xxii PRE FA TOR Y NO TE. 



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 constantly 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 the accumulation of 

 material within the lagoon would be less than in submerged or 

 incomplete 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 complete or nearly complete atolls the lagoon 

 would be deepening rather than shallowing ; yet at Diego 

 Garcia the lagoon is obviously shallowing in many places, and 

 has nowhere increased in depth since Captain Moresby's 

 survey in 1837. Indeed, the southern part seemed to have 

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

 able, 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 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. Speaker's Bank is very little 

 larger than Peros Banhos ; its lagoon is far shallower, having 

 a maximum depth of 24 fathoms. . . . Corals grow best in 

 places where a moderate current flows constantly over them. 

 They are killed in still water by the deposition of sediment, and 

 they will not grow in places where a strong current sets directly 



