Cu. VI. DISTRIBUTION OF CORAL-REEFS. 
177 
were evidently once reefs within 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 
100 feet, which probably marks a pause in its elevation. 
From these facts we may infer that the Cook and Austral 
Islands have been upheaved at a not very remote period. 
Savage Island (S.E. of the Friendly group) is according 
to Forster about 40 feet in height, and according to 
Williams about 100 feet. Forster 1 describes the plants as 
already growing 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 to 
the islands of the Friendly Group, which have been well 
described in the second and third voyages of Cook, and 
recently by Dana. The surface of Tongatabou is low and 
level, but with parts 50 or GO feet high ; the whole consists 
of coral-rock, ‘ which yet shows the cavities and irregular- 
ities worn into it by the action of the tides.’ 3 On Eoua 
the same appearances were noticed at an elevation of 
between 200 and 000 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 unequably elevated ; and 
Anamouka, an atoll equably elevated. This latter island 
contains 4 in its centre a salt-water lake, about a mile and a 
half in diameter, without any communication w 7 ith the sea, 
and around it the land rises gradually like a bank : the 
highest part is only between twenty and thirty feet; but 
1 Observations made during Voyage Round the World, p. 147. 
2 Voyage, vol. ii. p. 163. 
* Cook’s Third Voyage (4to edit.), vol. i. p. 314. 
4 Ibid. vol. i. p. 235. 
