When the channel has become broad and deep enough, coral growth may resume sway in it, sometimes to such an extent as almost to block it up again.
I need offer no proof of the formation of a reef flat and precipice by coral growth, the thing is obvious, at least in the case of ordinary fringing reefs. But the hollowing out of the boat channel, and with it that of other lagoons enclosed by coral, is less obvious, and it is natural to assign to a feature so distinctive of coral reefs an origin more directly dependent on the laws of coral growth. The proof comes from a consideration of the simplest case. Do we know of any reefs where solution and abrasion have formed these characteristic features without aid from coral growth? We do.
Plate XXXIII
Fig. 72. A portion of a reef flat shewing sections of contained shells
Fig. 73. The under surface of the same stone shewing that the reef is composed of a loosely cohering mass of shells and broken coral. In the centre of the mass is a shell easily recognisable as one living in abundance nowadays, Strombus fasciatus
The east coast of the island of Zanzibar is, like that of the Sudan on the Red Sea, composed entirely of elevated coral. But for some reason this abundant growth ceased shortly after the elevation of the island, and the reef edge bears now nothing but a little stony and filamentous seaweed, and in deeper water forests of sea-grass (Cymodocea). The reef is very wide, up to three miles, the edge regularly raised, and boat channel, as above mentioned, generally well developed. On the raised edge of the reef are numbers of stones, a foot or two in diameter, composed of the same recrystallised coral rock[51] as the shores and cliffs of the island. Now this rock differs very widely from that formed of recent coral in its hardness and weight. Its specific gravity totally forbids the assumption that these stones were torn from the reef by breakers and cast up in their present position above low tide level and indeed, though constantly among them and turning them over to search for specimens of marine life, I never saw one that had recently been moved by the waves, much less broken away from some projection of the submarine precipice.
In fact these are the hardest remnants of the mass of rock which has been removed in the cutting out of the reef, and their presence proves (1) that this was the mode of formation of the reefs, (2) that the addition by growth taking place since the elevation of the old reef has been either nothing or very inconsiderable. Here solution, attrition, and boring organisms alone have carved out from dead rock all the features of a reef which has grown up undisturbed[52].
The present flora of the reef edge may have been preceded by a flora and fauna capable of affording a more efficient protection, as is at present the case in the adjacent and similar island of Pemba, where the reef is narrower and consequently cleaner, and some stunted corals and Tubipora grow on the outer slope of the reef edge, a position where such species are never found in Zanzibar.