ct.
Leaving aside, then, all historical chronology, how far back can we
trace our own geological period, and the Species belonging to it? By
what means can we determine its duration? Within what limits, by what
standard, may it be measured? Shall hundreds, or thousands, or hundreds
of thousands, or millions of years be the unit from which we start?

I will begin this inquiry with a series of facts which I myself have
had an opportunity of investigating with especial care respecting the
formation and growth of the Coral Reefs of Florida. But first a few
words on Coral Reefs in general. They are living limestone walls that
are built up from certain depths in the ocean by the natural growth of a
variety of animals, but limited by the level of high-water, beyond which
they cannot rise, since the little beings that compose them die as soon
as they are removed from the vitalizing influence of the pure sea-water.
These walls have a variety of outlines: they may be straight, circular,
semicircular, oblong, according to the form of the coast along which
the little Reef-Builders establish themselves; and their height is, of
course, determined by the depth of the bottom on which they rest. If
they settle about an island on all sides of which the conditions for
their growth are equally favorable, they will raise a wall all around
it, thus encircling it with a ring of Coral growth. The Athols in the
Pacific Ocean, those circular islands inclosing sometimes a fresh-water
lake in mid-ocean, are Coral walls of this kind, that have formed a ring
around a central island. This is easily understood, if we remember that
the bottom of the Pacific Ocean is by no means a stable foundation
for such a structure. On the contrary, over a certain area, which has
already been surveyed with some accuracy by Professor Dana, during the
United States Exploring Expedition, it is subsiding; and if an island
upon which the Reef-Builders have established themselves be situated
in that area of subsidence, it will, of course, sink with the floor on
which it rests, carrying down also the Coral wall to a greater depth in
the sea. In such instances, if the rate of subsidence be more rapid than
the rate of growth in the Corals, the island and the wall itself will
disappear beneath the ocean. But whenever, on the contrary, the rate of
increase in the wall is greater than that of subsidence in the island,
while the latter gradually sinks below the surface, the forme