RATE OF GROW Til OF CORAL REEFS. 253 



rate of upward progress is one-sixteenth of an inch a year, it 

 would take for an addition of a single foot to its height, one 

 hundred and ninety years, and for five feet a thousand years. 



It is here to be considered, that the thickness of a srowin<y 

 reef could not exceed twenty fathoms (except by the few feet 

 added through beach and wind-drift accumulations), even if 

 existing for hundreds of thousands of years, unless there were 

 at the same time a slowly progressing subsidence ; so that if 

 we know the possible rate of increase in a reef, we cannot 

 infer from it the actual rate for any particular reef; for it may 

 have been very much slower than that. Without a subsidence 

 in progress, the reef would increase only its breadth. 



In order to obtain direct observations on the rate of in- 

 crease of reefs, a slab of rock was planted, by the order of Cap- 

 tain Wilkes, on Point Venus, Tahiti, and by soundings, the 

 depth of Dolphin shoal, below the level of this slab, was care- 

 fully 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 toward determining the average 

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

 condition 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 the shells of Tridacnas lie imbedded in 

 coral rock, has been supposed to afford some data for estima- 

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

 these mollusks 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 



