(DESCRIPTION OF THE PLATES.
PLATE III.--MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE VOLCANOES.
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[Inasmuch as the coloured map would have proved too costly to be given in this series, the indications of colour have been replaced by numbers referring to the dotted groups of reefs, etc.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.
