184 AGE OF CORAL REEFS. 
Some of my readers may ask why the Reef 
does not rise evenly to the level of the sea, and 
form a continuous line of land, instead of here 
and 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 differ- 
ent 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 influ- 
ences 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 
between them, while the inner one will long 
before that time become solidly united to the 
present shore-bluffs of Florida by the consolida- 
tion of the mud-flats, which will one day trans- 
form the inner channel into dry land. 
What is now the rate of growth of these Coral 
Reefs? We cannot, perhaps, estimate it with 
absolute accuracy, since they are now so nearly 
completed ; but Coral growth is constantly spring- 
ing 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, 
