On Coral Reofs and Islands. Al 
1s 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. 
he 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. hey occur in 
the dead rock,—generally where there are no growing corals, 
selves. But the shore-platform where they are found is not in- 
creasing in height. hey 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. 
Skcoxp Serres, Vol. XIII, No. 37.—Jan, 1852. 6 
