RATE OF GROWTH OF CORAL. 507 



the formation of coral reefs and islands is effected. Many 

 errors have prevailed upon this subject, both as to the rapidity 

 of their extension, and the depth from which they are built up 

 to the surface of the ocean. It has been commonly stated that 

 many channels and harbours in the Red Sea have been closed 

 up, within the memory of man, by the rapid increase of coral 

 limestone. But Ehrenberg, who carefully examined these 

 localities, attributes the obstruction rather, in some instances, to 

 the quantities 'of coral sand which have been washed into the 

 harbours, and in others to the accumulation of ballast (generally 

 composed of pieces of coral rock) thrown out from vessels. In 

 Captain Beechey's late Expedition to the Pacific, no positive 

 information could be obtained of any channel having been filled 

 up within a given period ; and he states, as an indisputable fact, 

 that several reefs had remained, for more than half a century, at 

 about the same depth from the surface. On the other hand, there 

 is evidence of the occasionally rapid growth Oi these structures. 

 In the Museum of the Bristol Institution is a mass of Agaricia, 

 weighing 2 Ibs. 9 oz., surrounding a species of Oyster, whose age 

 cannot be more than two years. Pieces of coral, detached from 

 a reef, and thrown into some other situation, soon become fixed 

 by the deposition of new stony matter, if the animal flesh have 

 not been too much destroyed ; and in this manner a sort of arti- 

 ficial reef may be formed in any spot desired, provided the depth 

 of water be suitable. The natives of the Polynesian Islands 

 have long employed this method for building their piers, wharfs, 

 fish-preserves, &c. Mr. Stutchbury mentions that he saw, at the 

 Island of Taapoto, in about seven fathoms water, the anchor of 

 a large ship, wrecked there not more than fifty years previously ; 

 this was completely incrusted by coral, though it preserved its 

 original form. One of the most interesting proofs of the occa- 

 sionally rapid growth of Coral, is afforded by the alteration in 

 form which is seen in two kinds of shells that inhabit the reefs 

 the Vermetus and Magilus ( 926). 



1075. There can be no doubt that, whether the growth of 

 Coral takes place as rapidly as some maintain, or as slowly as 

 it is believed to do by others, it is among the most important of 



