FORMATION OF CORAL REEFS. 151 



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 



