56 FLORIDA REEFS. 
by Captain Woodbury of Tortugas, with the view of determining the rate of 
growtla of corals, when taken up in 1858 had a crust of Ma?andrina upon it a 
little more than half an inch in thickness. Mr. Allen also sent me from Key 
West a number of fragments 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 vaiy 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 gathered 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 should 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 ago 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, 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 over- 
rating the rapidity of the progress, and our calculation of the period that 
must have elapsed in the formation of the reef will he far within the truth. 
The outer reef, still incomplete, as I have stated, and therefore of course 
somewhat lower tlian 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 oixter 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 
