450 Mr. G. C. Bourne. The Atoll of Diego Garcia [Mar. 22, 



more delicate branching species of the Madreporaria flourish in con- 

 siderable numbers, but true reef -building species, Porites, Mceandrina, 

 Pocillopora, and various stout species of Madrepora, are found there. 

 It is a mistake to suppose that certain species of corals are restricted 

 to the external shores, others to the lagoon. My collections proved 

 that many of the species growing in the lagoon at distances of 5 miles 

 and upwards from its outlet are identical with those growing on the 

 outer reef. In addition to them are numerous species, such as 

 Seriatopora striata, Mussa corymbosa, Favia lobata, Fungia dentata, 

 and many others that are not found on the outside. The reason is 

 that the last-named are either free forms such as Fungia, or are 

 attached by such slender and fragile stems to their supports that they 

 could not possibly obtain a foothold and maintain themselves among 

 the powerful currents and waves of the open ocean. 



These various species, numbers of which grow close together, form 

 knolls and patches within the lagoon, and it cannot be doubted that 

 their tendency is to fill it up. Again, in reefs which do not rise above 

 the surface, or are awash for the greater part of their extent at low 

 tides, great quantities of debris, torn from the outer slopes, are con- 

 stantly carried over the rim of the reef and tend to fill it up. 

 Hence it follows that in a lagoon entirely surrounded by dry land, or 

 nearly so, as is the case at Diego Garcia, the tendency to accumulation 

 of material within the lagoon would be less than in submerged or in- 

 complete atolls, for debris cannot be swept over into the lagoon, and 

 the only constructive agency is the growth of coral. If the power of 

 solution of sea-water is so great, it must be supposed that in com- 

 plete or nearly complete atolls the lagoon would be deepening rather 

 than shallowing ; yet at Diego Garcia the lagoon is obviously shallow- 

 ing in many places, and has nowhere increased in depth since Captain 

 Moresby's survey in 1837. Indeed, the southern part seems to have 

 shoaled a fathom since that time, and this is the more remarkable, 

 since the S.E. trade winds are by far the most constant and strongest 

 winds there, and tend to accumulate material at the northern rather 

 than at the southern end. The fact is, that these winds sweep the 

 sand out of the southern part, and thus leave an area particularly 

 favourably situated for the growth of corals. Mr. Murray points out 

 that larger atolls generally have deeper lagoons than small atolls, and 

 urges this fact in support of his theory ; but here again the facts in 

 the Chagos group are against him. Victory Bank is a submerged 

 atoll, the Solomons is an atoll with a large extent of dry land, in 

 each the lagoon attains a depth of 17 — 18 fathoms, and in Diego 

 Garcia the lagoon, although far larger, does not attain a greater 

 depth. Peros Banhos is far smaller than the Great Chagos Bank, yet 

 in both the lagoons attain nearly the same maximum depth, viz., 

 41 fathoms for Peros Banhos, 44 fathoms for the Great Chagos Bank. 



