CORAL REEFS AXD ISLANDS. 147 



is reef -rock, the strong waving line the surface of the living reef, and 

 the shaded portion the island. 



Conditions of Coral-Growth. — Reef-building corals do not grow in 

 all seas, nor over the whole bottom of the sea indiscriminately, but are 

 confined to certain seas, and in these to certain spots and lines. The 

 conditions of the growth are : 



1. A Winter Temperature of 68°. — This condition confines them al- 

 most entirely to the torrid zone. The most marked exception to this 

 is on the Florida coast and the Bahamas, where corals extend to 28° 

 north latitude, and in the Bermudas to 32° north latitude. This exten- 

 sion of the usual limits of reef -building corals is due to the warm tropi- 

 cal waters carried northward by the Gulf Stream. 



2. A Depth of not more than One Hundred Feet. — This condition 

 confines them to submarine banks, and especially to the shores of con- 

 tinents and islands. 



3. Clearness and Saltness of the Water. — On account of this condi- 

 tion corals will not grow on muddy shores, nor off the mouths of rivers, 

 being destroyed by the fresh and muddy water. 



4. Free Exposure to Waves. — Some species of corals grow in still 

 water, but the strongest reef-building species delight in the dash of the 

 surf. They will even flourish and build an almost perpendicular wall 

 in breakers which would wear away the hardest rock. The reason is, 

 that the immense profusion of life on a reef rapidly exhausts the water 

 of the oxygen necessary for respiration, and of the carbonate of lime 

 necessary for their stony structure, and therefore constant change of 

 water is necessary. 



All the conditions mentioned above apply only to reef-building 

 species. Some corals live in temperate regions, some in very deep 

 water, and some in sheltered places. 



Pacific Reefs. — The reefs of the Pacific Ocean are of three general 

 kinds, viz., fringing reefs, carrier reefs, and circular reefs or atolls. 

 "We will describe these in the order mentioned. 



Fringing Reefs, — In the tropical Pacific every high island or previ- 

 ously-existing land of any kind is surrounded by a reef which attaches 

 itself to the shore-line, and extends outward on every side just beneath 

 the water-level, as far as . _^_ ._ _,__ ___— _^^__ 



margin of this platform FlG< 117- 



the bottom drops off very suddenly, forming a slope of 50° to 60°, and 



sometimes almost perpendicularly. The position and extent of the 



