CORALS. 



521 



twelve fathoms), matches the blue of the surrounding ocean, but delicate apple- 

 green and yellow tints mingle with the blue wherever the sand or coral-rocks 

 approach the surface. Although the girdle of reef covered with vegetation 

 occasionally surrounds the lagoon, it is more often broken up into a ring of 

 separate islets of various sizes ; between some of which navigable channels are 

 found, giving admittance to the lagoon. 



The submarine fields of living coral spread along the coasts of the islands and 

 the mainland. Just as the accumulated remains of the primitive forests add layer 

 after layer to the soil, so the coral-reefs are added to by the breaking down of old 

 corals, by the shells of bivalves and of other organisms. These fragments keep filling 

 up the spaces between the separate living stocks, so that the level of the reef is 

 constantly rising towards the surface. The currents and waves also take part in 

 the building up of the reef. Masses of coral of all sizes, from great boulders to 

 minute sand grains are broken off by the waves, and are cast upon the reefs, and 

 then rolled about until quantities of fine detritus are produced, which, as calcareous 





CORAL-ISLAND OE ATOLL. 



mud, serves as a cement to bind the larger blocks together. A constant process of 

 destruction goes on ; some of the detritus being washed over the reef into the lagoon 

 or canal, and some filling the spaces between the corals along the edge of the reef, 

 while the rest remains upon the surface. 



The layer of dead coral-rock forming the foundation of the reef is bordered 

 by living coral. While this living coral is always extending the reef horizontally, 

 the waves are piling up the dead masses vertically, till they rise above the surface 

 of the water. Thus dry land begins to form, and by degrees islands arise well out 

 of the reach of the waves. The ocean is thus the builder of the coral-island as it 

 appears above the waves, the material having been supplied in the first place by 

 the coral-animals. The moment the island is above water, plant seeds reach it from 

 distant lands ; and ere long cover it with vegetation. The accompanying section 

 of a coral-reef shows the slope of the reef, both towards the lagoon on the 

 right and the open ocean on the left. At b-c is the steep slope from shallow 

 water to the land level on the outer side, and at d-e the gradual slope on the 

 inner side. The latter slope is then continued at almost the same angle (-/<), 

 the quiet water not disturbing the slow accumulation and growth of the lagoon or 

 canal shore. On the outer side of the reef, however, a broad terrace (a-6), succeeds 



