42 FLOlllDA KEEPS. 
early days. The stony ftice of the Sphinx is not more true to its past, nor 
the massive architecture of the Pyramids more unchanged, than they are. 
But the advocates of the mutabihty of species say truly enough that the 
most ancient traditions are but as yesterday in the world's history, and 
that what six thousand years could not do sixty thousand years might eifect. 
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 ? Witliin what limits, by what standard, 
may it be measured ? Shall hundreds, or thousands, or hundreds of thou- 
sands, 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 Avails, 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, or oblong, accord- 
ing 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 round it, thus encircling it with a ring of coral growth. 
The atolls in the Pacific Ocean, those circular islands enclosing sometimes 
a fresh-water lake in mid-ocean, are coral walls of this kind, that have 
formed a ring around a central island. 
Tills 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, already surveyed with some accuracy by 
Professor Dana, during the United States Exploring Expedition, it is sub- 
siding ; 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, 
