310 CORALS AND CORAL ISLANDS. 



The amount of subsidence determines in some cases the dis- 

 tance of barrier reefs from shore ; but it by no means accounts 

 for the difference in their extent in different parts of a single 

 group of islands. Indeed, if this cause be considered alone, 

 every grade of extent, from no subsidence to the largest amount, 

 might in many instances be proved as having occurred on a 

 single island. Of far greater importance, as has appeared, is 

 the volcanic character of the land. At whatever time the ex- 

 isting reefs in the Pacific commenced their growth, they be- 

 gan about those of the igneous islands whose fires had become 

 nearly or quite extinct ; and as others in succession were ex- 

 tinguished, these became in their turn, the sites of corals, and 

 of coral reefs. Those lands whose volcanoes still burn, are 

 yet without corals, or there are only limited patches on some 

 favored spots. Zoophytes and volcanoes are the land-making 

 agents of the Pacific. The latter prepare the way by pour- 

 ing forth the liquid rock, and building up the lofty summit. 

 Quiet succeeds, and then commences the work of the zoophyte 

 beneath the sea, while verdure covers the exposed heights. 



"We may add a few more illustrations from other parts of 

 the coral-reef seas. 



Along the north and northwest coast of Australia, there 

 appears to be little or no coral in the Gulf of Carpentaria, 

 while some extensive patches occur on the shores west of this 

 Gulf, as far as the northwest cape in latitude 23° S. 



In the East Indies, there are large, scattered reef-islands 

 south of Borneo and Celebes, about some of the Molluccas, 

 and near the west end of New Guinea. The islands of Timor- 

 laut, and Timor, with many of those intermediate, have large 

 reefs. The Arru Group consists wholly of coral. This sea, 

 from Arru, to the islands south of Borneo, is more thriving in 

 corals than anv other in the East Indies. 



