THE TORTUGAS AND FLORIDA REEFS. 125 



and prevailing winds. Next come the clusters of coral heads, huge masses of Astra3- 

 ans and of IVfeandrina, very limited in their distribution at the Tortugas, as well as 

 the more or less extensive patches of Madrepora puhmda, and finally what is known 

 as broken ground, namely, the outer edge of the reef occupied mainly by clusters 

 of Gorgonite, which also reach upward into the shallower region. Occasional patches 

 may be seen also of Astrasans, Madrepores, and other reef-builders, wdiich have 

 extended below the depths at which they generally flourish, and where they are 

 soon killed or choked by the accumulation of fine coral sand, and coralline sand or 

 ooze of the deeper waters. This sediment fills the broad and narrow flat channels 

 dividing the three great banks which compose the Tortugas, or separate the inner 

 shoals, banks, and islands. Finally come in the lines of broken coral heads and 

 branches, mixed with dead corallines, shells of Mollusks, old Serpulaj tubes,^ Gor- 

 gonia? stalks, and the like. These form a low dike, as it were, to be little by little 

 pounded up by the breakers into smaller fragments, and carried, either by the winds, 

 or waves, or currents, into the interior of the reefs, there to form sand flats of more 

 or less coarse materials, until on the western faces of the banks the finest detritus 

 is deposited in very steep slopes, constantly shifting like those of sand dunes, and, 

 like them, running forward and backward at the will of the winds and waves. 

 This continues until the particles have become cemented together by the action of 

 the carbonic acid contained in excess in the salt water surrounding the reefs, and 

 the gluing of the slight amount of animal matter which holds these particles together. 

 Some of the slopes (according to General Wright, of the Engineers) are as great as 

 thirty-three degrees. All this fine material, composed of fragments of all sizes of 

 every animal and plant with a calcareous skeleton, of course prevents the growth 

 of corals in positions which are not well scoured, either by the action of the tides 

 or of the prevailing winds. The corals when alive are gradually buried under this 

 mass of material constantly passing over them, and held in suspense. They flourish 

 therefore only where the disturbing elements are reduced to a minimum ; namely, on 

 steep banks or on the slopes which are scoured by tides, or on flats at considerable 

 depths, over which a large body of water can freely pass, whether brought by the 

 tides or driven by the winds. In such cases the corals can grow gradually towards 

 the surface as fast as the sediment deposited has closed up the circulation of the 

 lower levels. The quantity of calcareous matter held in suspense in the water in 

 the vicinity of a reef, and on the reef itself, is very great. The breakers pounding 



> Serpiilae often form incrasting masses of considerable extent, acting, as has been noticed by Darwin, much as the patches 

 of Nullipores do in protecting decayed and dead corals from being too rapidly broken to pieces by the action of tlie waves. 



