Chapter 1, Section I.II - Description of the plates

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SECTION 1.II.--GENERAL DESCRIPTION OF ATOLLS.








General form and size of atolls, their reefs and islets.--External slope.--
Zone of Nulliporae.--Conglomerate.--Depth of lagoons.--Sediment.--Reefs
submerged wholly or in part.--Breaches in the reef.--Ledge-formed shores
round certain lagoons.--Conversion of lagoons into land.

I will here give a sketch of the general form and structure of the many
atolls and atoll-formed reefs which occur in the Pacific and Indian Oceans,
comparing them with Keeling atoll.  The Maldiva atolls and the Great Chagos
Bank differ in so many respects, that I shall devote to them, besides
occasional references, a third section of this chapter.  Keeling atoll may
be considered as of moderate dimensions and of regular form.  Of the
thirty-two islands surveyed by Captain Beechey in the Low Archipelago, the
longest was found to be thirty miles, and the shortest less than a mile;
but Vliegen atoll, situated in another part of the same group, appears to
be sixty miles long and twenty broad.  Most of the atolls in this group are
of an elongated form; thus Bow Island is thirty miles in length, and on an
average only six in width (See Figure 4, Plate I.), and Clermont Tonnere
has nearly the same proportions.  In the Marshall Archipelago (the Ralick
and Radack group of Kotzebue) several of the atolls are more than thirty
miles in length, and Rimsky Korsacoff is fifty-four long, and twenty wide,
at the broadest part of its irregular outline.  Most of the atolls in the
Maldiva Archipelago are of great size, one of them (which, however, bears a
double name) measured in a medial and slightly curved line, is no less than
eighty-eight geographical miles long, its greatest width being under
twenty, and its least only nine and a half miles.  Some atolls have spurs
projecting from them; and in the Marshall group there are atolls united
together by linear reefs, for instance Menchikoff Island (See Figure 3,
Plate II.), which is sixty miles in length, and consists of three loops
tied together.  In far the greater number of cases an atoll consists of a
simple elongated ring, with its outline moderately regular.

The average width of the annular wreath may be taken as about a quarter of
a mile.  Captain Beechey (Beechey's "Voyage to the Pacific and Beering's
Straits," chapter viii.) says that in the atolls of the Low Archipelago it
exceeded in no instance half a mile.  The description given of the
structure and proportional dimensions of the reef and islets of Keeling
atoll, appears to apply perfectly to nearly all the atolls in the Pacific
and Indian Oceans.  The islets are first formed some way back either on the
projecting points of the reef, especially if its form be angular, or on the
sides of the main entrances into the lagoon--that is in both cases, on
points where the breakers can act during gales of wind in somewhat
different directions, so that the matter thrown up from one side may
accumulate against that before thrown up from another.  In Lutke's chart of
the Caroline atolls, we see many instances of the former case; and the
occurrence of islets, as if placed for beacons, on the points where there
is a gateway or breach through the reef, has been noticed by several
authors.  There are some atoll-formed reefs, rising to the surface of the
sea and partly dry at low water, on which from some cause islets have never
been formed; and there are others on which they have been formed, but have
subsequently been worn away.  In atolls of small dimensions the islets
frequently become united into a single horse-shoe or ring-formed strip; but
Diego Garcia, although an atoll of considerable size, being thirteen miles
and a half in length, has its lagoon entirely surrounded, except at the
northern end, by a belt of land, on an average a third of a mile in width.
To show how small the total area of the annular reef and the land is in
islands of this class, I may quote a remark from the voyage of Lutke,
namely, that if the forty-three rings, or atolls, in the Caroline
Archipelago, were put one within another, and over a steeple in the centre
of St. Petersburg, the whole world would not cover that city and its
suburbs.

