GEOGRAPHICAL DISTRIBUTION. 267 



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 Group is but one of the vol- 

 canic 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 

 anything more than isolated corals, for a long period, over 

 each of these 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 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, 

 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 distant from the scenes of igneous action, and from the 

 detritus and fresh water of island streams, remains to be deter- 

 mined. A sinking island becomes a more and more favourable 

 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. 

 Still the fact of a greater subsidence is not impossible or 

 improbable. 



In the India7i Ocean^ the Asiatic coast is mostly free from 

 growing coral. The great rivers of the continent are probably 



