After a time, an immense quantity of such materials is formed about a Coral Reef; tides and storms constantly throw them up on its surface, and at last a soil collects on the top of the Reef, wherever it has reached the surface of the water, formed chiefly of its own débris, of Coral sand, Coral fragments, even large masses of Coral rock, mingled with the remains of the animals that have had their home about the Reef, with sea-weeds, with mud from the neighboring land, and with the thousand loose substances always floating about in the vicinity of a coast and thrown upon the rocks or shore with every wave that breaks against them. Add to this the presence of a lime-cement in the water, resulting from the decomposition of some of these materials, and we have all that is needed to make a very compact deposit and fertile soil, on which a vegetation may spring up, whenever seeds floating from the shore or dropped by birds in their flight take root on the newly formed island.
There is one plant belonging to tropical or sub-tropical climates that is peculiarly adapted by its mode of growth to the soil of these islands, and contributes greatly to their increase. This is the Mangrove-tree. Its seeds germinate in the calyx of the flower, and, before they drop, grow to be little brown stems, some six or seven inches long and about as thick as a finger, with little rootlets at one end. Such Mangrove-seedlings, looking more like cigars than anything else, float in large numbers about the Reef. I have sometimes seen them in the water about the Florida Reef in such quantities that one would have said some vessel laden with Havana cigars had been wrecked there, and its precious cargo scattered in the ocean.
In consequence of their shape and the development of the root, one end is a little heavier than the other, so that they float unevenly, with the loaded end a little lower than the lighter one. When they are brought by the tide against such a cap of soil as I have described, they become stranded upon it by their heavier end, the rootlets attach themselves slightly to the soil, the advancing and retreating waves move the little plant up and down, till it works a hole in the sand, and having thus established itself more firmly, steadied itself as it were, it now stands upright, and, as it grows, throws out numerous roots, even from a height of several feet above the ground, till it has surrounded the lower part of its stem with a close net-work of roots. Against this natural trellis or screen all sorts of materials collect; sand, mud, and shells are caught in it; and as these Mangrove-trees grow in large numbers and to the height of thirty feet, they contribute greatly to the solidity and compactness of the shores on which they are stranded.
Such caps of soil on the summit of a Coral Reef are of course very insecure till they are consolidated by a long period of accumulation, and they may even be swept completely away by a violent storm. It is not many years since the light-house built on Sand Key for the greater security of navigation along the Reef was swept away with the whole island on which it stood. Thanks to the admirably conducted Investigations of the Coast-Survey, this part of our seaboard, formerly so dangerous on account of the Coral Reefs, is now better understood, and every precaution has been taken to insure the safety of vessels sailing along the coast of Florida.
I cannot deny myself the pleasure of paying a tribute here to the high scientific character of the distinguished superintendent of this survey, who has known so well how to combine the most important scientific aims with the most valuable practical results in his direction of it. If some have hitherto doubted the practical value of such researches,—and unhappily there are always those who estimate intellectual efforts only by their material results,—one would think that these doubts must be satisfied now that the Coast-Survey is seen to be the right arm of our navy. Most of the leaders in our late naval expeditions have been men trained in its service, and familiar with all the harbors, with every bay and inlet of our Southern coasts, from having been engaged in the extensive researches undertaken by Dr. Bache and carried out under his guidance. Many, even, of the pilots of our Southern fleets are men who have been employed upon this work, and owe their knowledge of the coast to their former occupation. It is a singular fact, that at this very time, when the whole country feels its obligation to the men who have devoted so many years of their lives to these investigations, a proposition should have been brought forward in Congress for the suspension of the Coast-Survey on economical grounds. Happily, the almost unanimous rejection of this proposition has shown the appreciation in which the work is held by our national legislature. Even without reference to their practical usefulness, it is a sad sign, when, in the hour of her distress, a nation sacrifices first her intellectual institutions. Then more than ever, when she needs all the culture, all the wisdom, all the comprehensiveness of her best intellects, should she foster the institution that have fostered them, in which they have been trained to do good service to their country in her time of need.