The form of the bottom off Keeling atoll, which gradually slopes to about
twenty fathoms at the distance of between one and two hundred yards from
the edge of the reef, and then plunges at an angle of 45 deg into
unfathomable depths, is exactly the same (The form of the bottom round the
Marshall atolls in the Northern Pacific is probably similar: Kotzebue
("First Voyage," volume ii., page 16) says: "We had at a small distance
from the reef, forty fathoms depth, which increased a little further so
much that we could find no bottom.") with that of the sections of the
atolls in the Low Archipelago given by Captain Beechey.  The nature,
however, of the bottom seems to differ, for this officer (I must be
permitted to express my obligation to Captain Beechey, for the very kind
manner in which he has given me information on several points, and to own
the great assistance I have derived from his excellent published work.)
informs me that all the soundings, even the deepest, were on coral, but he
does not know whether dead or alive.  The slope round Christmas atoll (Lat.
1 deg 4' N., 157 deg 45' W.), described by Cook (Cook's "Third Voyage,"
volume ii., chapter 10.), is considerably less, at about half a mile from
the edge of the reef, the average depth was about fourteen fathoms on a
fine sandy bottom, and at a mile, only between twenty and forty fathoms.
It has no doubt been owing to this gentle slope, that the strip of land
surrounding its lagoon, has increased in one part to the extraordinary
width of three miles; it is formed of successive ridges of broken shells
and corals, like those on the beach.  I know of no other instance of such
width in the reef of an atoll; but Mr. F.D. Bennett informs me that the
inclination of the bottom round Caroline atoll in the Pacific, is like that
off Christmas Island, very gentle.  Off the Maldiva and Chagos atolls, the
inclination is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell
(This fact is taken from a MS. account of these groups lent me by Captain
Moresby.  See also Captain Moresby's paper on the Maldiva atolls in the
"Geographical Journal", volume v., page 401.) found fifty and sixty fathoms
close to the edge of the reef, and at 300 yards distance there was no
bottom with a 300-yard line.  Captain Moresby informs me, that at 100
fathoms from the mouth of the lagoon of Diego Garcia, he found no bottom
with 150 fathoms; this is the more remarkable, as the slope is generally
less abrupt in front of channels through a reef, owing to the accumulation
of sediment.  At Egmont Island, also, at 150 fathoms from the reef,
soundings were struck with 150 fathoms.  Lastly, at Cardoo atoll, only
sixty yards from the reef, no bottom was obtained, as I am informed by
Captain Moresby, with a line of 200 fathoms!  The currents run with great
force round these atolls, and where they are strongest, the inclination
appears to be most abrupt.  I am informed by the same authority, that
wherever soundings were obtained off these islands, the bottom was
invariably sandy: nor was there any reason to suspect the existence of
submarine cliffs, as there was at Keeling Island.  (Off some of the islands
in the Low Archipelago the bottom appears to descend by ledges.  Off
Elizabeth Island, which, however, consists of raised coral, Captain Beechey
(page 45, 4to edition) describes three ledges: the first had an easy slope
from the beach to a distance of about fifty yards: the second extended two
hundred yards with twenty-five fathoms on it, and then ended abruptly, like
the first; and immediately beyond this there was no bottom with two hundred
fathoms.)  Here then occurs a difficulty; can sand accumulate on a slope,
which, in some cases, appears to exceed fifty-five degrees?  It must be
observed, that I speak of slopes where soundings were obtained, and not of
such cases, as that of Cardoo, where the nature of the bottom is unknown,
and where its inclination must be nearly vertical.  M. Elie de Beaumont
("Memoires pour servir a une description Geolog. de France," tome iv., page
216.) has argued, and there is no higher authority on this subject, from
the inclination at which snow slides down in avalanches, that a bed of sand
or mud cannot be formed at a greater angle than thirty degrees.
Considering the number of soundings on sand, obtained round the Maldiva and
Chagos atolls, which appears to indicate a greater angle, and the extreme
abruptness of the sand-banks in the West Indies, as will be mentioned in
the Appendix, I must conclude that the adhesive property of wet sand
counteracts its gravity, in a much greater ratio than has been allowed for
by M. Elie de Beaumont.  From the facility with which calcareous sand
becomes agglutinated, it is not necessary to suppose that the bed of loose
sand is thick.

