of the peninsula.

Outside the Keys, but not separated from them by so great a distance as
that which intervenes between them and the main-land, there stretches
beneath the water another Reef, abrupt, like the first, on its seaward
side, but sloping gently toward the inner Reef, and divided from it by
a channel. This outer Reef and channel are, however, in a much less
advanced state than the preceding ones; only here and there a sand-flat
large enough to afford a foundation for a beacon or a lighthouse shows
that this Reef also is gradually coming to the surface, and that a
series of islands corresponding to the Keys must eventually be formed
upon its summit. Some of my readers may ask why the Reef does not rise
evenly to the level of the sea, and form a continuous line of land,
instead of here and there an island. This is accounted for by the
sensitiveness of the Corals to any unfavorable circumstances impeding
their growth, as well as by the different rates of increase of the
different kinds. Wherever any current from the shore flows over the
Reef, bringing with it impurities from the land, there the growth of the
Corals will be less rapid, and consequently that portion of the Reef
will not reach the surface so soon as other parts, where no such
unfavorable influences have interrupted the growth. But in the course
of time the outer Reef will reach the surface for its whole length and
become united to the inner one by the filling up of the channel between
them, while the inner one will long before that time become solidly
united to the present shore-bluffs of Florida by the consolidation of
the mud-flats, which will one day transform the inner channel into dry
land.

What is now the rate of growth of these Coral Reefs? We cannot, perhaps,
estimate it with absolute accuracy, since they are now so nearly
completed; but Coral growth is constantly springing up wherever it can
find a foothold, and it is not difficult to ascertain approximately the
rate of growth of the different kinds. Even this, however, would give
us far too high a standard; for the rise of the Coral Reef is not in
proportion to the height of the living Corals, but to their solid parts
which never decompose. Add to this that there are many brittle delicate
kinds that have a considerable height when alive, but contribute to the
increase of the Reef only so much additional thickness as they would
have when broken and crushed down upon its surface. A fore