354 



NATURE 



\_Aitgnst 12, i8So 



living Sponges, Alcyonarians, Hydroids, Polyzoa, Foraminifera, 



From 350 to 500 yards from the edge of the reef we had a 

 slope with an angle of about 30°, and made up chiefly of coral 

 sand. Beyond 500 yards the angle of the slope decreased till n e 

 had at a distance of a mile from the reef an angle of 6°, a deplli 

 of 590 fathoms, and a mud composed of volcanic and c iral 

 sand, Pteropods, Pelagic and other Foraminifera, Coccoliths, 

 &c. 



In the lagoon channel the reefs were found to be fringed with 

 living coral, and to .slope downwards and outwards for a few 

 feet, and then plunge at once to a depth of 10 or 16 fathoms. 

 Many portions of these inner reefs were overhanging, and at 

 some places overhanging masses had recently fallen away. 

 Everywhere much dead coral rock «as exposed to the solvent 

 action of the sea-water. The reefs of Tahiti are at some 

 places fringing, at other places there is a boat passage within 

 the reef, and at Papiete tliere is a large ship channel with islet ; 

 within, and the outer edge of the reef is a mile distant from the 

 shore. The island itself is surrounded with a belt of fertile low 

 land, frequently three or four miles wide ; this shows that tlie 

 island has not in recent times undergone subsidence ; there are 

 indeed reasons for supposing it has recently been slightly 

 elevated. Everything appears to show that the reefs have com- 

 menced close to the shore and have extended seawards, first on a 

 foundation composed of the volcanic detritus of the island, and 

 afterwards on a talus composed of coral dSris and the shells 

 and skeletons of surface organisms. - 



The lagoon channel was subsequently slowly formed by the 

 solvent action of the sea-water thrown over the reefs at each 

 tide, and the islets in the lagoon channel are portions of the 

 original reef still left standing. The reefs have extended out- 

 wards from the island and have been disintegrated and removed 

 behind in the same way as the atoll has extended outwards after 

 reaching the surface. 



Where reefs rise quite to the surface, and are nearly continuous, 

 we find relatively few coral patches and heads in the lagoons and 

 lagoon channels. Where the outer reefs are much broken up, 

 the coral growths in the lagoon are relatively abundant. Where 

 the water was deep and the talus to be formed was great, the 

 outward growth has been relatively slow, 3 and the disintegrating 

 forces in the lagoons and lagoon channels gaining in the struggle, 

 the reefs would become very narrow, and might indeed be broken 

 up. This, however, would admit the oceanic waters and more 

 food, and growth would again commence on the inner as well as 

 the outer sides of the still remaining portions. In the great 

 barrier reef of Australia, where the openings are numerous and 

 wide, the reefs have a great width. Where the openings are few 

 and neither wide nor deep (as in lat. 12° 30') the reefs are very 

 narrow and "steep to " — on their inner side. 



At the Admiralty Islands, on the lagoon side of the islets on 

 the barrier reefs, the trees were found overhanging the water, 

 and in some cases the soil washed away from their roots. It is 

 a common observation in atolls that the islets on the reefs are 

 situated close to the lagoons. These facca point out the re- 

 moval of matter which is going on in the lagoons and lagoon 

 channels. 



Elcz'atioH aud Sulisidence. — Mr. Darwin has given many 

 reasons for believing tliat those islands and coasts which have 

 fringing reefs had recently been elevated, or had long remained 

 in a state of rest. Throughout the volcanic islands of the great 

 ocean basins the evidence of recent Elevations are everywhere 

 cons])icuous. Jukes has given most excellent reasons for believ- 

 ing that the coast of Australia fronted by the barrier reef, and 



' This ledge and steep slope beyond where a depth of jo or 40 fathoms was 

 reached, was characteristic of a large number of atoll and barrier reefs, and 

 seemed due to wave action. Experiments had been made with masses of 

 broken coral, and it was found that these could (on account of their rough 

 and jagged surface) be built up into a nearly perpendicular wall by letting 

 them fall on each other A talus formed in water deeper than 40 fathoms, 

 where there was little, if any, motion, would be different from one formed on 

 land. In the latter case the disintegrating forces at work always tended to 

 set the talus in motion ; in the former case evcrj-thirg tended to consolidate 

 and to fi.1 the blocks in the positions first assumed. A removal of lime in 

 solution would take place from the blocks forming this steep slope, but except 

 in very deep water this would not be sufficient to check the outward extension 

 of the reef. 



• ' ■j *'"=.<'g™g '" '55 fathoms, close to the barrierreef of Australia (between 

 '.,.* ".I,- j"°^ Island), gave a coral sand, which was, I estimate, more than 

 two-tnirds made up of the shells of surface animals. 



