122 CORAL FORMATIONS. 
of the sea, and are farther removed from the deleterious influences 
which may affect the latter. 
As stated above, no results were arrived at from observations made 
in the course of the voyage through the Pacific. The general impres- 
sion that their progress is slow, was fully sustained. The facts, with 
regard to the growth of zoophytes, give some data, though by no 
means satisfactory. 
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 retained 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 soundings, 
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 sta- 
tionary 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 condition of the reef should be ascertained, 
the species of corals upon it be identified, and the influence of the cur- 
rents investigated which sweep in that direction out of Matavai Bay. 
The depth to which Chamas or Tridacnas lie embedded 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,—gene- 
rally 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 themselves. But the shore platform where 
they are found is not increasing 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 
