ORIGIN of all THE TROPICAL LOW ISLES, over the whole South Sea." Captain Flinders, gives the following interesting ac count of the formation of Coral Islands, particularly of Half-way Island on the north coast of Terra Australis. Vol. ii. p. 114, 115, 116.
" This little island, or rather the surrounding reef, which is three or four miles long, affords shelter from the south-east winds ; and being at a moderate day's run from Murray's Isles, it forms a convenient anchor age for the night to a ship passing through Torres' Strait ; I named it Halfway Island. It is scarcely more than a mile in circumference, but appeal s to be increas ing both in elevation and extent. At no very distant period of time, it was one of those banks produced by the washing up of sand and broken coral, of which most reefs afford instances, and those of Torres' Strait, a great many. These banks are in different stages of progress; some, like this, are become islands, but not yet habitable ; some are above high-water mark, but destitute of vegetation ; whilst others are overflowed with every returning tide.
" It seems to me, that when the animalcules which form the corals at the bottom of the ocean, cease to live, their structures adhere to each other, by virtue either of the glutinous remains within, or of some pro perty in salt water ; and the interstices being gradually filled up with sand and broken pieces of coral washed by the sea, which also adhere, a mass of rock is at length formed. Future races of these animalcules erect their habitations upon the rising bank, and die in their turn, to increase, but principally to elevate, this monument of their wonderful labours. The care taken to work perpendicularly in the early stages, would mark a sur prising instinct in these diminutive creatures. Their wall of coral for the most part, in situations where the winds are constant, being arrived at the surface, affords a shelter, to leeward of which their infant colonies may be safely sent forth ; and to this their instinctive fore sight it seems to be owing, that the windward side of a reef exposed to the open sea, is generally, if not al ways, the highest part, and rises almost perpendicular, sometimes from the depth of 200, and perhaps many more fathoms. To be constantly covered with water,
seems necessary to the existence of the animalcules, for they do not work, except in holes upon the reef, beyond low-water mark ; but the coral sand and other broken remnants thrown up by the sea, adhere to the rock, and form a solid mass with it, as high as the com mon tides reach. That elevation surpassed, the future remnants, being rarely covered, lose their cohesive pro perty ; and remaining in a 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 pro gressive 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 sub stances in a loose state, with a little vegetable ; 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 vege tation.