DIEGO GARCIA ATOLL. 
303 
great quantities of ddbris, torn from tlie 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 accumulation of material within the lagoon 
would be less than in submerged or incomplete atolls, for 
ddbris 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 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 favour- 
ably 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 Cliagos 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 Banlios is far smaller than 
the Great Cliagos Bank, yet in both the lagoons attain 
nearly the same maximum depth, viz., 41 fathoms for 
Peros Banlios, 44 fathoms for the Great Chagos Bank. 
Speaker’s Bank is very little larger than Peros Banlios ; 
its lagoon is far shallower, having a maximum depth of 
24 fathoms.’ 
