Goose Green

Wednesday, August 06, 2008

Chapter 6 (beginning)




The principles, on which this map was coloured, are explained in the
beginning of Chapter VI.; and the authorities for each particular spot are
detailed in the Appendix to "Coral Reefs." The names not printed in upper
case in the Index refer to the Appendix.)

Description of the coloured map.--Proximity of atolls and barrier-reefs.--
Relation in form and position of atolls with ordinary islands.--Direct
evidence of subsidence difficult to be detected.--Proofs of recent
elevation where fringing-reefs occur.--Oscillations of level.--Absence of
active volcanoes in the areas of subsidence.--Immensity of the areas which
have been elevated and have subsided.--Their relation to the present
distribution of the land.--Areas of subsidence elongated, their
intersection and alternation with those of elevation.--Amount and slow rate
of the subsidence.--Recapitulation.

It will be convenient to give here a short account of the appended map
(Plate III.) [Inasmuch as the coloured map would have proved too costly to
be given in this series, the indications of colour have been replaced by
numbers referring to the dotted groups of reefs, etc. The author's
original wording, however, is retained in full, as it will be easy to refer
to the map by the numbers, and thus the flow of the narrative is
undisturbed.]: a fuller one, with the data for colouring each spot, is
reserved for the Appendix; and every place there referred to may be found
in the Index. A larger chart would have been desirable; but, small as the
adjoined one is, it is the result of many months' labour. I have
consulted, as far as I was able, every original voyage and map; and the
colours were first laid down on charts on a larger scale. The same blue
colour, with merely a difference in the depth of tint, is used for atolls
or lagoon-islands, and barrier-reefs, for we have seen, that as far as the
actual coral-formation is concerned, they have no distinguishing character.
Fringing-reefs have been coloured red, for between them on the one hand,
and barrier-reefs and atolls on the other, there is an important
distinction with respect to the depth beneath the surface, at which we are
compelled to believe their foundations lie. The two distinct colours,
therefore, mark two great types of structure.

The DARK BLUE COLOUR [represented by (3) in our plate] represents atolls
and submerged annular reefs, with deep water in their centres. I have
coloured as atolls, a few low and small coral-islands, without lagoons; but
this has been done only when it clearly appeared that they originally
contained lagoons, since filled up with sediment: when there were not good
grounds for this belief, they have been left uncoloured.

The PALE BLUE COLOUR [represented by (2)] represents barrier-reefs. The
most obvious character of reefs of this class is the broad and deep-water
moat within the reef: but this, like the lagoons of small atolls, is
liable to become filled up with detritus and with reefs of delicately
branched corals: when, therefore, a reef round the entire circumference of
an island extends very far into a profoundly deep sea, so that it can
hardly be confounded with a fringing-reef which must rest on a foundation
of rock within a small depth, it has been coloured pale blue, although it
does not include a deep-water moat: but this has only been done rarely,
and each case is distinctly mentioned in the Appendix.

The RED COLOUR (4) represents reefs fringing the land quite closely where
the sea is deep, and where the bottom is gently inclined extending to a
moderate distance from it, but not having a deep-water moat or lagoon-like
space parallel to the shore. It must be remembered that fringing-reefs are
frequently BREACHED in front of rivers and valleys by deepish channels,
where mud has been deposited. A space of thirty miles in width has been
coloured round or in front of the reefs of each class, in order that the
colours might be conspicuous on the appended map, which is reduced to so
small a scale.

The VERMILLION SPOTS, and streaks (1) represent volcanoes now in action, or
historically known to have been so. They are chiefly laid down from Von
Buch's work on the Canary Islands; and my reasons for making a few
alterations are given in the note below.

