CORAL ISLANDS, 195 
utmost depth at which the coral polyps can construct reefs is between 
twenty and thirty fathoms. Now, there are enormous areas in the 
Indian Ocean in which every island is a coral formation raised to the 
height to which the waves can throw up fragments and the winds pile 
up sand ; and the only theory which seems to account for all the cir- 
cumstances embraced, is that of the subsidence of vast regions in this 
ocean. ‘‘ As mountain after mountain and island after island slowly 
sunk beneath the water,” he says, “fresh bases would be successively 
afforded for the growth of the corals. I venture to defy any one to 
explain in any other manner how it is possible that numerous islands 
should be distributed throughout vast areas, all the islands being low, 
all built of coral absolutely requiring a foundation within a limited 
depth below the surface.” 
The orztes, according to Mr. Darwin, form the most elevated 
deposits of those which are situated nearer the level of the water: 
Millepora complanata also enters into the formation of the upper 
banks. Various other branched corals present themselves in great 
numbers in the cavities left by the Porites and Afillepora crossing 
each other. It is difficult to identify living species when they live in 
the deeper parts, but, according to Darwin, the lower parts of the reefs 
are occupied by polyps ot the same species as in the upper parts ; at 
the depth of eighteen fathoms and upwards, the bottom consists 
alternately of sand and corals. The total breadth of the circular reef 
or ring which constitutes the atoll of the Keeling or Cocos Island 
varies from 200 to 500 yards in breadth. Some little parasitic isles 
form themselves upon the reefs, at 200 or 300 yards from their 
exterior edge, by the accumulation of the fragments thrown up here 
during great storms. They rise from two to three yards above the 
sea-level, and consist of dead shells, corals, and sea urchins, the 
whole consolidated into hard and solid rock. 
The description of the Island of Cocos or Keeling is as follows :— 
“The ring-formed reef of the lagoon island is surmounted, in the 
greater part of its length, by linear islets. On the northern, or lee- 
ward side, there is an opening through which vessels can pass to the 
anchorage within. On entering, the scene was very curious, and 
rather pretty; its beauty, however, entirely depended on the brilliancy 
of the surrounding colours. The shallow, clear, and still water of the 
lagoon resting in its greater part on white sand, is, when illumined 
by a vertical sun, of the most vivid green. This brilliant expanse, 
several miles in width, is on all sides divided, either by a line of 
snow-white breakers from the dark heaving waters of the ocean, or 
from the blue vault of heaven by the strips of land crowned by the 
N 2 
