposit in which the shell and skeletons 

 of almost every surface organism is to be found. In the equa- 

 torial streams and calms the calcareous Algx', Pelagic Forami- 

 nifera, Pteropods, and Heteropods are more abundant in the 

 surface waters than elsewhere ; and it is in these same regions that 

 we found their dead shells at greater depths than in the deposits of 

 other parts of the ocean. Another circumstance influences the 

 bathymetrical distribution of these surface shells. When there 

 is a complete and free oceanic circulation from the top to the 

 bottom, these dead shells are found at greater depths in the 

 deposits than where the circulation is cut off by submarine 

 barriers. 



The agent by which these shells are removed is, as Sir Wyville 



^ Among the 



of Foraminifera recognised by Mr. Brady 



, , Micheliniana.. 



,, inmida. 



Pitltcitla obliqitUocitlata. 

 Spherrodma dehiscens. 

 Caudeina niiida. 

 Hastigerina Murrayi. 

 It is the dead shells of these PelagI 

 he calcareous oozes of the deep 



t l>uiloides. 

 aquilateralis. 

 sdcculi/era (hirsuta). 

 dubia. 



,, co'i^lcbata. 



„ injlata. 



Foraminifera which chiefly make up 

 The living shells of all the above 

 the tropical and sub-tropical waters near the surface. It 

 is especially in the region of the equatorial calms that the largest and thickest 

 shelled specimens are found. As we go narth or south into colder water 

 they become smaller, and many varieties die out. In the surface-waters of 

 the Arctic and Antarctic regions, only some dwarfed specimens of Globige- 

 rina iultoides are met with. The author is unable to agree with Dr. Car- 

 penter and Mr. Brady in thinking that these Pelagic Foraminifera also live 

 on the bottom. This question was made the subject of careful investigation 

 during the cruise. The shells from the surface and from the bottom were 

 compared at each locality, and it was found, by micrometric measurement, 

 that surface specimens were as large and as thick -shelled as any average 

 specimens from the soundings. It is quite unlikely that the same individuals 

 should pass a part of their lives in the warm sunny surface-waters, at a 

 temperature of from 70° to So° F., and another part in the cold dark waters 

 two or three miles beneath, at a temperature of 30° or 40" F._ The geogra- 

 phical distribution of these Pelagic forms over the bottom coincides exactly 

 Willi the distribution of the same forms on the surface, that is to say, both 

 on the surface and on the bottom the distribution is ruled by surface-tem- 

 perature. No specimens of these Pelagic varieties were ever obtained from 

 the bottom with the shells filled and surrounded with sarcode. Whereas 

 creeping and attached forms (like Truncatulina, Discorbina, Anomalina. and 

 some Textularia;) were taken in this condition in almost every dredge. These 

 last-mentioned forms, which we know live on the bottom, have a distribution 

 quite independent of surface-temperature. 



