Chapter XVIII: Appendix (3)

EAST COAST OF AFRICA.—Proceeding from the northern part, the coast appears, for a considerable space, without reefs. My information, I may here observe, is derived from the survey by Captain Owen, together with his narrative; and that by Lieutenant Boteler. At _Mukdeesha_ (10° 1′ N.) there is a coral-reef extending four or five miles along the shore (Owen’s “Narr.” volume i, page 357) which in the chart lies at the distance of a quarter of a mile from the shore, and has within it from six to ten feet water: this then is a fringing-reef, and is coloured red. From _Juba_, a little S. of the equator, to _Lamoo_ (in 2° 20′ S.) “the coast and islands are formed of madrepore” (Owen’s “Narrative,” volume i., page 363). The chart of this part (entitled _Dundas Islands_), presents an extraordinary appearance; the coast of the mainland is quite straight and it is fronted at the average distance of two miles, by exceedingly narrow, straight islets, fringed with reefs. Within the chain of islets, there are extensive tidal flats and muddy bays, into which many rivers enter; the depths of these spaces varies from one to four fathoms—the latter depth not being common, and about twelve feet the average. Outside the chain of islets, the sea, at the distance of a mile, varies in depth from eight to fifteen fathoms. Lieutenant Boteler (“Narr.” volume i., page 369) describes the muddy bay of _Patta_, which seems to resemble other parts of this coast, as fronted by small, narrow, level islets formed of decomposing coral, the margin of which is seldom of greater height than twelve feet, overhanging the rocky surface from which the islets rise. Knowing that the islets are formed of coral, it is, I think, scarcely possible to view the coast, and not at once conclude that we here see a fringing-reef, which has been upraised a few feet: the unusual depth of from two to four fathoms within some of these islets, is probably due to muddy rivers having prevented the growth of coral near the shore. There is, however, one difficulty on this view, namely, that before the elevation took place, which converted the reef into a chain of islets, the water must apparently have been still deeper; on the other hand it may be supposed that the formation of a nearly perfect barrier in front, of so large an extent of coast, would cause the currents (especially in front of the rivers), to deepen their muddy beds. When describing in the chapter on fringing-reefs, those of Mauritius, I have given my reasons for believing that the shoal spaces within reefs of this kind, must, in many instances, have been deepened. However this may be, as several parts of this line of coast are undoubtedly fringed by living reefs, I have coloured it red.— _Maleenda_ (3° 20′ S.). In the plan of the harbour, the south headland appears fringed; and in Owen’s chart on a larger scale, the reefs are seen to extend nearly thirty miles southward; coloured red.—_Mombas_ (4° 5′ S.). The island which forms the harbour, “is surrounded by cliffs of madrepore, capable of being rendered almost impregnable” (Owen’s “Narr.” volume i., page 412). The shore of the mainland N. and S. of the harbour, is most regularly fringed by a coral-reef at a distance from half a mile to one mile and a quarter from the land; within the reef the depth is from nine to fifteen feet; outside the reef the depth at rather less than half a mile is thirty fathoms. From the charts it appears that a space about thirty-six miles in length, is here fringed; coloured red.—_Pemba_ (5° S.) is an island of coral-formation, level, and about two hundred feet in height (Owen’s “Narr.” volume i., page 425); it is thirty-five miles long, and is separated from the mainland by a deep sea. The outer coast is represented in the chart as regularly fringed; coloured red. The mainland in front of Pemba is likewise fringed; but there also appear to be some outlying reefs with deep water between them and the shore. I do not understand their structure, either from the charts or the description, therefore have not coloured them.—_Zanzibar_ resembles Pemba in most respects; its southern half on the western side and the neighbouring islets are fringed; coloured red. On the mainland, a little S. of Zanzibar, there are some banks parallel to the coast, which I should have thought had been formed of coral, had it not been said (Boteler’s “Narr.” volume ii., page 39) that they were composed of sand; not coloured.—_Latham’s Bank_ is a small island, fringed by coral-reefs; but being only ten feet high, it has not been coloured.—_Monfeea_ is an island of the same character as Pemba; its outer shore is fringed, and its southern extremity is connected with Keelwa Point on the mainland by a chain of islands fringed by reefs; coloured red. The four last-mentioned islands resemble in many respects some of the islands in the Red Sea, which will presently be described.— _Keelwa_. In a plan of the shore, a space of twenty miles N. and S. of this place is fringed by reefs, apparently of coral: these reefs are prolonged still further southward in Owen’s general chart. The coast in the plans of the rivers _Lindy_ and _Monghow_ (9° 59′ and 10° 7′ S.) has the same structure; coloured red.—_Querimba Islands_ (from 10° 40′ to 13° S.). A chart on a large scale is given of these islands; they are low, and of coral-formation (Boteler’s “Narr.” volume ii., page 54); and generally have extensive reefs projecting from them which are dry at low water, and which on the outside rise abruptly from a deep sea: on their insides they are separated from the continent by a channel, or rather a succession of bays, with an average depth of ten fathoms. The small headlands on the continent also have coral-banks attached to them; and the Querimba islands and banks are placed on the lines of prolongation of these headlands, and are separated from them by very shallow channels. It is evident that whatever cause, whether the drifting of sediment or subterranean movements, produced the headlands, likewise produced, as might have been expected, submarine prolongations to them; and these towards their outer extremities, have since afforded a favourable basis for the growth of coral-reefs, and subsequently for the formation of islets. As these reefs clearly belong to the fringing class, the Querimba islands have been coloured red.—_Monabila_ (13° 32′ S.). In the plan of this harbour, the headlands outside are fringed by reefs apparently of coral; coloured red.—_Mozambique_ (150° S.) The outer part of the island on which the city is built, and the neighbouring islands, are fringed by coral-reefs; coloured red. From the description given in Owen’s “Narr.” (volume i., page 162), the shore from _Mozambique_ to _Delagoa Bay_ appears to be low and sandy; many of the shoals and islets off this line of coast are of coral-formation; but from their small size and lowness, it is not possible, from the charts, to know whether they are truly fringed. Hence this portion of coast is left uncoloured, as are likewise those parts more northward, of which no mention has been made in the foregoing pages from the want of information.

