1 693-] CORAL REEFS. 109 



and tlio' 'twas not good we were glad he had found it, hopmg 

 to reap some Benefit by it. 



When the Bark was in the "Water, we were all surpris'd to 

 find she did not obey the Eudder, and that to turn it we 

 must make use of an Oar. 



The day of our Departure, was fix'd to be Saturday the 

 19th of April 1693. The Moon being then near at the Full, 

 the Sea wou'd be high, and consequently the easier to pass 

 above the Shelves. The Eeason why we did not choose the 

 time when the Moon wou'd be quite at the Full was, because 

 we wou'd have as much as we cou'd of her Light. 



These Shelves of which I have often spoken,^ are (to inform, 

 en passant, those who do not understand the Term) Eocks 

 rising up in the Sea like a sort of Wall,^ with which the 



clocks and watches took their place. In the collection at the S. Ken- 

 sington Museum there are several of that period ; one in particular of 

 French make, with the hours and names of cities engraved on it, might 

 serve to illustrate our text. 



1 Cf. supra, p. 47, and note, ib. 



2 The island of Rodriguez is bound to the north, the south, and the 

 west with chains or reefs of rocks, nearly even with the water's edge, 

 on which are scattered the rocky islets mentioned above at page 88. 

 This bank or wall, as the text says, extends a league and a half from 

 the coast, and the north-east side is the least dangerous, as the reef 

 recedes sufficiently here to admit of ranging along the isle on that side. 

 (Grant's History of Mauritius, p. 310.) 



Mr. J. Murray, of the Challenger expedition, has shown that it is not 

 necessary to call in the subsidences required by Darwin's theory (J. 

 Naturalisfs Voyage, pp. 557 seqq.) to explain the characteristics of 

 these barrier reefs. He shows that the foundations for barrier reefs, such 

 as those at Rodriguez, have been prepared by the disintegration of 

 volcanic islands and by the building up of submarine volcanoes by the 

 deposition on their summits of organic sediments ; that barrier reefs have 

 been built up by coral plantations from the. shore on a foundation of 

 volcunic debris, or on a talus of coral blocks, coral sediments, and pelagic 

 shells and the lagoon channel formed by the solvent action of the sea- 

 water thrown over the reefs at each tide and by currents, etc. (See 

 Structure and Origin of Coral Reefs and Islands, by Mr. John Murray, 

 1880 ; Nature, vol. xxii, p. 351.) 



The coral animals by which the stony, cellular scaffoldings which com- 



