Chapter VIII: The Building of Reefs

Sea-water, besides containing comparatively large quantities of common salt, contains several other substances in solution in less quantity. One of these is the limestone[45] which the coral polyp extracts and renders solid as its stony skeleton, and of which, in essentially the same way, the “shell fish,” whether oyster, winkles or crabs make their hard coverings. Another constituent is magnesium carbonate, a substance rather similar to limestone, to which we refer later.

Having examined the individual stone formed by the growth of a coral colony we must consider how such stones are aggregated to form a reef.

It is obvious that colonies cannot live for ever, any more than do individuals, and we need to know the fate of a dead colony and how it is replaced by a living one which shall continue the building. So great is the competition among the crowds of the floating young of the fixed animals that any vacant spot is at once appropriated. When a coral colony dies the coloured film of flesh speedily rots away and the snow-white stony skeleton remains, washed clean by waves and currents[46]. In a few days this is covered with a film of the finest green seaweed, invisible among which are the embryos of several orders of animals, e.g. shell-fish, or, there may be the larvae of some other coral. There is a tense struggle for survival among these young creatures, but on a growing reef conditions of course generally favour the coral’s young (otherwise the reef would cease to grow), some of which grow at the expense of nearly everything else and cover the site. Many of the large hemispherical corals live on the reef like loose stones, but on turning them over one may find quite a small shell, or coral branch, attached to the centre of the underside. This is the foundation of the whole, the resting place of the tiny floating larva, the growth of which first covered the stone on which it settled by a vigorous colony which when large enough to be independent of support continued its growth until the mass exceeded by hundreds of times the bulk of the original foundation.

The more delicate of coral skeletons, such as those of the porous branched madrepores, rarely survive the death of the polyps that formed them. On losing its coat of living flesh the coral is exposed to the action of boring animals, as well as to the direct solvent action of the sea-water, and many are thus destroyed. Partly they go back into solution, but the greater portion is broken down to mud and sand. In shallow water branched colonies are broken up into pebbles and coarse sand by the waves, and these materials serve to fill in the spaces between the larger colonies and pack the whole together into a solid mass.

There are other constituents of coral reefs of not very much less importance than the corals themselves. Large masses are formed of the bivalve shells which live in the coral mud, and which, by the solidification of this mud, form with it a limestone, such as that of which the houses of Port Sudan are built. Although among the very numerous and conspicuous fossils of this stone coral branches are not the commonest, yet the mass of shells and hardened mud is every bit as much a part of the reef as anything else is. In some reefs too, even where coral is growing abundantly, the shells of the great clam _Tridacna_ are so abundant as to make up a considerable part of the total mass. Others again contain quantities of certain peculiar seaweeds (of which the “coralline” of British seas is one) which, though true plants in every detail, have the property of taking up limestone from the sea and forming therewith a skeleton, even harder and more compact than that of the corals. Plate XXX shews the appearance of these plants, and will enable the reader to identify some of those he meets with. These sometimes form a cement, by which the coral colonies and fragments are held together, and in some others the whole reef is formed of them[47]. Other organisms assist, but I observe my principle of dealing only with the most important features and desist from enumerating all.

This is the whole structure of the interior of such a reef as that which fringes the Red Sea coast, as seen e.g. during the excavation of the quay walls or slipway at Port Sudan. Great “stones” which are the more massive colonies, generally the genus _Porites_, are bedded in with smaller colonies whole or broken. In places are collections of grey mud and sand, also formed from coral by the action of boring organisms or perhaps as the residue left after partial solution by the sea.

“How fast does a coral reef grow?” is a question often asked, and never as yet truly answered. Probably each of the hundreds of species of coral has its own maximum rate of growth, which is however rarely attained, as it is certain that the rate of every colony of each species varies very widely with its position on the reef and its immediate surroundings. So taking the rate of growth of a few samples would go a very little way towards giving that of the corals in any given square yard of the reef edge. Although individual colonies may grow quite rapidly this is but half the question. We must know also full details of the action of eroding and transporting sea currents, solution, boring organisms (in coral, coral sand and mud) and subtract the total from that of the deposition of stone by living polyps, to obtain the net increase.