PERSIAN GULF.—From the charts lately published on a large scale by the East India Company, it appears that several parts, especially the southern shores of this gulf, are fringed by coral-reefs; but as the water is very shallow, and as there are numerous sandbanks, which are difficult to distinguish on the chart from reefs, I have not coloured the upper part red. Towards the mouth, however, where the water is rather deeper, the islands of _Ormuz_ and _Larrack_, appear so regularly fringed, that I have coloured them red. There are certainly no atolls in the Persian Gulf. The shores of _Immaum_, and of the promontory forming the southern headland of the Persian Gulf, seem to be without reefs. The whole S.W. part (except one or two small patches) of _Arabia Felix_, and the shores of _Socotra_ appear from the charts and memoir of Captain Haines (“Geographical Journal,” 1839, page 125) to be without any reefs. I believe there are no extensive coral-reefs on any part of the coasts of _India_, except on the low promontory of _Madura_ (as already mentioned) in front of Ceylon.

RED SEA.—My information is chiefly derived from the admirable charts published by the East India Company in 1836, from personal communication with Captain Moresby, one of the surveyors, and from the excellent memoir, “Über die Natur der Corallen-Bänken des Rothen Meeres,” by Ehrenberg. The plains immediately bordering the Red Sea seem chiefly to consist of a sedimentary formation of the newer tertiary period. The shore is, with the exception of a few parts, fringed by coral-reefs. The water is generally profoundly deep close to the shore; but this fact, which has attracted the attention of most voyagers, seems to have no necessary connection with the presence of reefs; for Captain Moresby particularly observed to me, that, in latitude 24° 10′ on the eastern side, there is a piece of coast, with very deep water close to it, without any reefs, but not differing in other respects from the usual nature of the coast-line. The most remarkable feature in the Red Sea is the chain of submerged banks, reefs, and islands, lying some way from the shore, chiefly on the eastern side; the space within being deep enough to admit a safe navigation in small vessels. The banks are generally of an oval form, and some miles in width; but some of them are very long in proportion to their width. Captain Moresby informs me that any one, who had not made actual plans of them, would be apt to think that they were much more elongated than they really are. Many of them rise to the surface, but the greater number lie from five to thirty fathoms beneath it, with irregular soundings on them. They consist of sand and living coral; coral on most of them, according to Captain Moresby, covering the greater part of their surface. They extend parallel to the shore, and they are not unfrequently connected in their middle parts by short transverse banks with the mainland. The sea is generally profoundly deep quite close to them, as it is near most parts of the coast of the mainland; but this is not universally the case, for between latitude 15° and 17° the water deepens quite gradually from the banks, both on the eastern and western shores, towards the middle of the sea. Islands in many parts arise from these banks; they are low, flat-topped, and consist of the same horizontally stratified formation with that forming the plain-like margin of the mainland. Some of the smaller and lower islands consist of mere sand. Captain Moresby informs me, that small masses of rock, the remnants of islands, are left on many banks where there is now no dry land. Ehrenberg also asserts that most of the islets, even the lowest, have a flat abraded basis, composed of the same tertiary formation: he believes that as soon as the surf wears down the protuberant parts of a bank, just beneath the level of the sea, the surface becomes protected from further abrasion by the growth of coral, and he thus accounts for the existence of so many banks standing on a level with the surface of this sea. It appears that most of the islands are certainly decreasing in size.

The form of the banks and islands is most singular in the part just referred to, namely, from latitude 15° to 17°, where the sea deepens quite gradually: the _Dhalac_ group, on the western coast, is surrounded by an intricate archipelago of islets and shoals; the main island is very irregularly shaped, and it includes a bay seven miles long, by four across, in which no bottom was found with 252 feet: there is only one entrance into this bay, half a mile wide, and with an island in front of it. The submerged banks on the eastern coast, within the same latitudes, round _Farsan_ Island, are, likewise, penetrated by many narrow creeks of deep water; one is twelve miles long, in the form of a hatchet, in which, close to its broad upper end, soundings were not struck with 360 feet, and its entrance is only half a mile wide: in another creek of the same nature, but even with a more irregular outline, there was no bottom with 480 feet. The island of Farsan, itself, has as singular a form as any of its surrounding banks. The bottom of the sea round the Dhalac and Farsan Islands consists chiefly of sand and agglutinated fragments, but, in the deep and narrow creeks, it consists of mud; the islands themselves consist of thin, horizontally stratified, modern tertiary beds, containing but little broken coral;[2] their shores are fringed by living coral-reefs.

[2] Rüppell, “Reise in Abyssinie,” Band. i., S. 247.

From the account given by Rüppell[3] of the manner in which Dhalac has been rent by fissures, the opposite sides of which have been unequally elevated (in one instance to the amount of fifty feet), it seems probable that its irregular form, as well as probably that of Farsan, may have been partly caused by unequal elevations; but, considering the general form of the banks, and of the deep-water creeks, together with the composition of the land, I think their configuration is more probably due in great part to strong currents having drifted sediment over an uneven bottom: it is almost certain that their form cannot be attributed to the growth of coral. Whatever may have been the precise origin of the Dhalac and Farsan Archipelagoes, the greater number of the banks on the eastern side of the Red Sea seem to have originated through nearly similar means. I judge of this from their similarity in configuration (in proof of which I may instance a bank on the east coast in latitude 22°; and although it is true that the northern banks generally have a less complicated outline), and from their similarity in composition, as may be observed in their upraised portions. The depth within the banks northward of latitude 17°, is usually greater, and their outer sides shelve more abruptly (circumstances which seem to go together) than in the Dhalac and Farsan Archipelagoes; but this might easily have been caused by a difference in the action of the currents during their formation: moreover, the greater quantity of living coral, which, according to Captain Moresby, exists on the northern banks, would tend to give them steeper margins.

[3] Rüppell, “Reise in Abyssinie,” Band. i., S. 245.

