AGE OF COKAL REEFS. 185 



however, would give us far too high a standard^ 

 for the rise of the Coral Reef is not in propor- 

 tion 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 in- 

 telligent head-workman watched the growth of 

 certain Corals that established themselves 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 



