FLORIDA KEEPS. 55 



there an island. This is accounted for by the sensitiveness of the corals to 

 any unfavorable circumstances impeding their growth, as well as by the 

 different rates of increase of their different kinds. Wherever any current 

 from the shore flows over the reef, bringing with it impurities from the land, 

 there the growth of the corals will be less rapid, and consequently that 

 portion of the reef will not reach the surface so soon as other parts, Avhere 

 no such unfavorable influences have interrupted the growth. But in the 

 course of time the outer reef will reach the surface for its whole length, 

 and become united to the inner one by the filling up of the channel be- 

 tween them, while the inner one will long before that time become solidly 

 united to the present shore-bluffs of Florida by the consolidation of the mud- 

 flats, Avhich will one day transform the inner channel into dry land. 



What is now the rate of growth of these coral reefs ? We cannot, per- 

 haps, estimate it Avith absolute accuracy, since they are now so nearly com- 

 pleted ; but coral growth is constantly springing up wherever it can find a 

 foothold, and it is not difficult to ascertain approximately the rate of growth 

 of the different kinds. Even this, however, would give us far too high a 

 standard ; for the rise of the coral reef is not in proportion to the height 

 of the living corals, but to their solid parts which never decompose. Add 

 to this that there are many brittle, delicate kinds that have a considerable 

 height when alive, but contribute to the increase of the reef only so much 

 additional thickness as their branches would have if broken and crushed 

 down upon its surface. A forest in its decay does not add to the soil of the 

 earth a thickness corresponding to the height of its trees, but only such a 

 thin layer as would be left by the decomposition of its whole vegetation. 

 In the coral reef, also, we must allow not only for the deduction of the 

 soft parts, but also for the comminution of all these little branches, which 

 would be broken and crushed by the action of the storms and tides, and add, 

 therefore, but little to the reef in proportion to their size when alive. 



The foundations of Fort Jefferson, which is built entirely of coral rock, 

 were laid on the Tortugas Islands in the year 1846. A very intelligent 

 head-workman watched the growth of certain corals that established them- 

 selves on these foundations, and recorded their rate of increase. He has 

 shown me the rocks on which corals had been growing for some dozen 

 years, during which they had increased at the rate of about half an inch in 

 ten years. I have collected facts from a variety of sources and localities 

 that confirm this testimony. A brick placed under water, in the year 1850, 