From this account, brief and imperfect as it is, we can see that the great chain of banks on the eastern coast, and on the western side in the southern portion, differ greatly from true barrier-reefs wholly formed by the growth of coral. It is indeed the direct conclusion of Ehrenberg (“Uber die,” etc., pages 45 and 51), that they are connected in their origin quite secondarily with the growth of coral; and he remarks that the islands off the coast of Norway, if worn down level with the sea, and merely coated with living coral, would present a nearly similar appearance. I cannot, however, avoid suspecting, from information given me by Dr. Malcolmson and Captain Moresby, that Ehrenberg has rather under-rated the influence of corals, in some places at least, on the formation of the tertiary deposits of the Red Sea.

_The West Coast of the Red Sea between lat._ 19° _and_ 22°.—There are, in this space, reefs, which, if I had known nothing of those in other parts of the Red Sea, I should unhesitatingly have considered as barrier-reefs; and, after deliberation, I have come to the same conclusion. One of these reefs, in 20° 15′, is twenty miles long, less than a mile in width (but expanding at the northern end into a disc), slightly sinuous, and extending parallel to the mainland at the distance of five miles from it, with very deep water within; in one spot soundings were not obtained with 205 fathoms. Some leagues further south, there is another linear reef, very narrow, ten miles long, with other small portions of reef, north and south, almost connected with it; and within this line of reefs (as well as outside) the water is profoundly deep. There are also some small linear and sickle-formed reefs, lying a little way out at sea. All these reefs are covered, as I am informed by Captain Moresby, by living corals. Here, then, we have all the characters of reefs of the barrier class; and in some outlying reefs we have an approach to the structure of atolls. The source of my doubts about the classification of these reefs, arises from having observed in the Dhalac and Farsan groups the narrowness and straightness of several spits of sand and rock: one of these spits in the Dhalac group is nearly fifteen miles long, only two broad, and it is bordered on each side with deep water; so that, if worn down by the surf, and coated with living corals, it would form a reef nearly similar to those within the space under consideration. There is, also, in this space (latitude 21°) a peninsula, bordered by cliffs, with its extremity worn down to the level of the sea, and its basis fringed with reefs: in the line of prolongation of this peninsula, there lies the island of _Macowa_ (formed, according to Captain Moresby, of the usual tertiary deposit), and some smaller islands, large parts of which likewise appear to have been worn down, and are now coated with living corals. If the removal of the strata in these several cases had been more complete, the reefs thus formed would have nearly resembled those barrier-like ones now under discussion. Notwithstanding these facts, I cannot persuade myself that the many very small, isolated, and sickle-formed reefs and others, long, nearly straight, and very narrow, with the water unfathomably deep close round them, could possibly have been formed by corals merely coating banks of sediment, or the abraded surfaces of irregularly shaped islands. I feel compelled to believe that the foundations of these reefs have subsided, and that the corals, during their upward growth, have given to these reefs their present forms: I may remark that the subsidence of narrow and irregularly-shaped peninsulas and islands, such as those existing on the coasts of the Red Sea, would afford the requisite foundations for the reefs in question.

_The West Coast from lat._ 22° _to_ 24°.—This part of the coast (north of the space coloured blue on the map) is fronted by an irregularly shelving bank, from about ten to thirty fathoms deep; numerous little reefs, some of which have the most singular shapes, rise from this bank. It may be observed, respecting one of them, in latitude 23° 10′, that if the promontory in latitude 24° were worn down to the level of the sea, and coated with corals, a very similar and grotesquely formed reef would be produced. Many of the reefs on this part of the coast may thus have originated; but there are some sickle, and almost atoll-formed reefs lying in deep water off the promontory in latitude 24°, which lead me to suppose that all these reefs are more probably allied to the barrier or atoll classes. I have not, however, ventured to colour this portion of coast.—_On the west coast from lat._ 19° _to_ 17° (south of space coloured blue on the map), there are many low islets of very small dimensions, not much elongated, and rising out of great depths at a distance from the coast; these cannot be classed either with atolls, or barrier- or fringing-reefs. I may here remark that the outlying reefs on the west coast, between latitude 19° and 24°, are the only ones in the Red Sea, which approach in structure to the true atolls of the Indian and Pacific Oceans, but they present only imperfect miniature likenesses of them.

_Eastern Coast_.—I have felt the greatest doubt about colouring any portion of this coast, north of the fringing-reefs round the Farsan Islands in 16° 10′. There are many small outlying coral-reefs along the whole line of coast; but as the greater number rise from banks not very deeply submerged (the formation of which has been shown to be only secondarily connected with the growth of coral), their origin may be due simply to the growth of knolls of corals, from an irregular foundation situated within a limited depth. But between latitude 18° and 20°, there are so many linear, elliptic, and extremely small reefs, rising abruptly out of profound depths, that the same reasons, which led me to colour blue a portion of the west coast, have induced me to do the same in this part. There exist some small outlying reefs rising from deep water, north of latitude 20° (the northern limit coloured blue), on the east coast; but as they are not very numerous and scarcely any of them linear, I have thought it right to leave them uncoloured.

In the _southern parts_ of the Red Sea, considerable spaces of the mainland, and of some of the Dhalac islands, are skirted by reefs, which, as I am informed by Captain Moresby, are of living coral, and have all the characters of the fringing class. As in these latitudes, there are no outlying linear or sickle-formed reefs, rising out of unfathomable depths, I have coloured these parts of the coast red. On similar grounds, I have coloured red the _northern parts of the western coast_ (north of latitude 24° 30′), and likewise the shores of the chief part of the _Gulf of Suez_. In the _Gulf of Acaba_, as I am informed by Captain Moresby there are no coral-reefs, and the water is profoundly deep.

