98 , RATE OF GROWTH. Ch. IV. 



masses of great thickness have been formed by the 

 growth of coral. In the case of Vanikoro, judging 

 only from the depth of the moat between the land 

 and the reef, the wall of coral-rock must be at least 

 300 feet in vertical thickness. 



So again some of the upraised islands in the Pacific 

 show what thick masses of coral-rock have been 

 formed. Dana 1 states that Metia, in the Paumotu 

 or Low Archipelago, consists of white solid limestone 

 with some disseminated corals ; and this island once 

 existed as an atoll, though now surrounded by cliffs 

 250 feet in height. The cliffs round Elizabeth Island 

 in the same archipelago are 80 feet high, and are 

 composed, according to Beechey, of homogeneous coral- 

 rock. Mangaia in the Hervey Group, and Eurutu, 

 appear both to have once existed as encircled islands, 

 and their barrier-reefs are now in parts 300 feet above 

 the level of the sea. 2 



Some attempts have been made, with but little 

 success, to ascertain by boring the thickness of coral 

 formations. At Bow Island, in the Low Archipelago, 

 Sir E. Belcher 3 bored to a depth of 45 feet, and 

 below the first 20 found only coral-sand. During 

 Wilke's Expedition 4 , in a boring of 21 feet in depth 



1 Corals and Coral Islands, 1872, p. 193. See also Mr. Couthouy's 

 pamphlet above referred to. 



2 Dana, Corals and Coral Islands, p. 336. Also Forster's Voyage 

 round the World with Cook, vol. ii. pp. 163, 167. Williams's Narrative 

 of Missionary Enterprise, pp. 30, 48, and 249. 



3 Voyage Round the World, vol. i. 1843, p. 369. 



4 Narrative U.S. Exploring Expedition, vol. iv. p. 268. Dana, 

 Corals and Coral Islands, p. 184. 



