. 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. 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 be
. 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. 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,. Please note that these images are extracted from scanned page
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Keywords: ., bookcentury1800, bookdecade1840, booksubjectgeology, bookyear1846
