126 



NA TURE 



[Dec. 6, 1883 



landmarks furnished by the islands can be relied upon 

 for the location of buoys. 



A still more perfect example of an atoll formed under 

 similar conditions is that of Alacranon the opposite coast 

 of Yucatan. Its eastern face is a great arc of about 

 20 miles, where, exposed to the open sea and easterly 

 winds, the corals flourish vigorously. On the eastern or 

 interior face of the western chord of the reef, however, the 

 silt derived from the pounding of the breakers to the east- 

 ward ha-, already killed the corals The lagoon is occu- 

 pied by detached coral-heads with lanes of deep w'ater 

 between them ' 



To the east of the Tortugas, neirer the mainland of 

 Florida, older stages of development among coral-reefs 

 •may be traced. By the westward drift of the calcareous 

 sand and silt the lagoons have beer, converted into flats, 

 and these in succession hive been turned into more or 

 less continuous dry land. There is no evidence of sub- 

 sidence. The area seems to have remained stationary 

 for a long period, or if there has been movement at all, it 

 has been in an upward direction. Should the present 

 condition of things be prolonged, there will be a further 

 extension of the Florida coast-line. By the heapin^ uu 

 of the shells of dead organisms in the track of the counter 

 current, the submarine bank will continue to be brought 

 up within the depth at which reef-building corals can 

 flourish. Successive clumps of reef-builders, springing up 

 and growing outward, will build up atoll-shaped reefs. 

 The abrading action of the waves upon these reefs will 

 furnish detritus to be drifted into the lagoons and channels, 

 which will eventually be silted up into dry land. 



An interesting indication of the progress of these 

 changes is furnished by the terrestrial flora and fauna of 

 the reefs. The plants of the mainland are found likewise 

 on the reefs, but become fewer in number as they are 

 followed southward, until on the Tortugas, — the last 

 addition to the dry land, — the flora consists of a few Bay- 

 cedars, a hop-vine with a thick white fljwer, Bermuda 

 grass, and a solitary mangrove tree. One of the species 

 of land-shells common at Key West has found its way to 

 the Tortugas. No terrestrial reptiles have yet reached that 

 furthest atoll, though at Key West, less than 100 statute 

 miles to the east, many of the frogs, toads, lizards, and 

 snakes common to the southern mainland have already 

 established themselves. 



It will be ob <erved that the conclusions arrived at by 

 Prof Agassiz from his own independent researches en- 

 tirely confirm tho-e previously announced by Mr. Murray. 

 That two observers, who have enjoyed exceptional advan- 

 tages in the investigation of this subject, should come to 

 practical agreement must be admitted to be a strong 

 argument in favour of the views which they have adopted. 

 Putting together all the data which have here been sum- 

 marised, 1 think we are driven to admit that barrier reefs 

 and atolls may be formed without subsidence of the sea- 

 floor. Whether this has been the usual or only an excep- 

 tional manner of their origin is a question that will depend 

 for its solution upon whether or not it can be shown that 

 there are general phenomena which can only be explained 

 by subsidence. Three such phenomena may be adduced : 

 I am not aware of any others that deserve serious con- 

 sideration. 



I. One of the early difficulties which Darwin's expla- 

 nation satisfactorily solved was the necessity tor the 

 existence of so many peaks, coming up from the depths of 

 ocean just to the zone in which reef-building corals live. 

 No cause was conceivable which should have so generally 

 arrested the upward growth or upheaval of these sub- 

 marine heights at the limit where coral-reefs might begin. 

 And this difficulty has always been' looked upon as fur- 

 nishing one of the strongest arguments in favour of the 

 theory of subsidence, for that theory completely removes 

 it, by showing how, in a general submergence, peak after 



' Bulletin A/iis. Com^. Zool , v. No. i. 



peak wouli sink, and come within the sphere of the opera- 

 tions of the reef-builders. 



The difficulty is met in a totally diflTerent way by those 

 who believe it to be more formidable in appearance than 

 in reality. They contend that, while it must not be for- 

 gotten that many peaks dorise above the sea-level, and many 

 submarine banks still fall far short of the coral-zone, two 

 powerful causes conspire to bring subaiarine banks to a 

 common uniformity of level at a short distance below the 

 surface of the ocean. On the one hand, those portions of 

 volcanic mountains that rise above the sea-level are 

 worn down by the atmosphere and the waves, and unless 

 otherwise preserved, must inevitably be reduced to the 

 lower limit of wave-action, which is probably nearly coin- 

 cident with the lo.ver limit of reef-builders. On the 

 other hand, submarine banks in tropical seas are built 

 up towards the surface by the accumulation of the aggre- 

 gated remains of plants and animals which live on the 

 bottom or fall down to it from upper waters, and the 

 magnitude of this upward growth is hardly yet adequately 

 realised. 



In balancing these opposite views, we must, I think, 

 admit that subsidence is adequate to provide platforms 

 for coral-reefs, but that these platforms could likewise be 

 furnished by the two other processes just referred to. 

 Subsidence has been invoked because no other solution 

 of the problem seemed admissible. But as another 

 solution has been found the argument in favour of sub- 

 sidence has no longer the same force. The new solution, 

 being based upon facts which are everywdiere observable 

 in the coral regions, appears to me to be more probable 

 than the older one, wdiich is only an inference resting on 

 no positive proofs. 



2. The precipitous descent of the outer face of the reefs 

 to depths far below those at which corals can live is 

 another difficulty which finds a ready explanation on the 

 theory of subsidence. If it were true, as is popularly 

 assumed, that a coral reef presents towards the ocean a 

 vast perpendicular wall of limestone, entirely composed of 

 solid cor.il, there could be no escape from the conclusion 

 that subsidence must have occurred, to permit of such an 

 aggregation of coral-rock- We learn, however, that much 

 misconception exists on this subject. Some of the earlier 

 accounts of coral-islands speaV of " unfathomable " depths 

 at a short distance seawards from the reefs ; but more 

 recent soundings afford no confirmation of these state- 

 ments. Instead of being the summits of vast submarine 

 pillars of limestone, atolls, as well as barrier-reefs, appear 

 to be really planted on the tops of submarine peaks and 

 ridges. The outer face of the reef is undoubtedly steep, 

 in some places vertical. .At Tahiti, for example, as 

 shown in Fig. 2, the living face of coral may extend to a 

 depth of 30 to 35 fathoms, beneath which huge detached 

 blocks of coral are piled up and cemented together, form- 

 ing a steep face, which descends to about 150 fathoms 

 at a distance of 180 fathoms from the upper edge of the 

 reef The sea-bottom beyond that point is covered with 

 coral sand and slopes at 25" to 30°, after which the angle 

 lessens to 6^ By the abrading action of the breakers in 

 tearing off blocks of coral, and strewing them down in 

 steep talus-slopes, a platform is prepared on which the 

 actually growing part of the reef can build outwards. 



In Darwin's section of the Gambler Islands the thick- 

 ness of the encircling reef is made to be about 2000 feet.' 

 Prof. Dana by one estimate puts it at 1150, and by 

 another at 1750 feet. He assumes that in general the 

 thickness of solid coral must be considerable, though he 

 admits that calculations based on the seaward continua- 

 tion of the slope of the land are liable to error from many 

 causes.- Even if we admit (what cannot be proved) that 

 the calcareous mass of any coral-reef does attain a thick- 

 ness of many hundred feet, it would not necessarily con- 



