349 6 



CCELENTERA TES 



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 

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

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

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

 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 



CORAL ISLAND OR ATOLL. 



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

 pears 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 toward 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 disturb- 

 ing the slow accumulation and growth of the lagoon or canal shore. On the outer 

 side of the reef, however, a broad terrace (a-b) succeeds the steep slope, and 

 surrounds the land which has risen above the sea, this terrace being exposed at low 

 tide. 



We have still to mention some of the causes of modification in the form and 

 growth of the coral reefs. The presence of harbors in reefs and atolls can, as a rule, 

 be traced to the tides and to local ocean currents. There is generally an outflow 

 through the canals and openings in the reefs. This is apparently due to the fact 



