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 Astrzeas, 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 lable 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 
fect, 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, 
