Pie CORALS AND CORAL ISLANDS. 
oms under water more or less covered with growing corals. 
The seas about these sunken reefs of the Indian Ocean are 
2,000 to 2,600 fathoms in depth. Nearer to northern Mada- 
gascar there are coral islands that are not submerged. 
The following are other evidences in favor of the theory 
of subsidence. 
The theory explains all the varying depths of lagoons, 
from the condition of near obliteration to that of a basin 
one to three hundred feet deep. 
It gives a satisfactory reason for the existence of great 
and abrupt depths about many atolls, and off great barriers, 
and the steepest of submarine declivities. The powers of 
growth in the reef, through the limestone material derived 
from the waters by the polyps, enable it to keep itself at the 
water-level in spite of the deepening that is going on. Such 
facts as those from the depths alongside of the eastern Baha- 
mas, the reef-islands southwest of Cuba, and many atolls in 
the deeper oceans have here their full elucidation. 
The sunken atolls of the ocean, like the Chagos Bank 
(page 191), derive from Darwin’s theory their only explana- 
tion. The basin-shaped reef with high borders, the bottom 
dead because of the too great depth, the borders in places 
growing corals and having some surface islets over spots 
of more luxuriant growth where rate of progress was suffi- 
cient to keep up with the subsidence, —all the facts are a 
natural consequence of the method of origin. A model of 
such a lagoon-bank with its raised margin and few tall and 
steep islet towers, 20 to 30 feet above the rest of the border, 
would be one of the best of demonstrations for the subsidence 
theory. The coral-growing areas over the great lagoons of 
atolls and the barrier-bounded channels of the Feejees and 
other archipelagoes and those of the outer waters about 
