THE OCEAN AND ITS WORK 



243 



It is interesting to note that no deposits of this sort have ever been 

 found on the continents, showing perhaps that the great depths of 

 the ocean have never been raised to form dry land. It is rare that 

 any rock is found on the continent which implies water deeper than a 

 few hundred feet. 



Coral Reefs and Islands 



Coral islands have long excited the interest of mariners, both be- 

 cause of their location far from land and because of their beauty. 

 They are also of great scientific interest because of their origin. 



Reef-building corals grow best in seas (1) with a minimum tem- 

 perature of not less than 6o° F. ; (2) at a depth of not more than 150 

 feet; (3) where the salt water is free from sediment; and (4) where 

 they are exposed to the dash of the waves. Free exposure to the 

 waves is of advantage, since the profusion of life on a coral reef soon 

 exhausts the oxygen needed for respiration and the calcium carbonate 

 necessary for their stony structure. Since they do not thrive in 

 muddy or fresh waters they are not developed near the mouths of 

 rivers. 



In tropical regions where the above favorable conditions prevail, 

 the shores of the continents are bordered by coral reefs, called fring- 

 ing reefs, which have 



a steep slope of 50 

 to 6o° on the sea- 

 ward side. In many 

 cases in addition to 

 the fringing reef 

 there is another reef, 

 surrounding the is- 

 land or paralleling 

 the land, several miles 

 from shore. Such a 

 reef is termed a 

 barrier reef (Fig. 

 235). Circular reefs 

 or atolls, without 

 islands in the center, and lagoonless coral islands also occur. 



The geological importance of coral animals lies in the fact that they 

 have the power of extracting calcium carbonate from sea water and 

 depositing it within their own bodies. Upon the death of the 



Fig. 235. — Barrier reef off the coast of the island 

 of Curacao, Dutch West Indies. 



