FORMATION OF 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 J F. in the North 

 Pacific and 70 F. in the 

 South. Coral reefs are abun- 

 dant in the West Indies, but 

 still more so in the Central 

 Pacific, where there are a 

 much greater number of spe- 

 cies of corals (Dana). Along 

 the Brazilian coast, as far 

 south as Cape Frio, are coral 

 reefs (Hartt). In depth living 

 coral-reef-builders do not ex- 

 tend more than fifteen or 

 twenty fathoms below the sur- |' 

 face. 



Coral reefs are divided by | 

 Dana into outer or barrier 

 reefs (Fig. 57) and inner reefs. 

 The barrier reefs are formed 

 from the growth of corals ex- 

 posed to the open seas, while 

 the inner or fringing reefs 

 (Fig. 57) are formed in quiet 

 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 kc-^s at the level of 



