34 ZOOLOGY. 



in quiet water between a barrier reef and the island. As 

 coral reefs are usually built upon islands which are slow- 

 ly sinking, barrier reefs are simply ancient fringing reefs 

 foruiod when the island stood higher above the sea, hence 

 they are built up as rapidly as the laud sinks, and thus the 

 top of the reef keeps at the level of the sea. The reefs are 

 often of great thickness, for, as Dana says, "could .ve raise 

 one of these coral-bound islands from the waves, we should 

 find that the reefs stand upon the submarine slopes, like 

 massy structures of artificial masonry; some forming a 

 broad flat ])latform or shelf ranging around the land, and 

 others encircling it like vast ramparts, perhaps a hundred 

 miles or more in circuit." Darwin has estimated that some 

 reefs in the Pacific Ocean are at least 2000 feet in thickness. 

 Thus far we have spoken of reefs surrounding moun- 

 tainous islands; coral islands or atolls (Fig. 34, A) resemble 

 such reefs, except that they surround a lake or lagoon in- 

 stead of a high island, the coral island itself being seldom 

 more than ten or twelve feet above the sea, and usually 

 supporting a growth of cocoanut trees, while the sea may 

 be of great depth very near the outer edge of the atoll, 

 which " usually seems to stand as if stilted up in a fathom- 

 less sea" (Dana). These reefs and atolls are formed and 

 raised above the sea by the action of the winds and waves, 

 in breaking up the living corals, comminuting it, and form- 

 ing, with the debris of shells and other limestone-secreting 

 animals and plants, banks or de]iosits of coral mixed with 

 a chalky limestone, as the base of the reef. When it rises 

 above the waves, cocoanuts and other seeds are caught and 

 washed up on the top, and gradually the island becomes 

 large enough to support a few human beings. The Ber- 

 mudas are the remnants of a single atoll, and are situated 

 farther from the equator than any other reefs. Some bar- 

 rier reefs and coral islands or atolls are foi'nicd in an area 

 of subsidence, where the bottom of tlie ocean is gradually 

 sinking; this accounts for the peculiar foini and great 

 thickness of many reefs. On the other hand, the coral 



