. The elements of geology, for popular use : containing a description of the geological formations and mineral resources of the United States. Geology; Geology. 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 bee
. The elements of geology, for popular use : containing a description of the geological formations and mineral resources of the United States. Geology; Geology. 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,. Please note that these images are extracted from scanned page i
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