CORAL-REEFS. 167 



there are in the central hollow, formerly the bed of the lagoon, 

 many scattered patches of coral-rock, some of them raised to a 

 height of forty feet." These knolls of coral-rock were evidently 

 . once separate reefs in the lagoon of an atoll. Mr. Martens, at 

 Sydney, informed me that this island is surrounded by a 

 terrace-like plain at about the height of a hundred feet, which 

 probably marks a pause in its elevation. From these facts we 

 may infer, perhaps, that the Cook and Austral Islands have 

 been upheaved at a period probably not very remote. 



Savage Island (S.E. of the Friendly group) is about forty 

 feet in height. Forster x describes the plants as already grow- 

 ing out of the dead, but still upright and spreading trees of 

 coral ; and the younger Forster 2 believes that an ancient 

 lagoon is now represented by a central plain ; here we cannot 

 doubt that the elevatory forces have recently acted. The 

 same conclusion may be extended, though with somewhat less 

 certainty, to the islands of the Friendly Groups which have 

 been well described in the second and third voyages of Cook. 

 The surface of Tongatabou is low and level, but with some 

 parts a hundred feet high ; the whole consists of coral-rock, 

 " which yet shows the cavities and irregularities worn into it 

 by the action of the tides." 3 On Eoua the same appearances 

 were noticed at an elevation of between 200 and 300 feet. 

 Vavao, also, at the opposite or northern end of the group, 

 consists, according to the Rev. J. Williams, of coral-rock. 

 Tongatabou, with its northern extensive reefs, resembles either 

 an upraised atoll with one half originally imperfect, or one 

 unequally elevated ; and Anamouka, an atoll equally elevated. 

 This latter island contains 4 in its centre a salt-water lake, about 

 a mile and a half in diameter, without any communication with 

 the sea, and around it the land rises gradually like a bank ; the 

 highest part is only between twenty and thirty feet ; but on 

 this part, as well as on the rest of the land (which, as Cook 



1 Observations made during Voyage round the World, p. 147. 



2 Voyage^ vol. ii. p. 163. 



3 Cook's Third Voyage (4to edition), vol. i. p. 314. 



4 J bid. t vol. i. p. 235. 