WEST INDIES.—My information regarding the reefs of this area, is derived from various sources, and from an examination of numerous charts; especially of those lately executed during the survey under Captain Owen, R.N. I lay under particular obligation to Captain Bird Allen, R.N., one of the members of the late survey, for many personal communications on this subject. As in the case of the Red Sea, it is necessary to make some preliminary remarks on the submerged banks of the West Indies, which are in some degree connected with coral-reefs, and cause considerable doubts in their classification. That large accumulations of sediment are in progress on the West Indian shores, will be evident to any one who examines the charts of that sea, especially of the portion north of a line joining Yucutan and Florida. The area of deposition seems less intimately connected with the debouchement of the great rivers, than with the course of the sea-currents; as is evident from the vast extension of the banks from the promontories of Yucutan and Mosquito.

Besides the coast-banks, there are many of various dimensions which stand quite isolated; these closely resemble each other, they lie from two or three to twenty or thirty fathoms under water, and are composed of sand, sometimes firmly agglutinated, with little or no coral; their surfaces are smooth and nearly level, shelving only to the amount of a few fathoms, very gradually all round towards their edges, where they plunge abruptly into the unfathomable sea. This steep inclination of their sides, which is likewise characteristic of the coast-banks, is very remarkable: I may give as an instance, the Misteriosa Bank, on the edges of which the soundings change in 250 fathoms horizontal distance, from 11 to 210 fathoms; off the northern point of the bank of Old Providence, in 200 fathoms horizontal distance, the change is from 19 to 152 fathoms; off the Great Bahama Bank, in 160 fathoms horizontal distance, the inclination is in many places from 10 fathoms to no bottom with 190 fathoms. On coasts in all parts of the world, where sediment is accumulating, something of this kind may be observed; the banks shelve very gently far out to sea, and then terminate abruptly. The form and composition of the banks standing in the middle parts of the W. Indian Sea, clearly show that their origin must be chiefly attributed to the accumulation of sediment; and the only obvious explanation of their isolated position is the presence of a nucleus, round which the currents have collected fine drift matter. Any one who will compare the character of the bank surrounding the hilly island of Old Providence, with those banks in its neighbourhood which stand isolated, will scarcely doubt that they surround submerged mountains. We are led to the same conclusion by examining the bank called Thunder Knoll, which is separated from the Great Mosquito Bank by a channel only seven miles wide, and 145 fathoms deep. There cannot be any doubt that the Mosquito Bank has been formed by the accumulation of sediment round the promontory of the same name; and Thunder Knoll resembles the Mosquito Bank, in the state of its surface submerged twenty fathoms, in the inclinations of its sides, in composition, and in every other respect. I may observe, although the remark is here irrelevant, that geologists should be cautious in concluding that all the outlyers of any formation have once been connected together, for we here see that deposits, doubtless of exactly the same nature, may be deposited with large valley-like spaces between them.

Linear strips of coral-reefs and small knolls project from many of the isolated, as well as coast-banks; sometimes they occur quite irregularly placed, as on the Mosquito Bank, but more generally they form crescents on the windward side, situated some little distance within the outer edge of the banks:—thus on the Serranilla Bank they form an interrupted chain which ranges between two and three miles within the windward margin: generally they occur, as on Roncador, Courtown, and Anegada Banks, nearer the line of deep water. Their occurrence on the windward side is conformable to the general rule, of the efficient kinds of corals flourishing best where most exposed; but their position some way within the line of deep water I cannot explain, without it be, that a depth somewhat less than that close to the outer margin of the banks, is most favourable to their growth. Where the corals have formed a nearly continuous rim, close to the windward edge of a bank some fathoms submerged, the reef closely resembles an atoll; but if the bank surrounds an island (as in the case of Old Providence), the reef resembles an encircling barrier-reef. I should undoubtedly have classed some of these fringed banks as imperfect atolls, or barrier-reefs, if the sedimentary nature of their foundations had not been evident from the presence of other neighbouring banks, of similar forms and of similar composition, but without the crescent-like marginal reef: in the third chapter, I observed that probably some atoll-like reefs did exist, which had originated in the manner here supposed.

Proofs of elevation within recent tertiary periods abound, as referred to in the sixth chapter, over nearly the whole area of the West Indies. Hence it is easy to understand the origin of the low land on the coasts, where sediment is now accumulating; for instance on the northern part of Yucutan, and on the N.E. part of Mosquito, where the land is low, and where extensive banks appear to be in progressive formation. Hence, also, the origin of the Great Bahama Banks, which are bordered on their western and southern edges by very narrow, long, singularly shaped islands, formed of sand, shells, and coral-rock, and some of them about a hundred feet in height, is easily explained by the elevation of banks fringed on their windward (western and southern) sides by coral-reefs. On this view, however, we must suppose either that the chief part of the surfaces of the great Bahama sandbanks were all originally deeply submerged, and were brought up to their present level by the same elevatory action, which formed the linear islands; or that during the elevation of the banks, the superficial currents and swell of the waves continued wearing them down and keeping them at a nearly uniform level: the level is not quite uniform; for, in proceeding from the N.W. end of the Bahama group towards the S.E. end, the depth of the banks increases, and the area of land decreases, in a very gradual and remarkable manner. The latter view, namely, that these banks have been worn down by the currents and swell during their elevation, seems to me the most probable one. It is, also, I believe, applicable to many banks, situated in widely distant parts of the West Indian Sea, which are wholly submerged; for, on any other view, we must suppose, that the elevatory forces have acted with astonishing uniformity.

The shore of the Gulf of Mexico, for the space of many hundred miles, is formed by a chain of lagoons, from one to twenty miles in breadth (“Columbian Navigator,” page 178, etc.), containing either fresh or salt water, and separated from the sea by linear strips of sand. Great spaces of the shores of Southern Brazil.[4] and of the United States from Long Island (as observed by Professor Rogers) to Florida have the same character. Professor Rogers, in his “Report to the British Association” (volume iii., page 13), speculates on the origin of these low, sandy, linear islets; he states that the layers of which they are composed are too homogeneous, and contain too large a proportion of shells, to permit the common supposition of their formation being simply due to matter thrown up, where it now lies, by the surf: he considers these islands as upheaved bars or shoals, which were deposited in lines where opposed currents met. It is evident that these islands and spits of sand parallel to the coast, and separated from it by shallow lagoons, have no necessary connection with coral-formations. But in Southern Florida, from the accounts I have received from persons who have resided there, the upraised islands seem to be formed of strata, containing a good deal of coral, and they are extensively fringed by living reefs; the channels within these islands are in some places between two and three miles wide, and five or six fathoms deep, though generally[5] they are less in depth than width. After having seen how frequently banks of sediment in the West Indian Sea are fringed by reefs, we can readily conceive that bars of sediment might be greatly aided in their formation along a line of coast, by the growth of corals; and such bars would, in that case, have a deceptive resemblance with true barrier-reefs.

