AGE OF CORAL REEFS. 185 
however, would give us far too high a standard ; 
for the rise of the Uoral Reef is not in propor- 
tion to the height of the living Corals, but to 
their solid parts which never decgmpose. 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 