_Plate XXX_

The boring animals mentioned as reducing coral stone to mud are very easily found and examined. Almost any old worn piece of coral, and many still living colonies, are found to be studded with small slit-like holes with slightly raised borders. On breaking into the stone each hole is found to lead at once into an oval cavity, say an inch long by three-eighths in diameter, containing a bivalve shell of about the same size, _Lithodomus_ by name, from its appearance known as the “date shell,” which has made the hollow and is continually enlarging it. Other colonies when broken across instead of shewing pure white limestone are found to be honeycombed with yellow or red spongy matter. This is the sponge _Clione_, which has the property, especially astonishing in a sponge, of boring into any limestone, whether coral or shell, making it quite rotten and so, finally, reducing it to mud and sand.

Certain worms live in the same way. The largest species of these, by name _Eunice siciliensis_, attains a length of a yard or so, and the thickness of a quarter of an inch, but so intricate is its boring that it is practically impossible to extract a full-grown specimen entire. The head end is at the innermost part of the burrow, and when extracted the two white gouge-like teeth, by which the burrow is cut out, are easily seen.

There is a fish too, _Pseudoscarus_ by name, which actually lives on coral! It is commonly taken by fishermen and is easily recognised by its gorgeous green, blue and pink colours, but particularly by its teeth, which are fused into two pairs of chisels, with which the surface of the coral, and with it its living matter, is browsed away. Cut open a specimen of this fish and you find its guts full of fragments of coral[48].

Figs. 66 and 67. Two specimens of _Porites_.

In 66 the flesh has been left on the coral and where it is broken
it is seen that the dark coloured living matter penetrates the
mass only to the bottoms of the coral cups. In this small piece
the openings of 14 _Lithodomus_ burrows are seen, and in four of
them the lips of the shell are visible.

In 67 the base of the still living coral is rotting away, being full
of small holes formed by the sponge _Clione_ and small boring worms.

Fig. 68. An old piece of coral in which so much of the surface
has rotted away that the numerous _Lithodomus_ borings, of which
only the small openings can be seen in living corals, are fully
exposed. This in spite of partial protection by growth of encrusting
stony seaweed as at the point marked _a_.

Fig. 69. Section of a large shell, a material very much harder than
any coral, yet bored in the same way by both mollusca and sponge.

Fig. 70. The mollusc _Pholas_ lying in its burrow in coral.

Fig. 70 about natural size, the rest about half this.

From specimens in the Cambridge University Museum of Zoology.

_Plate XXXI_

Borings of molluscs and sponges]

_Pholas_, another boring bivalve mollusc, common in shales on some British coasts, is less often seen. Its burrow is deeply buried in a solid living colony separated from the outside by a comparatively long passage. But it is not nearly so common as the preceding forms, and lives more solitary, _Lithodomus_ generally occurring in numbers together.

After the coral has been broken down by these means, the sand is further reduced to fine mud by the action of those animals which live by burrowing in it, and passing large quantities through their guts, after the manner of earthworms. Just as there is a great fauna which lives on the nourishment filtered from large quantities of sea-water[49] so there is another great and varied community of sand eaters. There are first of all the worms, next, but more important in the tropics, great numbers of large holothurians or “sea slugs” (though slugs they are _not_), some of which crawl on and eat only the surface sand, but one species burrows deeply and raises casts like an earthworm, but a hundred times the size. Considering the large effects produced by the ordinary earthworm in a year, that resulting from the presence of animals hundreds of times their bulk, whose casts in many lagoons entirely cover the bottom, must be very considerable indeed.

One observation that can be made by anybody is to note how long it is before corals reappear once a reef has been cleared out, e.g. for the foundations of a quay wall. Two small portions of an apparently growing reef at Port Sudan were buried under a pile of stones for the foundations of the east and west Customs landings, and four years later there was no growth of coral on the artificial slopes thus made, though every condition apparently remains as favourable as before. Again at a point inside Dongonab Bay, where coral growth is luxuriant in shallows, the coral was some years ago collected from one spot and a sea wall built with it. A few small colonies have established themselves upon the sides of the wall after an interval of twelve years; they are perfectly healthy, yet their bulk is an infinitesimal fraction of that removed from the wall by solution and attrition. This shews how even among growing coral one cannot be sure that the degradation of rock to sand and mud is not in excess of aggradation, i.e. its building up by coral organisms, and how a lagoon may be rapidly eating away its encircling reefs and yet contain comparatively luxuriant coral gardens.

