CORAL REEFS AND ISLANDS. 145 



We will hereafter make use of these facts and principles in the ex- 

 planation of beds of iron-ore. 



Section 3. — Lime Accumulations. 

 Coral Reefs and Islands. 



Interest and Importance. — The subject of corals and coral reefs is 

 one of much popular as well as scientific interest. The strange forms 

 and often splendid colors of the living animals ; the number and ex- 

 treme beauty of the coral islands which gem the surface of certain 

 seas ; the large amount of habitable land which owes its existence to 

 the agency of these minute animals ; the fact that a large area, prob- 

 ably several thousand square miles, has been thus added to our own 

 territory ; the great dangers connected with the navigation of coral 

 seas, strikingly displayed on our own coast by the fact that the consid- 

 erable town of Key West is almost wholly dependent on the wrecking 

 business for its existence — these and many other facts invest the sub- 

 ject with popular interest, while the great importance of corals as a 

 geological agent gives the subject a scientific interest no less strong. 



Coral Polyp. — The animal which secretes coralline stone is no insect, 

 as generally supposed, but belongs to one of the lowest divisions of the 

 animal kingdom, viz., the class of polyps. Like most of the lowest 

 animals, it is composed of soft, gelatinous, and almost transparent 

 tissue. The animal, however, has the power of extracting carbonate of 

 lime from sea- water, and depositing it within its own body. The lime 

 carbonate is deposited only in the lower portion of the animal, leaving 

 thus the upper part and the tentacles free to move. The radiated 

 structure of the polyp is pefectly reproduced in the coralline axis. 

 This is a purely vital function, having no more connection with voli- 

 tion than the secretion of the shell of an oyster or the bones of the 

 higher animals. The limestone thus deposited within the animal con- 

 stitutes 90 to 95 per cent of its whole weight. 



Compound Coral, or Corallnm. — A single coral potyp is very small, 

 but, like many of the lower animals, it has the power of multiplying 

 indefinitely by buds and branches. Thus are formed compound corals. 

 These may branch profusely, and then may be called coral-trees ; or 

 may grow in hemispherical masses, and are then called coral-heads. 

 Coral-trees are sometimes six or eight feet high, and coral-heads fifteen 

 to twenty feet in diameter. They consist of millions of individual 

 coral polyps. Only the upper and outer portions of a coral-tree, and 

 outer portion of a coral-head, are living ; the lower and interior por- 

 tions consist only of coralline limestone without life. 



Coral Forests. — Coral polyps, however, reproduce not only by bud- 

 ding, but also by eggs. These eggs have the power of locomotion. 

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