CORAL-REEFS. 97 



the Dolphin Reef off Tahiti, which has remained at the 

 same depth beneath the surface, namely, about two fathoms 

 and a half, for a period of sixty-seven years. There are 

 reefs in the Red Sea, which certainly do not appear 1 to 

 have increased in dimensions during the last half-century, 

 and from the comparison of old charts with recent surveys, 

 probably not during the last two hundred years. These, 

 and other similar facts, have so strongly impressed many 

 with the belief of the extreme slowness of the growth of 

 corals, that they have even doubted the possibility of 

 islands in the great oceans having been formed by their 

 agency. Others, again, who have not been overwhelmed 

 by this difficulty, have admitted that it would require thou- 

 sands, and tens of thousands of years, to form a mass, 

 even of inconsiderable thickness ; but the subject has not, 

 I believe, been viewed in the proper light. 



That masses of considerable thickness have been formed 

 by the growth of coral, may be inferred with certainty from 

 the following facts : — In the deep lagoons of Peros Banhos 

 and of the Great Chagos Bank, there are, as already 

 described, small steep-sided knolls covered with living 

 coral. There are similar knolls in the southern Maldiva 

 atolls, some of which, as Captain Moresby assures me, are 

 less than a hundred yards in diameter, and rise to the 

 surface from a depth of between 250 and 300 feet. Con- 

 sidering their number, form, and position, it would be 

 preposterous to suppose that they are based on pinnacles of 

 any rock, not of coral formation ; or that sediment could 

 have been heaped up into such small and steep isolated 

 cones. As no kind of living coral grows above the height 

 of a few feet, we are compelled to suppose that these knolls 

 have been formed by the successive growth and death of 



1 Ehrenberg, ut sup. , p. 43. 



872 



