MODERN CORAL REEFS 389 



oceans. ( )f the Pacific atolls the largest group forms the Low 

 Archipelago in the center of the Pacific, south of the_ ecjuator, and 

 next the Caroline Archipelago in the western part of the Pacific, 

 north of the equator. The Marshall group, the Gilbert Islands, the 

 Ellis Island group with Funafuti, and a number of scattered atolls, 

 lie between the two first mentioned archipelagoes. In the Indian 

 Ocean the Laccadives, the Maldives, the Chagos, and the Sava de 

 Malha groups form the largest of the associations of atolls. The 

 Cocos-Keeling islands may also be mentioned as isolated but typical 

 atolls in the eastern part of the Indian Ocean (lat. 12° S., long. 

 96° E.). 



From the geological point of view, a classification based on the 

 relationship of the reef to the continental block is perhaps of even 

 more fundamental significance. On such a basis reefs may be di- 

 vided into oceanic, those encircling oceanic islands or resting on 

 oceanic platforms, and epicontinental or neritic, those growing upon 

 or on the border of the continental shelf and around the islands per- 

 taining thereto. Each group may be represented by fringing and 

 barrier reefs, as well as by atolls, though the latter are more com- 

 monly confined to the oceanic type. 



In general only the epicontinental type is represented in the 

 older deposits of the known earth's crust, though some of these may 

 well have had deep, marginal depressions similar to those found off 

 many of the western Pacific reefs. The dolomite reefs of the Tyrol 

 have, however, been regarded as atolls. Of existing examples of 

 epicontinental reefs the Great Barrier Reef of Australia may be 

 considered the best representative. In their composition and in- 

 ternal structure and in the factors influencing their distribution, all 

 reefs are more or less alike. 



Factors Limiting the Distribution of Modern Coral Reefs. 

 The geographic distribution of modern coral reefs falls within that 

 portion of the oceans lying approximately between 28° north and 

 south latitudes, in those depths in which the mean annual tempera- 

 ture does not fall below + 68° F. (-(- 20° C), nor rise very much 

 above that point. Most of the existing coral reefs are found in the 

 Pacific Ocean, whose average surface temperature is the highest of 

 the four oceans (19.1° C, 66.38° F., maximum 27.60° C, 80.69° 

 F.). (See Chapter IV.) The Indian Ocean comes next with a 

 mean annual temperature of 17° C, 62.6° F. (27.88° C, 82.18° F: 

 maximum), and it is next in importance with reference to the dis- 

 tribution of coral reefs. The Atlantic (mean annual temperature 

 16.9° C. 62.42° F., maximum 26.83° C., 80.38° F.), finally, is sur- 

 prisingly poor in coral reefs, though numerous fringing reefs occur 



