On the Formation mid Growth of Coral Reefs and Islands. 5 I 



most regular order, with comparatively little or no admixture of other 

 substances, which may be accounted for by considering their separation to 

 have taken place by the mechanical action of the waves, according to their 

 different degrees of gravity.* 



Islands often occur of a flat or tabular form, generally oval or irregu- 

 larly circular at their circumference : of this form may be mentioned 

 the groupe, named by Cook the Friendly Isles, consisting of numerous 

 islands, the majority of which are of the tabular form. 



There are also many crescent shaped reefs, with the most convex por- 

 tion of their arc the highest, often denoting themselves to the mariner only 

 by the breaking of the waves, and here and there a rock above the level of 

 the ocean, while the horns of the crescent are depressed, and gradually lost 

 in the greater depths ; in a few instances, as at the groupe called Gambler's 

 Islands, they are sufficiently raised to have become verdant and inhabited. 



We may next refer to those which form long narrow slips of land; for 

 example, Tethuroa,t or the great reef which takes the course of the north- 

 eastern shore of New Holland, which Captain Flinders describes as being 

 more than one thousand miles in extent; in the course of which, there is 

 a continued portion, exceeding three hundred and fifty miles, with scarcely 

 a break or passage through it. 



Among these groupes are many high mountainous mineral-formed islands, 

 which are surrounded by coral ; of these it will only be necessary to refer 

 to the Society Isles, which include Tahiti, and other islands familiar to us, 

 from the beautiful descriptions we have been favoured with, by our intrepid 

 circumnavigators, Wallis, Cook, and others, Of these islands, at the 

 present, we need only state, that they are surrounded by coral reefs, gene- 

 rally situate four or five hundred yards off" shore, with a deep channel 

 between, having numerous openings, through which ships can enter, and 

 lie at anchor in perfect safety. These breaks in the coral barrier, are in 

 most instances opposite the mouths of fresh water rivulets. 



• At Bow Island may be observed beds of cardium fragum, from six to eight feet 

 thick, and more than two miles in extent, without even the broken portion of any 

 other shells l)eing (le])0sited in the same stratum. 



t Tethuroa is a coral island, situate a fow leagues from Tahiti, and uninhabited. 



