GEOGRAPHICAL DISTRIBUTION. 349 
doubted that the frequent eruptions prevented the growth of 
any thing more than isolated corals, for a long period, over 
each of these areas. For other causes we must look to the na- 
ture of the coasts, fresh-water streams, and marine currents ; 
we leave it for other investigators to apply the explanation to 
particular coasts. 
The coast of China owes its freedom from corals to the 
cool temperature of the waters, the coast being wholly outside, 
as has been stated, of the coral-reef seas. 
One interesting fact should be noted:—the most extensive 
reefs in the East Indies are to be found in the open seas, be- 
tween the large islands; these islands, at the same time, often 
being without proper reefs, or with mere traces of coral. This 
is the case between Borneoand the range of large islands south ; 
the China Sea is another instance of it; north of New Guinea, 
a few degrees, is another. How far this is due to their being 
distant from the scenes of igneous action, and from the detri- 
tus and fresh water of island streams, remains to be deter- 
mined. A sinking island becomes a more and more favorable 
spot for the growth of coral, as it descends; for as its extent 
diminishes, its streams of fresh water and detritus also de- | 
crease. It might, therefore, be expected, on this account alone, 
that such isolated spots of land, away from all impure waters, 
in the open ocean, should become the bases of large reefs. 
The existence of these reef-islands is, therefore, no necessary 
proof of greater subsidence than the coast adjoining has un- 
dergone. Still the fact of a greater subsidence is not im- 
possible or improbable. 
In the Indian Ocean, the Asiatic coast is mostly free from 
growing coral. The great rivers of the continent are probably 
the most efficient cause of their absence, both directly, through 
their fresh waters, and through the detritus they transport and 
