16 ATOLLS. Ch. I. 



sively consist of those kinds which live on the outer 

 coast. The highest part of the islets (excepting hil- 

 locks of blown sand, some of which are 30 feet high), is 

 close to the outer beach (E of the wood-cut) and aver- 

 ages from six to ten feet above ordinary high-water 

 water. From the outer beach the surface slopes gently 

 to the shores of the lagoon ; and this slope no doubt 

 is due to the breakers, the further they have rolled 

 over the reef, having had less power to throw up 

 fragments. The little waves of the lagoon heap up 

 sand and fragments of thinly-branched corals on the 

 inner side of the islets on the leeward side of the atoll ; 

 and these islets are broader than those to windward, 

 some being even 800 yards in width ; but the land thus 

 added is very low. The fragments beneath the surface 

 are cemented into a solid mass, which is exposed as a 

 ledge (D of the wood-cut), projecting some yards in 

 front of the outer shore, and from two to four feet high. 

 This ledge is just reached by the waves at ordinary 

 high-water : it extends in front of all the islets, and 

 everywhere has a water-worn and scooped appearance. 

 The fragments of coral which are occasionally cast on 

 the ' flat ' are during gales of unusual violence swept 

 together on the beach, where the waves each day at 

 high-water tend to remove and gradually wear them 

 down ; but the lower fragments are firmly cemented 

 together by percolated calcareous matter, and they resist 

 the daily tides longer than the loose upper fragments ; 

 and thus a projecting ledge is formed. The cemented 

 mass is generally of a white colour, but in some few 



