20 CORAL-REEFS. 



B.— Outer edge of that flat part of the reef, which dries at low water : the 

 edge either consists of a convex mound, as represented, or of rugged points, 

 like those a little farther seaward, beneath the water. 



C. — A flat of coral-rock, covered at high water. 



B.— A low projecting ledge of brecciated 1 coral-rock washed by the waves at 

 high water. 



E. — A slope of loose fragments, reached by the sea only during gales : the 

 upper part, which is from six to twelve feet high, is clothed with vegetation. 

 The surface of the islet gently slopes to the lagoon. 



F.— Level of the lagoon at low water. 



The section is true to the scale in a horizontal line, but 

 it could not be made so in a vertical one, as the average 

 greatest height of the land is only between six and twelve 

 feet above high-water mark. I will describe the section, 

 commencing with the outer margin, I must first observe 

 that the reef-building polypifers, not being tidal animals, 

 require to be constantly submerged or washed by the 

 breakers. I was assured by Air. Liesk, a very intelligent 

 resident on these islands, as well as by some chiefs at 

 Tahiti (Otaheite), that an exposure to the rays of the sun 

 for a very short time invariably causes their destruction. 

 Hence it is possible only under the most favourable circum- 

 stances, afforded by an unusually low tide and smooth 

 water, to reach the outer margin, where the coral is alive. 

 I succeeded only twice in gaining this part, and found it 

 almost entirely composed of a living Pontes, which forms 

 great irregularly rounded masses (like those of an Astraea, 

 but larger) from four to eight feet broad, and little less in 

 thickness. These mounds are separated from each other 

 by narrow crooked channels, about six feet deep, most of 

 which intersect the line of reef at right angles. On the 

 furthest mound, which I was able to reach by the aid of a 

 leaping-pole, and over which the sea broke with some 



1 That is, sharp angular fragments of coral-rock partially or wholly 

 cemented together by calcium carbonate. — Ed. 



