For a long time it was supposed that the Coral animals inhabited very deep waters, for they were sometimes brought up on sounding-lines from a depth of many hundreds or even thousands of feet, and it was taken for granted that they must have had their home where they were found; but the facts recently ascertained respecting the subsidence of ocean-bottoms have shown that the foundation of a Coral wall may have sunk far below the place where it was laid, and it is now proved beyond a doubt that no Reef-Building Coral can thrive at a depth of more than fifteen fathoms, though Corals of other kinds occur far lower, and that the dead Reef-Corals sometimes brought to the surface from much greater depths are only broken fragments of some Reef that has subsided with the bottom on which it was growing.
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