
RATE OF GROWTH OF CORAL REEFS. DRG 
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 growing 
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. Betore, 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 Matava 
Bay. See the map, page 247, and Appendix, page 417, 
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 
i aa 
