2i6 CORALS AND CORAL LS LANDS. 



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 condition of 

 the reef should be ascertained, the species of corals growing 

 upon it be identified, and the influence of the currents investi- 

 gated 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 afiford some data for estimat- 

 ing 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 



^ Since the above was written, a memoir, by MM. Le Clere and de 

 Benaze, has appeared in Paris (1872), on their attempts to make use of the 

 stone planted by Captain Wilkes in determining the rate of growth of the 

 Dolphin Shoal. They made various measurements ; but they observe that 

 Wilkes does not state whether he measured from the top of a head of coral or 

 from a solid bank on which the corals were growing, and further, that the 

 use of an "excellent spirit level,'' from a stone of so little length is not 

 sufficiently exact for correct results ; and hence, were not able to draw any 

 satisfactory conclusion from their results. Before leaving the region, they 

 made the following arrangements with reference to future measurements. 

 They planted two blocks of coral, cementing them below and nearly burying 

 them in the soil, placing them 0'2I metres above the Wilkes stone which is 

 between them ; they then put a mark upon them on plates of metal, 

 directed toward the place of observation on the shoal. A third stone was 

 placed 40 metres from the south-west angle of the Point Venus lighthouse, 

 in order to give a second observation on the position of the spot on which 

 soundings were to be made. This spot was found to bear from the two new 

 stones N. 77° 30' E. ; from the third stone N. 70" 55' E. ; from the bell of 

 the new mission church S. 81° 40' E. A horizontal line passing from the 

 mark on the new stone is 7 "460'" above the madreporic heads. This obser- 

 vation they leave for comparison with future measurements. They farther 

 made observ tions that satisfied them that Tahiti was not at present under- 

 going any general elevation. Two maps accompany the pamphlet ; one is 

 copied from Wilkes; the other (see opposite page) is from a chart by MM. Le 

 Clerc and Minier, lieutenants of the vessel, and contains hnes showing the 

 position of the points refeired to above. 



