CORALS. 133 



reefs and the Bahamas, are explicable without any theoretical supposition of submerg- 

 ence or elevation of the foundation upon which they rest. 



The origin of an atoll in mid-ocean where currents are not confined as in the 

 triangle between Cuba, Florida, and the Bahamas, presents a similar although 

 somewhat modified problem. Let us suppose a submarine mountain situated in mid- 

 ocean upon which for ages has rained successive depositions of sediment from the 

 waters above. This sediment is composed in part of shells of pelagic animals, and 

 plants, with which the waters of the tropics or currents from the same are filled. 

 Even at the greatest depths life exists upon such a bank, and the hard portions of the 

 animals which live and die there are being continually added to a growing submarine 

 bank. By increments made for many years the bank slowly rises to the surface of the 

 water. As it rises higher and higher the activity of the life on its crest increases, and 

 when it rises into the bathymetric zone of the reef-builders, between one hundred 

 and one hundred and fifty feet below the surface, a more rapid growth awaits it. 

 The coral plantation as it develops is washed by an ocean current, probably on one 

 side. Such a current in fapt is divided by the bank, so that each of the bifurcations 

 flow tangentially along its sides. It is evident in the first place that it is around the 

 border of the bank washed by the current that the most active coral life is to be 

 looked for, and in this region also that the predominant upward growth must be sought. 

 If the bank with which we started from the sea depths is circular the resulting atoll 

 will have the same form, and the part which first raises itself above the water will be 

 ring-shaped, containing a central lagoon. 



The luxuriance of the growth of the animals, which form a coral reef, is directly de- 

 pendent upon the amount of food which the oceanic currents bring to the growing col- 

 onies. The waters of the ocean are filled with a wealth of microscopic and other life 

 floating in it upon which the living coral animals feed. It is evident that the possibilities 

 of prolific coral life are greatest where their food is most abundant, and there too we 

 must look for the greatest amount of coral growth. It is clearly in the line of ocean 

 currents that this food, being constantly renewed by the flow of the stream, is most 

 abundant, and along its borders the possibilities of the coral animals of obtaining their 

 food are the greatest. This cause alone is not^ capable of determining the outlines of 

 coral islands, but it is a most important factor in regulating their growth. 



Coral islands in two different conditions ought to be mentioned. We have coral 

 islands in process of formation, and those in conditions of destruction by erosion. The 

 Florida reefs are examples of the former, the Bermudas of the latter. Coral islands, 

 in progress of formation, seldom rise to any great height above the ocean, are composed 

 of coral fragments, sand, or half-formed coral rock. They are but very slightly eroded, 

 have no extensive caves with stalactitic formation, and no red soil. Fully formed 

 coral islands in which the erosive power of the water has had its full effect are gen- 

 erally elevated, honey-combed throughout by caves, and possess a soil of red earth. 

 This last characteristic of coral islands in the progress of erosion is of a problem- 

 atical nature and origin. By some authors it is regarded as the heavier residuum 

 resulting from the wearing away of the coral rocks, while others have even gone so far 

 as to look upon it as the excrement of birds, mingled with coral sand. The former 

 theory seems most i-ational, but there is, if this theory be adopted, great difiiculty in 

 accounting for its reddish color. Red soil is very abundant in the Bahamas, the 

 Bermudas, and several of the West Indian islands. In the Bermudas it is contained 

 in pockets in the rocks, and in it are grown the well-known early vegetables, potatoes 



