FRINGING AND BARRIER REEFS. 37 
a foot or less of the surface. In these shallow waters the various 
zoophytes at times are crowded over extensive areas; yet very often 
they occur only in patches scattered throughout large fields of coral 
debris. The top of the reef is mostly destitute of life, and consists 
of the naked coral rock, more or less covered with coral sand. Yet 
there are some shallow pools, especially towards the outer limits, 
which abound in corals. 
The exposed edge of the reef is commonly raised a few inches 
above the general surface, and is, therefore, the first part laid bare by 
the retreating tide, although a dangerous place for a ramble, on ac- 
count of the heavy breakers. ‘Though very uneven, the surface has 
generally a smooth, water-worn appearance, and is spotted with various 
shades of pink and purple. ‘These colours, as observed by Chamisso, 
are due to incrusting Nullipores, that grow like lichens over the 
rock: they are vegetable in nature, though composed mostly of lime. 
Other nodular and branching Nullipores, some sprigs of Madrepores, 
and a few Astreas grow in the more sheltered cavities, where they 
are not easily dislodged by the waves; and among them, despite the 
breakers, cling numerous echini, asterias, and actinie. The gradual 
wear of the reefs by the wash of the sea is prevented, to a great 
extent, by these Nullipore incrustations, as was pointed out by Dar- 
win.* He states that on Keeling’s Island they constitute a layer two 
or three feet in thickness, with a breadth of twenty feet. They are 
abundant on the Paumotu reefs. 
The outer reefs are distinguished in many parts from the inner by 
becoming covered with accumulations of coral fragments and sand, 
which are thrown up by the waves: finding a lodgment some dis- 
tance back from the margin of the reef, they gradually increase, till in 
many instances they form dry land, and thus commence the foundation 
of islands. Such effects are mostly confined, however, to the sides 
open to the prevailing wind, and are generally of limited extent. Oc- 
casionally, as at Bolabola, the reef for miles in length is changed from 
the submerged coral bank into a habitable islet—a green belt to the 
island of rocks and forests within. The causes and the result are much 
the same as in the case of the lagoon island, and the steps in the pro- 
cess will be more particularly described when treating of the coral atoll. 
The rock of the reef, wherever broken, exhibits a compact texture. 
In some parts it consists of coral fragments of quite large size, firmly 
* Op. cit. page 9, and elsewhere. 
10 
