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 simi)ly ancient fringing reefs 
formed when the island stood higher above the sea, hence 
they are built up as rapidly as the land 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 re 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 platform 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 arc at least 2000 feet in thickness. 
Thus far we have spoken of reefs surrounding moun- 
tainous islands; coral islands or aiolls (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 stilled 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 deposits 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 formed in an area 
of subsideiice, 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 