(I have also made considerable use of the geological part of Berghaus'
"Physical Atlas." Beginning at the eastern side of the Pacific, I have
added to the number of the volcanoes in the southern part of the
Cordillera, and have coloured Juan Fernandez according to observations
collected during the voyage of the "Beagle" ("Geological Transactions,"
volume v., page 601.) I have added a volcano to Albemarle Island, one of
the Galapagos Archipelago (the author's "Journal of Researches," page 457).
In the Sandwich group there are no active volcanoes, except at Hawaii; but
the Rev. W. Ellis informs me, there are streams of lava apparently modern
on Maui, having a very recent appearance, which can be traced to the
craters whence they flowed. The same gentleman informs me, that there is
no reason to believe that any active volcano exists in the Society
Archipelago; nor are there any known in the Samoa or Navigator group,
although some of the streams of lava and craters there appear recent. In
the Friendly group, the Rev. J. Williams says ("Narrative of Missionary
Enterprise," page 29) that Toofoa and Proby Islands are active volcanoes.
I infer from Hamilton's "Voyage in the 'Pandora'" (Page 95), that Proby
Island is synonymous with Onouafou, but I have not ventured to colour it.
There can be no doubt respecting Toofoa, and Captain Edwards (Von Buch,
page 386) found the lava of recent eruption at Amargura still smoking.
Berghaus marks four active volcanoes actually within the Friendly group;
but I do not know on what authority: I may mention that Maurelle describes
Latte as having a burnt-up appearance: I have marked only Toofoa and
Amargura. South of the New Hebrides lies Matthews Rock, which is drawn and
described as an active crater in the "Voyage of the 'Astrolabe'." Between
it and the volcano on the eastern side of New Zealand, lies Brimstone
Island, which from the high temperature of the water in the crater, may be
ranked as active (Berghaus "Vorbemerk," II Lief. S. 56). Malte Brun,
volume xii., page 231, says that there is a volcano near port St. Vincent
in New Caledonia. I believe this to be an error, arising from a smoke seen

on the OPPOSITE coast by Cook ("Second Voyage," volume ii., page 23) which
smoke went out at night. The Mariana Islands, especially the northern
ones, contain many craters (see Freycinet's "Hydrog. Descript.") which are
not active. Von Buch, however, states (page 462) on the authority of La
Peyrouse, that there are no less than seven volcanoes between these islands
and Japan. Gemelli Creri (Churchill's "Collect." volume iv., page 458),
says there are two active volcanoes in latitude 23 deg 30', and in latitude
24 deg: but I have not coloured them. From the statements in Beechey's
"Voyage" (page 518, 4to edition) I have coloured one in the northern part
of the Bonin group. M. S. Julien has clearly made out from Chinese
manuscripts not very ancient ("Comptes Rendus," 1840, page 832), that there
are two active volcanoes on the eastern side of Formosa. In Torres
Straits, on Cap Island (9 deg 48' S., 142 deg 39' E.) a volcano was seen
burning with great violence in 1793 by Captain Bampton (see Introduction to
Flinders' "Voyage," page 41). Mr. M'Clelland (Report of Committee for
investigating Coal in India, page 39) has shown that the volcanic band
passing through Barren Island must be extended northwards. It appears by
an old chart, that Cheduba was once an active volcano (see also "Silliman's
North American Journal", volume xxxviii., page 385). In Berghaus'
"Physical Atlas," 1840, No. 7 of Geological Part, a volcano on the coast of
Pondicherry is said to have burst forth in 1757. Ordinaire ("Hist. Nat.
des Volcans," page 218) says that there is one at the mouth of the Persian
Gulf, but I have not coloured it, as he gives no particulars. A volcano in
Amsterdam, or St. Paul's, in the southern part of the Indian Ocean, has
been seen ("Naut. Mag." 1838, page 842) in action. Dr. J. Allan, of
Forres, informs me in a letter, that when he was at Joanna, he saw at night
flames apparently volcanic, issuing from the chief Comoro Island, and that
the Arabs assured him that they were volcanic, adding that the volcano
burned more during the wet season. I have marked this as a volcano, though
with some hesitation, on account of the possibility of the flame arising
from gaseous sources.)

