584 



FOEMATIOxN OF CORAL EEEFS. 



[Ch. XLIX. 



washed away bv a change in the currents, and it is clear that 

 a coating of growing coral had been formed in a short time.* 

 Experiments, also, of Dr. Allan, on the east coast of Mada- 

 gascar, prove the possibility of coral growing to a thickness of 

 three feet in about half a year ; f so that the rate of increase 

 may, under favourable circumstances, be very far from slow. 

 It must not be supposed that the calcareous masses termed 

 coral reefs are exclusively the work of zoophytes : a great 

 variety of shells, and, among them 



heaviest of known species, contribute to augment the mass. 

 In the South Pacific, great beds of Serpulas, oysters, mussels, 

 Pi7ince marince, Chamce (or Tridacnm), and other shells, 

 cover in profusion almost every' reef; and on the beach of 

 coral islands are seen the shells of echini and broken frao-- 

 ments of crustaceous animals. 



discernible through the clear blue water, and their teeth and 

 hard palates cannot fail to be often preserved although their 

 soft cartilaginous bones may decay. 



It was the opinion of the German naturalist Forster, in 

 1 780, after his voyage round the world with Captain Cook, 

 that coral animals had the power of building up steep and 



depths in the sea, a 



Large shoals of fish are also 



almost perpendicular walls from 



notion afterwards adopted by Captain Flinders and others ; 

 but it is now very generally believed that most of these zoo- 

 phytes cannot live in water of great depths. 



Mr. Darwin has come to the conclusion, that those species 

 which are most effective in the construction of reefs, rarely 



fathoms 



In 



some lagoons, however, where the water is but little agitated. 



Kot 



fathoms' water, or 150 feet; but these may perhaps have 

 begim to live in shallower water, and may have been carried 

 downwards by the subsidence of the reef. There are also 

 various species of zoophytes, and among them some which 

 are provided with calcareous as well as horny stems, which 

 live in much deeper water, even in some cases to a, dpnth of 



fathoms 



reefs. 



* Darwin's Coral Eeefs, p. 77. 



t Ibid. p. 78. 



