86 



ON CORAL REEFS AND ISLANDS. 



If we allow that Madrepores may grow three inches a year, it 

 is far from admitting that a reef may increase as rapidly. In the 

 best coral plantations, not over one-third of the surface is covered 

 with growing zoophytes. It would therefore follow, supposing 

 all the species to grow at this rate, and all the material to be re- 

 tained on the plantation, that in twelve months the reef might 

 possibly increase one inch in height ; including shells and other 

 animal remains, it might, perhaps, be one and one-quarter inches. 

 This estimate is based on too many assumptions to be received 

 with any confidence, except it be the confidence that the result 

 is overrated. 



With reference to this subject, by the order of Captain Wilkes 

 a slab of rock was planted on Point Venus, Tahiti, and by sound- 

 ings, the depth of Dolphin shoal, below the level of this slab, 

 was carefully ascertained. By adopting this precaution, any 

 error from change of level in the island, was guarded against : 

 the slab remains as a stationary mark for future voyagers to test 

 the rate of increase of the shoal. Before, however, the results 

 can be of any general value towards determining the average 

 rate of growing reefs, it is still necessary that the growing condi- 

 tion of the reef should be ascertained, the species of corals upon 

 it be identified, and the influence of the currents investigated 

 which sweep in that direction out of Matavai bay. 



The depth to which Chamas or Tridacnas lie imbedded in 

 coral rock, has been supposed to afford some data for estimating 

 the growth of reefs. But Mr. Darwin rightly argues that these 

 molluscs have the power of sinking themselves in the rock, as 

 they grow, by removing the lime about them. They occur in 

 the dead rock, — generally where there are no growing corals, 

 except rarely some small tufts. If they indicate anything, it 

 must be the growth of the reef rock, and not of the corals them- 

 selves. But the shore-platform where they are found is not in- 

 creasing in height. They resemble, in fact, other saxicavous 

 molluscs, several species of which are found in the same seas, 

 some buried in the solid masses of dead coral lying on the reef. 

 The bed they excavate for themselves is usually so complete 

 that only an inch or two in breadth of their ponderous shells are 

 exposed to view. Without some means like this of securing 

 their habitations, these molluscs would be destroyed by the waves ; 

 a tuft of byssus, however strong, which answers for some small 

 bivalves, would be an imperfect security against the force of the 

 sea for shells weighing one to five hundred pounds. 



