APPENDIX II. 
800 
(with or without organisms), which however forms also at 
all depths, from a few feet beneath the low tide level, in 
the case of large islands, the coasts of which, owing to 
the sediment brought down by streams, are bare of reefs. 
In a paper printed in the ‘ Proceedings of the Royal 
Society ’ (vol. xliii. p. 440, 1888), Mr. G. C. Bourne gives a 
minute description of the atoll of Diego Garcia, and discusses 
the theories of coral-reef formation in connection with the 
Chagos group. In the Laccadive, Maidive and Chagoa 
group, ‘ there is no instance of a fringing or a barrier reef; 
nothing but coral structure rises above the waves ; all the 
islands are atolls.’ The three groups are believed to stand 
on a submarine bank lying 1,000 fathoms 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 of 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 19 fathoms. 
Mr. Bourne describes the different kinds of coral rock, 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 waves and currents, and to the 
effect of the latter upon the growth of coral : showing 
how the living coral may be killed by a change in a current 
which, formerly clear, now brings sand. This material 
proceeds to entomb the dead coral, and then, on a return 
to the former conditions, a new growth of coral may take 
place upon the stratum of sand. He is of opinion that the 
subsidence theory cannot be applied to explain the Great 
Chagos Bank (see p. 53), because its rim 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 