It is in its external form that a coral reef shews features which give it an individuality above that of a mere heap of stones. Generally it rises with a steep slope from the sea bottom which ends in a low precipice, above which is another and more gentle slope to the highest point of the reef, a foot or two above lowest water, which is near its outer edge. Passing landwards the reef level is lower again, and we may have a boat channel or series of lagoons, where the native canoes can travel on calm water however the sea may rage outside. This is succeeded by a flat of bare rock, which rises slowly up to the base of the undercut coral cliffs as in imaginary section in Diagram 1 and the Photograph on Plate XXXII.

We have here three striking features, viz. precipitous reef edge, raised border and reef flat with boat channel, strongly differentiating the shore of a coral sea from the more or less even slope we are accustomed to at home, resulting in a nearly waveless shore and breakers out at sea, an endless line of purest white dividing the green of the shallows from the blue-black of the deep water.

_Plate XXXII_

Imagine land newly raised from the sea upon which coral growth is only beginning. In section its coastline would be a more or less gradual slope (to take the simplest case) as the line A, B in Diagram 2, sea level being represented by the line C, D. Suppose the scale to be such that the depth C to A is about 50 fathoms. Now it is found that under the best of conditions reef corals do not grow at this depth; if the conditions are less favourable so the maximum depth at which

these corals grow is decreased. (That this fact is the crux of the problems to be discussed later may as well be noted at once.) Coral growth will be most luxuriant in the shallow water, and the first stage of our reef will be a mound of coral of the shape shewn in section by the dotted area. Between E and D this comes to the surface, and the corals, projecting above the water at lowest spring tides, are killed at the top, so that E to D becomes an almost flat surface of dead corals, some of which may, however, be still living where their bases are immersed in clear sea-water. A continuation of this process gives us a reef flat of considerable area, indicated by the line F, D, the slope A, F becoming correspondingly steepened. At the point F the waves have thrown up a long low mound of coral fragments and shells, which, in the way described below, may be consolidated into a ridge of solid rock.

It is easy to see how an extension of coral growth would make A, F a regular precipice, as F approximates to C. What happens after F, the reef edge, grows out to water 50 fathoms deep, where no living foundation can be laid for

the support of the still growing reef above? Diagram 3 explains. Passing seawards from F is the gentle slope formed by the breaking waves, next a precipice, followed by a very steep slope to the sea bottom beyond A formed of the broken and dead corals fallen from the growing zone above, and which forms the foundation on which the shallow water corals extend the reef seawards.

Now at the same time as growth has added to the reef seawards the waves have cut down the land on the other side. Consider this case separately and then combine with that above. As before, A, B in Diagram 4 is the outline in section of recently formed land upon which the sea has as yet had no action, and C, D is the level of lowest tides, C′, D′ that of the highest. Between these two levels, upon the land mass lying between D and D′, is the never ceasing beat of waves and the wear of silt-carrying currents, so that in time the land is eaten away along a line a little above C, D, say X, Y, and a cliff, Y, Z, is formed. We are assuming that the material of the land is sufficiently coherent to form such a flat and cliff, but even so in general X, Y becomes a sloping shore, not a flat. It only remains as a flat if for some reason the seaward surface at X is protected against further detrition by waves, e.g. by the growth of corals and stony seaweeds. If they are present, even if their growth adds nothing to the mass of the rock, it hinders its decay and causes the formation of a reef flat in place of a sloping shore.

