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Updated: May 17, 2025
At the Society Archipelago, on the other hand, where a slight tremor is only rarely felt, the shoaliness of the lagoon-channels round some of the islands, the number of islets formed on the reefs of others, and the broad belt of low land at the foot of the mountains, indicate that, although there must have been great subsidence to have produced the barrier-reefs, there has since elapsed a long stationary period.
Their position in front of valleys, although often separated from the land by deep water lagoon-channels, which it might be thought would entirely remove the injurious effects both of the fresh water and the sediment, will receive a simple explanation when we discuss the origin of barrier-reefs.
I have not thought it worth while to introduce all those small and very numerous reefs, which occur within the lagoons of most atolls and within the lagoon-channels of most barrier-reefs, and which stand either isolated, or are attached to the shores of the reef or land.
It is remarkable, that the lagoon-channels round mountainous islands have not in every instance been long ago filled up with coral and sediment; but it is more easily accounted for than appears at first sight.
Closely resemble in general form and structure atoll-reefs. Width and depth of the lagoon-channels. Breaches through the reef in front of valleys, and generally on the leeward side. Checks to the filling up of the lagoon-channels. Size and constitution of the encircled islands. Number of islands within the same reef. Barrier-reefs of New Caledonia and Australia.
From this same cause, the strata which were formed within the lagoons of atolls and lagoon-channels of barrier-reefs, and which must consist in a large part of sedimentary matter, would more often be preserved to future ages, than the exterior solid reef, composed of massive corals in an upright position; although it is on this exterior part that the present existence and further growth of atolls and barrier-reefs entirely depend.
The horizontal shading shows the barrier-reefs and lagoon-channels. It should be observed that the sections might have been taken in any direction through these islands, or through many other encircled islands, and the general features would have been the same.
I may first observe, that the reefs within the lagoons of atolls and within lagoon-channels, would, if favourably circumstanced, grow upwards during subsidence in the same manner as the annular rim; and, therefore, we might expect that such lagoon-reefs, when not surrounded and buried by an accumulation of sediment more rapid than the rate of subsidence, would rise abruptly from a greater depth than that at which the efficient polypifers can flourish: we see this well exemplified in the small abruptly-sided reefs, with which the deep lagoons of the Chagos and Southern Maldiva atolls are studded.
In the Society Archipelago, on the other hand, where the lagoon-channels are almost choked up, where much low alluvial land has accumulated, and where in some cases long islets have been formed on the barrier-reefs facts all showing that the islands have not very lately subsided only feeble shocks are most rarely felt.
In the Society archipelago, on the other hand, where the lagoon-channels are almost choked up, where much low alluvial land has accumulated, and where in some cases long islets have been formed on the barrier-reefs facts all showing that the islands have not very lately subsided only feeble shocks are most rarely felt.
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