Several of the Florida Keys, such as Key West and Indian Key, are already large, inhabited islands, several miles in extent. The interval between them and the main-land is gradually filling up by a process similar to that by which the islands themselves were formed. The gentle landward slope of the Reef and the channel between it and the shore are covered with a growth of the more branching lighter Corals, such as Sea-Fans, Coral-lines, etc., answering the same purpose as the intricate roots of the Mangrove-tree. All the débris of the Reef, as well as the sand and mud washed from the shore, collect in this net-work of Coral growth within the channel, and soon transform it into a continuous mass, with a certain degree of consistence and solidity. This forms the foundation of the mud-flats which are now rapidly filling the channel and must eventually connect the Keys of Florida with the present shore of the peninsula.
Outside the Keys, but not separated from them by so great a distance as that which intervenes between them and the main-land, there stretches beneath the water another Reef, abrupt, like the first, on its seaward side, but sloping gently toward the inner Reef, and divided from it by a channel. This outer Reef and channel are, however, in a much less advanced state than the preceding ones; only here and there a sand-flat large enough to afford a foundation for a beacon or a lighthouse shows that this Reef also is gradually coming to the surface, and that a series of islands corresponding to the Keys must eventually be formed upon its summit. Some of my readers may ask why the Reef does not rise evenly to the level of the sea, and form a continuous line of land, instead of here and there an island. This is accounted for by the sensitiveness of the Corals to any unfavorable circumstances impeding their growth, as well as by the different rates of increase of the different kinds. Wherever any current from the shore flows over the Reef, bringing with it impurities from the land, there the growth of the Corals will be less rapid, and consequently that portion of the Reef will not reach the surface so soon as other parts, where no such unfavorable influences have interrupted the growth. But in the course of time the outer Reef will reach the surface for its whole length and become united to the inner one by the filling up of the channel between them, while the inner one will long before that time become solidly united to the present shore-bluffs of Florida by the consolidation of the mud-flats, which will one day transform the inner channel into dry land.
What is now the rate of growth of these Coral Reefs? We cannot, perhaps, estimate it with absolute accuracy, since they are now so nearly completed; but Coral growth is constantly springing up wherever it can find a foothold, and it is not difficult to ascertain approximately the rate of growth of the different kinds. Even this, however, would give us far too high a standard; for the rise of the Coral Reef is not in proportion to the height of the living Corals, but to their solid parts which never decompose. Add to this that there are many brittle delicate kinds that have a considerable height when alive, but contribute to the increase of the Reef only so much additional thickness as they would have when broken and crushed down upon its surface. A forest in its decay does not add to the soil of the earth a thickness corresponding to the height of its trees, but only such a thin layer as would be left by the decomposition of its whole vegetation. In the Coral Reef, also, we must allow not only for the deduction of the soft parts, but also for the comminution of all these brittle branches, which would be broken and crushed by the action of the storms and tides, and add, therefore, but little to the Reef in proportion to their size when alive.
The foundations of Fort Jefferson, which is built entirely of Coral rock, were laid on the Tortugas Islands in the year 1846. A very intelligent head-work man watched the growth of certain Corals that established themselves on these foundations, and recorded their rate of increase. He has shown me the rocks on which Corals had been growing for some dozen years, during which they had increased at the rate of about half an inch in ten years. I have collected facts from a variety of sources and localities that confirm this testimony. A brick placed under water in the year 1850 by Captain Woodbury of Tortugas, with the view of determining the rate of growth of Corals, when taken up in 1858 had a crust of Maeandrina upon it a little more than half an inch in thickness. Mr. Allen also sent me from Key West a number of fragments of Maeandrina from the breakwater at Fort Taylor; they had been growing from twelve to fifteen years, and have an average thickness of about an inch. The specimens vary in this respect,—some of them being a little more than an inch in thickness, others not more than half an inch. Fragments of Oculina gathered at the same place and of the same age are from one to three inches in length; but these belong to the lighter, more branching kinds of Corals, which, as we have seen, cannot, from their brittle character, be supposed to add their whole height to the solid mass of the Coral wall. Millepore gives a similar result.
Estimating the growth of the Coral Reef according to these and other data of the same character, it should be about half a foot in a century; and a careful comparison which I have made of the condition of the Reef as recorded in an English survey made about a century ago with its present state would justify this conclusion. But allowing a wide margin for inaccuracy of observation or for any circumstances that might accelerate the growth, and leaving out of consideration the decay of the soft parts and the comminution of the brittle ones, which would subtract so largely from the actual rate of growth, let us double this estimate and call the average increase a foot for every century. In so doing, we are no doubt greatly overrating the rapidity of the progress, and our calculation of the period that must have elapsed in the formation of the Reef will be far within the truth.