106 ON CORAL REEFS AND ISLANDS. 



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 

 region so extensively and so recently igneous, the coral polyp 

 would have found little chance to develop 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 instance of it ; north of New Guinea, a 

 few degrees, is another. How far this is due to their being dis- 

 tant from the scenes of igneous action, and from the detritus and 

 fresh- water of island streams, remains to be determined. A sink- 

 ing 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 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 undergone, though the fact of a greater sub- 

 sidence is by no means impossible. 



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 

 distribute along the shores. It will be observed that this agent, 

 so ineffectual on small islands, is one of vast influence upon 

 larger lands. Ceylon has some fringing reefs. 



* Mr. Darwin alludes to small patches in the Persian Gulf. 



