No More Learning

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 But though fifteen fathoms is the maximum depth at
which any reef-builder can prosper, there are many which will
not sustain even that degree of pressure; and this fact has, as
we shall see, an important influence on the           of the reef.