CORAL REEFS. 



If a map of the world is studied 

 carefully a peculiar fact will present 

 itself. With several minor exceptions 

 the Coral Reefs and coral islands of the 

 globe are confined to a belt-like area 

 bounded on the north by the Tropic of 

 Cancer and on the south by the Tropic 

 of Capricorn. This singular distribution 

 arises from the fact that reef-building 

 corals live only in seas in which the 

 temperature during the winter months 

 does not fall below sixty-eight degrees 

 Fahrenheit. It has been ascertained 

 that the mean temperatvires for the Pa- 

 cific Ocean are seventy-three degrees for 

 the North Pacific and seventy degrees 

 for the South Pacific. It will thus easily 

 be understood why such a restricted area 

 should be given to Coral Reefs. An in- 

 teresting exception to this rule is the 

 Bermudas, the reefs of which are made 

 possible by the warm waters of the Gulf 

 Stream. 



When we examine the area within the 

 two tropics we find the condition still 

 more interesting and peculiar. The coral 

 belt, if we may so designate this portion 

 of the globe, is divisible into three main 

 areas of distribution. The first is the 

 Pacific Ocean, which is the largest. The 

 second is the Indian Ocean, and the third 

 includes the Gulf of Mexico, the Carib- 

 bean Sea and the Brazilian coast. The 

 Pacific Ocean is the metropolis of Coral 

 Reefs. Here the temperature and depth 

 of water is particularly favorable. The 

 Atlantic Ocean, on the contrary, is the 

 least adapted for the growth of corals 

 and consequently but few reefs of im- 

 portance are found there. The west 

 coasts of both the Americas and of Af- 

 rica are almost devoid of Coral Reef';, 

 while the eastern shores of those conti- 

 nents embrace large areas. This is due 

 to the cold extra-tropical currents of 

 water which flow from the arctic and ant- 

 arctic toward the equator on those sides 

 of the continents. The coast of China 

 has no Coral Reefs for the same reason. 

 Another barrier to the growth of Coral 

 Reefs is found along muddy shores or 

 the vicinity of the mouths of large rivers, 



for coral polyps cannot live except in 

 clear sea water. Regions of volcanic ac- 

 tion are also devoid of Coral Reefs, for 

 evident reasons. Depth plays an import- 

 ant part in the distribution of Coral 

 Reefs. It has been found that coral- 

 building polyps do not flourish below one 

 hundred and fifty feet. Hence the Coral 

 Reefs are always found on top of sub- 

 marine islands and on the shallow mar- 

 gins of islands and continents. 



Of the larger corals the Madrepores 

 are in the main the true reef-builders. 

 There are over sixty species of this genus, 

 each colony of which grows to a large 

 size. The largest number of species is 

 found in the Pacific Ocean, where they 

 also attain their greatest perfection. In 

 Florida and the West Indies the Mean- 

 drinas or brain-corals are the most con- 

 spicuous reef-builders, although several 

 large species of Madrepores also form 

 reefs, an example being the Madrepora 

 cervicornis shown on the plate. 



Coral formations may be conveniently 

 divided into two kinds or classes : First, 

 coral islands and, second. Coral Reefs. 

 The first class includes those isolated 

 formations which occur in the open sea 

 and which are formed on top of sub- 

 merged mountain peaks. The coral island 

 is usually in the form of an atoll, which 

 is a rim of coral rock surrounding a cen- 

 tral salt water lake or lagoon, communi- 

 cating with the ocean by one or more 

 openings. Such lagoons are always of 

 an irregular form, generally longer than 

 wide, and, curiously enough, are never 

 circular. In size these atolls vary from 

 a few miles to over fifty miles in length, 

 the smaller ones lacking the lagoon 

 Some oi these islands are perfectly bare 

 and are swept by the sea at high tide, 

 while others are well wooded. The wood- 

 ed portion generally lies to the wind- 

 ward and afifords an excellent protec- 

 tion for shipping if the lagoon is large 

 enough for a harbor, as is frequently the 

 case. Each side of the rim of the atoll 

 forms a coral sand beach, which is gen- 

 erally bare at low tide and beyond this 

 the edge is bordered by living corals to 



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