10 ON CORAL REEFS AND ISLANDS. 



be inferred from the varied character of submarine slopes; in 

 some cases the shallow waters may extend for two or three miles 

 beyond the reef, but it is far more common to meet with the op- 

 posite extreme — unfathomable depths within a few hundred feet. 



The growing corals are mostly confined to the shallow waters 

 of the reef, and to its sloping margin up which they extend to 

 within 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 

 account 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 colors, as observ- 

 ed by Chamisso, are due to incrusting Nullipores, that grow like 

 lichens over the rock : they are vegetable in nature, though com- 

 posed mostly of lime. Other nodular and branching Nullipores, 

 some sprigs of Madrepores, and a few of Astraeas 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 actiniae. The gradual wear of the reefs by 

 the wash of the sea is prevented, to a great extent, by these Nul- 

 lipore incrustations, as was pointed out by Darwin.* He states 

 that on Reeling'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 distance back from the margin of the reef, the accumula- 

 tions gradually increase, till in many instances they form dry land, 

 and prepare the way for vegetation. Such effects are mostly con- 

 lined, however, to the sides open to the prevailing wind, and are 

 generally of limited extent. Occasionally, 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 for- 

 ests within. The causes and the result are much the same as in 

 the case of the lagoon island, and the steps in the process will be 

 more particularly described when treating of the coral atoll. 



* Darwin on Coral Reefs, London, 1842, page 9, and elsewhere. 



