172 



CORAL ISLANDS. 



Fig. 39. 



a a, the habitable part of the island. 6, the lagoon. 



islands are owing to their being the crusts of sub- 

 marine volcanoes, Mr. Lyell states that every island 

 yet examined in the w^ide region termed Eastern 

 bceanica, consists either of volcanic rocks or coral 

 limestones ; and that in some of them, as in Gam- 

 bier's Group, rocks of porous lava actually rise up 

 in the centre. That many of these islands have 

 been raised from the sea by volcanic action, there 

 can be no doubt ; for on the sumtnit of the highest 

 mountain in Tahiti, an island composed almost en- 

 tirely of volcanic rocks, there is a distinct stratum 

 of fossil coral, resembling that of modern reefs. In 

 addition, we may state that MM. Quoy and Gaim- 

 ard describe the shores of Coupang and Timon as 

 formed of coral beds twenty-five or thirty feet in 

 thickness, and that above these repose vertical beds 

 of slate, traversed by quartz ; also, that in the Isle 

 of France, a coral bed ten feet thick occurs be- 

 tween two lava-currents. 



One circumstance in relation to these coral isl- 

 ands is worth remarking, and that is the deep, nar- 

 row passage which almost invariably leads from the 

 sea to the lagoon in the centre. This is kept open 

 by the water rushing in during high tide and rush- 

 ing out again at low tide, and with such force 

 as to prevent the coral animals from raising their 

 structure. In the same manner, the deepest chan- 

 nel of our harbour (Gedney's Channel) is kept open 

 by the strong ebb tide which sweeps out the sand, 

 which would otherwise soon block it up, and de- 

 posites it in the ocean. But if we follow up the 

 Hudson to the Overslaugh, near Albany, where the 

 tide is scarcely felt, we find the channel constantly 

 blocking up v/ith sand, and obstructing navigation, 



