may ask why the Reef does not rise evenly to the level of the sea, and form a continuous line of land, instead of here and there an island.  This is accounted for by the sensitiveness of the Corals to any unfavorable circumstances impeding their growth, as well as by the different rates of increase of the different kinds.  Wherever any current from the shore flows over the Reef, bringing with it impurities from the land, there the growth of the Corals will be less rapid, and consequently that portion of the Reef will not reach the surface so soon as other parts, where no such unfavorable influences have interrupted the growth.  But in the course of time the outer Reef will reach the surface for its whole length and become united to the inner one by the filling up of the channel between them, while the inner one will long before that time become solidly united to the present shore-bluffs of Florida by the consolidation of the mud-flats, which will one day transform the inner channel into dry land.

What is now the rate of growth of these Coral Reefs?  We cannot, perhaps, estimate it with absolute accuracy, since they are now so nearly completed; but Coral growth is constantly springing up wherever it can find a foothold, and it is not difficult to ascertain approximately the rate of growth of the different kinds.  Even this, however, would give us far too high a standard; for the rise of the Coral Reef is not in proportion to the height of the living Corals, but to their solid parts which never decompose.  Add to this that there are many brittle delicate kinds that have a considerable height when alive, but contribute to the increase of the Reef only so much additional thickness as they would have when broken and crushed down upon its surface.  A forest in its decay does not add to the soil of the earth a thickness corresponding to the height of its trees, but only such a thin layer as would be left by the decomposition of its whole vegetation.  In the Coral Reef, also, we must allow not only for the deduction of the soft parts, but also for the comminution of all these brittle branches, which would be broken and crushed by the action of the storms and tides, and add, therefore, but little to the Reef in proportion to their size when alive.

The foundations of Fort Jefferson, which is built entirely of Coral rock, were laid on the Tortugas Islands in the year 1846.  A very intelligent head-work man watched the growth of certain Corals that established 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 Woodbury of Tortugas, with the view of determining the rate of growth of Corals, when taken up in 1858 had a crust