172 



CORAL ISLANDS. 



Fig. 39, 



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



islands are owing to their being the crusts of sub- 

 marine volcanoes, Mr. Lyell states that every island 

 yet examined in the wide region termed Eastern 

 Oceanica, 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 summit 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 with sand, and obstructing navigation, 



