Reef-making corals can not endure exposure to the air and they can not thrive at a depth of more than 20 fathoms, so that their vertical range is about 115 feet; yet hooks and anchors brought up coral rock and sand from many hundreds of feet below the limit of growth; in a great number of instances, the atolls or ring-like reefs are mere peaks rising with abrupt slopes from "fathomless" abysses. Coral-bearing areas within the Indian and Pacific Oceans are of vast extent, there being chains of archipelagos, 1,000 to 1,500 miles long. The reefs are rudely circular or elliptical in the islands but are linear along the coasts; in the one case, the reef encloses a lagoon, in the other, a lagoon-like channel separates the reef from the coast. These are fundamental elements of the problem, not one of which may be neglected in the solution. A clue to the explanation was found by this keen observer when he saw an islet of old rock, fringed with coral, rising from the lagoon of an atoll, so that the atoll-ring resembled in many respects the barrier reef of a continent and the lagoon itself resembled the lagoon-like channel seen on the Australian and other coasts.
Chamisso's suggestion that coral reefs had been formed on banks of sedimentary material seemed wholly incompetent to meet the conditions, for the areas are too vast, and Darwin was compelled to believe that the atolls rest on rocky bases; but even on this supposition, it appears incredible that peaks of several great mountain chains should all come to within less than 180 feet of the surface and that not one rose any higher. The long study in South America had prepared him to seek an explanation in mobility of the earth's crust; but it was clear that elevation could not bring about the conditions, as that would destroy the corals themselves; subsidence alone can account for the phenomena. And thus Darwin presents his case:
The origin of the ring as well as that of the barrier reef seemed to be easily explained by this hypothesis. The corals on the outer side of the reef grew with greater rapidity than did those within, as the supply of food is constant; those on the inner side became starved and eventually the interior growth ceased and the lagoon was shallowed by wind-drifted material from the shores.