Captain Beechey has observed, that the submarine slope is much less at the
extremities of the more elongated atolls in the Low Archipelago, than at
their sides; in speaking of Ducie's Island he says (Beechey's "Voyage," 4to
edition, page 44.) the buttress, as it may be called, which "has the most
powerful enemy (the S.W. swell) to oppose, is carried out much further, and
with less abruptness than the other."  In some cases, the less inclination
of a certain part of the external slope, for instance of the northern
extremities of the two Keeling atolls, is caused by a prevailing current
which there accumulates a bed of sand.  Where the water is perfectly
tranquil, as within a lagoon, the reefs generally grow up perpendicularly,
and sometimes even overhang their bases; on the other hand, on the leeward
side of Mauritius, where the water is generally tranquil, although not
invariably so, the reef is very gently inclined.  Hence it appears that the
exterior angle varies much; nevertheless in the close similarity in form
between the sections of Keeling atoll and of the atolls in the Low
Archipelago, in the general steepness of the reefs of the Maldiva and
Chagos atolls, and in the perpendicularity of those rising out of water
always tranquil, we may discern the effects of uniform laws; but from the
complex action of the surf and currents, on the growing powers of the coral
and on the deposition of sediment, we can by no means follow out all the
results.

Where islets have been formed on the reef, that part which I have sometimes
called the "flat" and which is partly dry at low water, appears similar in
every atoll.  In the Marshall group in the North Pacific, it may be
inferred from Chamisso's description, that the reef, where islets have not
been formed on it, slopes gently from the external margin to the shores of
the lagoon; Flinders states that the Australian barrier has a similar
inclination inwards, and I have no doubt it is of general occurrence,
although, according to Ehrenberg, the reefs of the Red Sea offer an
exception.  Chamisso observes that "the red colour of the reef (at the
Marshall atolls) under the breakers is caused by a Nullipora, which covers
the stone WHEREVER THE WAVES BEAT; and, under favourable circumstances,
assumes a stalactical form,"--a description perfectly applicable to the
margin of Keeling atoll.  (Kotzebue's "First Voyage," volume iii., page
142.  Near Porto Praya, in the Cape de Verde Islands, some basaltic rocks,
lashed by no inconsiderable surf, were completely enveloped with a layer of
Nulliporae.  The entire surface over many square inches, was coloured of a
peach-blossomed red; the layer, however, was of no greater thickness than
paper.  Another kind, in the form of projecting knobs, grew in the same
situation.  These Nulliporae are closely related to those described on the
coral-reefs, but I believe are of different species.)  Although Chamisso
does not state that the masses of Nulliporae form points or a mound, higher
than the flat, yet I believe that this is the case; for Kotzebue (Kotzebue,
"First Voyage," volume ii., page 16.  Lieutenant Nelson, in his excellent
memoir in the Geological Transactions (volume ii., page 105), alludes to
the rocky points mentioned by Kotzebue, and infers that they consist of
Serpulae, which compose incrusting masses on the reefs of Bermudas, as they
likewise do on a sandstone bar off the coast of Brazil (which I have
described in "London Phil. Journal," October 1841).  These masses of
Serpulae hold the same position, relatively to the action of the sea, with
the Nulliporae on the coral-reefs in the Indian and Pacific Oceans.), in
another part, speaks of the rocks on the edge of the reef "as visible for
about two feet at low water," and these rocks we may feel quite certain are
not formed of true coral (Captain Moresby, in his valuable paper "on the
Northern atolls of Maldivas" ("Geographical Journal", volume v.), says that
the edges of the reefs there stand above water at low spring-tides.)
Whether a smooth convex mound of Nulliporae, like that which appears as if
artificially constructed to protect the margin of Keeling Island, is of
frequent occurrence round atolls, I know not; but we shall presently meet
with it, under precisely the same form, on the outer edge of the
"barrier-reefs" which encircle the Society Islands.

There appears to be scarcely a feature in the structure of Keeling reef,
which is not of common, if not of universal occurrence, in other atolls.
Thus Chamisso describes (Kotzebue's "First Voyage," volume iii., page 144.)
a layer of coarse conglomerate, outside the islets round the Marshall
atolls which "appears on its upper surface uneven and eaten away."  From
drawings, with appended remarks, of Diego Garcia in the Chagos group and of
several of the Maldiva atolls, shown me by Captain Moresby (see also
Moresby on the Northern atolls of the Maldivas, "Geographical Journal",
volume v., page 400.), it is evident that their outer coasts are subject to
the same round of decay and renovation as those of Keeling atoll.  From the
description of the atolls in the Low Archipelago, given in Captain
Beechey's "Voyage," it is not apparent that any conglomerate coral-rock was
there observed.

