56 FLORIDA REEFS. 



by Captain AVooclbury of Tortugas, with the view of determining the rate of 

 growth of corals, when taken up in 1858 had a crust of Ma^andrina upon it a 

 Httle more than half an inch in thickness. Mr. Allen also sent me from Key 

 West a number of fras-ments of Ma^andrina from the breakwater at Fort 

 Taylor ; they had been growing from twelve to fifteen years, and have an 

 average thickness of about an inch. The specimens vary in this respect, — 

 some of them being a little more than an inch in thickness, others not more 

 than half an inch. Fragments of Oculina gatliered at the same place and 

 of the same age are from one to three inches in height and width ; but these 

 belong to the lighter, more branching kinds of corals, which, as we have seen, 

 cannot, from their brittle character, be supposed to add their whole height to 

 the solid mass of the coral wall. Millepora gives a similar result. 



Estimating the growth of the coral reef according to these and other 

 data of the same character, it shoidd be about half a foot in a century ; and 

 a careful comparison which I have made of the condition of the reef as 

 recorded in an English survey made about a century jigo with its present 

 state would justify this conclusion. 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, ^vhich 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 over- 

 rating the rapidity of the progress, and our calculation of the period that 

 must have elapsed in the formation of the reef w^ill 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 seventj'' 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 transformed into 

 islands, with here and there a narrow 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. 



This brings us to the shore-bluffs, consisting simply of another reef ex- 

 actly like those already described, except that in course of time it has been 



