CHAPTER IV. 



GEOLOGICAL CONCLUSIONS FROM THE STRUCTURE AND COM- 

 POSITION OF CORAL REEFS AND ISLANDS. 



The geological bearing of the facts that have been detailed, 

 has probably been already perceived by our readers. A brief re- 

 capitulation, however, may afford a convenient review of the 

 subject. The following are the points of more special interest. 



I. The coral reef -rock has been described as solid limestone of 

 coral origin. In some parts it is a coral conglomerate, or breccia, 

 made up of fragments firmly cemented. Over much larger areas 

 it is a fine white limestone, as compact as any secondary marble, 

 and as homogeneous in texture. It is often free from any traces 

 of organic life, or proofs of an organic origin. Only now and 

 then an imbedded shell or some other relic evinces that animals 

 of any kind were living in the seas. This white limestone 

 breaks with a conchoidal fracture, a splintery surface, and rings 

 under the hammer. These facts are of great importance in de- 

 ciding upon the origin of the older limestone strata. Other por- 

 tions of the rock, of less extent, are made of standing corals with 

 the intervals filled in by reef-debris, and the whole cemented solid. 

 The latter variety here mentioned prevails in the inner patches 

 growing in quiet waters. The former kind is common about outer 

 reefs, since large areas in the coral plantation are mere sand. It is 

 still more abundant, forming the bottom among the inner patches, 

 or in the lagoons, where the finer detritus is washed by the sea. 

 A glance at the chart of the Feejees or at the Kingsmills, (a copy 

 of which, from Capt. Wilkes's Narrative is inserted in this vol- 

 ume,) will show how large a portion of the reef increases from 

 these fine accumulations. The exterior of a coral island, for a 

 few hundred yards, excepting some islets within, is the only part 

 which is the proper growth of the living reef. Within the ex- 

 terior reef the coral structure may consist almost wholly of the 

 compact homogeneous white limestone we have described. The 

 elevated island of Metia was for a long time after elevation ex- 

 posed to the ravages of the sea, before the present shore-reefs ac- 

 cumulated to give it protection. Proofs of degradation along the 

 coast have been referred to. There is much reason, therefore, 

 for believing that the Metia now existing, exposes on its eastern 

 and southern sides at least (where particularly examined by us) 

 the interior of the original structure ; and this view is supported 

 by the compact character of the rock. 