Hence in barrier reefs, where the depth outside is very great, we find 

 the reefs running closer to the shore than where the depth is less, and conse- 

 quently the talus to be formed is smaller. s , a u t.oi s 



even the barrier reef itself, have recently been elevated. 1 Dana 

 and Conthouy have given a list of islands in almost every barrier 

 reef and atoll region which have recently been elevated.' 



This is what we should expect. Generally speaking, all the 

 volcanic regions which we know have in the main been areas of 

 elevation, and we should expect the same to hold good in those 

 v.ist and permanent hollows of tlie earth which are occupied by 

 the waters of the ocean. It must be remembered tliat protablv 

 all atolls were seated on submarine volcanoes. Areas of loci'l 

 depression are to be looked for in the ocean basins on either side 

 of and between groups of volcanic islands and atolls, and not on 

 the very site of tliese islands. This is what the deep-sea sound- 

 ings show if they show any depression at all. Subsidence has 

 been called in in order to account for the existence of lagoons 

 and lagoon channels, and the narrow bands of reef which inclose 

 these ; but it has been shown that these were produced by quite 

 other causes — by the vigorous grow-th of the corals where most 

 nourishment was to be had, and their death solution and disinte- 

 gration by the action of sea- water and currents^ at those parts 

 which cannot be, on account of their situation, sufficiently sup- 

 plied with food. 



All the chief and characteristic features of barrier reefs and 

 atolls may indeed exist with slow elevation, for the removal of 

 lime from the lagoons and the dead upper surface of the reefs 

 by currents, and in solution by rain and sea-water, might keep 

 pace with the upward movement. 



The most recent charts of all coral reef regions have been 

 examined, and it is found possible to explain all the phenomena 

 by the principles here advanced ; Mhile on the subsidence theory 

 it is most difficult to explain the appearances and struc- 

 tures met with in many groups ; for instance, in the Fiji islands, 

 where fringing reefs, barrier reefs, and atolls all occur in close 

 proximity, and where all the other evidence seems to point to 

 elevation, or at least a long period of rest. In instances like the 

 Gambler group the reefs situated on the seaward side of the outer 

 islands would grow more vigorously than those towards the 

 interior ; they \iould extend in the direction of the shallower 

 water, and ultimately would form a continuous barrier around 

 the whole group. The distinguishing feature of the views now 

 advanced is that they do away with the great and general sub- 

 sidences required by Darwin's theory," and are in harmony w'itli 

 Dana's views of the great antiquity and permanence of the great 

 ocean basin, which all recent deep-sea researches appear to 

 support. 



Summary. — It was shown (i) that foundations have been pre- 

 pared for barrier reefs and atolls by the disintegration of 

 volcanic islands, and by the building up of submarine volcanoes 

 by the deposition on their summits of organic and other 

 sediments. 



(2) That the chief food of the corals consists of the abundant 

 Pelagic life of the tropical regions, and the extensive solvent 

 action of seawater is shown by the removal of the carbonate of 

 lime-shells of these surface organisms from all the greater depths 

 of the ocean. 



(3) That when coral plantations build up from submarine 

 banks they assume an atoll form, owing to the more abundant 

 supply of food to the outer margins, and the removal of dead 

 coral rock from the interior portions by currents and by the 

 action of the carbonic acid dissolved in sea-water. 



(4) That barrier reefs have built out from the shore on a 

 foundation of volcanic debris or on a talus of coral blocks,^coral 

 sediment, and Pelagic shells, and the lagoon channel is formed 

 in the same way as a lagoon. 



(5) That it is not necessary to call in subsidence to explain any 

 of the characteristic features of barrier reefs or atolls, and that 

 all these features would exist alike in areas of slow elevation, of 

 rest, or of slow subsidence. 



In conclusion it was pointed out that all the causes here 

 appealed to for an explanation of the structure of coral reefs 

 are proximate, relatively well known, and continuous in their 

 action. 



" " Voyage of the /'/>'," vol i p. 335. 



• Dana's *' Corals and Coral Islands," p. 345 : Couthouy's '"Remarks on 

 Coral Formations," Bost. Jauni. Nat. Hist. See also Stutchbury, West 0/ 

 Eugtand Journal. 



3 Very strong currents run out of the entrances into lagoons and lagoon 

 channels, and when the tow-net was used in these entrances it showed that 

 a large quantity of coral tietritus was being carried seawards. 



" " We may conclude that immense areas have subsided, to an amount 

 sufficient to bury not only any formerly existinn; lofty table-land, but even 

 the heights formed by fractured strata and erupted matter." — "Coral 

 Reefs," p. 190. 