The lagoon in Keeling atoll is shallow; in the atolls of the Low
Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall
Group, according to Chamisso, from 30 to 35; in the Caroline atolls it is
only a little less.  Within the Maldiva atolls there are large spaces with
45 fathoms, and some soundings are laid down of 49 fathoms.  The greater
part of the bottom in most lagoons, is formed of sediment; large spaces
have exactly the same depth, or the depth varies so insensibly, that it is
evident that no other means, excepting aqueous deposition, could have
leveled the surface so equally.  In the Maldiva atolls this is very
conspicuous, and likewise in some of the Caroline and Marshall Islands.  In
the former large spaces consist of sand and SOFT CLAY; and Kotzebue speaks
of clay having been found within one of the Marshall atolls.  No doubt this
clay is calcareous mud, similar to that at Keeling Island, and to that at
Bermuda already referred to, as undistinguishable from disintegrated chalk,
and which Lieutenant Nelson says is called there pipe-clay.  (I may here
observe that on the coast of Brazil, where there is much coral, the
soundings near the land are described by Admiral Roussin, in the "Pilote du
Bresil", as siliceous sand, mingled with much finely comminuted particles
of shells and coral.  Further in the offing, for a space of 1,300 miles
along the coast, from the Abrolhos Islands to Maranham, the bottom in many
places is composed of "tuf blanc, mele ou forme de madrepores broyes."
This white substance, probably, is analogous to that which occurs within
the above-mentioned lagoons; it is sometimes, according to Roussin, firm,
and he compares it to mortar.)

Where the waves act with unequal force on the two sides of an atoll, the
islets appear to be first formed, and are generally of greater continuity
on the more exposed shore.  The islets, also, which are placed to leeward,
are in most parts of the Pacific liable to be occasionally swept entirely
away by gales, equalling hurricanes in violence, which blow in an opposite
direction to the ordinary trade-wind.  The absence of the islets on the
leeward side of atolls, or when present their lesser dimensions compared
with those to windward, is a comparatively unimportant fact; but in several
instances the reef itself on the leeward side, retaining its usual defined
outline, does not rise to the surface by several fathoms.  This is the case
with the southern side of Peros Banhos (Plate I., Figure 9) in the Chagos
group, with Mourileu atoll (Frederick Lutke's "Voyage autour du Monde,"
volume ii., page 291.  See also his account of Namonouito, below, and the
chart of Oulleay in the Atlas.) in the Caroline Archipelago, and with the
barrier-reef (Plate I., Figure 8) of the Gambier Islands.  I allude to the
latter reef, although belonging to another class, because Captain Beechey
was first led by it to observe the peculiarity in the question.  At Peros
Banhos the submerged part is nine miles in length, and lies at an average
depth of about five fathoms; its surface is nearly level, and consists of
hard stone, with a thin covering of loose sand.  There is scarcely any
living coral on it, even on the outer margin, as I have been particularly
assured by Captain Moresby; it is, in fact, a wall of dead coral-rock,
having the same width and transverse section with the reef in its ordinary
state, of which it is a continuous portion.  The living and perfect parts
terminate abruptly, and abut on the submerged portions, in the same manner
as on the sides of an ordinary passage through the reef.  The reef to
leeward in other cases is nearly or quite obliterated, and one side of the
lagoon is left open; for instance, at Oulleay (Caroline Archipelago), where
a crescent-formed reef is fronted by an irregular bank, on which the other
half of the annular reef probably once stood.  At Namonouito, in the same
Archipelago, both these modifications of the reef concur; it consists of a
great flat bank, with from twenty to twenty-five fathoms water on it; for a
length of more than forty miles on its southern side it is open and without
any reef, whilst on the other sides it is bounded by a reef, in parts
rising to the surface and perfectly characterised, in parts lying some
fathoms submerged.  In the Chagos group there are annular reefs, entirely
submerged, which have the same structure as the submerged and defined
portions just described.  The Speaker's Bank offers an excellent example of
this structure; its central expanse, which is about twenty-two fathoms
deep, is twenty-four miles across; the external rim is of the usual width
of annular reefs, and is well-defined; it lies between six and eight
fathoms beneath the surface, and at the same depth there are scattered
knolls in the lagoon.  Captain Moresby believes the rim consists of dead
rock, thinly covered with sand, and he is certain this is the case with the
external rim of the Great Chagos Bank, which is also essentially a
submerged atoll.  In both these cases, as in the submerged portion of the
reef at Peros Banhos, Captain Moresby feels sure that the quantity of
living coral, even on the outer edge overhanging the deep-sea water, is
quite insignificant.  Lastly, in several parts of the Pacific and Indian
Oceans there are banks, lying at greater depths than in the cases just
mentioned, of the same form and size with the neighbouring atolls, but with
their atoll-like structure wholly obliterated.  It appears from the survey
of Freycinet, that there are banks of this kind in the Caroline
Archipelago, and, as is reported, in the Low Archipelago.  When we discuss
the origin of the different classes of coral formations, we shall see that
the submerged state of the whole of some atoll-formed reefs, and of
portions of others, generally but not invariably on the leeward side, and
the existence of more deeply submerged banks now possessing little or no
signs of their original atoll-like structure, are probably the effects of a
uniform cause,--namely, the death of the coral, during the subsidence of
the area, in which the atolls or banks are situated.

