Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . island becomes lower and smaller, and the space between the edge ofthe reef, r, and the beach proportionately broader. A section of thereef and island, after a subsidence of several hundred feet, is given in figure 69. The formerliving margin of the reef,r, is now dead coral,dragged down to depthsat which the polypescease to exist; but their progeny continue in active life at r, now themargin of a barrier reef, separated by the lagoon channel n, from theremnant of the l
Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . island becomes lower and smaller, and the space between the edge ofthe reef, r, and the beach proportionately broader. A section of thereef and island, after a subsidence of several hundred feet, is given in figure 69. The formerliving margin of the reef,r, is now dead coral,dragged down to depthsat which the polypescease to exist; but their progeny continue in active life at r, now themargin of a barrier reef, separated by the lagoon channel n, from theremnant of the land b. Let the island go on subsiding, and thecoral reef will continue growing up on its own foundation, whilst thewater gains on the land, until the highest point is covered, and there remains a perfect atoll,of which figure 70. re-presents a vertical sec-tion. In this diagramr is the living and^*°- growing outer margin of the encircling reef, and the lagoon channel is now converted into thecalm central lake n, of the atoll. Thus by the process of subsidence /• ? X n .- 65 .. b# \W^ ^-^snui^Q ^-^^^-^ Barrier * CXXXII. POLYPI. 143 the fringing reef (^figs 68.) is converted into the barrier reef {^fig. 69.),and this into the atoll (^^. 70.). If the movement of the land should now be reversed, and the levelof the sea be again brought back by elevation of the island, to theline (s. \fig. 70.), an island apparently composed exclusively of coralrock, like Elizabeth Island, would be the result. The prodigious extent of the combined and unintermitting laboursof these little world-architects must be witnessed in order to beadequately conceived. They have built up a barrier-reef alongthe shores of New Caledonia for a length of 400 miles, andanother which runs along the north-east coast of Australia 1000 milesin extent. To take a small example, a single atoll may be 50 milesin length by 20 in breadth; so that if the ledge of coral rock formingthe ring were extended in one line
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