s and the comminution of the brittle ones, which would subtract
so largely from the actual rate of growth, let us double this estimate
and call the average increase a foot for every century. In so doing, we
are no doubt greatly overrating the rapidity of the progress, and our
calculation of the period that must have elapsed in the formation of the
Reef will be far within the truth.

The outer Reef, still incomplete, as I have stated, and therefore of
course somewhat lower than the inner one, measures about seventy feet in
height. Allowing a foot of growth for every century, not less than seven
thousand years must have elapsed since this Reef began to grow. Some
miles nearer the main-land are the Keys, or the inner Reef; and though
this must have been longer in the process of formation than the outer
one, since its growth is completed, and nearly the whole extent of its
surface is transformed into islands, with here and there a narrow break
separating them, yet, in order to keep fully within the evidence of the
facts, I will allow only seven thousand years for the formation of this
Reef also, making fourteen thousand for the two.

This brings us to the shore-bluffs, consisting simply of another Reef
exactly like those already described, except that the lapse of time has
united it to the main-land by the complete filling up and consolidation
of the channel which once divided it from the extremity of the
peninsula, as a channel now separates the Keys from the shore-bluffs,
and the outer Reef, again, from the Keys. These three concentric Reefs,
then, the outer Reef, the Keys, and the shore-bluffs, if we measure
the growth of the two latter on the same low estimate by which I have
calculated the rate of progress of the former, cannot have reached their
present condition in less than twenty thousand years. Their growth must
have been successive, since, as we have seen, all Corals need the fresh
action of the open sea upon them, and if either of the outer Reefs had
begun to grow before the completion of the inner one, it would have
effectually checked the growth of the latter. The absence of an
incipient Reef outside of the outer Reef shows these conclusions to be
well founded. The islands capping these three do not exceed in height
the level to which the fragments accumulated upon their summits may have
been thrown by the heaviest storms. The highest hills of this part of
Florida are not over ten or twelve feet above the leve