474 THEORY OF CORAL-REEFS,. [cHap. xx. 
lidated fragments. Thus the difficulty on this head, which ap- 
peared so great, disappears. 
If, instead of an island, we had taken the shore of a continent 
fringed swith reefs, and had imagined it to have subsided, a great 
straight} barrier, like that of Australia or New Caledonia, sepa- ° 
rated from’the land by a wide and deep channel, would evidently 
have been the result. 
Let us take our new encircling barrier-reef, of which the sec- 
tion is now represented by unbroken lines, and which, as I have 
said, is a real section through Bolabola, and let it go on sub- 
new atoll. 
N.B. According to the true scale, the depths of the lagoon-channel and lagoon are much 
exagger.ted. ‘ 
go on vigoreusly growing upwards; but as the island sinks, the 
water will gain inch by inch on the shore—the separate moup 
tains first forming separate islands within one great reef—and 
finally, the last and highest pincacle disappearing. The instant 
this takes place, a perfect atoll is formed: I have said, remove 
the high land from within an encircling barrier-reef, and an atoll 
is left, and the land has been removed. We can now per- 
ceive how it comes that atolls, having sprung from encircling 
barrier-reefs, resemble them in general size, form, in the manner 
in which they are grouped together, and in their arrangement 
in single or double lines; for they may be called rude outline 
charts of the sunken islands over which they stand. We can 
further see how it arises that the atolls in the Pacific and Indian 
oceans extend in lines parallel to the generally prevailing strike of 
the high islands and great coast-lines of those oceans. T venture, 