There is seldom, with the exception of the Maldiva atolls, more than two or
three channels, and generally only one leading into the lagoon, of
sufficient depth for a ship to enter.  in small atolls, there is usually
not even one.  Where there is deep water, for instance above twenty
fathoms, in the middle of the lagoon, the channels through the reef are
seldom as deep as the centre,--it may be said that the rim only of the
saucer-shaped hollow forming the lagoon is notched.  Mr. Lyell ("Principles
of Geology," volume iii., page 289.) has observed that the growth of the
coral would tend to obstruct all the channels through a reef, except those
kept open by discharging the water, which during high tide and the greater
part of each ebb is thrown over its circumference.  Several facts indicate
that a considerable quantity of sediment is likewise discharged through
these channels; and Captain Moresby informs me that he has observed, during
the change of the monsoon, the sea discoloured to a distance off the
entrances into the Maldiva and Chagos atolls.  This, probably, would check
the growth of the coral in them, far more effectually than a mere current
of water.  In the many small atolls without any channel, these causes have
not prevented the entire ring attaining the surface.  The channels, like
the submerged and effaced parts of the reef, very generally though not
invariably occur on the leeward side of the atoll, or on that side,
according to Beechey (Beechey's "Voyage," 4to edition, volume i., page
189.), which, from running in the same direction with the prevalent wind,
is not fully exposed to it.  Passages between the islets on the reef,
through which boats can pass at high water, must not be confounded with
ship-channels, by which the annular reef itself is breached.  The passages
between the islets occur, of course, on the windward as well as on the
leeward side; but they are more frequent and broader to leeward, owing to
the lesser dimensions of the islets on that side.

At Keeling atoll the shores of the lagoon shelve gradually, where the
bottom is of sediment, and irregularly or abruptly where there are
coral-reefs; but this is by no means the universal structure in other atolls.
Chamisso (Kotzebue's "First Voyage," volume iii., page 142.), speaking in
general terms of the lagoons in the Marshall atolls, says the lead
generally sinks "from a depth of two or three fathoms to twenty or
twenty-four, and you may pursue a line in which on one side of the boat you
may see the bottom, and on the other the azure-blue deep water."  The shores
of the lagoon-like channel within the barrier-reef at Vanikoro have a similar
structure.  Captain Beechey has described a modification of this structure
(and he believes it is not uncommon) in two atolls in the Low Archipelago,
in which the shores of the lagoon descend by a few, broad, slightly
inclined ledges or steps: thus at Matilda atoll (Beechey's "Voyage," 4to
edition, volume i, page 160.  At Whitsunday Island the bottom of the lagoon
slopes gradually towards the centre, and then deepens suddenly, the edge of
the bank being nearly perpendicular.  This bank is formed of coral and dead
shells.), the great exterior reef, the surface of which is gently inclined
towards and beneath the surface of the lagoon, ends abruptly in a little
cliff three fathoms deep; at its foot, a ledge forty yards wide extends,
shelving gently inwards like the surface-reef, and terminated by a second
little cliff five fathoms deep; beyond this, the bottom of the lagoon
slopes to twenty fathoms, which is the average depth of its centre.  These
ledges seem to be formed of coral-rock; and Captain Beechey says that the
lead often descended several fathoms through holes in them.  In some
atolls, all the coral reefs or knolls in the lagoon come to the surface at
low water; in other cases of rarer occurrence, all lie at nearly the same
depth beneath it, but most frequently they are quite irregular,--some with
perpendicular, some with sloping sides,--some rising to the surface, and
others lying at all intermediate depths from the bottom upwards.  I cannot,
therefore, suppose that the union of such reefs could produce even one
uniformly sloping ledge, and much less two or three, one beneath the other,
and each terminated by an abrupt wall.  At Matilda Island, which offers the
best example of the step-like structure, Captain Beechey observes that the
coral-knolls within the lagoon are quite irregular in their height.  We
shall hereafter see that the theory which accounts for the ordinary form of
atolls, apparently includes this occasional peculiarity in their structure.