[4] In the “London and Edinburgh Philosophical Journal,” 1841, page 257, I have described a singular bar of sandstone lying parallel to the coast off Pernambuco in Brazil, which probably is an analogous formation.

[5] In the ordinary sea-charts, no lagoons appear on the coast of Florida, north of 26°; but Major Whiting (“Silliman’s Journal,” volume xxxv., page 54) says that many are formed by sand thrown up along the whole line of coast from St. Augustine’s to Jupiter Inlet.

Having now endeavoured to remove some sources of doubt in classifying the reefs of the West Indies, I will give my authorities for colouring such portions of the coast as I have thought myself warranted in doing. Captain Bird Allen informs me, that most of the islands on the _Bahama Banks_ are fringed, especially on their windward sides, with living reefs; and hence I have coloured those, which are thus represented in Captain Owen’s late chart, red. The same officer informs me, that the islands along the southern part of _Florida_ are similarly fringed; coloured red. CUBA: Proceeding along the northern coast, at the distance of forty miles from the extreme S.E. point, the shores are fringed by reefs, which extend westward for a space of 160 miles, with only a few breaks. Parts of these reefs are represented in the plans of the harbours on this coast by Captain Owen; and an excellent description is given of them by Mr. Taylor (Loudon’s “Mag. of Nat. Hist.” volume ix., page 449); he states that they enclosed a space called the “_baxo_,” from half to three-quarters of a mile in width, with a sandy bottom, and a little coral. In most parts people can wade, at low water, to the reef; but in some parts the depth is between two and three fathoms. Close outside the reef, the depth is between six and seven fathoms; these well-characterised fringing-reefs are coloured red. Westward of longitude 77° 30′, on the northern side of Cuba, a great bank commences, which extends along the coast for nearly four degrees of longitude. In the place of its commencement, in its structure, and in the “_cays_,” or low islands on its edge, there is a marked correspondence (as observed by Humboldt, “Pers. Narr.” volume vii., page 88) between it and the Great Bahama and Sal Banks, which lie directly in front. Hence one is led to attribute the same origin to both these sets of banks; namely, the accumulation of sediment, conjoined with an elevatory movement, and the growth of coral on their outward edges; those parts which appear fringed by living reefs are coloured red. Westward of these banks, there is a portion of coast apparently without reefs, except in the harbours, the shores of which seem in the published plans to be fringed. The _Colorado Shoals_ (see Captain Owen’s charts), and the low land at the western end of Cuba, correspond as closely in relative position and structure to the banks at the extreme point of Florida, as the banks above described on the north side of Cuba, do to the Bahamas, the depth within the islets and reefs on the outer edge of the _Colorados_, is generally between two and three fathoms, increasing to twelve fathoms in the southern part, where the bank becomes nearly open, without islets or coral-reefs; the portions which are fringed are coloured red. The southern shore of Cuba is deeply concave, and the included space is filled up with mud and sandbanks, low islands and coral-reefs. Between the mountainous _Isle of Pines_ and the southern shore of Cuba, the general depth is only between two and three fathoms; and in this part small islands, formed of fragmentary rock and broken madrepores (Humboldt, “Pers. Narr.” volume vii. pages 51, 86 to 90, 291, 309, 320), rise abruptly, and just reach the surface of the sea. From some expressions used in the “Columbian Navigator” (volume i., part ii., page 94), it appears that considerable spaces along the outer coast of Southern Cuba are bounded by cliffs of coral-rock, formed probably by the upheaval of coral-reefs and sandbanks. The charts represent the southern part of the Isle of Pines as fringed by reefs, which the “Columb. Navig.” says extend some way from the coast, but have only from nine to twelve feet water on them; these are coloured red.—I have not been able to procure any detailed description of the large groups of banks and “cays” further eastward on the southern side of Cuba; within them there is a large expanse, with a muddy bottom, from eight to twelve fathoms deep; although some parts of this line of coast are represented in the general charts of the West Indies, as fringed, I have not thought it prudent to colour them. The remaining portion of the south coast of Cuba appears to be without coral-reefs.

YUCUTAN.—The N.E. part of the promontory appears in Captain Owen’s charts to be fringed; coloured red. The eastern coast, from 20° to 18° is fringed. South of latitude 18°, there commences the most remarkable reef in the West Indies: it is about one hundred and thirty miles in length, ranging in a N. and S. line, at an average distance of fifteen miles from the coast. The islets on it are all low, as I have been informed by Captain B. Allen; the water deepens suddenly on the outside of the reef, but not more abruptly than off many of the sedimentary banks: within its southern extremity (off _Honduras_) the depth is twenty-five fathoms; but in the more northern parts, the depth soon increases to ten fathoms, and within the northernmost part, for a space of twenty miles, the depth is only from one to two fathoms. In most of these respects we have the characteristics of a barrier-reef; nevertheless, from observing, first, that the channel within the reef is a continuation of a great irregular bay, which penetrates the mainland to the depth of fifty miles; and secondly, that considerable spaces of this barrier-like reef are described in the charts (for instance, in latitude 16° 45′ and 16° 12′) as formed of pure sand; and thirdly, from knowing that sediment is accumulating in many parts of the West Indies in banks parallel to the shore; I have not ventured to colour this reef as a barrier, without further evidence that it has really been formed by the growth of corals, and that it is not merely in parts a spit of sand, and in other parts a worn down promontory, partially coated and fringed by reefs; I lean, however, to the probability of its being a barrier-reef, produced by subsidence. To add to my doubts, immediately on the outside of this barrier-like reef, _Turneffe_, _Lighthouse_, and _Glover_ reefs are situated, and these reefs have so completely the form of atolls, that if they had occurred in the Pacific, I should not have hesitated about colouring them blue. _turneffe Reef_ seems almost entirely filled up with low mud islets; and the depth within the other two reefs is only from one to three fathoms. From this circumstance and from their similarity in form, structure, and relative position, both to the bank called _Northern Triangles_, on which there is an islet between seventy and eighty feet, and to _Cozumel_ Island, the level surface of which is likewise between seventy and eighty feet in height, I consider it more probable that the three foregoing banks are the worn down bases of upheaved shoals, fringed with corals, than that they are true atolls, wholly produced by the growth of coral during subsidence; left uncoloured.

