384 PHYSICAL GEOGRAPHY. 



loose state, form what is usually called a key upon the top of the reef. The new bank is 

 not long in being visited by sea-birds ; salt-plants take root upon it, and a soil begins to 

 be formed ; a cocoa-nut or the drupe of a pandanus is thrown on shore ; land-birds visit 

 it, and deposit the seeds of shrubs and trees ; every high tide, and still more every gale, 

 adds something to the bank ; the form of an island is gradually assumed ; and last of all 

 comes man to take possession. 



" Half-way Island is well advanced in the above progressive state ; having been many 

 years, probably some ages, above the reach of the highest spring tides, or the wash of the 

 surf in the heaviest gales. I distinguished, however, in the rock which forms its basis, 

 the sand, coral, and shells, formerly thrown up, in a more or less perfect state of cohesion, 

 small pieces of wood, pumice stone, and other extraneous bodies which chance had mixed 

 with the calcareous substances when the cohesion began, were inclosed in the rock, and in 

 some cases were still separable from it without much force. The upper part of the island 

 is a mixture of the same substances in a loose state with a little vegetable soil ; and is 

 covered with the casuarina and a variety of other trees and shrubs, which give food to 

 parroquets, pigeons, and some other birds, to whose ancestors, it is probable, the island 

 was originally indebted for this vegetation." 



Captain Beechey, during his expedition to the Pacific in the ship Blossom, collected a 

 large mass of interesting information respecting the peculiarities of form and structure 

 exhibited by the coral islands. Of thirty-two examined by him the largest was thirty 

 miles in diameter, and the smallest somewhat less than a mile. They were of various 

 shapes, and all formed of living coral, except one, called Henderson's Island, which was 

 partially surrounded by it. They all appeared to be increasing their dimensions by the 

 active operations of the zoophytes, which are gradually extending their structures, and 

 bringing the immersed part to the surface of the water. Twenty-nine of the number had 

 lagoons or lakes in their centres, a proportion sufficiently large to render it highly 

 probable that the remainder also had them in the early period of their formation, and 

 that this is a peculiar structure common to the coral islands. The depth of these lagoons 

 is various ; in some which were entered, it was from twenty to thirty-eight fathoms, but 

 in others, to which no access was gained, it appeared from the light blue colour of the 

 water to be very small. The bases of the lagoons are formed of coral, and are gradually 

 filling up by the labours of the insects, and by the deposition of sand and zoophytic 

 substances ; so that the lakes will in process of time vanish, and one connected mass of 

 land present itself. At Ducie's Island, the lagoon in the centre was partly enclosed by 

 trees, and owing to the transparency of the water, the bottom presented a submarine 

 picture of extreme beauty. The corallines were of various colours, pink, blue, white, 

 lilac, and yellow ; and numerous small fish of brilliant hues, threading the labyrinths of the 

 coral branches, or, when alarmed, darting rapidly for shelter into the recesses of the stony 

 thickets, afforded a singularly pleasing and almost kaleidoscopic effect. It is remarkable, 

 that as almost all th'ese islands are situated within the action of the trade winds, they 

 follow one general rule in having their windward sides higher and more protected than 

 the others, and not unfrequently well wooded, while the opposite ones are only half- 

 drowned reefs, or wholly under water. At Gambia and Matilda islands this inequality 

 was very conspicuous, the weather sides of both being wooded, and of the former inhabited, 

 while the other sides were from twenty to thirty feet under water. It would seem that 

 the coral insects pursued their labours under the guidance of a surprising instinct, 

 apprehending from what quarter danger threatened their structures, and hence erecting 

 buttresses to oppose the action of the waves impelled by the trade wind. The observations 

 of Captain Horsburgh and other hydrographers upon the coral islands of other seas are 

 in harmony with the preceding statements, chiefly derived from Captain Beechey. The 



