FLORIDA REEFS. 55 
tliere an island. This is accounted for by the sensitiveness of the corals to 
any unfavorable circumstances imiieding 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, where 
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, w-hich will one day transform the inner channel into diy land. 
What is now the rate of growth of these coral reefs ? We cannot, per- 
haps, estimate it with 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 decomj)ose. 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, Avhich is built entirely of coral rock, 
were laid on the Tortugas Islands in the ^-ear 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, 
