On Coral Reefs and Islands. 347 
cones; and while some have the broken declivities that are 
marks of age, others have regular slopes, as if but just now ex- 
tinguished’; a dozen of these cones may sometimes be seen on a 
single island. These volcanic peaks often rise out of the sea, as 
if their formation had begun with a submarine eruption. In a 
_Tegion so extensively and so recently igneous, the coral polyp 
would have found little chance to develop: itself, until volcanic 
action had become comparatively _ and deluges of hot water 
ceased. There appears, therefore, to be some reason for-the fact 
that the reefs are small, and have sinc reached the surface. 
he Sooloo Sea is ‘but one of the volcatie clusters in these 
as. Java, several of the Philippines, and other mye south 
of these yet with the northern shore of New Guine up 
a wide region of fires, and it cannot be doubted that me fredteat 
eruptions buen the growth of coral, for a long period, over 
large areas. For other causes we must look to the nature of the 
coasts, fresh-water streams and marine currents; we leave it for 
other a to apply the explanation to particular coasts. 
The coast of China probably owes its freedom from corals to 
its ery aden and its fresh-water streams. 
One interesting fact should be noted:—the most extensive 
reefs in the East Indies are to be found in the open seas, between 
the large islands; these islands, at the same time, often being 
Without proper reefs, or with mere traces of coral. . This is the 
& 
few degrees, is another. How far this is due to their being dis- 
tant from the scenes of igneous action, and from the detritus a 
fresh-water of island streams, remains to be determined. A sin 
ing island becomes a more and more favorable spot for the reer 
of coral, as it descends; for as its extent diminishes, its streams 
of fresh-water and decile also decrease. 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 medargone thong the fact of a greater sub- 
Sidence is by no means impossi 
In the Indian Ocean, the Asiatic coast is mostly free from 
growing coral.* ‘I'he great rivers of the Continent are probabl 
the most efficient cause of their absence, both directly, through 
their fresh waters, aud through the detritus they transport t and 
distribute _— the shores. It will be observed that this agent, 
80 ineffectual on small islands, is one of vast influence ae 
larger taeda: Ceylon has some feinging reefs. 
* Mr. Darwin alludes to small patches in the Persian Gulf. 
