In the first chapter, Darwin describes atolls and lagoon islands, taking as his typical example his own detailed findings and the Beagle survey findings on the Keeling Islands.
A brief introduction sets out the aims of the book. One possibility was a lack of calcareous matter around the islands, but his main proposal, which FitzRoy had suggested to him, was that the seas were too cold.Some dynamic relationship between the reefs and their foundations seemed to shape them. Darwin's Theory of Coral Atoll Formation. Coral islets are supposed to have been formed on the reef; and a ship is anchored in the lagoon-channel. To establish the thickness of coral barrier reefs, he relied on the old nautical rule of thumb to project the slope of the land to that below sea level, and then applied his idea that the coral reef would slope much more steeply than the underlying land. Having heard that parrotfish browsed on the living coral, he dissected specimens to find finely ground coral in their intestines. Should the land subside too quickly or sea level rise too fast, the coral dies as it is below its habitable depth. Returning to London six months later, he told and wrote not of species variation, but of coral reef variation. Now as the island sinks down, either a few feet at a time or quite insensibly, we may safely infer from what is known of the conditions favourable to the growth of coral, that the living masses, bathed by the surf on the margin of the reef, will soon regain the surface. Subsequent sections in each chapter then describe other reefs in comparison with the typical example. And the Pacific's many atolls — collections of smaller coral islands — often took ringlike or looplike forms themselves. Meanwhile, Lyell incorporated Darwin's subsidence theory into his edition of Principles, making it quite literally the textbook explanation. The structure and distribution of coral reefs. Though Eschscholtz overextended this idea to unlikely deeps because the depth limit of coral growth wasn't established until just after he offered his theory, his hypothesis seemed to account for many reef forms and helped to explain lagoons.
This chapter ends with a recapitulation which summarises the findings of each chapter and concludes by describing the global image as "a magnificent and harmonious picture of the movements, which the crust of the earth has within a late period undergone". FitzRoy noted the smooth and solid rock-like outer wall of the atoll, with most life thriving where the surf was most violent.
London and New York: Continuum.
The likenesses startle. All three types of reefs are common in the South Pacific. But the charts showed that those taller islands were vastly outnumbered by the thousands of coral isles and atolls, including archipelagos hundreds of miles long, that barely cleared the water.
His botched explanation of that valley's parallel roads, published two years after his initial coral reef presentation, also sought to explain a geologic mystery by proposing a series of changes over long periods. Not sure?
The coral islands would thus furnish proof both of that subsidence and, indirectly, of the elevation of South America. By the time he wrote his Autobiography, inhe could accurately say of The Structure and Distribution of Coral Reefs that it was "thought highly of by scientific men, and the theory therein given is, I think, now well established.