In front of the eastern _Mosquito_ coast, there are between latitude 12° and 16° some extensive banks (already mentioned, page 148), with high islands rising from their centres; and there are other banks wholly submerged, both of which kinds of banks are bordered, near their windward margins, by crescent-shaped coral-reefs. But it can hardly be doubted, as was observed in the preliminary remarks, that these banks owe their origin, like the great bank extending from the Mosquito promontory, almost entirely to the accumulation of sediment, and not to the growth of corals; hence I have not coloured them.

_Cayman Island_: this island appears in the charts to be fringed; and Captain B. Allen informs me that the reefs extend about a mile from the shore, and have only from five to twelve feet water within them; coloured red.—_Jamaica_: judging from the charts, about fifteen miles of the S.E. extremity, and about twice that length on the S.W. extremity, and some portions on the S. side near Kingston and Port Royal, are regularly fringed, and therefore are coloured red. From the plans of some harbours on the N. side of Jamaica, parts of the coast appear to be fringed; but as these are not represented in the charts of the whole island, I have not coloured them.—_St. Domingo_: I have not been able to obtain sufficient information, either from plans of the harbours, or from general charts, to enable me to colour any part of the coast, except sixty miles from Port de Plata westward, which seems very regularly fringed; many other parts, however, of the coast are probably fringed, especially towards the eastern end of the island.—_Puerto Rico_: considerable portions of the southern, western, and eastern coasts, and some parts of the northern coast, appear in the charts to be fringed; coloured red.—Some miles in length of the southern side of the Island of _St, Thomas_ is fringed; most of the _Virgin Gorda_ Islands, as I am informed by Mr. Schomburgk, are fringed; the shores of _Anegada_, as well as the bank on which it stands, are likewise fringed; these islands have been coloured red. The greater part of the southern side of _Santa Cruz_ appears in the Danish survey to be fringed (see also Prof. Hovey’s account of this island, in “Silliman’s Journal,” volume xxxv., page 74); the reefs extend along the shore for a considerable space, and project rather more than a mile; the depth within the reef is three fathoms; coloured red.—The _Antilles_, as remarked by Von Buch (“Descrip. Iles Canaries,” page 494), may be divided into two linear groups, the western row being volcanic, and the eastern of modern calcareous origin; my information is very defective on the whole group. Of the eastern islands, _Barbuda_ and the western coasts of _Antigua_ and _Mariagalante_ appear to be fringed: this is also the case with _Barbadoes_, as I have been informed by a resident; these islands are coloured red. On the shores of the Western Antilles, of volcanic origin, very few coral-reefs appear to exist. The island of _Martinique_, of which there are beautifully executed French charts, on a very large scale, alone presents any appearance worthy of special notice. The south-western, southern, and eastern coasts, together forming about half the circumference of the island, are skirted by very irregular banks, projecting generally rather less than a mile from the shore, and lying from two to five fathoms submerged. In front of almost every valley, they are breached by narrow, crooked, steep-sided passages. The French engineers ascertained by boring, that these submerged banks consisted of madreporitic rocks, which were covered in many parts by thin layers of mud or sand. From this fact, and especially from the structure of the narrow breaches, I think there can be little doubt that these banks once formed living reefs, which fringed the shores of the island, and like other reefs probably reached the surface. From some of these submerged banks reefs of living coral rise abruptly, either in small detached patches, or in lines parallel to, but some way within the outer edges of the banks on which they are based. Besides the above banks which skirt the shores of the island, there is on the eastern side a range of linear banks, similarly constituted, twenty miles in length, extending parallel to the coast line, and separated from it by a space between two and four miles in width, and from five to fifteen fathoms in depth. From this range of detached banks, some linear reefs of living coral likewise rise abruptly; and if they had been of greater length (for they do not front more than a sixth part of the circumference of the island), they would necessarily from their position have been coloured as barrier-reefs; as the case stands they are left uncoloured. I suspect that after a small amount of subsidence, the corals were killed by sand and mud being deposited on them, and the reefs being thus prevented from growing upwards, the banks of madreporitic rock were left in their present submerged condition.

THE BERMUDA ISLANDS have been carefully described by Lieutenant Nelson, in an excellent Memoir in the “Geological Transactions” (volume v., part i., page 103). In the form of the bank or reef, on one side of which the islands stand, there is a close general resemblance to an atoll; but in the following respects there is a considerable difference,—first, in the margin of the reef not forming (as I have been informed by Mr. Chaffers, R.N.) a flat, solid surface, laid bare at low water, and regularly bounding the internal space of shallow water or lagoon; secondly, in the border of gradually shoaling water, nearly a mile and a half in width, which surrounds the entire outside of the reef (as is laid down in Captain Hurd’s chart); and thirdly, in the size, height, and extraordinary form of the islands, which present little resemblance to the long, narrow, simple islets, seldom exceeding half a mile in breadth, which surmount the annular reefs of almost all the atolls in the Indian and Pacific Oceans. Moreover, there are evident proofs (Nelson, Ibid., page 118), that islands similar to the existing ones, formerly extended over other parts of the reef. It would, I believe, be difficult to find a true atoll with land exceeding thirty feet in height; whereas, Mr. Nelson estimates the highest point of the Bermuda Islands to be 260 feet; if, however, Mr. Nelson’s view, that the whole of the land consists of sand drifted by the winds, and agglutinated together, were proved correct, this difference would be immaterial; but, from his own account (page 118), there occur in one place, five or six layers of red earth, interstratified with the ordinary calcareous rock, and including stones too heavy for the wind to have moved, without having at the same time utterly dispersed every grain of the accompanying drifted matter. Mr. Nelson attributes the origin of these several layers, with their embedded stones, to as many violent catastrophes; but further investigation in such cases has generally succeeded in explaining phenomena of this kind by ordinary and simpler means. Finally, I may remark, that these islands have a considerable resemblance in shape to Barbuda in the West Indies, and to Pemba on the eastern coast of Africa, which latter island is about two hundred feet in height, and consists of coral-rock. I believe that the Bermuda Islands, from being fringed by living reefs, ought to have been coloured red; but I have left them uncoloured, on account of their general resemblance in external form to a lagoon-island or atoll.

