FORMATION OF CORAL REEFS. 151 
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have brought down to us unchanged all the char- 
acters that superstition hallowed in those early 
days. The stony face 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 mutability 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 effect. 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 deter- 
mine its duration? Within what limits, by what 
standard, may it be measured? Shall hundreds, 
or thousands, or hundreds of thousands, or mil- 
lions 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 in- 
vestigating with especial care respecting the for- 
mation and growth of the Coral Reefs of Florida. 
But first a few words on Coral Reefs in general. 
They are living limestone walls 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 
