XV.] CORAL LAND. 255 



of the mass of coral is usually the highest, for it is here that 

 the coral-polypes flourish most luxuriantly ; while the dash 

 of the breakers, during storms, tears off fragments of the 

 coral rock, and heaps them up on this side. It should be 

 borne in mind that the land is not entirely formed of corals, 

 since other creatures living in the lagoon, and on the banks 

 of the reef, contribute their remains to swell the mass. 

 Vegetable life too is not without its effect on the formation 

 of the new land ; and, indeed, the outer edge of a reef is 

 often formed, in large measure, of iiuHipores, a kind of sea- 

 weed, the tissues of which are strongly impregnated with 

 carbonate of lime. 



Although corals of some kinds may be found in almost all 

 seas, those particular species which grow together in masses, 

 and thus form reefs and islands, are limited to the warmer 

 parts of the world. Prof Dana,, who has had ample 

 opportunity of observation, believes that the reef-forming 

 coral-animals are restricted to waters in which the mean tem- 

 perature for the month, even in the coldest season, never 

 falls below 68° F.i If, then, a line be drawn through all 

 parts of the ocean north of the equator, where the coldest 

 month has this average temperature, and a similar line south 

 of the equator, they will include a zone within which all the 

 coral-reefs of the world are situated. It need hardly be said 

 that these lines M'ill not be straight lines running in circles, 

 round the world, like parallels of latitude, but will be irre- 

 gular lines, rising in one part and falling in another, accord- 

 ing as the temperature is locally affected by the presence of 

 ocean-currents or by the proximity of land. This belt of 

 warm water, congenial to the coral-makers, never extends 

 beyond about 30 degrees from the equator. 



Though the reef-building corals abound in many parts of 

 ' Corals and Coral Islands. By James D. Dana, LL. D. 1875. 



