398 



MINERALOGY. 



Geognosy, sembles the plains on the dry land ; it makes the tran- 

 ^"""V^^ sition from the dry land to the submarine, and will no 

 doubt one day be changed into a plain. Deep subma- 

 rine plains also often occur. Sometimes the bottom of 

 the sea has a very uneven surface, and is composed of 

 hills, either of sand and gravel, particularly near the 

 coast, (as is the case on the coast of Holland), or of 

 rocky hills, or of cliffs and other irregularities. The 

 summits of these submarine hills form islands, and these 

 sometimes appear as continuations of the high country 

 on the dry land. We sometimes also meet with great 

 hollows, which are unfathomable. 



Coral reefs. The coral reef's that surround the islands in the South 

 Sea and Indian Ocean, are to be viewed as a variety of 

 the shoal surface already mentioned. These are the 

 work of very minute animals. They occur in vast 

 abundance along the east coast of New Holland, and in 

 the South Sea, particularly to the eastward of the 

 Friendly Islands, from the 10 to 15 South Latitude. 

 A reef of this kind surrounds the island of Otaheite, and 

 rises like a wall from unfathomable depths. Many of 

 the other islands are encompassed with similar reefs, in 

 particular New Holland, New Caledonia, &c. Captain 

 Flinders says, the quantity of coral reefs between New 

 Holland and New Caledonia and New Guinea is such, 

 that this might be called the coral sea. Thus for 350 

 miles, in a straight line from south-east to north-west, 

 in the east coast of New Holland, is a coral reef unin- 

 terrupted by any large opening into the sea, and this 

 reef is probably connected with others, so as altogether 

 to form an extent of upwards of 1000 miles, and hav- 

 ing a mean breadth of from 20 to 50 miles. These 

 reefs sometimes give rise to islands. The coral rises 

 above the surface of the water, disintegrates, becomes 

 mixed with seeds of different kinds, carried by birds, 

 floated by the waves, or wafted by the winds. These 

 grow and decay, and thus afford a more or less deep 

 covering of soil, on which the cocoa nut and other large 

 trees take root and grow. Palmerston island, the island 

 of Tanea, the island of Middleburg, the island of Ton- 

 gataboo, the island of Mangea, and others in the South 

 Sea, have a base of coral. 



The following details in regard to the formation 

 of coral reefs and islands, as given by Dr. Forster 

 and Captain Flinders, will increase the interest of 

 the preceding account. Dr. Forster remarks, " All 

 the low isles seem to me to be a production of the 

 sea, or rather its inhabitants, the polype-like animals 

 forming the lithophytes. These animalcules raise 

 their habitation gradually from a small base, always 

 spreading more and more, in proportion as the struc- 

 ture grows higher. The materials are a kind of 

 lime mixed with some animal substance. I have 

 seen these large structures in all stages, and of va- 

 rious extent. Near Turtle Island, we found, at a 

 few miles distance, and to leeward of it, a considerable 

 large circular reef, over which the sea broke everywhere, 

 and no part of it was above water ; it included a large 

 deep lagoon. To the east and north-east of the Socie- 

 ty Isles, are a great many isles, which, in some parts, 

 are above water ; in others, the elevated parts are con- 

 nected by reefs, some of which are dry at low-water, 

 and others are constantly under water. The elevated 

 parts consist of a soil formed by a sand of shells and 

 coral rocks, mixed with a light black mould, produced 

 from putrined vegetables, and the dung of sea-fowls ; 

 and are commonly covered by cocoa-nut trees and other 

 shrubs, and a few anti-scorbutic plants. The lower parts 

 have only a few shrubs, and the above plants; others still 

 lower, are washed by the sea at high water. All these 



isles are connected, and include a lagoon in the middle, Geognosy, 

 which is full of the finest fish ; and sometimes there is "^ "V' 

 an opening, admitting a boat or canoe in the reef, but 

 I never saw or heard of an opening that would admit a 

 ship. 



" The reef, or the first origin of these isles, is formed 

 by the animalcules inhabiting the lithophytes. They 

 raise their habitation within a little of the surface of the 

 sea, which gradually throws shells, weeds, sand, small 

 bits of corals, and other things, on the tops of the ; e 

 coral rocks, and at last fairly raises them above water ; 

 where the above things continue to be accumulated by 

 the sea, till by a bird, or by the sea, a few seeds of 

 plants, that commonly grow on the seashore, are thrown 

 up and begin to vegetate; and by their annual decay 

 and reproduction from seeds, create a little mould, year- 

 ly accumulated by the mixture with sand, increasing 

 the dry spot on every side ; till another sea happens to 

 carry a cocoa-nut hither, which preserves its vegetative 

 power a long time in the sea, and therefore will soon 

 begin to grow on this soil, especially as t thrives 

 equally in all kinds of soil ; and thus may all these low 

 isles have become covered with the finest cocoa-nut 

 trees. 



" The animalcules forming these reefs, want to shel- 

 ter their habitation from the impetuosity of the winds, 

 and the power and rage of the ocean ; but as, within 

 the tropics, the winds blow commonly from one quar- 

 ter, they, by instinct, endeavour to stretch only a ledge, 

 within which is a lagoon, which is certainly entirely 

 screened against the power of both ; this therefore 

 might account for the method employed by the animal- 

 cules in building only narrow ledges of coral rocks, to 

 secure in their middle a calm and sheltered place ; and 

 this seems to me to be the most probable cause of THE 

 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 anchorage 

 for the night to a ship passing through Torres' Strait ; 

 I named it Half-way Island. It is scarcely more than 

 a mile in circumference, but appears to be increasing 

 both in elevation and extent. At no very distant pe- 

 riod 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 habita- 

 ble ; some are above high-water mark, but destitute of 

 vegetation ; whilst others are overflowed with every re- 

 turning 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 washeil 

 by the sea, which also adhere, a mass of rock is at length 

 formed. Future races of these animalcules erect thek 1 

 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 



