CORAL-REEFS. 27 



outer coast. The highest part of the islets (excepting 

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

 close to the outer beach (E of the woodcut), and averages 

 from six to ten feet above ordinary high-water mark. From 

 the outer beach the surface slopes gently to the shores of 

 the lagoon, which no doubt has been caused by 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 wind- 

 ward, some being even 800 yards in width ; but the land 

 thus added is very low. The fragments beneath the sur- 

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

 ledge (D of the woodcut), 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 

 having become firmly cemented together by the percolation 

 of calcareous matter, resist the daily tides longer, and hence 

 project as a ledge. The cemented mass is generally of a 

 white colour, but in some few parts reddish from ferruginous 

 matter; it is very hard, and is sonorous under the hammer; 

 it is obscurely divided by seams, dipping at a small angle 

 seaward ; it consists of fragments of the corals which grow 

 on the outer margin, some quite and others partially 

 rounded, some small and others between two and three 

 feet across ; and of masses of previously formed 



