MAN AND THE OCEAN 



289 



the warm currents flow past continually, bringing a fresh 

 supply of food. In tropical seas many islands are fringed 

 by reefs built by these animals. 



In the tropical Pacific circular coral reefs are found 

 nearly inclosing large shallow lagoons. Soundings out- 

 side of these reefs show that the sea rapidly sinks to great 

 depths. Islands of this kind are called atolls. How these 

 atolls were formed is still an open question. As reef- 



CANAL. 



Two great oceans artificially united. 



building corals cannot live below 20 fathoms, it is evident 

 that corals could not have built up the reefs from the bot- 

 tom of the deep sea. 



137. Man and the Ocean. At first thought it would 

 seem better for the life of the world if the proportion 

 of land and water were reversed. Yet when we consider 

 that almost barren wastes constitute many continental 

 interiors and that plenty of rainfall is necessary to make 



