THE FLORIDA REEFS 83 
stated already by Darwin, Dana, and others, that по coral reef can 
grow above the level of the lowest tides, and that all subsequent 
additions of material must be due to accumulation of sediment 
transported by the action of the tides and prevailing winds. 
Next come the clusters of coral heads, huge masses of astraans 
and of meandrina, very limited in their distribution at the Tor- 
tugas, as well as the more or less extensive patches of Madrepora 
palmata (Fig. 51), and finally what is known as broken ground, 
namely, the outer edge of the reef occupied mainly by clusters 
of gorgonie (Fig. 52), which also reach upward into the shal- 
lower region. Occasional patches may be seen of astraeans, ma- 
drepores, and other reef-builders, which have extended below the 
depths at which they generally flourish, where they are soon 
killed or choked by the accumulation of fine coral sand and cor- 
alline 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 the lines of broken coral heads 
and branches, mixed with dead corallines, shells of mollusks, old 
serpule tubes, gorgonie 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 ce- 
mented together by the action of the carbonic acid contained in 
excess in the salt water surrounding the reefs, and by the gluing 
of the slight amount of animal matter which holds these parti- 
cles 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 every animal 
! Serpule often form inerusting masses and dead corals from being too rapidly 
of considerable extent, acting, as has been broken to pieces by the action of the 
noticed by Darwin, much as the patches waves. 
of nullipores do in protecting decayed 
