42 FLORIDA REEFS. 



early days. The stony fiice 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 historj^, and 

 that what six thousand years could not do sixty thousand years might effect. 

 Leavino- aside, then, all historical chronology, how far back can Ave 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 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 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 have a variety 

 of outlines : they may be straight, circidar, semicircular, or oblong, accord- 

 in"- 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. 



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, 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 grow^th in the corals, the island and the wall 

 itself will disappear beneath the ocean. But whenever, on the contrary, 



