ORGANIC AGENCIES. 



97 



plants growing and dying on the same spot, so a reef is 

 similarly formed by successive generations of corals. As 



FIG. 50. -Corals iu the Great Barrier Reef, Australia. 



peat-ground may rise above the surrounding country, so 

 a coral reef rises far above the surrounding sea-bottom. 

 As peat represents so much carbon taken from the air 

 and added to the ground, so a reef represents so much 

 carbonate of lime taken from the sea-water and added to 

 the sea-bottom. The limestone thus formed by the 

 broken remains of corals cemented together is called the 

 reef-rock. Thus a reef is a submarine bank composed of 

 reef-rock, crowned with the present generation of living 

 corals. 



Coral Islands. But even coral reefs are not yet coral 

 islands, since corals can not grow above the sea-level. 

 Coral islands are made by the action of waves. Waves 



LE CONTE, GEOL. 7 



