244 PHYSICAL GEOLOGY 



animal this " skeleton " is left as a firm calcareous deposit. The 

 reef-building corals live principally in colonies and because of this 

 assume many forms ; some are in great head-like masses (brain corals), 

 others are branching like trees (staghorn corals), while others are 

 in flat masses. 



Coral reefs are not built up entirely of the remains of coral animals, 

 but a large part is contributed by other lime-secreting organisms 

 which live in association with the corals. These reefs are not built 

 above the level of the water by the coral animals, but by storm waves 

 which tear masses of coral from the reef and pile them up above sea 

 level. The building above the sea is thus seen to be accomplished 

 in the same way as is the formation of sand reefs (p. 221). As soon 

 as the broken coral rock is above the sea, some of it is dissolved by 

 rain water and spray, and upon being redeposited cements the frag- 

 ments into firm rock. The coral reefs thus built above the sea are 

 consolidated into compact limestone. 



The most extensive barrier reef in the world is the Great Barrier 

 Reef of Australia which borders the coast of that continent for about 

 1000 miles at a distance of 20 to 50 miles from the mainland. Its 

 breadth beneath the surface of the sea varies from 10 to 90 miles, 

 although but little is exposed above the water. The channel between 

 the Great Barrier Reef and the shore is from 60 to 240 feet deep, but 

 the outside of the reef has a steep slope, so that in short distances 

 depths of 1800 feet are encountered. This difference of slope on 

 the two sides is due to the fact that growth on the outside is better 

 assured, as it receives the full sweep of the waves, so that aeration 

 is better realized there, food is more abundant, and the washing 

 away of dead parts more quickly accomplished. This rapid growth 

 toward the open sea causes a very jagged contour and an abrupt 

 slope. 



Coral-reef Problem. — The origin of fringing reefs is evident, 

 since all of the conditions favorable to coral growth, such as a warm 

 temperature, a depth of water not greater than 150 feet, and free ex- 

 posure to the waves, are present on portions of the shores of the 

 islands and continents of the tropics, or where the ocean currents bring 

 water with a temperature of at least 68° F. The origin of barrier 

 reefs, atolls, and lagoonless islands is not so clear, since barrier reefs 

 are separated from the land by channels sometimes several miles wide 

 and from 120 to 180 feet deep, and the lagoons of the atolls some- 

 times have a depth of 350 feet. 



