ture of the ocean, there are other and important conditions which influence the distribution of corals. One of these is the proximity of the mouths of rivers on account of the sediment which they bring down, and distribute over the sea bottom and the neighbouring coast. No coral reefs can be formed under such conditions. Too steep a shore and too deep water, is another obstruction to the growth of coral. Hereafter it will be shown that corals flourish in comparatively shallow water. Finally, the proximity of volcanic action seems to be a great deterrent to the growth of coral and the development of reefs.. For instance, although other conditions are favourable, the island of Hawaii has active volcanoes, and but few traces of coral about it, whereas neighbouring islands, which have long been free from volcanic action, have considerable coral reefs. The depth at which living corals may be found in the tropics, though varying somewhat in the estimates.... of different naturalists, is generally admitted to be comparatively small. There was a time when coral.. reefs were alluded to by voyagers as standing ins unfathomable ocean. This might have been only a poetical licence, or, more probably, hazarded without an attempt at sounding. Quoy and Gaymard, the naturalists who explored the Pacific in 1817-1820, were the first to show that the guess was unfounded. According to their observations the limit of distribu-tion downward was from 30 feet to 36 feet. Ehrenberg concluded that in the Red Sea living coral did not occur below 36 feet. Stutchbury, after a visit to the coral groups of the Paumotas and Tahiti, fixed the depth at from 96 feet to 100 feet. Darwin writes that, "Although the limit of depth, at which each particular kind of coral ceases to exist, is far from being accurately known, yet, when we bear in mind the manner in which the clumps of coral gradually became infrequent at about the same depth, and wholly disappeared at a greater depth than 120 feet, on the siope round Keeling atoll, on the leeward side of the Mauritius, and at rather less depth, both without and within the atolls of the Maldive and Chagos Archipelagos, and when we know that the reefs round these islands do not differ from other coral formations, in their form and structure, we may, I, think conclude that in ordinary cases reef-building polypifers do not flourish at greater depths than between 120 feet and 180 feet." 1 Captain Moresby reported to Darwin that he found only decayed coral on Padua Bank, north of the Laccadives which has an average depth of between 150 feet and 210 feet, but that on some other banks in the same group, with only 60 feet to 72 feet of water on them, the coral was living. Lieutenant Wellstead has also affirmed that, in the more northern parts of the Red Sea, there are extensive beds of living coral at a depth of 150 feet. Within the lagoons of some of the Marshall atolls, where the water can be but little agitated, there are, according to Kotzebue, living beds of coral in 150 feet. Captain Beechey states that branches of pink and yellow coral were frequently brought up from between 120 feet and 150 feet off the Low atolls, and Lieutenant Stokes, writing from the north-west coast of Australia, says that a strongly-branched coral was procured there from 80 feet. Professor Agassiz observes that, about the Florida reefs, the reef-building corals do not extend below 60 feet. Professor Dana remarks that in the Wilkes's Exploring Expedition the soundings, in the course of the various and extensive surveys, afforded no evidence of growing coral beyond 120 feet. Among the Fiji Islands the extent of coral reef grounds surveyed was many hundreds of square miles. The reefs of the Navigator's Islands were also sounded out, with others of the Society group, and, through all these regions, no evidence was obtained of corals living at a greater depth than 90 feet to 120 feet. He concludes that, "there is hence little room to doubt that twenty fathoms (or 120 feet) may be received as the ordinary limit in depth of reef corals in the tropics." of the growth, or extension of coral, either laterally or vertically, in the formation of reefs, as considered apart from the growth of individual species. On this point all the evidence we can produce is that collected by Darwin, and that of a very limited character. It has been assumed that the vertical growth of coral must be slow, as inferred from a few instances often cited, but which Darwin declares are inconclusive. Ehrenberg has said, for instance, that in the Red Sea, corals only coat other rocks in a layer from one to two feet in thickness, or at most to nine feet, and he did not believe that in any case they would, of their own proper growth, form stratified masses. He alludes to certain large massive corals in the Red Sea, which he imagined of such vast antiquity as to have been cotemporaneous with Pharoah. On this point Darwin admits that there are reefs in the Red Sea which do not appear to have increased in dimensions during half a century, and from comparison of old charts, probably not during the last two hundred years. These, and similar facts, have strongly impressed many with the belief of the extreme slowness of the growth of corals, so that they have even doubted the possibility of islands in the ocean being formed by this agency. On the contrary, there are facts, from which it may be inferred with certainty, that masses of considerable thickness have been formed by the growth of coral. There are knolls in the Southern Maldive atolls, some of which, according to Captain Moresby, are less than a hundred yards in diameter, and rise to the surface from a depth of between 250 feet and 300 feet. "Considering their number, form, and position," says Darwin, "it would be preposterous to suppose that they are based on pinnacles of any rock, not of coral formation; or that sediment could have been heaped up into such small and steep isolated cones. As no kind of living coral grows above the height of a few feet, we are compelled to suppose that these knolls have been formed by the successive growth and death of many individuals, first one being broken off or killed by some accident, and then another, and one set of species being replaced by another set with different habits, as the reef rose nearer the surface, or as other changes supervened. In reefs of the barrier class we may feel sure that masses of great thickness have been formed by the growth of coral; in the case of Vanikoro, judging only from the depth of the moat between the land and the reef the wall of coral rock must be at least 300 feet in vertical thickness." 1 There can be little doubt that Matilda atoll, in the Low Archipelago, has been converted, in the space of thirty-four years, from a "reef of rocks" into a lagoon island, fourteen miles in |
