THE FLORIDA REEFS. 85 
tom river carrying its silt to the steep slope south of Hatteras, 
depositing occasionally a few patches of green sand along the 
sides of its course, while the upper waters are perfectly clear and 
of the deepest blue. 
Corals alone cannot supply all the sand we know to be carried. 
by the Gulf Stream. We must add to this the mass of silt, mud, 
and sand which come from pelagic animals, and which is dis- 
tributed by the winds and waves, to be spread uniformly over 
large areas, as is well shown from the distribution of the immense 
mass of caleareous ooze over the whole of the bottom between 
Florida and Cuba. This mass undoubtedly owes its existence 
in part to the silt which the Gulf Stream brings from the south- 
eastern edge and slope of the Yucatan Bank, in part to the ac- 
cumulation of the pelagic fauna which the same great current 
sweeps along its course. The amount of work done by the ani- 
mals living upon the reef in preparation for the grinding process 
of the breakers is very great. All writers upon the reefs have 
referred to the destructive agency of boring mollusks, annelids, 
and echinoderms, that riddle the coral branches and heads with 
holes, and prepare the way for their fracture into larger or 
smaller fragments. 
The echinoderms found upon the flats seem to live almost 
exclusively upon the organic matter and foraminifera they find 
mixed with the coral sand, upon which they feed, and which 
fills their digestive cavity. Their action, however, while an 
important one, in that they reduce the sand to a smaller size, is 
yet very slight as compared to the action of the breakers upon 
the sea-face of the reef. Darwin and others have referred to 
this agency of the echinoderms as one among those at work in 
triturating the corals. By some observers, these animals are 
supposed to be browsing on the living coral. This is not the 
case either with holothurians and Diadematide or with elypeas- 
teroids: living on flats, they swallow the sand as they find it. 
But with Cidaris and Echinometra, which dig out holes in the 
coral rock, the case is different. H. H. Guppy has also ob- 
served the holothurians full of sand on the flats of the reefs of 
the Solomon Islands. 
The Loggerhead, Bird, and Bush Key banks, which protect 
