548 



NATURE 



\ April s, 1888 



The latter is covered at depths of 3 or 4 fathoms and upwards by a 

 layer of fine sand, which may attain a thickness of 2 or 3 feet. 

 In protected parts of the lagoon and in spots where the change- 

 able currents have ceased to deposit any quantity of sand, corals 

 will grow in considerable quantities, chiefly those wide-spreading 

 ing species of Madrepora which cannot find a lodging on the 

 exterior of the reef, where they would be dashed to pieces by the 

 waves. By the continual growth of new colonies on the top of 

 the old ones which have died, a layer of solid rock of consider- 

 able thickness may be formed. Whilst diving for corals at the 

 lower part of the lagoon, I often noticed such layers of half- 

 formed rock on which living coral was growing or not, according 

 as the constantly changing currents were at that time throwing 

 up sand in the locality or not. Thus on the west side of the 

 lagoon, off Point Marianne, there are large tracts of recently 

 formed coral rock, on which no living corals are to be seen, 

 whilst on the east side of the lagoon, exactly opposite to Point 

 Marianne, a similar basis of rock is luxuriantly covered with 

 growing coral. 



Now, as the currents are constantly changing, and as the 

 changes may, as I saw, affect an area some miles in extent, one 

 may suppose that an area was first covered with corals growing 

 on the sand, which everywhere covers the reef rock, when the 

 latter lies more than a fathom below the surface. A change in 

 the currents brought abundant sand to the spot, killed the corals, 

 and deposited an even layer of sand of some little thickness over 

 the rock formed by the skeletons of the dead corals. A further 

 change in the currents would again render the spot suitable for 

 coral growth, and a new layer of rock would be formed over the 

 last layer of sand. I have seen quite analogous formations in 

 progress in a fathom of water a little way above Point Marianne. 

 Raise the formation to the surface, and you get that stratification 

 which occurs in so many parts of the island, a stratification 

 which cannot be explained on any theory of subsidence, and is 

 scarcely less difficult to explain on the supposition of rest. At 

 first I had some hesitation in extending to an island on the 

 borders of the lagoon, as is East Island, a view of the formation 

 of layers of sand and rock derived from an inspection of the 

 interior of the lagoon, but afterwards I saw that similar layers 

 were being formed just within the large reef known as Spur's 

 Reef, west of Middle Island, so that no objection can be raised 

 on that score. The whole character of the Chagos Group is very 

 much opposed to the theory that atolls and barrier reefs are 

 formed during subsidence. There are several atolls rising above 

 the waves, that of Peros Banhos being 55 miles in circuit, and 

 composed of numerous small islets placed upon a ring-shaped 

 reef through which there are several large and deep channels. 

 Egmont or Six Islands is an instance of an atoll in which the 

 encircling reef is perfect and unbroken by any channels, the 

 land consisting of six islets placed for the most part on the 

 southern and western sides of the reef. There are several sub- 

 merged banks, nearly all of wliich have an atoll form. Of these 

 the best known is the Great Chagos Bank, a huge submerged 

 atoll 95 miles long and 65 miles broad, having a depth of 4 to 10 

 fathoms over a narrow rim around its periphery, and a central 

 lagoon of a depth varying up to 45 fathoms. South-west of the 

 Great Chagos Bank, distant less than 15 miles, lies the atoll of 

 Six Islands, and on the other side of these, scarcely 12 miles 

 distant, lies another submerged atoll, known as Pitt's Bank. 

