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Appendix 5.
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, "Uber die
Natur der Corallen-Banken 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 deg 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 deg
and 17 deg 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 deg to 17 deg, 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 (Ruppell, "Reise in Abyssinie," Band. i., S. 247.),
their shores are fringed by living coral-reefs.
From the account given by Ruppell (Ibid., S. 245.) 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 deg; 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
deg, 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.
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 LATITUDE 19 DEG AND 22 DEG.
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 deg 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 deg) 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 LATITUDE 22 DEG TO 24 DEG.
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 deg 10', that if the promontory in latitude 24 deg 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 deg, 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 LATITUDE
19 DEG TO 17 DEG (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 deg and 24 deg, 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 deg 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 deg and 20 deg, 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 deg (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
deg 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 shores 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 (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.), 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 (In the ordinary sea-charts, no lagoons appear on
the coast of Florida, north of 26 deg; 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.)
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.
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 deg 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 deg to 18 deg is
fringed. South of latitude 18 deg, 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 deg 45' and 16 deg 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 deg
and 16 deg 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.
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