Now these two processes, addition by growth and abrasion by wave action, go on simultaneously, and to get at the true method of formation of a reef flat in the Red Sea the two diagrams must be combined, as in Diagram 5, where as before F is the raised reef edge and F, Y the whole extent of the reef flat, and the cliff Z, Y is undermined as shewn. Where, as in some seas, F to D is recent growth and X to Y is rather older coral rock, it is impossible to locate accurately the dividing line between reef formed by recent growth and that cut out of the land, but near shore, where the flat is free from mud and sand, its surface is seen to consist of _sections_ of the constituent shells and corals, cut as cleanly as if done by a stonemason (see Plate XXXIII). Even such hard shells as those of the “giant clam,” _Tridacna_, are cut across at the same level, thus shewing very clearly the origin of the surface by the planing down of a mass of rock to that level.

The boat channel indicated between F and G remains to be accounted for. As the reef flat widens it results in a great area covered by shallow water at high tide level and partly bare at low. Exposed to a tropic sun life is impossible for any but a few specialised forms, the rock is unprotected from such wave motion as there is, and from boring organisms, which agencies quickly reduce its level. Strong currents also flow over the surface, for the breakers throw water over the raised edge which may have to travel several miles before reaching a gap through which it can return to the sea, and this with tidal currents cause swift rivers of muddy water to flow over the reef flat parallel with the shore. The obvious result is the hollowing out of a boat channel[50], and the accumulation in it of great quantities of mud and sand, which in many places form the greater part of its actual surface, but which eventually are swept out to sea.

The presence of certain marine flowering plants of grass-like form (_Cymodocea_ and other genera) assists, if not wholly responsible for, the formation of such accumulations by binding the mass together by their strong and tangled roots and rhizomes.

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_

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.

The absence of corals from the outer slope of the reef edge is remarkable seeing that they flourish in a few places in the boat channel, so much so in one place as to almost block it up and form a new reef surface. They flourish too round all the many sandbanks and islets of the channel which separates Zanzibar from the mainland of Africa. The mud from the broad reef flat together with the strong currents that impinge upon these coasts are amply sufficient to prevent the settlement of the delicate coral larvae, if not to destroy full-grown colonies.

Another case from the Cape Verde Islands, where reef corals do not exist at all, is shewn on Plate XXXIV. A reef flat, with raised definite edge and miniature boat channel complete, has been cut out of sandstone, the edge of which was protected by a growth of stony seaweed (lithothamnia), and vast numbers of the shelly tubes of that strange animal _Vermetus_[53]. These two organisms combine to form a continuous crust over the whole surface of the seaward edge of the sandstone, and so greatly delay its removal by the sea, but landwards, this protection being absent, the reef flat is hollowed out into a “boat channel.” This sandstone is a local deposit just to the south-west of the town of St Vincent, but the volcanic rocks of which the island is composed are cut down to a narrow flat in the same way, but less regularly.

_Plate XXXIV_

A third case, from the Mediterranean near Alexandria, is so striking as to be worth illustrating, though only the embryo of a reef, as it were a ledge a few yards wide, has been formed as yet. The rock is a calcareous sandstone, a consolidated dune, and the protecting organisms are much the same as those found in the Cape Verde Islands, but here forming a less coherent coating to the rock. The regularity of the ledge laid bare by the retreat of a wave is very striking.

Reefs may shew other features, no one arrangement can be taken as typical of all. Instead of the smooth slope and rounded ridge which compose the reef edge on this coast and that of Zanzibar it is usual, in many oceanic reefs, for the growing edge to be cut into by deep and narrow fissures, up which the great breakers send violent torrents of water.