In the midst of a group of atolls, there sometimes occur small, flat, very
low islands of coral formation, which probably once included a lagoon,
since filled up with sediment and coral-reefs.  Captain Beechey entertains
no doubt that this has been the case with the two small islands, which
alone of thirty-one surveyed by him in the Low Archipelago, did not contain
lagoons.  Romanzoff Island (in lat. 15 deg S.) is described by Chamisso
(Kotzebue's "First Voyage," volume iii., page 221.) as formed by a dam of
madreporitic rock inclosing a flat space, thinly covered with trees, into
which the sea on the leeward side occasionally breaks.  North Keeling atoll
appears to be in a rather less forward stage of conversion into land; it
consists of a horse-shoe shaped strip of land surrounding a muddy flat, one
mile in its longest axis, which is covered by the sea only at high water.
When describing South Keeling atoll, I endeavoured to show how slow the
final process of filling up a lagoon must be; nevertheless, as all causes
do tend to produce this effect, it is very remarkable that not one
instance, as I believe, is known of a moderately sized lagoon being filled
up even to the low water-line at spring-tides, much less of such a one
being converted into land.  It is, likewise, in some degree remarkable, how
few atolls, except small ones, are surrounded by a single linear strip of
land, formed by the union of separate islets.  We cannot suppose that the
many atolls in the Pacific and Indian Oceans all have had a late origin,
and yet should they remain at their present level, subjected only to the
action of the sea and to the growing powers of the coral, during as many
centuries as must have elapsed since any of the earlier tertiary epochs, it
cannot, I think, be doubted that their lagoons and the islets on their
reef, would present a totally different appearance from what they now do.
This consideration leads to the suspicion that some renovating agency
(namely subsidence) comes into play at intervals, and perpetuates their
original structure.

(DESCRIPTION OF THE PLATES.

PLATE II.--GREAT CHAGOS BANK, NEW CALEDONIA,MENCHIKOFF ATOLL, ETC.

FIGURE 1.--GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey by
Captain Moresby and Lieutenant Powell; the parts which are shaded, with the
exception of two or three islets on the western and northern sides, do not
rise to the surface, but are submerged from four to ten fathoms; the banks
bounded by the dotted lines lie from fifteen to twenty fathoms beneath the
surface, and are formed of sand; the central space is of mud, and from
thirty to fifty fathoms deep.

FIGURE 2.--A vertical section, on the same scale, in an eastern and western
line across the Great Chagos Bank, given for the sake of exhibiting more
clearly its structure.

FIGURE 3.--MENCHIKOFF ATOLL (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern's "Atlas of the
Pacific;" originally surveyed by Captain Hagemeister; the depth within the
lagoons is unknown.

FIGURE 4.--MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
Powell; the white spaces in the middle of the separate small reefs, both on
the margin and in the middle part, are meant to represent little lagoons;
but it was found not possible to distinguish them clearly from the small
islets, which have been formed on these same small reefs; many of the
smaller reefs could not be introduced; the nautical mark (dot over a dash)
over the figures 250 and 200, between Mahlos Mahdoo and Horsburgh atoll and
Powell's island, signifies that soundings were not obtained at these
depths.

FIGURE 5.--NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern's "Atlas," compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the "Atlas of
the Voyage of the 'Astrolabe'."  In Krusenstern's "Atlas," the reef is
represented by a single line with crosses; I have for the sake of
uniformity added an interior line.

FIGURE 6.--MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
Captain Moresby and Lieutenant Powell.)