146 ORGANIC AGENCIES. 



As soon as they are extruded, they swim and float away, and, if they 

 fall on sea-bottom favorable for their growth, they soon form first a 

 coral polyp, and finally a coral-tree or coral-head. Thus from one 

 coral-tree other coral-trees spring up all around and form a coral for- 

 est, which spreads in every direction where they find conditions favor- 

 able. 



Coral Reef, — Finally, the limestone accumulation of thousands of 

 successive generations of coral forests growing and dying on the same 

 spot, together with the shells of mollusks and the bones of fishes which 

 live in swarms preying on the corals, the whole, of course, crowned with 

 the living forest of the present generation, constitute the coral reef. It 

 is evident, then, that a reef is formed somewhat after the manner of a 

 peat-bog. As a peat-bog represents so much matter taken from the air, 

 so a coral reef represents so much matter taken from the sea-water. 

 As each generation adds itself to the ancestral funeral-pile, the ground 

 upon which the corals grow steadily rises until it becomes elevated far 

 above the surrounding sea-bottom. 



Coral Islands. — These are due to the action of waves upon the coral 

 reefs. We have already seen how low islands are formed on submarine 



banks by this agency. 

 Now, reefs are also a 

 kind of submarine 

 _. bank. On these, there- 

 fore, islands are also 

 formed by waves. Fig. 115 represents an ideal section across a reef, as 

 it would be if no wave-action interfered, 1 1 being the sea-level. But by 

 the action of the beating waves during storms large masses of reef- 

 rock, often six or eight 

 feet in diameter, or great 

 coral-heads, are broken 

 off from the outer or 

 seaward side of the reef 

 and rolled over to the 

 leeward side. These 

 form a nucleus about which collect similar or smaller fragments, and 

 among these still smaller fragments, and these again are filled in and 

 made firm with coral-sand, and the whole cemented into solid limestone 

 rock (breccia) by the carbonate of lime in the sea-water. 



Islands thus formed, like all wave-formed islands, are low (twelve 

 to fifteen feet high) and narrow (one quarter to one half mile wide), 

 but long in the direction of the reef. They are at first perfectly bare, 

 but become in time covered with vegetation, and even teeming with 

 population. They are celebrated for their gem-like beauty. The final 

 result is shown in ideal section in Fig. 116, in which the dotted portion 



Fig. 116. 



