98 
RATE OF GROWTH. 
Cn. IV. 
of many individuals, — first one being broken off or 
killed by some accident, and then another, and one set 
of species being replaced by another set with different 
habits, as the reef rose nearer the surface, or as other 
changes supervened. The spaces between the corals 
would become filled up with fragments and sand, and 
such matter would probably soon be consolidated, for 
we learn from Lieut. Nelson’s 1 observations at Bermuda 
that a process of this kind takes place beneath water, 
without the aid of evaporation. In reefs, also, of the 
barrier class, we may feel sure, as I have shown, that 
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 2 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 Rurutu, 
appear both to have once existed as encircled islands, 
1 Geological Transactions, vol. v. p. 118. 
2 Corals and Coral Islands, 1872, p. 193. See also Mr. Couthouy’s 
pamphlet above referred to. 