The uncoloured coasts consist, first and chiefly, of those, where there are
no coral-reefs, or such small portions as to be quite insignificant.
Secondly, of those coasts where there are reefs, but where the sea is very
shallow, for in this case the reefs generally lie far from the land, and
become very irregular, in their forms: where they have not become
irregular, they have been coloured. thirdly, if I had the means of
ascertaining the fact, I should not colour a reef merely coating the edges
of a submarine crater, or of a level submerged bank; for such superficial
formations differ essentially, even when not in external appearance, from
reefs whose foundations as well as superficies have been wholly formed by
the growth of coral. Fourthly, in the Red Sea, and within some parts of
the East Indian Archipelago (if the imperfect charts of the latter can be
trusted), there are many scattered reefs, of small size, represented in the
chart by mere dots, which rise out of deep water: these cannot be arranged
under either of the three classes: in the Red Sea, however, some of these
little reefs, from their position, seem once to have formed parts of a
continuous barrier. There exist, also, scattered in the open ocean, some
linear and irregularly formed strips of coral-reef, which, as shown in the
last chapter, are probably allied in their origin to atolls; but as they do
not belong to that class, they have not been coloured; they are very few in
number and of insignificant dimensions. Lastly, some reefs are left
uncoloured from the want of information respecting them, and some because
they are of an intermediate structure between the barrier and fringing
classes. The value of the map is lessened, in proportion to the number of
reefs which I have been obliged to leave uncoloured, although, in a
theoretical point of view, few of them present any great difficulty: but
their number is not very great, as will be found by comparing the map with
the statements in the Appendix. I have experienced more difficulty in
colouring fringing-reefs than in colouring barrier-reefs, as the former,
from their much less dimensions, have less attracted the attention of
navigators. As I have had to seek my information from all kinds of
sources, and often from indirect ones, I do not venture to hope that the
map is free from many errors. Nevertheless, I trust it will give an
approximately correct view of the general distribution of the coral-reefs
over the whole world (with the exception of some fringing-reefs on the
coast of Brazil, not included within the limits of the map), and of their
arrangement into the three great classes, which, though necessarily very
imperfect from the nature of the objects classified, have been adopted by
most voyagers. I may further remark, that the dark blue colour represents
land entirely composed of coral-rock; the pale blue, land with a wide and
thick border of coral-rock; and the red, a mere narrow fringe of

Looking now at the map under the theoretical point of view indicated in the
last chapter, the two blue tints signify that the foundations of the reefs
thus coloured have subsided to a considerable amount, at a slower rate than
that of the upward growth of the corals, and that probably in many cases
they are still subsiding. The red signifies that the shores which support
fringing-reefs have not subsided (at least to any considerable amount, for
the effects of a subsidence on a small scale would in no case be
distinguishable); but that they have remained nearly stationary since the
period when they first became fringed by reefs; or that they are now rising
or have been upraised, with new lines of reefs successively formed on them:
these latter alternatives are obviously implied, as newly formed lines of
shore, after elevations of the land, would be in the same state with
respect to the growth of fringing-reefs, as stationary coasts. If during
the prolonged subsidence of a shore, coral-reefs grew for the first time on
it, or if an old barrier-reef were destroyed and submerged, and new reefs
became attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of error,
any coast has been coloured wrongly with respect to movement indicated.
Well characterised atolls and encircling barrier-reefs, where several occur
in a group, or a single barrier-reef if of large dimensions, leave scarcely
any doubt on the mind respecting the movement by which they have been
produced; and even a small amount of subsequent elevation is soon betrayed.
The evidence from a single atoll or a single encircling barrier-reef, must
be received with some caution, for the former may possibly be based upon a
submerged crater or bank, and the latter on a submerged margin of sediment,
or of worn-down rock. From these remarks we may with greater certainty
infer that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or have
been upraised.
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