144 CORAL FORMATIONS. 
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 region so extensively and so recently igneous, the coral 
polyp would have found little chance to develope itself, until volcanic 
action had become comparatively quiet, and deluges of hot water 
ceased. ‘There appears, therefore, to be some reason for the fact that 
the reefs are small, and have seldom reached the surface. 
The Sooloo Sea is but one of the volcanic clusters in these seas. 
Java, several of the Philippines, and other islands south of these last, 
with the northern shore of New Guinea, make up a wide region of 
fires, and it cannot be doubted that the frequent eruptions prevented 
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 investigators to apply the 
explanation to particular coasts. 
The coast of China probably owes its freedom from corals to its 
alluvial character 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 case between Borneo 
and the range of large islands south: the China Sea is another in- 
stance 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 detritus and fresh water of island streams, remains to be 
determined. A sinking island becomes a more and more favourable 
spot for the growth of coral, as it descends; for as its extent dimi- 
nishes, its streams of fresh water and detritus 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 be- 
come the bases of large reefs. ‘The existence of these reef-islands is, 
therefore, no necessary proof of greater subsidence than the coast ad- 
joining has undergone, though the fact of a greater subsidence is by 
no meaus impossible. 
In the Indian Ocean, the Asiatic coast is mostly free from growing 
coral.* The great rivers of the continent are probably the most efh- 
cient cause of their absence, both directly, through their fresh waters, 
and through the detritus they transport and distribute along the shores. 
* Mr. Darwin alludes to small patches in the Persian Gulf. 
