AGE OF CORAL REEFS. 187 
with its present state would justify this conclu- 
sion. But, allowing a wide margin for inaccu- 
racy of observation or for any circumstances that 
might accelerate the growth, and leaving out of 
consideration the decay of the soft parts and the 
comminution of the brittle ones, which would 
subtract so largely from the actual rate of growth, 
let us double this estimate and call the average 
increase a foot for every century. In so doing, 
we are no doubt greatly overrating the rapidity 
of the progress, and our calculation of the period 
that must have elapsed in the formation of the 
Reef will be far within the truth. 
The outer Reef, still incomplete, as I have 
stated, and therefore of course somewhat lower 
than the inner one, measures about seventy feet 
in height. Allowing a foot of growth for every 
century, not less than seven thousand years must 
have elapsed since this Reef began to grow. 
Some miles nearer the main-land are the Keys, 
or the inner Reef; and though this must have 
been longer in the process of formation than the 
outer one, since its growth is completed, and 
nearly the whole extent of its surface is trans- 
formed into islands, with here and there a nar- 
row break separating them, yet, in order to keep 
fully within the evidence of the facts, I will allow 
only seven thousand years for the formation of 
this Reef also, making fourteen thousand for 
the two. 
