138 CORALS AND CORAL ISLANDS. 



outer reef, and are not always present; the outer margin is 

 higher than the rest of the reef when they are absent. 



The Nullipores are not alone on this outer edge, for there 

 are always sprigs of Madrepores, small Astraaas, and some oth- 

 er corals, lodged in the cavities, with many Echini, star-fishes 

 and sea-anemones, besides barnacles and serpulas ; and fish of 

 many colors dart in and out of the numerous recesses. 



Outer reefs are far more liable than the inner to become 

 covered with accumulations of coral fragments and sand 

 through the force and inward movement of the waves. The 

 debris gathered up by the waters finds a lodgment some dis- 

 tance back from the margin — it may be one or two hundred 

 feet, or as many yards, and gradually increases, until in many 

 instances dry land is formed, and an islet covered with vegeta- 

 tion appears. Such effects are confined chiefly to the reef on 

 the sides open to the prevailing wind, and the final result, a 

 green islet, is not of common occurrence. But occasionally, 

 the reef for miles has become changed from the coral bank, 

 bare at low or middle tide, to habitable land, and makes liter- 

 ally, as at Bolabola, a green belt to the island of volcanic rocks 

 and lofty hills within. The causes and the result are much the 

 same as in a coral island, and the steps in the process are 

 more particularly described beyond where treating of atolls. 



The rock of the outer reef, wherever broken, exhibits usu- 

 ally a compact texture. In some parts it consists of coral 

 fragments, rounded or angular, of quite large size, firmly ce- 

 mented. Other portions are a finer coral breccia or conglom- 

 erate. Still others, more common, are solid white limestones, 

 as impalpable and homogeneous in texture as the old limestones 

 of our continents. There are also other regions where the 

 corals in the rock retain the original position of growth. But 

 the rock in general consists of the debris of the coral fields, 