The land, or reef islands, may be either portions of the reef elevated above sea level, containing fossil corals in the positions in which they grow, or it may be partly formed of a mass of corals thrown up by storms backed generally by an accumulation of sand. The coral rock thus elevated may be, as in the Red Sea, but little different from the original material of which it was formed, but more generally it is much altered. The continual wetting by spray or rain and drying under the tropical sun has a very marked effect in hardening and consolidating elevated coral, or coral sand. The upper parts are dissolved, and as the water sinks into the porous corals and becomes supersaturated with lime, the latter is crystallised out, thus filling up all cavities with _crystalline_ limestone. Thus in the end the highly porous heterogeneous limestone becomes a rock of exceeding hardness, crystalline and homogeneous. All the more delicate organisms are dissolved, only the largest remaining recognisable. At the same time as sea-water contains magnesium carbonate as well as limestone, and the former is less soluble than the latter, it tends to be deposited more quickly, so that it comes to replace the original limestone to some extent[54]. The alteration in the external appearance of the rock is very marked. Instead of the yellow, rather shapeless, cliffs of the Red Sea coast, in most other parts of the world, where tides supply spray and there is a considerable rainfall, we have coal-black rock with a very peculiar surface, all covered with sharp points and knife edges separating depressions left by the solution of the stone by water, hence the name “coral rag” applied to such rock. Where it forms the shore of a sheltered bay its homogeneity causes the undermining by the sea to go on to an astonishing extent before the unsupported piece falls away from the cliff to which it is attached. Such projections of the rocks which may be much longer than those shewn on Plate XXXV, also illustrate the hardness of this recrystallised material, for on striking one with a hammer a loud clear bell-like note is produced. Given the right conditions and we have the same peculiar result in the Red Sea and even in the Mediterranean. For instance, a considerable swell breaks at times on the narrow reef fringing the east side of the Tella Tella Kebir Islands, thus keeping the cliff behind it drenched with spray. In consequence the rock has become like that of Zanzibar and British East Africa. And generally, wherever the coral rock is exposed to spray it takes on these characters partially or completely, as is the case at the bases of all the cliffs along a narrow band just about sea level, where the rock is “’twixt wind and water.” Here the outer part is converted into a black, hard, and pitted crust, higher up it is harder than normal but above gradually passes into the slightly altered rock of the normal cliffs. Such a crust also covers the reef flats of the Red Sea, the reef within consisting, as before noted, of loose masses of coral bedded in with shells and sand. A portion of this crust is photographed on Plate XXXIII; the upper surface (Fig. 72) with its _sections_ of contained shells has already been referred to. It is nearly smooth and very hard. The under side of the same fragment is shewn in the next figure and is seen to consist of an irregular mass of shells and coral branches lightly cemented to the crust, from between which the sand, which has not been consolidated, has fallen away. The formation of beach sandstone is practically the same process of cementation, by alternate solution and deposition of lime, taking place in a mass of shell and coral sand instead of larger fragments, the rock following exactly the curve of the sandbank, of which it is obviously a part which has been consolidated _in situ_.

_Plate XXXV_

Fig. 76. Chuaka Bay. Note undermining of fallen fragments

„ 77. Bawi Island. Rock masses supported by narrow stalks

Coral reefs are classified into three sets according to their relation with other land[55].

I. _Fringing reefs_, which, as the name implies, border the land, are continuous with it, and the seaward edge of which can be reached by wading.

II. _Barrier reefs_, which run parallel to the coast but separated from it by deep water navigable for coasting vessels larger than canoes.

III. _Atolls_, ring- or crescent-shaped reefs having no obvious relation to any land and typically found far out in the ocean, from the great depths of which they rise with steep slopes to, at most, a few feet above high tide level.

Fringing reefs we have already dealt with; the two agents described—growth and abrasion of coral—will account for all of them. Barriers and atolls are more puzzling. Why should the barrier form its line parallel to the coast, though at a distance from it, and the very existence of atolls is one of the most striking phenomena of Nature.

The problem is complicated by the fact that ordinary reef corals die out at a depth of 50 fathoms or so. Now 50 fathoms is a mere nothing compared to the depths from which the Pacific atolls rise, and is only a quarter the depth often found within a few hundred yards of the Red Sea reefs. How then to account for the building of reefs in deep water?

One suggestion was that atoll rings were formed by the growth of a mere cap of coral round the edge of the craters of huge submarine volcanoes. But that postulates far too large a number of such immense volcanoes[56], and the early stages of these formations have not been found. Darwin’s hypothesis was hailed with joy as the obvious solution, and held the field against all rivals for many years. Briefly it is that corals formed a reef by direct growth in shallow water on the coast of an island, forming a fringe thereto in the way explained above. Now is postulated one of those great, slow earth movements such as have very often occurred in the past and are occurring at the present day. In this case the island is to sink slowly, at such a rate that the reef grows upwards as fast as it is submerged. The result is obviously a mass of corals of a thickness equal to the total sinking movement of our island, though every individual coral grew while in water under 50 fathoms deep.