INDEX.

The names not in capitals are all names of places, and refer exclusively to the Appendix: in well-defined archipelagoes, or groups of islands, the name of each separate island is not given.

ABROLHOS, Brazil, coated by corals.

Abrolhos (Australia).

ABSENCE of coral-reefs from certain coasts.

Acaba, gulf of.

Admiralty group.

AFRICA, east coast, fringing-reef of. Madreporitic rock of.

Africa, east coast.

AGE of individual corals.

Aiou.

Aitutaki.

Aldabra.

Alert reef.

Alexander, Grand Duke, island.

ALLAN, Dr., on Holuthuriæ feeding on corals. On quick growth of corals at Madagascar. On reefs affected by currents.

Alloufatou.

Alphonse.

Amargoura. (Amargura.)

Amboina.

America, west coast.

Amirantes.

Anachorites.

Anambas.

ANAMOUKA, description of.

Anamouka.

Anadaman islands.

Antilles.

Appoo reef.

Arabia Felix.

AREAS, great extent of, interspersed with low islands. Of subsidence and of elevation. Of subsidence appear to be elongated. Of subsidence alternating with areas of elevation.

Arru group.

Arzobispo.

ASCIDIA, depth at which found.

Assomption.

Astova.

Atlantic islands.

ATOLLS, breaches in their reefs. Dimensions of. Dimensions of groups of. Not based on craters or on banks of sediment, or of rock. Of irregular forms. Steepness of their flanks. Width of their reef and islets. Their lowness. Lagoons. General range. With part of their reef submerged, and theory of.

Augustine, St.

AURORA island, an upraised atoll.

Aurora.

AUSTRAL islands, recently elevated.

Austral islands.

Australia, N.W. coast.

AUSTRALIAN barrier-reef.

Australian barrier.

Babuyan group.

Bahama banks.

Balahac.

Bally.

Baring.

BARRIER-REEF of Australia. Of New Caledonia.

BARRIER-REEFS, breaches through. Not based on worn down margin of rock. On banks of sediment. On submarine craters. Steepness of their flanks. Their probable vertical thickness. Theory of their formation.

Bampton shoal.

Banks islands.

Banks in the West Indies.

Bashee islands.

Bass island.

Batoa.

Beaupre reef.

BEECHEY, Captain, obligations of the author to. On submerged reefs. Account of Matilda island.

BELCHER, Captain, on boring through coral-reef.

Belize reef, off.

Bellinghausen.

Bermuda islands.

Beveridge reef.

Bligh.

BOLABOLA, view of.

Bombay shoal.

Bonin Bay.

Bonin group.

BORINGS through coral-reefs.

BORNEO, W. coast, recently elevated.

Borneo, E. coast. S.W. and W. coast N. coast. Western bank.

Boscawen.

Boston.

Bouka.

Bourbon.

Bourou.

Bouton.

BRAZIL, fringing-reefs on coast of.

BREACHES through barrier-reefs.

Brook.

Bunker.

Bunoa.

BYRON.

Cagayanes.

Candelaria.

Cargados Carajos.

Caroline archipelago.

Caroline island.

Carteret shoal.

CARYOPHYLLIA, depth at which it lives.

Cavilli.

Cayman island.

Celebes.

Ceram.

CEYLON, recently elevated.

Ceylon.

CHAGOS Great Bank, description and theory of.

CHAGOS group.

Chagos group.

CHAMA-SHELLS embedded in coral-rock.

CHAMISSO, on corals preferring the surf.

CHANGES in the state of Keeling atoll. Of atolls.

CHANNELS leading into the lagoons of atolls. Into the Maldiva atolls. Through barrier-reefs.

Chase.

China sea.

CHRISTMAS atoll.

Christmas atoll.

Christmas island (Indian Ocean).

Clarence.

Clipperton rock.

COCOS, or Keeling atoll.

Cocos (or Keeling).

Cocos island (Pacific).

COCHIN China, encroachments of the sea on the coast.

Cochin China.

Coetivi.

Comoro group.

COMPOSITION of coral-formations.

CONGLOMERATE coral-rock on Keeling atoll. On other atolls. Coral-rock.

COOK islands, recently elevated.

Cook islands.

CORAL-BLOCKS bored by vermiform animals.

CORAL-REEFS, their distribution and absence from certain areas. Destroyed by loose sediment.

CORAL-ROCK at Keeling atoll. Mauritius. Organic remains of.

CORALS dead but upright in Keeling lagoon. Depths at which they live. Off Keeling atoll. Killed by a short exposure. Living in the lagoon of Keeling atoll. Quick growth of, in Keeling lagoon. Merely coating the bottom of the sea. Standing exposed in the Low archipelago.

CORALLIAN sea.

Corallian sea.

Cornwallis.

Cosmoledo.

COUTHOUY, Mr., alleged proofs of recent elevation of the Low archipelago. On coral-rock at Mangaia and Aurora islands. On external ledges round coral-islands. Remarks confirmatory of the author’s theory.

CRESCENT-FORMED reefs.

