THE FLORIDA REEFS. 85 



torn 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 

 larofe areas, as is well shown from the distribution of the immense 

 mass of calcareous ooze over the whole of the bottom between 

 Florida and Cuba. This mass undovibtedly 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 frao^ments. 



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 Diadematidse or with clypeas- 

 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 