When our island is half submerged the fringing reef has become a barrier, when wholly gone the reef ring remains enclosing an empty lagoon, and is the only mark of the grave of a drowned island. Thus Darwin’s theory has the further merit of referring the two forms of reef, barrier and atoll, to one common cause, the sinking of the land. But we have no idea of how the original islands were formed in such numbers, and many believe that no such vast sinking of the ocean basins has occurred since they were formed. Also, if solution be ignored, it is difficult to see why, as the island sank, coral growth did not close in over the submerged land, and so form a vast reef flat instead of leaving a lagoon up to 50 fathoms deep.

To settle the matter an expedition was sent to a typical atoll, Funafuti, and a boring 1200 feet deep was made to find out what the interior of the reef is made of. The material brought out of the bore hole has been carefully examined by experts, and reported to consist of the remains of exactly similar corals to those found near the surface, and this result was taken by one or two geologists as complete vindication of Darwin’s theory. But apart from the extreme difficulty of the identification of all coral species, especially those which have been subject to partial crystallisation and so on, one remembers that a considerable part of the foundations in deep water are formed of corals which have fallen down the steep slope from the growing reef above, so that their presence buried a thousand fathoms deep proves nothing, while the boring at Funafuti only went to about 200 fathoms.

After all it is easier to imagine that the atoll grew up from the bottom of the deep sea. The only postulate is a chance elevation on the sea bottom. On such elevations it is found that the remains of marine organisms, including deep sea corals (as distinct from reef builders), tend to accumulate much more rapidly than on the floor of the surrounding depths. The elevation is consequently slowly but surely raised, and the higher it grows the more rapid the accumulation, until at last reef corals obtain a footing forming a cap or pinnacle reaching to the surface. From this masses of coral, sand, stones or large boulders, are always falling on to the foundation slopes, forming successive sloping layers indicated by the dotted lines, upon which fresh growth takes its rise[57]. When the coral reef has become of some breadth (and atoll rings may be 30 miles or more across) a boring at the edge might descend for a thousand fathoms and never meet with the original foundations, but would pass only through recent corals fallen from the shallow zone.

We should expect to find a continuous surface of coral at sea level. As a matter of fact there is a broad lagoon, generally of considerable depth, one or two gaps through the encircling reef giving communication with the open ocean. This is the natural result of the causes described when dealing with the boat channel of a fringing reef; it is the same thing on a much larger scale. Seeing that the rate of growth of coral masses is always only the excess of growth over destruction and solution, the presence of growing corals is no evidence against the fact that the lagoon shores may be undergoing destruction, and that such coral growth as is present may add nothing to the inner sides of the reef in the end. No more does the accumulation of great quantities of mud prove that the lagoon will in time be quite filled in. Mud and sand[58] are but stages in the destruction of coral rock, and its presence where that process is going on is to be expected. An abnormal tide, a shift of the currents, and vast quantities are swept out through the gaps in the reefs. My home on the Red Sea is beside a large landlocked lagoon in which coral gardens of great luxuriance, whence collections of many species can be procured, are frequent. Spite of this, the evidence is as clear as possible that its shores and islands are undergoing rapid denudation, and its reefs are being cut down by currents to banks below water level. As in the Red Sea the level rarely alters by more than a foot once in the twenty-four hours, and often the rise or fall is much less, the action of tidal currents is at a minimum, yet even so they produce well-marked effects.

Barrier reefs may be formed from fringing reefs by the enlargement of the boat channel, while the reef is extending seawards.

The island of Zanzibar, 60 miles long by 20 wide, and 20 miles from the mainland of Africa, seems to be a part of the East African barrier system, and it certainly was separated from the mainland by the destruction of the intervening land; the shallow dividing channel being full of shoals and sandbanks formed by cutting down of islands. The fauna of Zanzibar, including leopards, serval cats, &c., can be accounted for in no other way. The Great Barrier of Australia, a thousand miles long, is the same thing on a vastly greater scale. But, as described in the next chapter, the Barrier system of the Red Sea is quite another thing, and its mode of formation may possibly be unique in the world.