 South-west of Pitt's Bank are two smaller banks, Ganges and 

 Centurion's Banks. Darwin considered that the Great Chagos 

 Bank afforded particularly good evidence of the truth of the 

 subsidence theory. He regarded it as an atoll carried down by 

 a too rapid subsidence below the depth at which reef-building 

 corals flourish. The same would be the case for Pitt's Bank and 

 the two others just mentioned. A more intimate knowledge of the 

 Great Chagos Bank, and of the relations of it and other submerged 

 banks of existing land, shows this view to be untenable. In the 

 first place, the rim of the Great Chagos Bank is on an average 

 not more than 6 fathoms below the surface, and therefore situated 

 in a depth eminently favourable for coral growth, and there are 

 actually six islets on the northern and western edges rising above 

 the water, and some of them inhabited. Secondly, any such 

 rapid subsidence could not have affected areas only 30 miles 

 apart without involving the Six Islands atoll lying directly 

 between them. A similar argument might be extended to the 

 more northern islands of the Chagos Group, and even to Diego 

 Garcia itself, although it lies somewhat apart from the rest of the 

 group. Again, if atolls and barrier reefs are formed around 

 subsiding peaks, it is at least curious that throughout the Lacca- 



dive, Maldive, and Chagos Groups there are no instances of high 

 islands surrounded by barrier reefs, marking the last remnants 

 of pre-existing land. In the more western parts of the Indian 

 Ocean, between Madagascar and the Seychelles, there are 

 numerous atoll islands, and in long. 60° E. there lie the sub- 

 merged Saya de Malha Bank and the reef known as Cargados 

 Carajos. Between the?e two lies the extensive Nazareth Bank, 

 having over it depths of from 14 to 45 fathoms. The Saya de 

 Malha Bank appears to have the characters of a submerged 

 atoll, having a central depression of 65 fathoms, surrounded by 

 a rim which has only 8 to 16 fathoms on its eastern side, but 

 22 fathoms on the western. Some of the groups north of Mada- 

 gascar! afford very good evidence of upheaval. Aldabra Island, 

 situated in lat. 9° 22' S., long. 46° 14' E., is a perfect instance of 

 an upraised atoll. Captain Wharton describes the external 

 shores as consisting of low coral cliffs, about 20 feet high, the 

 surface of the land being composed of jagged coral rock. The 

 lagoon is entered by a passage varying from 1 1 to 5 fathoms in 

 depth, but its internal portions are either very shallow or partly 

 dry at low water. Not far distant is the Cosmo Ledo Group, a 

 perfect atoll, with a lagoon some 4 fathoms deep, or less. There 

 are ten islets of various sizes on the reef, and all of them appear 

 to have been elevated some 10 feet. There are some hills 40 and 

 50 feet high on the two largest islands, but these appear, accord- 

 ing to Captain Wharton, to be formed of blown sand. The 

 Farquhar Group and Assumption Island, situated within the 

 same area, have been raised, according to the same authority, 

 some 10 feet. Providence Island, in lat. 9° 14' S., long. 

 51° 2' E., appears to be a low island situated upon the edge of 

 the atoll-shaped Providence Reef. At a distance of 19 miles from 

 Providence Island is the island of St. Pierre, which has no 

 fringing reef. It is particularly interesting, for although it is in 

 close proximity to the low Providence atoll, it has been raised 

 about 40 feet above high water, and in the absence of a fringing 

 reef the sea breaks with great violence against a low cliffy coast, 

 hollowing out a number of caverns which, from the description 

 given in the sailing directions for Mauritus and its islands, 

 appear to open inshore by " blow-holes." ^ 



Near and among these raised coral formations are several sub- 

 merged banks, the most important of which is the McLeod Bank, 

 situated in lat, 9° 57' S., long. 50°2o' E., between Providence 

 Island and the Cosmo Ledo Group. The details show that there 

 is a group of coral formations, situated in lat. 10° S. , north of 

 Madagascar,in which are foundraised atolls — atollswhose dry land 

 just rises above the waves and submerged banks. There can be 

 no clearer proof that atolls are formed in areas of elevation, and, 

 if the facts which I have already stated concerning Diego Garcia 

 are of any weight, it would seem that most of the coral forma- 

 tions of the Indian Ocean mark areas of elevation rather than of 

 rest, certainly they are not evidence of subsidence. 



Those who have felt that the evidence brought against Darwin's 

 subsidence theory is too strong to be resisted, must often have 

 felt that no satisfactory explanation of the lagoons of atolls or 

 the lagoon channels of barrier reefs has been given in its place. 

 Semper was the first to suggest that the lagoon was formed by a 

 solution of the interior parts of the reef, and more recently this 

 view has been urged with great force by Murray, who points out, 

 in addition, that corals on the periphery of a reef must, from 

 their position, get the advantage over those more interiorly 

 situated, being more directly in the track of food-bearing cur- 

 rents. Neither of these explanations has completely satisfied 

 me. That sea-water exercises a solvent action upon carbonate of 

 lime does not admit of doubt, and that the scour of tides, com- 

 bined with this solvent action of the water, does affect the 

 extent and depth of a lagoon is obvious. But I challenge the 

 statement that the destructive agencies within an atoll or a sub- 

 merged bank are in excess of the construction. It would be 

 nearer the mark to say that they nearly balance one another. In 

 the first place, the carbonate of lime held in solution by sea- 

 water is deposited as crystalline limestone in the interstices of 

 dead corals or coral debHs. Anyone who is acquainted with the 

 structure of coralline rock knows how such a porous mass as a 

 Mseandrina head becomes perfectly solid by the deposition of 

 lime within its mass. This deposition can only be effected by 

 the infiltration of sea-water. In reckoning the solvent action of 

 sea-water, therefore, account must be taken of the fact that a 

 not inconsiderable proportion of the carbonate of lime held in 

 solution is redeposited in the form of crystalline limestone. Of 



' For the information on the islands mrth of Madgascar I am indebted to 

 the courtesy of Captain W. J. L. Wharton, R.N., F.R S. 



