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themselves on these foundations, and recorded their rate of increase. He 

 has shown me the rocks on which Corals had been growing for some dozen 

 years, during which they had increased at the rate of about half-an-inch in 

 ten years. I have collected facts from a variety of sources and localities 

 that confirm this testimony. A brick placed under water, in the year 1850, 

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

 of growth of Corals, when taken up, in 1858, had a crust of Mseandrina 

 upon it a little more than half-an-inch in thickness. Mr. Allen also sent me 

 from Key West a nimiber of fragments of Mseandrina 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 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. Millepore 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 

 inaccuracy of obsei-vation, 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 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 70 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 

 mUes nearer the mainland 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 

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



