FORMATION OP CORAL REEFS. 87 



Polyzoa, and certain plants called Nullipores, the Madrepo- 

 raria in the main are the true reef-builders. They are con- 

 fined to waters in which through the coldest winter month 

 the temperature of the water 

 does not fall below 68 F., 

 though usually the waters are 

 much warmer than this, the 

 mean annual temperature be- 

 ing about 73 F. in the North 

 Pacific and 70 F. in the 5 

 South. Coral reefs are abun- 3 

 dant in the West Indies, but J^ 

 still more so in the Central 

 Pacific, where there are a o 

 much greater number of spe- 

 cies of corals (Dana). Along 2. 

 the Brazilian coast, as far g 

 south as Cape Frio, are coral ^ 

 reefs (Hartt). In depth living & 

 coral-reef-builders do not ex- . 

 tend more than fifteen or 3. 

 twenty fathoms below the sur- 

 face. 



Coral reefs are divided by ? 

 Dana into outer or barrier 5- 

 reefs (Fig. 57) and inner reefs. ^ 

 The barrier reefs are formed 

 from the growth of corals ex- 3 

 posed to the open seas, while 1 

 the inner or fringing reefs 

 (Fig. 57) are formed in quiet P 

 water between a barrier reef 

 and the island. As coral 

 reefs are usually built upon 

 islands which are slowly sink- 

 ing, barrier reefs are simply 

 ancient fringing reefs formed when the island stood higher 

 above the sea, hence they are built up as rapidly as the land 

 sinks, and thus the top of the reef keeps at the level of 



