STRUCTURE OF CORAL REEFS. 109 



coral, for it is without a pore ; and layer is added to layer until 

 it has considerable thickness. It is thus an important pro- 

 tection to the reef against the wash of the waters. 



Darwin states that on Reeling's Island, the Nullipore bed 

 has a thickness of two or three feet and a breadth of twenty 

 feet. NuUipores are abundant on the Paumotu reefs. Still, 

 they are not essential to the formation or protection of an 

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

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



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

 are always sprigs of Madrepores, small Astraeas, and some 

 other corals, lodged in the cavities, with many echini, star- 

 fishes and sea-anemones, besides barnacles and serpulas ; 

 and fish of many colours 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 

 processare more particularly described further on, where treat- 

 ing of atolls. 



The rock of the outer reef, wherever broken, exhibits 

 usually 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 conglo- 

 merate. Still others, more common, are solid v/hite limestones, 



