386 PRINCIPLES OF STRATIGRAPHY 



Characters and Development of Modern Coral Reefs. 



Types of Modern Coral Reefs. — Three types of coral reefs 

 are recognized in the modern seas : the fringing reef, harrier 

 reef, and atoll. The Fringing Reef, or shore reef (Figs. 71-2), is 

 a platform of coral rock closely bordering a continent or island of 

 earlier origin and extending some distance from the shore. Sea- 

 ward the platform of coral rock is often somewhat higher than 

 the inner end, rising into low ridges or mounds of living or dead 

 coral and coral sand often awash at low tide. It is intersected by 

 channels and a pronounced but shallow channel extends between it 

 and the shore. Its seaward side is often rather steep, but it descends 

 into comparatively shallow water. This is the zone of active coral 

 growth as well as destruction, for here wave activity is most marked. 

 Within the outer rim conditions of life are less favorable, since 



Fig. 71. Island with fringing reef, 

 and submarine platform over 

 which the surf breaks. (After 

 Le Conte.) 



Fig. 72. Diagrammatic cross-section 

 of the island and reef shown in 

 preceding figure. a, a, Dead 

 coral material added to island. 

 (After Le Conte.) 



here the water is more stagnant, and the chamiels are scoured out 

 by the tidal currents and the solvent action of the water. The 

 influx of fresh water and clastic sediment from the land also hin- 

 ders coral growth to a considerable extent within the outer rim of 

 the fringe. Typical examples of fringing reefs are found on the 

 borders of the Sandwich, Solomon, Friendly, and Navigator islands, 

 the New Hebrides and Mariana islands and many others, all in the 

 Pacific, also in the Seychelles and Nicobar islands and in Mau- 

 ritius, as well as parts of the coast of Madagascar and Mozambique 

 in the Indian Ocean. In the Red Sea they occur on both the 

 Arabian and African shores, and in the Atlantic on the coast of 

 Florida and around nearly the whole of the islands of the West 

 Indies. 



The Barrier Reef (Figs. 73-4) is situated further from shore 

 and consequently has a greater depth of water both between it and 

 the land and on the seaward side. It may be formed from the 

 fringing type by the subsidence of the land and the simultaneous up- 

 ward growth of the corals on the outer border of the reef (Darwin) ; 



