st in its
decay does not add to the soil of the earth a thickness corresponding to
the height of its trees, but only such a thin layer as would be left by
the decomposition of its whole vegetation. In the Coral Reef, also, we
must allow not only for the deduction of the soft parts, but also for
the comminution of all these brittle branches, which would be broken and
crushed by the action of the storms and tides, and add, therefore, but
little to the Reef in proportion to their size when alive.

The foundations of Fort Jefferson, which is built entirely of Coral
rock, were laid on the Tortugas Islands in the year 1846. A very
intelligent head-work man watched the growth of certain Corals that
established themselves on these foundations, and recorded their rate of
increase. He has shown me the rocks on which Corals had been growing for
some dozen years, during which they had increased at the rate of about
half an inch in ten years. I have collected facts from a variety of
sources and localities that confirm this testimony. A brick placed under
water in the year 1850 by Captain Woodbury of Tortugas, with the view of
determining the rate of growth of Corals, when taken up in 1858 had
a crust of Maeandrina upon it a little more than half an inch in
thickness. Mr. Allen also sent me from Key West a number of fragments
of Maeandrina from the breakwater at Fort Taylor; they had been growing
from twelve to fifteen years, and have an average thickness of about an
inch. The specimens vary in this respect,--some of them being a little
more than an inch in thickness, others not more than half an inch.
Fragments of Oculina gathered at the same place and of the same age are
from one to three inches in length; but these belong to the lighter,
more branching kinds of Corals, which, as we have seen, cannot, from
their brittle character, be supposed to add their whole height to the
solid mass of the Coral wall. Millepore gives a similar result.

Estimating the growth of the Coral Reef according to these and other
data of the same character, it should be about half a foot in a century;
and a careful comparison which I have made of the condition of the Reef
as recorded in an English survey made about a century ago with its
present state would justify this conclusion. But allowing a wide margin
for inaccuracy of observation or for any circumstances that might
accelerate the growth, and leaving out of consideration the decay of the
soft part