8 ATOLLS. Ch. I. 



the outer margin. But 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 Mr. Liesk, an intelligent resident on 

 these islands, as well as by some chiefs at Tahiti (Ota- 

 heite), that an exposure to the rays of the sun for a very 



No, 3. 



E 



159 Yards 



a, ^ ) 



A — Level of the sea at low water : where the letter A is placed, the 

 depth is 25 fathoms, and the distance rather more than 150 yards from 

 the edge of the reef. 



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. 



D — A low projecting ledge of brecciated coral-rock, washed by the 

 waves at high water* 



E-*-k. 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. 



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 Porites, 



