54 On the Formation and Growth of Coral Reefs and Islands. 



before mentioned lagoons. Tongataboo, Vavaa, and others of the Tonga 

 and Hapai isles, are standing from ten to thirty feet above the surface of 

 the water. 



The crescent-shaped reefs appear to have been formed under similar 

 circumstances to the lagoons j but at the time of their elevation, their 

 projectile force was unequally exerted, thus depressing one portion of the 

 circle, while the other was in a slight degree elevated. 



Of the long slips of coral, or coral banks, such as Tethuroa, and the 

 great barrier of New Holland, the most probable solution will be, to con- 

 sider them as built upon elevated ridges, or the tops of a vast chain of 

 mountains, the corals having formed upon the apices, until arrived at the 

 water's surface, when they enlarged themselves laterally, until one con- 

 tinued wall of coral presented itself, with occasional openings, which may 

 have happened by the chain of mountains being broken by champaign 

 country. Captain King states, that the continuity of the great barrier, 

 taking seven hundred and fifty miles of its course, is broken by the inter- 

 vention of gaps, at about thirty or forty miles distance from each other; 

 this would answer very well for the distance from one cone to the other of 

 any chain of mountains, supposing them to be submerged. 



Lastly, we arrive at the high basaltic islands, the Society Isles, for in- 

 stance, of which we will take Tahiti. The highest point of this island is 

 computed to be 12,000 feet above the level of the ocean ; the whole of the 

 island is formed of basalt, with scarcely any other substance, except the 

 common minerals which occur in such rocks, some portions being very 

 compact, and of whicli the natives form their tools, while other parts are 

 vesicular, or scoriated, bearing decided evidences of volcanic origin. 



Here a reef surrounds tiie shores, sometimes forming a connected flat 

 coral platform ; but in most instances it appears to have raised itself ab- 

 ruptly from the sides of the land, leaving a space, as we have before stated, 

 between the reef and the shore. 



This island may be considered as a favourable example for explaining, 

 or rather corroborating, the views before taken ; for upon the apex of one 

 of the highest mountains, there is a distinct and regular stratum of semi- 

 fossil coral ; while at the summit of an adjacent mountain, at a much 

 lower elevation, there is a basin considered by most who have visited it to 

 be an extinct volcanic crater, now filled with rain water and forming a 

 fresh water lake : this basin or crater has its outer edge broken by two 

 notches or gorges.* 



We are thus furnished with the best possible proof of the state in which 

 this and the other high lands of the group once stood j it has been stated 



* The lake is named by the natives V'ai-hiria, from vai, water, and hiria, to swim, 

 in consequence of the onlj' mode of getting from the one side to the other being by 

 swimming across by the two gorges ; the edges of the basin being in other parts ex- 

 ceedingly perpendicnlar. Its diameter in the rainy season is about one mile. 



