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As the barrier-reef slowly sinks down, the corals will go on vigorously growing upwards; but as the island sinks, the water will gain inch by inch on the shore the separate mountains first forming separate islands within one great reef and finally, the last and highest pinnacle disappearing.
The reef-constructing corals have indeed reared and preserved wonderful memorials of the subterranean oscillations of level; we see in each barrier-reef a proof that the land has there subsided, and in each atoll a monument over an island now lost.
At Keeling atoll the shores of the lagoon shelve gradually, where the bottom is of sediment, and irregularly or abruptly where there are coral-reefs; but this is by no means the universal structure in other atolls. The shores of the lagoon-like channel within the barrier-reef at Vanikoro have a similar structure.
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.
Well characterised atolls and encircling barrier-reefs, where several occur in a group, or a single barrier-reef if of large dimensions, leave scarcely any doubt on the mind respecting the movement by which they have been produced; and even a small amount of subsequent elevation is soon betrayed.
Also a chart of the passage or channel through the barrier-reef, for saving this passage, there is no landing upon the island that I know of. Nor shall you attempt this passage except at the flood and the seas calm. And if I join you not within two months, then shall this mighty treasure three-quarters thereof be yours.
During the enormous period necessary to effect an elevation of the kind just alluded to, the surface would necessarily be denuded to a great thickness; hence it is highly improbable that any fringing-reef, or even any barrier-reef, at least of those encircling small islands, would be preserved.
When they had passed the barrier-reef, and were sailing over the calm waters of the enclosed lagoon in the direction of Sandy Cove, the young officer went up to the pirate captain with a perplexed air and a degree of hesitation that was very foreign to his character. Gascoyne flushed deeply when he observed him. "I know what you would say to me," he said, quickly. "You have a duty to perform.
In the same chart, the space of open water within the reef in some parts is even more than three miles wide: the land is bold and peaked; this island, therefore, is encircled by a well-characterised barrier-reef, and is coloured pale blue. Its height, as I am informed by Dr. Allan, is about 3,500 feet; it is very precipitous, and is composed of granite, greenstone, and quartz; coloured blue.
I may here remark that the reefs were all coloured before the volcanoes were added to the map, or indeed before I knew of the existence of several of them. The volcano in Torres Strait, at the northern point of Australia, is that which lies nearest to a large subsiding area, although situated 125 miles within the outer margin of the actual barrier-reef.
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