Cuba.

CUMING, Mr., on the recent elevation of the Philippines.

Dangerous, or Low archipelago.

Danger islands.

DEPTHS at which reef-building corals live. At Mauritius, the Red Sea, and in the Maldiva archipelago. At which other corals and corallines can live.

Dhalac group.

DIEGO GARCIA, slow growth of reef.

DIMENSIONS of the larger groups of atolls.

DISSEVERMENT of the Maldiva atolls, and theory of.

DISTRIBUTION of coral-reefs.

Domingo, St.

DORY, Port, recently elevated.

Dory, Port.

Duff islands.

Durour.

Eap.

EARTHQUAKES at Keeling atoll. In groups of atolls. In Navigator archipelago.

EAST INDIAN ARCHIPELAGO, recently elevated.

Easter.

Echequier.

EHRENBERG, on the banks of the Red Sea. On depths at which corals live in the Red Sea. On corals preferring the surf. On the antiquity of certain corals.

Eimeo.

ELEVATED reef of Mauritius.

ELEVATIONS, recent proofs of. Immense areas of.

Elivi.

ELIZABETH island. Recently elevated.

Elizabeth island.

Ellice group.

ENCIRCLED ISLANDS, their height. Geological composition.

EOUA, description of.

Eoua.

ERUPTED MATTER probably not associated with thick masses of coral-rock.

FAIS, recently elevated.

Fais.

Fanning.

Farallon de Medinilla.

Farson group.

Fataka.

FIJI archipelago.

FISH, feeding on corals. Killed in Keeling lagoon by heavy rain.

FISSURES across coral-islands.

FITZROY, Captain, on a submerged shed at Keeling atoll. On an inundation in the Low archipelago.

Flint.

Flores.

Florida.

Folger.

Formosa.

FORSTER, theory of coral-formations.

Frederick reef.

Freewill.

FRIENDLY group recently elevated.

Friendly archipelago.

FRINGING-REEFS, absent where coast precipitous. Breached in front of streams. Described by MM. Quoy and Gaimard. Not closely attached to shelving coasts. Of east coast of Africa. Of Cuba. Of Mauritius. On worn down banks of rock. On banks of sediment. Their appearance when elevated. Their growth influenced by currents. By shallowness of sea.

Galapagos archipelago.

Galega.

GAMBIER islands, section of.

Gambier islands.

Gardner.

Gaspar rico.

GEOLOGICAL COMPOSITION of coral-formations.

Gilbert archipelago.

Gilolo.

Glorioso.

GLOUCESTER Island.

Glover reef.

Gomez.

Gouap.

Goulou.

Grampus.

Gran Cocal.

GREAT CHAGOS BANK, description and theory of.

GREY, Captain, on sandbars.

GROUPING of the different classes of reefs.

Guedes.

HALL, Captain B., on Loo Choo.

HARVEY islands, recently elevated.

HEIGHT of encircled islands.

Hermites.

Hervey or Cook islands.

Hogoleu.

HOLOTHURIAE (Holuthuriæ) feeding on coral.

HOUDEN island, height of.

Honduras, reef off.

Horn.

Houtman Abrolhos.

HUAHEINE; alleged proofs of its recent elevation.

Huaheine.

Humphrey.

Hunter.

HURRICANES, effects of, on coral-islands.

Immaum.

Independence.

INDIA, west coast, recently elevated.

India.

IRREGULAR REEFS in shallow seas.

ISLETS of coral-rock, their formation. Their destruction in the Maldiva atolls.

Jamaica.

Jarvis.

JAVA, recently elevated.

Java.

Johnston island.

Juan de Nova.

Juan de Nova (Madagascar).

Kalatoa.

KAMTSCHATKA, proofs of its recent elevation.

Karkalang.

KEELING atoll, section of reef.

Keeling, south atoll. North atoll.

Keffing.

Kemin.

Kennedy.

Keppel.

Kumi.

Laccadive group.

LADRONES, or Marianas, recently elevated.

Ladrones archipelago.

LAGOON of Keeling atoll.

LAGOONS bordered by inclined ledges and walls, and theory of their formation. Of small atolls filled up with sediment.

LAGOON-CHANNELS within barrier-reefs.

LAGOON-REEFS, all submerged in some atolls, and rising to the surface in others.

Lancaster reef.

Latte.

Lauglan islands.

LEDGES round certain lagoons.

Lette.

Lighthouse reef.

LLOYD, Mr., on corals refixing themselves.

LOO CHOO, recently elevated.

Loo Choo.

Louisiade.

LOW ARCHIPELAGO, alleged proofs of its recent elevation.

Low archipelago.

LOWNESS of coral-islands.

Loyalty group.

Lucepara.

LUTKE, Admiral, on fissures across coral-islands.

LUZON, recently elevated.

Luzon.

LYELL, Mr., on channels into the lagoons of atolls. On the lowness of their leeward sides. On the antiquity of certain corals. On the apparent continuity of distinct coral-islands. On the recently elevated beds of the Red Sea. On the outline of the areas of subsidence.

Macassar strait.

Macclesfield bank.

MADAGASCAR, quick growth of corals at. Madreporitic rock of.

Madagascar.

Madjiko-sima.

Madura (Java).

Madura (India).

MAHLOS MAHDOO, theory of formation.

MALACCA, recently elevated.

Malacca.

MALCOLMSON, Dr., on recent elevation of W. coast of India. On recent elevation of Camaran island.

Malden.

MALDIVA atolls, and theory of their formation. Steepness of their flanks. Growth of coral at.

Maldiva archipelago.

MANGAIA island. Recently elevated.

Mangaia.

Mangs.

MARIANAS, recently elevated.

Mariana archipelago.

Mariere.

Marquesas archipelago.

Marshall archipelago.

Marshall island.

Martinique.

Martires.

MARY’S ST., in Madagascar, harbour made in reefs.

Mary island.

Matia, or Aurora.

MATILDA atoll.

MAURITIUS, fringing-reefs of. Depths at which corals live there. Recently elevated.

Mauritius.