FORMATION OF CORAL REEFS. 89 
rampurts, 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 mountainous 
fslands ; coral islands or atolls (Fig. 58) resemble such reefs, 
except that they surround a lake or lagoon instead 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 scems to stand’as 
if stilted up in a fathomless 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 forming with the débris of shells and 
other limestone-secreting animals and plants, banks or de- 
posits 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 Bermudas are the remnants of a single atoll, 
and are situated farther from the equator than any other 
reefs. Most barrier reefs and coral islands or atolls are 
formed in an area of subsidence, where the bottom of the 
ocean is gradually sinking ; this accounts for the peculiar 
form and great thickness of many reefs. On the other 
hand, the coral reefs of the West Indies are, generally 
speaking, in an area of elevation. 
A section of a coral reef is shown by Fig. 59: 7 is the point 
where the shore slopes rapidly down within the lagoon 
(which lies to the right), and m is where the reef suddenly 
descends toward the open ocean. Between 0 ¢ and d e lies 
the higher part of the reef. The shore toward the lagoon 
slopes away regularly from d to n ; while toward the open 
ocean there is a broad horizontal terrace (a to dc) which 
becomes uncovered at low water. 
The theory of the formation of barrier reefs is shown by 
the diagram, Fig. 60. The island, for example, the volcanic 
island Coro, which is slowly sinking, at the ancient sea-level 
Tis surrounded by a fringing reef ff, a small rock-terrace 
