MR. G. C. BOURNES EJ'/DENCE. 87 



scooping out is concerned, seems indisputable. I may, nevertheless, offer a suggestion : 

 This condition of reef is apparently only to be accounted for in two waj's — either by 

 subsidence, or by assuming that the animals, be they corals or other lime-secreting 

 organisms that settle on the bank, do, when it gets, by their accumulation, within a certain 

 distance of the surface, and under certain conditions of current and food supply, intercept so 

 much of the food borne in by the currents, that similar life, suitable to that depth zone, 

 cannot be supported in the central area. Thenceforward, the rim alone will grow, and the 

 organisms fitted to live in the successively shallower zones to the surface will alone find 

 foothold on it. This would be the perfect atoll, but, with less nicely-balanced conditions, 

 growth would also take place in patches in the central area, as is often the case." 



The foregoing evidence by Captain Wharton concerning the Tizard and Macclesfield 

 can be scarcely said to invalidate Mr. Darwin's subsidence theory, since it is frankly ad- 

 mitted that the existing condition of their associated reefs may be, alternatively, accounted 

 for by subsidence. 



Testimony of a similar somewhat negative description regarding the subsidence theory, 

 and in direct opposition to Dr. Murray's chemical-solution interpretation, was adduced about 

 the same time by Mr. G. C. Bourne, in the form of a paper printed in the " Proceedings 

 of the Royal Society," Vol. XLIII., 1888, dealing with the coral atoll of Diego Garcia, and 

 other coral-reef formations of the Chagos Archipelago. Here, again, it has been found most 

 desirable to submit a precis of Dr. Bonney's longer abstract published in Appendix II. of the 

 third edition of Mr. Darwin's treatise. — 



" Mr. G. C. Bourne gives a minute description of the atoll of Diego Garcia, and di.scusses the theories of coral-reef 

 formation in connection with the Chagos group. In the Laccadive, Maldive, and Chagos group, there is no instance of 

 a fringing or a barrier reef ; nothing but coral structures rise above the waves ; and all the islands are atolls. The three 

 groups are believed to stand on a. submarine bank lying i,ooo feet below the surface in an ocean of an average depth of 

 2,000 fathoms. At Diego Garcia, the shores externally slope away very rapidly to considerable depths, the sounding line 

 giving depths 250 fathoms and upwards at a distance of a few hundred yards from the edge of the reef, except in one 

 case. The depths inside the lagoon vary up to nineteen fathoms. Mr. Bourne describes the different kinds of coral-rocks, 

 and gives reasons for supposing that there has been a recent elevation of a few feet. He calls attention to the changes 

 produced by the action of the waves and currents, and to the effect of the latter upon the growth of the coral ; showing 

 how the living coral may be killed by a change in a current which, formerly clear, now brings sand. This material pro- 

 ceeds to entomb the dead coral, and then, on a return to the former conditions, a new growth of coral may take place 

 upon a stratum of sand. He is of opinion that the subsidence theory cannot be applied to explain the Great Chagos 

 Bai.k, because its rim is on an average not more than six 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 ! He indicates further difficulties in applying the theory of subsidence to the Chagos 

 Bank, especially pointing out that the six-island atoll, within a few miles' distance, has not been affected ; still, he admits 

 that the Saya de Malha Bank appears to have the character of a submerged atoll, having a central depression of sixty-five 

 fathoms, surrounded by a rim which has only eight to sixteen fathoms on its eastern side, but twenty-two fathoms on the 



