AGE OF CORAL REEFS. 187 



jnth 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 vears 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 

 tht; two. 



