. Elementary physical geography;. LAND AND WATER 49 cles it. The islands themselves are popularly known ascoral islands. The coral polyps, of whose mineral remains these islands and reefsare formed, are an animal growth not unlike a tree with its mouths of the polyp completely cover its upper surface somewhatas the flowers of the hollyhock or mullein cluster about the stem. Ina single community the growth of the polyp is chiefly upward, but wherethe communities are thickly clustered, their branches interlock andfinally form a compact mass. The living portion of the coral reef isat
. Elementary physical geography;. LAND AND WATER 49 cles it. The islands themselves are popularly known ascoral islands. The coral polyps, of whose mineral remains these islands and reefsare formed, are an animal growth not unlike a tree with its mouths of the polyp completely cover its upper surface somewhatas the flowers of the hollyhock or mullein cluster about the stem. Ina single community the growth of the polyp is chiefly upward, but wherethe communities are thickly clustered, their branches interlock andfinally form a compact mass. The living portion of the coral reef isat the surface of the water or a few feet below it; the dead parts mayextend a hundred fathoms or more below the AN ATOLL, ENCLOSING A LAGOON It is thought that the coral polyps began their growthon the slopes of the volcanic peaks, and that the lattergradually subsided until they were covered by the while the peak was slowly sinking the coral polypssteadily built their reefs upward, keeping the top alwayseven with the wash of the waves. This opinion, first madeprominent by Darwin, is borne out by the fact that, whilethe coral polyp cannot live more than twenty fathoms 50 PHYSICAL GEOGRAPHY below the surface of the sea, the feets sometimes extendalmost vertically to a depth of several hundred fathoms. In other instances it seems certain that the accumulationof marine remains raised the ocean floor to a level uponwhich the coral polyp could live and grow. Whateverbuilding may have been clone above the surface of thereefs and islands, is the work of the waves. As a rule, these islands consist of an irregular ring of reefwaste, broken and tossed up by the waves. The reef is callei Ian atoll
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